Shahin Internship Report on Non-Destructive Testing in Saj Engineering and Trading Company

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Practicum Report On Non-Destructive Testing (NDT) Submitted To Registrar IUBAT—International University of Business Agriculture and Technology

Submitted By

1. Md. Shahin Manjurul Alam

ID# 07207013

Program: BSME

December 14, 2010

IUBAT-International University of Business Agriculture & Technology 1 [email protected]

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Non-Destructive Testing (NDT)

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Request for the Report December 14, 2010

Engr. Abdul Wadud Faculty and Course Coordinator Department of Mechanical Engineering CEAT- College of Engineering and Technology IUBAT- International University of Business Agriculture and Technology 4, Embankment Drive Road, Uttara Model Town, Sector 10, Dhaka 1230, Bangladesh.

Subject: Request for the report.

Dear Sir With due respect, I would like to submit this report as partial fulfillment of the BSME program, the topic of ―Non-Destructive Testing (NDT)‖. It was superlative opportunity for me to work on this topic to actualize my theoretical knowledge in the practical area and to have an enormous experience on that system. Now I am looking forward for your kind assessment regarding this report.

I would be very kind of you, if you please take the trouble of going through the report and evaluate my performance regarding this report.

Sincerely Yours, ….………………… 1. Md. Shahin Manjurul Alam ID # 07207014 BSME 3 [email protected]

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Letter of Transmittal December 14, 2010

Engr. Abdul Wadud Faculty and Course Coordinator Department of Mechanical Engineering CEAT- College of Engineering and Technology IUBAT- International University of Business Agriculture and Technology 4, Embankment Drive Road, Uttara Model Town, Sector 10, Dhaka 1230, Bangladesh.

Subject: Letter of Transmittal of the Practicum Report.

Dear Sir I have pleasure in submitting the practicum report on “Non-Destructive Testing (NDT)”. According to your requirement I had worked in Saj Engineering & Trading Company. It was a challenging work because, in our country Saj Engineering & Trading Company is the only one company which has the latest equipments for NDT services and expert NDT practitioners. It was certainly a great opportunity for me to work on this paper to actualize my theoretical knowledge in the practical arena.

Though there were many hindrances arose during I was conducting data and information for this project, I tried my level best to achieve our goal to make a realistic and informative research paper.

Thank you, Sir Sincerely Yours, ….…………………….. 2. Md. Shahin Manjurul Alam ID # 07207014 BSME

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SAJ ENGINEERING & TRADING COMPANY 205/5 Elephant Road (1st floor) Dhaka-1205, Bangladesh Phone: +88 02 9677628, +88 02 8616859 Fax: +88 02 9677625 www.sajetc.com

To Whom It May Concern

This is to certify that Md. Shahin Manjurul Alam, student of IUBAT has continuing his Internship program with us from 1st October, 2010 till today. The subject matter of the internship program was Non-Destructive Testing (NDT).

During his internship he followed instructions according to the satisfaction of the management. He was very keen to learn the lessons and enthusiastic in completing any assignment that was given to him time to time.

We wish all the best for his future endeavors.

----------------------------Jahangir Kabir CEO Saj Engineering & Trading Company

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Student's Declaration This is to inform that the Practicum Report on “Non-Destructive Testing (NDT)” has only been prepared as a partial fulfillment of the Bachelor of Science in Mechanical Engineering (BSME) Program. I hereby declare that the project embodied in this report in the result of my own handwork and has not been submitted for another degree to another university.

Authors,

….……………………… 1. Md. Shahin Manjurul Alam ID # 07207014 BSME

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Acknowledgement This Practicum Report which is entitled as “Non-Destructive Testing (NDT)” is the concrete effort of a number of people. In the process of conducting this research project, I would like to express my gratitude and respect to some generous persons for their immense help and enormous cooperation.

First of all I would like to pay my gratitude to Honorable Vice Chancellor Prof. Dr. M. Alimullah Miyan for giving me chance to prepare my research about this splendid topic.

I am very much grateful to some of my faculties specially Engr. Abdul Wadud, Respected Course Coordinator of ME department of IUBAT, for his helping hand. I also say my warmest thanks to Engr. Sarwar Iqbal, respected Faculty of ME department of IUBAT who had taken many courses. I would like to thanks Engr. Amirul Islam for his painstaking guidance and constant inspiration to do this report. After that I would like to express my special gratitude to Md. Jahangir Kabir, CEO of Saj Engineering & Trading Company, Engr. Amit Hasan, Service Engineer and Ferdous Ahmed Marketing Executive of Saj Engineering & Trading Company for their keen interest and valuable suggestions regarding preparing this report. I will never forget Engr. Md. Rashedul Alam, Marico Bangladesh Ltd, who recommended us to do our internship on Non-Destructive Testing in Saj Engineering Trading Company.

Finally I also feel it is important to acknowledge and thanks to my classmates especially to those who participated in the data collection and who helped a lot to provide a valuable forum for the exchange of ideas and information.

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Company Profile Saj Engineering & trading company was established on 1998 and trying to develop specializing

on the supply full range of Non-Destructive Testing (NDT) equipments and its consumables to Aviation, fertilizer, power generation, shipbuilding, Defense, training institute and NDT service provider etc.

Company Goal It is our goal to provide NDT practitioners with quality on the latest technology at reasonable prices and ensure that these are readily available at our customer's convenience and satisfaction. Through the years, we have endeavored to represent only the well-known manufacturers of NDT equipment in the world.

Our Customer 

BIMAN BANGLADESH AIR LINES.



BANGLADESH AIR FORCE.



BANGLADESH ARMY.



BANGLADESH NAVY.



BANGLADESH SHIPPING CORPORATION.



BANGLADESH INLAND WATER TRANSPORT AUTHORITY.



BANGLADESH INLAND WATER TRANSPORT CORPORATION



DOCKYARD & ENGINEERING WORKS LTD.



KHULNA SHIPYARD LTD.



BANGLADESH POWER DEVELOPMENT BOARD.



CHITTAGONG PORT AUTHORITY.



ATOMIC ENERGY CENTER



ENGINEERING UNIVERSITY



ALL INSPECTION COMPANY IN BANGLADESH

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Our Sole Agent 

MITSUBISHI HEAVY INDUSTRIES MARINE LTD. JAPAN



DAIKAI ENGINEERING PTE LTD. SINGAPORE (Daihatsu Diesel co ltd.)



IHI MARINE CO LTD. JAPAN



ISS MACHINERY CO LTD. JAPAN



MacGREGOR (SGP) PTE LTD. SINGAPORE



SKL MOTOREN-UND SYSTEMTECHNIK,GERMANY



ROSTOCK DIESEL GmbH, GERMANY



RS ―UNISCHIFF GmbH‖ GERMANY



VRM SERVICES, SINGAPORE



OLYMPUS SINGAPORE PTE LTD.



OLYMPUS NDT , CANADA



SIMPLEX MARINE PTE LTD(BLOM +VOSS,GERMANY)



BRANDNER ENGINEERING –AIRCON SAVER, GERMANY



PT SELAMAT SEMPURNA TBK(SAKURA), INDONESIA



ITW, INDIA(ITW, USA- FORTUNE 200 COMPANY)

 ICM, BELGIUM  Emerson, USA

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Table of Contents Topic Name 1. Preparatory Part:

Page

A. Title Fly…………………………………………………………………………..…… 10 B. Topic Name…………………………………..…………………………….………… 10 C. Request for Report ………………………………………………………...………… 10 D. Letter of Transmittal…………………………............................................................. 10 E. To whom it May we concern …………………………................................................ 10 F. Student Declaration …………………………...............................................................10 G. Acknowledgement …………………………………………………………...…….…10 H. Table of Content ………………………………………………………………… 10- 10 J. Executive Summary …………………………………………………………………...10

2. Text of the Report Introduction 2.1 2.2 2.2.1 2.2.2 2.3 2.4 2.5

Origin of the report Objective Broad Objective Specific Objectives Background Methodology Limitations

3.

Company Overview 3.1 Company Profile…………………………………………………………….1

4.

3.1

Company Goal………………………………………………………..1

3.2

Our Customer……………………………………………………….…………..1

3.3

Our Sole Agent…………………………………………………………………2

Introduction of Non-destructive Testing (NDT)………………………….3 4.1 4.3 4.5

History of NDT……………………………………………………….…3 Importance of NDT……………………………………………………...4 NDT Methods…………………………………………………………...5 10 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com 4.6

Six Most Common NDT Methods……………………………………....5 4.6.1 Visual and Optic Testing (VT)…………………………………………..6 4.6.2 Dye Penetrant Testing (DPT)……………………………………………6 4.6.3 Magnetic Particle Testing (MPT)………………………….……………..6 4.6.4 Eddy Current Testing (ECT)…………………………………….………6 4.6.5 Radiography Testing (RT)……………………………………….………7 4.6.6

5.

6.

7.

Ultrasonic Testing (UT)…………………………….………………7

Visual Testing (VT) 5.1

Introduction………………………………………………………………7

5.1

Physical Principle………………………………………………………..8

5.3

Inspection Requirements………………………………………………..9

5.4

Practical Considerations………………………………………………..10

Dye Penetrant Testing (DPT) 6.1

Introduction Dye Penetrant Testing…………………………………………...10

6.2

History Dye Penetrant Testing………………………………………..………..11

6.3

DPT for Detectability of Flaws…………………………………..…………….11

6.4

Process for Dye Pretrant Testing………………………………………………..12

6.5

Common Uses of Dye Penetrant Testing………………………………………..14

6.6

Effectiveness of Dye Penetrant Testing…………………………………………15

6.7

Advantages of Dye Penetrant Testing………………………………………….16

6.8

Disadvantages of Dye Penetrant Testing ……………………………………….17

Magnetic Particle Testing (MPT) 7.1

Introduction of Magnetic Particle Testing………………………………………17

7.2

History of Magnetic Particle Testing……………………………………………18 11 [email protected]

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8.

7.3

Basic Principles…………………………………………………………………19

7.4

Magnetic Field Orientation and Flaw Detectability…………………………….20

7.5

Portable Equipments for Magnetic Particle Testing……………………….……22

7.6

Permanent magnets……………………………………………………………..22

7.7 7.8

Electromagnets…………………………………………………………………..23 Prods……………………………………………………………………………..24

7.9

Portable Coils and Conductive Cables………………………………………….25

7.10

Portable Power Supplies………………………………………………………...25

7.11

Lights for Magnetic Particle Inspection…………….………………………….26

7.12

Dry Particle Inspection…………………………………………………………27

7.13

Examples of Dry Magnetic Particle Inspection…………………………………27

7.14 7.15

Advantages of Magnetic Particle Testing………………………………………28 Disadvantages of Magnetic Particle Testing……………………………………28

Eddy Current Testing (ECT) 8.1

Introduction of Eddy Current Testing…………………………………………29

8.2

History of Eddy Current Testing……………………………………………….30

8.3

Present State of Eddy Current Inspection………………………………………30

8.4

Research to Improve Eddy current measurements………………………………31

8.4.1 Photoinductive Imaging (PI)……………………………………………………32 8.4.1 Pulse Eddy Current……………………………………………………………..32 8.5

Eddy Current Instruments………………………………………………………32

8.6

Probes - Mode of Operation…………………………………………………….33 8.6.1 Absolute Probes…………………………………………………………34 8.6.2 Differential Probes……………………………………………………...34 12 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com 8.6.3 Reflection Probes………………………………………………………..35 8.6.4 Hybrid Probes…………………………………………………………..35 8.8.5 Probes – Configurations………………………………………………..36 8.8.6 Surface Probes………………………………………………………….36 8.8.7 Bolt Hole Probes……………………………………….……………….37 8.8.8 ID or Bobbin Probes……………………………………………………37 8.8.9 OD or Encircling Coils………………………………………………….38 8.9 8.10

Surface Breaking Cracks…………………………………………………………38 Surface Crack Detection Using Sliding Probes………………………………….40

8.11

Probe Types……………………………………………………………………...40 8.11.1 Fixed Sliding Probes…………………………………………………….40 8.11.2 Adjustable Sliding Probes……………………………………………….40

8.12

Reference Standards………………………………………………………….41

8.13

Inspection Variables……………………………………………………………42 8.13.1 Liftoff signal Adjustment……………………………………………...42 8.13.2 Scan Patterns…………………………………………………………..42 8.13.3 Signal Interpretation…………………………………………...………42 8.13.4 Probe Scan Deviation…………………………………………………..43 8.13.5 Crack Angle Deviation…………………………………………………43 8.13.6 Electrical Contact……………………………………………………....44

8.14

Tube Inspection by Eddy Current…………………………….………………..44

8.15

Thickness Measurements of Thin Material…………………….……………….45

8.16

Corrosion Thinning of Aircraft Skins………………………….……………….45 13 [email protected]

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9.

8.17

Thickness Measurement of Thin Conductive Sheet, Strip and Foil…….……...46

8.18

Thickness Measurement of Thin Conductive Layers…………………………..47

8.19

Pulsed Eddy Current Inspection……………………………………………….47

8.20

EC Standards and Methods…………………………………………………….48

Radiography Testing (RT) 9.1

History of Radiography…………………………………………………………50

9.2

A Second Source of Radiation…………………………………………………51

9.3

Health Concerns………………………………………………………………..52

9.4

Present State of Radiography……………………………………………….….53

9.5

Future Direction of Radiographic Education…………………………………..54

9.6

Properties of X-Rays and Gamma Rays………………………………………..55 9.6.1 X-Radiation……………………………………………………………..55 9.6.1 Bremsstrahlung Radiation………………………………………………56 9.6.1 Gamma Radiation……………………………………………………….57

9.7

Types Radiation Produced by Radioactive Decay………………………………57 9.7.1

Alpha Particles…………………………………………………………58

9.7.2

Beta Particles…………………………………………………………..58

9.7.3

Gamma-rays…………………………………………………………..58

9.8

Filters in Radiography………………………………………………………….58

9.9

Radiation Safety………………………………………………………………..59

9.10

Radiographic Film………………………………………………………………60 9.10.1

Film Selection………………………………………………………….61

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Internship report on Non-Destructive Testing www.sajetc.com 9.10.2

Film Packaging…………………………………………………………62

9.10.3

Film Handling………………………………………………………….63

9.10.4

Film Processing………………………………………………………..63 9.10.4.1

Manual Processing & Darkrooms…………………………..64

9.10.4.2

Automatic Processor Evaluation……………………………65

9.11

Radiograph Interpretation – Welds……………………………………………..65

9.12

Discontinuities…………………………………………………………………..66

9.13

Welding Discontinuities…………………………………………………………66 9.13.1

Cold Lap……………………………………………………………….66

9.13.2

Porosity………………………………………………………………...67

9.13.3

Cluster porosity…………………………………………………………67

9.13.4

Slag inclusions…………………………………………………………68

9.13.5

IP and LOP……………………………………………………………..69

9.13.6

Incomplete fusion………………………………………………………69

9.13.7

Internal concavity or suck back………………………………………..70

9.13.8

Internal or root undercut……………………………………………….70

9.13.9

External or crown undercut…………………………………………….71

9.13.10 Offset or mismatch…………………………………………………….72 9.13.11 Inadequate weld reinforcement………………………………………72 9.13.12 Excess weld reinforcement……………………………………………73 9.13.13 Cracks…………………………………………………………………73 9.13.14

Discontinuities in TIG welds………………………………………...74

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Tungsten inclusions…………………………………………………..74

9.13.16

Oxide inclusions……………………………………………………...74

9.13.17

Discontinuities in Gas Metal Arc Welds (GMAW)…………………..75

9.13.18 Burn-Through…………………………………………………………75

10.

9.14

Real-time Radiography…………………………………………………………76

9.15

Advantages of Radiography……………………………………………………76

9.16

Disadvantages of Radiography…………………………………………………76

Ultrasonic Testing (UT) 10.1

Introduction of Ultrasonic Testing………………………………………………77

10.2

Basic Principles of Ultrasonic Testing…………………………………………..77

10.3

History of Ultrasonics…………………………………………………………..79

10.4

Present State of Ultrasonics…………………………………………………….80

10.5

Future Direction of Ultrasonic Inspection………………………………………82

10.6

Wavelength and Defect Detection………………………………………………83

10.7

Sound Propagation in Elastic Materials…………………………………………85

10.8

Speed of Sound…………………………………………………………………85

10.9

Applications of Non-Destructive Testing………………………………………..86

10.10 Piezoelectric Transducers……………………………………………………….88 10.11 Characteristics of Piezoelectric Transducers……………………………………90 10.12

Radiated Fields of Ultrasonic Transducers…………………………………….91

10.13

Transducer Types………………………………………………………………93 10.13.1

Contact Transducers…………………………………………………94

10.13.2

Immersion transducers……………………………………………….94

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Internship report on Non-Destructive Testing www.sajetc.com 10.13.3

More on Contact Transducers………………………………………..94

10.13.4

Dual element transducer…………………………………………….95

10.13.5

Delay line transducers……………………………………………….95

10.13.6

Angle beam transducers……………………………………………..96

10.13.7

Normal incidence shear wave transducers……………….………….96

10.13.8

Paint brush transducers……………………………………………...96

10.14

Couplant………………………………………………………………………..97

10.15

Pulser-Receivers………………………………………………………………..97

10.16

Angle Beams I………………………………………………………………….99

10.17 Angle Beams II………………………………………………………………….99

11.

10.18

Calibration Methods……………………………………………………………100

10.19

Weldments (Welded Joints)……………………………………………………101

10.20

Advantages of Ultrasonic Flaw Detection…………………………………….103

10.21

Disadvantages of Ultrasonic Flaw Detection…………………………………104

Applications of Non-Destructive Testing………………………………104 11.1

Aerospace Industry……………………………………….………………….104

11.2

Aircraft Overhaul………………………………………………….…………104

11.3

Automotive Industry……………………………………………….………...104

11.4

Petrochemical & Gas Industries………………………………………….…..104

11.5

Railway Industry…………………………………………………….…..…...104

11.6

Mining Industry………………………………………………………….…...104

11.7

Agricultural Engineering. ………………………………………….………....104

11.8

Power Generation…………………………………………………………….105

11.9

Iron Foundry………………………………………………………………….105

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Internship report on Non-Destructive Testing www.sajetc.com 11.10 Shipbuilding Industry…………………………………………………………105 11.11 Steel Industry…………………………………………………………………105 11.12 Pipe & Tube Manufacturing Industry………………………………………...105

12.

Recommendation…………………………………………………………………..105

13.

Conclusion…………………………………………………………………………..106

4

References…………………………………………………………………………...106

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Executive Summary Non-destructive testing in its present form has been carried out, by specialised service companies and manufacturers, for decades. Röntgen Technische Dienst bv in The Netherlands was established more than 60 years ago, in 1937, and that year marked the beginning of radiographic inspection of welds in The Netherlands. Similar situations exist in other countries. In fact, welding industry would not have experienced the growth and the wide range of applications it has today if there were no such thing as NDT. NDT has a very important formal status. Requirements for performance of NDT, acceptance criteria and requirements for personnel qualification are implemented in codes and standards. The NDT procedure is part of the contract. During the many years that NDT methods have been used in industry a well-established situation has evolved, enabling the use of NDT for the evaluation of welds against Good Workmanship Criteria on a routine basis, thus maintaining workmanship standards and minimising the risks of component failure. In addition, NDT plays an important part in industrial maintenance. During plant shutdowns for instance, many thousands of ultrasonic wall thickness measurements are taken on piping, vessels, furnace tubes etc. All these thickness readings have to go into extensive data bases, and this process is, thanks to modern computers and data loggers, ever more automated. The ultimate aim was, to find a way to accept and reject weld defects on the basis of their significance for weld integrity. For let us be honest: in conventional NDT we are doing something completely different. We base our judgement on density differences on a film, or on echo amplitudes on a screen. Parameters that have very little to do indeed with significance of defects for weld integrity. In maintenance practice, we base our decisions on NDT that is performed during shutdowns. A significant amount of money could be saved if we would have NDT methods that minimize the time required for that shutdown, or, a step further.

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4. Introduction of Non-destructive Testing (NDT) Non-destructive Testing is one part of the function of Quality Control and is complementary to other long established methods. By definition non-destructive testing the use of noninvasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object. It is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. The technique can be applied on a sampling basis for individual investigation or may be used for 100% checking of material in a production quality control system. Whilst being a high technology concept, evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture - from steel making to site inspection of components already in service. A certain degree of skill is required to apply the techniques properly in order to obtain the maximum amount of information concerning the product, with consequent feed back to the production facility. Non-destructive Testing is not just a method for rejecting substandard material; it is also an assurance that the supposedly good is good. The technique uses a variety of principles; there is no single method around which a black box may be built to satisfy all requirements in all circumstances.

4.1 History of NDT Nondestructive testing has been practiced for many decades, with initial rapid developments in instrumentation spurred by the technological advances that occurred during World War II and the subsequent defense effort. During the earlier days, the primary purpose was the detection of defects. As a part of "safe life" design, it was intended that a structure should not develop macroscopic defects during its life, with the detection of such defects being a cause for removal of the component from service. In the early 1970's, two events occurred which caused a major change in the NDT field. First, improvements in the technology led to the ability to detect small flaws, which caused more parts to be rejected even though the probability of component failure had not changed. However, the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size will fail under a particular load when a material's fracture toughness properties are known. Other laws were developed to predict the growth rate of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for the new philosophy of "damage tolerant" design. Components having known defects could 20 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com continue in service as long as it could be established that those defects would not grow to a critical, failure producing size. A new challenge was thus presented to the nondestructive testing community. Detection was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics based predictions of remaining life. The need for quantitative information was particularly strongly in the defense and nuclear power industries and led to the emergence of quantitative nondestructive evaluation (QNDE) as a new engineering/research discipline. A number of research programs around the world were started, such as the Center for Nondestructive Evaluation at Iowa State University (growing out of a major research effort at the Rockwell International Science Center); the Electric Power Research Institute in Charlotte, North Carolina; the Fraunhofer Institute for Nondestructive Testing in Saarbrucken, Germany; and the Nondestructive Testing Centre in Harwell, England.

4.3 Importance of NDT NDT plays an important role in the quality control of a product. It is used during all the stages of manufacturing of a product. It is used to monitor the quality of the: 1. Raw materials which are used in the construction of the product. 2. Fabrication processes which are used to manufacture the product. 3. Finished product before it is put into service. Use of NDT during all stages of manufacturing results in the following benefits: 1. It increases the safety and reliability of the product during operation. 2. It decreases the cost of the product by reducing scrap and conserving materials, labor and energy. 3. It enhances the reputation of the manufacturer as producer of quality goods. All of the above factors boost the sales of the product which bring more economical benefits to the manufacturer. NDT is also used widely for routine or periodic determination of quality of the plants and structures during service. This not only increases the safety of operation but also eliminates any forced shut down of the plants.

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4.5 NDT Methods The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. The methods covered are:  Visual Testing  Microwave  Thermography  Magnetic Particle Testing  Tap Testing  Radiography Testing  Acoustic Microscopy  Acoustic Emission  Magnetic Measurements  Ultrasonic Testing  Flux Leakage  Laser Interferometry  Eddy Current  Dye Penetrant Testing

4.6 Six Most Common NDT Methods There are six NDT methods that are used most often. They are 1. Visual and Optical Testing (VT) 2. Dye Penetrant Testing (DPT) 3. Magnetic Particle Testing(MPT) 4. Eddy Current Testing(ECT) 5. Radiography Testing (RT) 6. Ultrasonic Testing (UT)

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4.6.1 Visual and Optic Testing (VT) Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.

4.6.2 Dye Penetrant Testing (DPT) Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.

4.6.3 Magnetic Particle Testing (MPT) This NDT method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and nearsurface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw.

4.6.4 Eddy Current Testing (ECT) Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the materials conductive and permeability properties, can be detected with the proper equipment.

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4.6.5 Radiography Testing (RT) Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections. An X-ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other imaging media. The resulting shadowgraph shows the dimensional features of the part. Possible imperfections are indicated as density changes on the film in the same manner as medical X-ray shows broken bones.

4.6.6 Ultrasonic Testing (UT) Ultrasonic use transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.

5. Visual Testing (VT) 5.1

Introduction of Visual Testing

Visual inspection is by far the most common nondestructive testing (NDT) technique. When attempting to determine the soundness of any part or specimen for its intended application, visual inspection is normally the first step in the examination process. Generally, almost any specimen can be visually examined to determine the accuracy of its fabrication. For example, visual inspection can be used to determine whether the part was fabricated to the correct size, whether the part is complete, or whether all of the parts have been appropriately incorporated into the device. While direct visual inspection is the most common nondestructive testing technique, many other NDT methods require visual intervention to interpret images obtained while carrying out the examination. For instance, penetrant inspection using visible red or fluorescent dye relies on the inspector‘s ability to visually identify surface indications. In arriving at a definition of visual inspection, it has been noted in the literature that experience in visual inspection and discussion with experienced visual inspectors revealed that this NDT method includes more than 24 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com use of the eye, but also includes other sensory and cognitive processes used by inspectors. Thus, there is now an expanded definition of visual inspection in the literature: ―Visual inspection is the process of examination and evaluation of systems and components by use of human sensory systems aided only by mechanical enhancements to sensory input as magnifiers, dental picks, stethoscopes, and the like. The inspection process may be done using such behaviors as looking, listening, feeling, smelling, shaking, and twisting. It included a cognitive component wherein observations are correlated with knowledge of structure and with descriptions and diagrams from service literature.‖ The human eye is one of mankind‘s most fascinating tools and is capable of assessing many visual characteristics and identifying various types of discontinuities.

5.2

Physical Principle

The human eye is one of mankind‘s most fascinating tools. It has greater precision and accuracy than many of the most sophisticated cameras. It has unique focusing capabilities and has the ability to work in conjunction with the human brain so that it can be trained to find specific details or characteristics in a part or test piece. It has the ability to differentiate and distinguish between colors and hues as well. The human eye is capable of assessing many visual characteristics and identifying various types of discontinuities. The eye can perform accurate inspections to detect size, shape, color, depth, brightness, contrast, and texture. Visual testing is essentially used to detect any visible discontinuities, and in many cases, visual testing may locate portions of a specimen that should be inspected further by other NDT techniques. Many inspection factors have been standardized so that categorizing them as major and minor characteristics has become common. Surface finish verification of machined parts has even been developed, and classification can be performed by visual comparison to manufactured finish standards. In the fabrication industry, weld size, contour, length, and inspection for surface discontinuities are routinely specified many companies have mandated the need for qualified and certified visual weld inspection. This is the case particularly in the power industry, which requires documentation of training and qualification of the inspector. Forgings and castings are normally inspected for surface indications such as laps, seams, and other various surface conditions.

5.3

Inspection Requirements 25 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com Requirements for visual inspection typically pertain to the vision of the inspector; the amount of light falling on the specimen, which can be measured with a light meter; and whether the area being inspected is in anyway obstructed from view.In many cases, each of these requirements is detailed in regulatory code or other inspection criteria. Mechanical and/or optical aids may be necessary to perform visual testing. Because visual inspection is so frequently used, several companies now manufacture gages to assist visual inspection examinations. Mechanical aids include: measuring rules and tapes, calipers and micrometers, squares and angle measuring devices, thread, pitch and thickness gages, level gages, and plumb lines. Welding fabrication uses fillet gages to determine the width of the weld fillet, undercut gages, angle gages, skew fillet weld gages, pit gages, contour gages, and a host of other specialty items to ensure product quality. At times direct observation is impossible and remote viewing is necessary which requires the use of optical aids. Optical aids for visual testing range from simple mirrors or magnifying glasses to sophisticated devices, such as closed circuit television and coupled fiber optic scopes. The following list includes most optical aids currently in use :  Mirrors (especially small, angled mirrors).  Magnifying glasses, eye loupes, multilens magnifiers, measuring magnifiers.  Microscopes (optical and electron).  Optical flats (for surface flatness measurement).  Borescopes and fiber optic borescopes.  Optical comparators.  Photographic records  Closed circuit television (CCTV) systems (alone and coupled to borescopes/microscopes).  Machine vision systems.  Positioning and transport systems (often used with CCTV systems).  Image enhancement (computer analysis and enhancement).

5.4

Practical Considerations 26 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com Visual inspection is applicable to most surfaces, but is most effective where the surfaces have been cleaned prior to examination, for example, any scale or loose paint should be removed by wire brushing, etc. Vision testing of an inspector often requires eye examinations with standard vision acuity cards such as Jaeger, Snellen, and color charts. Vision testing of inspectors has been in use for about 40 years. Although many changes in NDT methods have taken place over the years and new technologies have been developed, vision testing has changed little over time. Also little has been done to standardize vision tests used in the industrial sector. For those seeking certification in the area of visual testing, (Visual and Optical Testing) provides a useful reference.

6.

Dye Penetrant Testing (DPT)

6.1

Introduction Dye Penetrant Testing

This method is frequently used for the detection of surface breaking flaws in non ferromagnetic materials. The subject to be examined is first of all chemically cleaned, usually by vapors phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks. Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrant into the crack during this period. The surplus penetrant on the surface is then removed completely and thin coating of powdered chalk is applied. After a further period (development time) the chalk Fig: Dye Penetrant Testing draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background. The process is purely a mechanical/chemical one and the various substances used may be applied in a large variety of ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic basis at the other end. The latter system requires sophisticated tanks, spraying and drying 27 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com equipment but the principle remains the same. Dye Penetrant Testing is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

6.2

History of Dye Penetrant Testing

A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, a heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. A different (though related) method was introduced in the 1940's. The surface under examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects.

6.3

DPT for Detectability of Flaws

The advantage that a Dye Penetrant Testing (DPT) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, DPT produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye. Due to the physical features of the eye, 28 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with 20/20 vision.The second way that DPT improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background also heDPTng to make the indication more easily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast between the white developer. In other words, the developer serves as a high contrast background as well as a blotter to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions.

6.4

Process for Dye Pretrant Testing 1. Surface Preparation: One of the most critical steps of a Dye Penetrant Testing is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering. 2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath. 3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

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Internship report on Non-Destructive Testing www.sajetc.com 4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water. 5. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers). 6. Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks. 7. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present. 8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

6.5

Common Uses of Dye Penetrant Testing

Dye penetrant Testing (DPT) is one of the most widely used nondestructive Testing (NDT) methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. DPT can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using DPT include the following:  Metals (aluminum, copper, steel, titanium, etc.)  Glass

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Internship report on Non-Destructive Testing www.sajetc.com  Many ceramic materials  Rubber  Plastics

DPT offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant materials can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. In the image above, visible dye penetrant is being locally applied to a highly loaded connecting point to check for fatigue cracking. Dye Penetrant Testing can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:  Fatigue cracks  Quench cracks  Grinding cracks  Overload and impact fractures  Porosity  Laps  Seams  Pin holes in welds  Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface. To learn more about the advantages and disadvantages of DPT, proceed to the next page.

6.6

Effectiveness of Dye Penetrant Testing 

small round defects than small linear defects. Small round defects are generally easier to detect for several reasons. First, they are typically volumetric defects that can trap significant amounts of penetrant. Second, round defects fill with penetrant faster than 32 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com linear defects. One research effort found that elliptical flaw with length to width ratio of 100, will take the penetrant nearly 10 times longer to fill than a cylindrical flaw with the same volume. 

deeper flaws than shallow flaws. Deeper flaws will trap more penetrant than shallow flaws, and they are less prone to over washing.



flaws with a narrow opening at the surface than wide open flaws. Flaws with narrow surface openings are less prone to over washing.



flaws on smooth surfaces than on rough surfaces. The surface roughness of the part primarily affects the removability of a penetrant. Rough surfaces tend to trap more penetrant in the various tool marks, scratches, and pits that make up the surface.



flaws with rough fracture surfaces than smooth fracture surfaces. The surface roughness that the fracture faces is a factor in the speed at which a penetrant enters a defect. In general, the penetrant spreads faster over a surface as the surface roughness increases. It should be noted that a particular penetrant may spread slower than others on a smooth surface but faster than the rest on a rougher surface.



flaws under tensile or no loading than flaws under compression loading. In a 1987 study at the University College London, the effect of crack closure on detectability was evaluated. Researchers used a four-point bend fixture to place tension and compression loads on specimens that were fabricated to contain fatigue cracks. All cracks were detected with no load and with tensile loads placed on the parts. However, as compressive loads were placed on the parts, the crack length steadily decreased as load increased until a load was reached when the crack was no longer detectable.

6.7

Advantages of Dye Penetrant Testing 

The method has high sensitivity to small surface discontinuities.



The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.



Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.



Parts with complex geometric shapes are routinely inspected.

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Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.



Aerosol spray cans make penetrant materials very portable.



Penetrant materials and associated equipment are relatively inexpensive.

6.8

Disadvantages of Dye Penetrant Testing 

Only surface breaking defects can be detected.



Only materials with a relatively nonporous surface can be inspected.



Precleaning is critical since contaminants can mask defects.



Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.



The inspector must have direct access to the surface being inspected.



Surface finish and roughness can affect inspection sensitivity.



Multiple process operations must be performed and controlled.



Post cleaning of acceptable parts or materials is required.



Chemical handling and proper disposal is required.

7.

Magnetic Particle Testing

7.1

Introduction of Magnetic Particle Testing

This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferrite steel and iron. Magnetic particle Testing (MPI) is a nondestructive testing method used for defect detection. MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective. The method is used to inspect a variety of product forms including 34 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

7.2

History of Magnetic Particle Testing

Magnetism is the ability of matter to attract other matter to itself. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent. The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not commonly used until sometime after World War I. In the early 1920‘s, William Hoke realized that magnetic particles (colored metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. The image shows a 1928 Electyro-Magnetic Steel Testing Device (MPI) made by the Equipment and Engineering Company Ltd. (ECO) of Strand, England. In the early 1930‘s, magnetic particle inspection was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad industry to inspect steam engine boilers, wheels, axles, and tracks. Today, the MPI inspection

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Internship report on Non-Destructive Testing www.sajetc.com method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check materials such as steel bar stock for seams and other flaws prior to investing machining time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in-service for a period of time. For example, many components of high performance racecars are inspected whenever the engine, drive train or another system undergoes an overhaul. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical structures.

7.3

Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole. When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.

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Internship report on Non-Destructive Testing www.sajetc.com If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

7.4

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to understand that the orientation between the magnetic lines of force and the flaw is very important. There are two general types of magnetic fields that can be established within a component. A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or electromagnets.

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Internship report on Non-Destructive Testing www.sajetc.com A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component. The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. As can be seen in the image below, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable. Alternately, transverse-type defects will not be detectable with circular magnetization. 38 [email protected]

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7.5

Portable Equipments for Magnetic Particle Testing

To properly inspect a part for cracks or other defects, it is important to become familiar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detecting a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degree angle. Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications because they disrupt more of the magnet flux.

Therefore, for proper inspection of a component, it is important to be able to establish a magnetic field in at least two directions. A variety of equipment exists to establish the magnetic field for MPI. One way to classify equipment is based on its portability. Some equipment is designed to be portable so that inspections can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility.

7.6

Permanent magnets

Permanent magnets are sometimes used for magnetic particle inspection as the source of magnetism. The two primary types of permanent magnets are bar magnets and horseshoe (yoke) magnets. These industrial magnets are 39 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com usually very strong and may require significant strength to remove them from a piece of metal. Some permanent magnets require over 50 pounds of force to remove them from the surface. Because it is difficult to remove the magnets from the component being inspected, and sometimes difficult and dangerous to place the magnets, their use is not particularly popular. However, permanent magnets are sometimes used by divers for inspection in underwater environments or other areas, such as explosive environments, where electromagnets cannot be used. Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit.

7.7

Electromagnets

Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism.

That

is,

using

an

electrical current to produce the magnetic field. An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off. They can be powered with alternating current from a wall socket or by direct current from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected. Some yokes can lift weights in excess of 40 pounds.

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P ortable yoke with battery pack 7.8

P ortable magnetic particle kit

Prods

Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods that can be used in magnetic particle inspection. Prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off. Sometimes the two prods are connected by any insulator (as shown in the image) to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections. If proper contact is not maintained between the prods and the component surface, electrical arcing can occur and cause damage to the component. For this reason, the use of prods are not allowed when inspecting aerospace and other critical components. To help prevent arcing, the prod tips should be inspected frequently to ensure that they are not oxidized, covered with scale or other contaminant, or damaged.

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Internship report on Non-Destructive Testing www.sajetc.com The following applet shows two prods used to create a current through a conducting part. The resultant magnetic field roughly depicts the patterns expected from an magnetic particle inspection of an unflawed surface. The user is encouraged to manipulate the prods to orient the magnetic field to "cut across" suspected defects.

7.9

Portable Coils and Conductive Cables

Coils and conductive cables are used to establish a longitudinal magnetic field within a component. When a preformed coil is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used to energize the coil. Conductive cables are wrapped around the component. The cable used is typically 00 extra flexible or 0000 extra flexible. The number of wraps is determined by the magnetizing force needed and of course, the length of the cable. Normally, the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere-turns. Ampere-turns is the amperage shown on the amp meter times the number of turns in the coil.

Conductive Cable

Portable Coil

7.10 Portable Power Supplies

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Internship report on Non-Destructive Testing www.sajetc.com Portable power supplies are used to provide the necessary electricity to the prods, coils or cables. Power supplies are commercially available in a variety of sizes. Small power supplies generally provide up to 1,500A of half-wave direct current or alternating current when used with a 4.5 meter 0000 cable. They are small and light enough to be carried and operate on either 120V or 240V electrical service. When more power is necessary, mobile power supplies can be used. These units come with wheels so that they can be rolled where needed. These units also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half-wave DC.

7.11 Lights for Magnetic Particle Inspection Magnetic particle inspection can be performed using particles that are highly visible under white light conditions or particles that are highly visible under ultraviolet light conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is required as long as the area of inspection is well lit. A light intensity of at least 1000 lux (100 fc) is recommended when visible particles are used, but a variety of light sources can be used. When fluorescent particles are used, special ultraviolet light must be used. Fluorescenc e is defined as the property of emitting radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic particle inspections are coated with a material that produces light in the visible spectrum when exposed to near-ultraviolet light. This "particle glow" provides high contrast indications on the component anywhere particles collect. Particles that fluoresce yellow-green are most common because this color matches the peak sensitivity of the human eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available.

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7.12 Dry Particle Inspection In this magnetic particle testing technique, dry particles are dusted onto the surface of the test object as the item is magnetized. Dry particle inspection is well suited for the inspections conducted on rough surfaces. When an electromagnetic yoke is used, the AC or half wave DC current creates a pulsating magnetic field that provides mobility to the powder. The primary applications for dry powders are unground welds and rough as-cast surfaces. Dry particle inspection is also used to detect shallow subsurface cracks. Dry particles with half wave DC is the best approach when inspecting for lack of root penetration in welds of thin materials. Half wave DC with prods and dry particles is commonly used when inspecting large castings for hot tears and cracks.

7.13 Examples of Dry Magnetic Particle Inspection One of the advantages that a magnetic particle inspection has over some of the other nondestructive evaluation methods is that flaw indications generally resemble the actual flaw. This is not the case with NDT methods such as ultrasonic and eddy current inspection, where an electronic signal must be interpreted. When magnetic particle inspection is used, cracks on the surface of the part appear as sharp lines that follow the path of the crack. Flaws that exist below the surface of the part are less defined and more difficult to detect. Below are some examples of magnetic particle indications produced using dry particles.

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Fig: Before and after inspection pictures of cracks emanating from a hole 7.14 Advantages of Magnetic Particle Testing (1) It does not need very stringent pre-cleaning operation. (2) Best method for the detection of fine, shallow surface cracks in ferromagnetic material. (3) Fast and relatively simple NDT method. (4) Generally inexpensive. (5) Will work through thin coating. (6) Few limitations regarding the size/shape of test specimens. (7) Highly portable NDT method. (8) It is quicker.

(9) Simplicity of operation and application.

7.15 Disadvantages of Magnetic Particle Testing (1) Material must be ferromagnetic. (2) Orientation and strength of magnetic field is critical. (3) Detects surface and near-to-surface discontinuities only. (4) Large currents sometimes required. (5) ―Burning‖ of test parts a possibility. (6) Parts must often be demagnetized, which may be difficult.

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8.

Eddy Current Testing (ECT)

8.1

Introduction of Eddy Current Testing

This method is widely used to detect surface flaws, to sort materials, to measure thin walls from one surface only, to measure thin coatings and in some applications to measure case depth. This method is applicable to electrically conductive materials only. In the method eddy currents are produced in the product by bringing it close to an alternating current carrying coil. The main applications of the eddy current technique are for the detection of surface or subsurface flaws, conductivity measurement and coating thickness measurement. The technique is sensitive to the material conductivity, permeability and dimensions of a product. Eddy currents can be produced in any electrically conducting material that is subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating magnetic field is normally generated by passing an alternating current through a coil. The coil can have many shapes and can between 10 and 500 turns of wire. The magnitude of the eddy currents generated in the product is dependent on conductivity, permeability and the set up geometry. Any change in the material or geometry can be detected by the excitation coil as a change in the coil impedance. The most simple coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a crack, for example, occurs in the product surface the eddy currents must travel farther around the crack and this is detected by the impedance change.

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8.2

History of Eddy Current Testing

Eddy Current testing has its origins with Michael Faraday's discovery of electromagnetic induction in 1831. Faraday was a chemist in England during the early 1800's and is credited with the discovery of electromagnetic induction, electromagnetic rotations, the magneto-optical effect, diamagnetism, and other phenomena. In 1879, another scientist named Hughes recorded changes in the properties of a coil when placed in contact with metals of different conductivity and permeability. However, it was not until the Second World War that these effects were put to practical use for testing materials. Much work was done in the 1950's and 60's, particularly in the aircraft and nuclear industries. Eddy current testing is now a widely used and well-understood inspection technique.

8.3

Present State of Eddy Current Inspection

Eddy current inspection is used in a variety of industries

to

find

defects

and

make

measurements. One of the primary uses of eddy current testing is for defect detection when the nature of the defect is well understood. In general, the technique is used to inspect a relatively small area and the probe design and test parameters must be established with a good understanding of the flaw that is to be detected. Since eddy currents tend to concentrate at the surface of Eddy current inspection is used in a

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Internship report on Non-Destructive Testing www.sajetc.com variety of industries to find defects and make measurements. One of the primary uses of eddy current testing is for defect detection when the nature of the defect is well understood. In general, the technique is used to inspect a relatively small area and the probe design and test parameters must be established with a good understanding of the flaw that is to be detected. Since eddy currents tend to concentrate at the surface of a material, they can only be used to detect surface and near surface defects. In thin materials such as tubing and sheet stock, eddy currents can be used to measure the thickness of the material. This makes eddy current a useful tool for detecting corrosion damage and other damage that causes a thinning of the material. The technique is used to make corrosion thinning measurements on aircraft skins and in the walls of tubing used in assemblies such as heat exchangers. Eddy current testing is also used to measure the thickness of paints and other coatings. Eddy currents are also affected by the electrical conductivity and magnetic permeability of materials. Therefore, eddy current measurements can be used to sort materials and to tell if a material has seen high temperatures or been heat treated, which changes the conductivity of some materials. Eddy current equipment and probes can be purchased in a wide variety of configurations. Eddyscopes and a conductivity tester come packaged in very small and battery operated units for easy portability. Computer based systems are also available that provide easy data manipulation features for the laboratory. Signal processing software has also been developed for trend removal, background subtraction, and noise reduction. Impedance analyzers are also sometimes used to allow improved quantitative eddy-current measurements. Some laboratories have multidimensional scanning capabilities that are used to produce images of the scan regions. A few portable scanning systems also exist for special applications, such as scanning regions of aircraft fuselages.

8.4

Research to Improve Eddy current measurements

A great deal of research continues to be done to improve eddy current measurement techniques. A few of these activities, which are being conducted at Iowa State University, are described below. 48 [email protected]

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8.4.1 Photoinductive Imaging (PI) A technique known as photoinductive imaging (PI) was pioneered at CNDE and provides a powerful, high-resolution scanning and imaging tool. Microscopic resolution is available using standard-sized eddy-current sensors. Development of probes and instrumentation for photoinductive (PI) imaging is based on the use of a medium-power (5 W nominal power) argon ion laser. This probe provides high resolution images and has been used to study cracks, welds, and diffusion bonds in metallic specimens. The PI technique is being studied as a way to image local stress variations in steel.

8.4.2 Pulse Eddy Current Research is currently being conducted on the use of a technique called pulsed eddy current (PEC) testing. This technique can be used for the detection and quantification of corrosion and cracking in multi-layer aluminum aircraft structures. Pulsed eddy-current signals consist of a spectrum of frequencies meaning that, because of the skin effect, each pulse signal contains information from a range of depths within a given test specimen. In addition, the pulse signals are very low-frequency rich which provides excellent depth penetration. Unlike multi-frequency approaches, the pulse-signals lend themselves to convenient analysis. . Measurements have been carried out both in the laboratory and in the field. Corrosion trials have demonstrated how material loss can be detected and quantified in multi-layer aluminum structures. More recently, studies carried out on three and four layer structures show the ability to locate cracks emerging from fasteners. Pulsed eddy-current measurements have also been applied to ferromagnetic materials. Recent work has been involved with measuring the case depth in hardened steel samples.

8.5

Eddy Current Instruments

Eddy current instruments can be purchased in a large variety of configurations. Both analog and digital instruments are available. Instruments are

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Internship report on Non-Destructive Testing www.sajetc.com commonly classified by the type of display used to present the data. The common display types are analog meter, digital readout, impedance plane and time versus signal amplitude. Some instruments are capable of presenting data in several display formats. The most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter. While it might actually be possible to detect some types of defects with this type of equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a few of the more important aspects of eddy current instrumentation will be discussed.

8.6

Probes - Mode of Operation

Eddy current probes are available in a large variety of shapes and sizes. In fact, one of the major advantages of eddy current inspection is that probes can be custom designed for a wide variety of applications. Eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe generally refers to the way the coil or coils are packaged to best "couple" to the test area of interest. An example of different configurations of probes would be bobbin probes, which are inserted into a piece of pipe to inspect from the inside out, versus encircling probes, in which the coil or coils encircle the pipe to inspect from the outside in. The mode of operation refers to the way the coil or coils are wired and interface with the test equipment. The mode of operation of a probe generally falls into one of four categories: absolute, differential, reflection and hybrid. Each of these classifications will be discussed in more detail below.

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8.6.1 Absolute Probes Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field. As discussed in the physics section, AC is passed through the coil and this sets up an expanding and collapsing magnetic field in and around the coil. When the probe is positioned next to a conductive material, the changing magnetic field generates eddy currents within the material. The generation of the eddy currents take energy from the coil and this appears as an increase in the electrical resistance of the coil. The eddy currents generate their own magnetic field that opposes the magnetic field of the coil and this changes the inductive reactance of the coil. By measuring the absolute change in impedance of the test coil, much information can be gained about the test material. Absolute coils can be used for flaw detection, conductivity measurements, liftoff measurements and thickness measurements. They are widely used due to their versatility. Since absolute probes are sensitive to things such as conductivity, permeability liftoff and temperature, steps must be taken to minimize these variables when they are not important to the inspection being performed. It is very common for commercially available absolute probes to have a fixed "air loaded" reference coil that compensates for ambient temperature variations.

8.6.2 Differential Probes Differential probes have two active coils usually wound in opposition, although they could be wound in addition with similar results. When the two coils are over a flaw -free area of test sample, there is no differential signal developed between the coils since they are both inspecting identical material. However, when one coil is over a defect and the other is over good material, a differential signal is produced. They have the 51 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com advantage of being very sensitive to defects yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations. Probe wobble signals are also reduced with this probe type. There are also disadvantages to using differential probes. Most notably, the signals may be difficult to interpret. For example, if a flaw is longer than the spacing between the two coils, only the leading and trailing edges will be detected due to signal cancellation when both coils sense the flaw equally.

8.6.3 Reflection Probes Reflection probes have two coils similar to a differential probe, but one coil is used to excite the eddy currents and the other is used to sense changes in the test material. Probes of this arrangement are often referred to as driver/pickup probes. The advantage of reflection probes is that the driver and pickup coils can be separately optimized for their intended purpose. The driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small defects.

8.6.4 Hybrid Probes An example of a hybrid probe is the split D, differential probe shown to the right. This probe has a driver coil that surrounds two D shaped sensing coils. It operates in the reflection mode but additionally, its sensing coils operate in the differential mode. This type of probe is very sensitive to surface cracks. Another example of a hybrid probe is one that uses a conventional coil to generate eddy currents in the material but then uses a different type of sensor to detect changes on the surface and within the test material. An example of a hybrid probe is one that uses a Hall effect sensor to detect changes in the magnetic flux leaking from the test surface. Hybrid probes are usually specially designed for a specific inspection application.

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8.7

Probes - Configurations

As mentioned on the previous page, eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe generally refers to the way the coil or coils are packaged to best "couple" to the test area of interest. Some of the common classifications of probes based on their configuration include surface probes, bolt hole probes, inside diameter (ID) probes, and outside diameter (OD) probes.

8.7.1 Surface Probes Surface probes are usually designed to be handheld and are intended to be used in contact with the test surface. Surface probes generally consist of a coil of very fine wire encased in a protective housing. The size of the coil and shape of the housing are determined by the intended use of the probe. Most of the coils are wound so that the axis of the coil is perpendicular to the test surface. This coil configuration is sometimes referred to as a pancake coil and is good for detecting surface discontinuities that are oriented perpendicular to the

test

surface.

Discontinuities,

such

as

delaminations, that are in a parallel plane to the test surface will likely go undetected with this coil configuration. Wide surface coils are used when scanning large areas for relatively large defects. They sample a relatively large area and allow for deeper penetration. Since they do sample a large area, they are often used for conductivity tests to get more of a bulk material measurement. However, their large sampling area limits their ability to detect small discontinuities.

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Internship report on Non-Destructive Testing www.sajetc.com Pencil probes have a small surface coil that is encased in a long slender housing to permit inspection in restricted spaces. They are available with a straight shaft or with a bent shaft, which facilitates easier handling and use in applications such as the inspection of small diameter bores. Pencil probes are prone to wobble due to their small base and sleeves are sometimes used to provide a wider base.

8.7.2 Bolt Hole Probes Bolt hole probes are a special type of surface probe that is designed to be used with a bolt hole scanner. They have a surface coil that is mounted inside a housing that matches the diameter of the hole being inspected. The probe is inserted in the hole and the scanner rotates the probe within the hole.

8.7.3 ID or Bobbin Probes ID probes, which are also referred to as Bobbin probes or feed-through probes, are inserted into hollow products, such as pipes, to inspect from the inside out. The ID probes have a housing that keep the probe centered in the product and the coil(s) orientation somewhat constant relative to the test surface. The coils

are

most

commonly wound around

the

circumference of the probe so that the probe inspects an area around the entire circumference of the test object at one time.

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8.7.3 OD or Encircling Coils OD probes are often called encircling coils. They are similar to ID probes except that the coil(s) encircle the material to inspect from the outside in. OD probes are commonly used to inspect solid products, such as bars.

8.8

Surface Breaking Cracks

Eddy current equipment can be used for a variety of applications such as the detection of cracks (discontinuities), measurement of metal thickness, detection of metal thinning due to corrosion and erosion, determination of coating thickness, and the measurement of electrical conductivity and magnetic permeability. Eddy current inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known. Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength. The images to the right show an eddy current surface probe on the surface of a conductive component. The strength of the eddy currents under the coil of the probe ins indicated by color. In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area. Of course, factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires: 1. A knowledge of probable defect type, position, and orientation. 2. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw. 3. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, 55 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration. 4. Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for. The basic steps in performing an inspection with a surface probe are the following: 1. Select and setup the instrument and probe. 2. Select a frequency to produce the desired depth of penetration. 3. Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen. 4. Place the inspection probe (coil) on the component surface and null the instrument. 5. Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required. 6. Monitor the signal for a local change in impedance that will occur as the probe moves over a discontinuity. Move the probe over the surface of the specimen and compare the signal responses from a surface breaking crack with the signals from the calibration notches. The inspection can be made at a couple of different frequencies to get a feel for the effect that frequency has on sensitivity in this application.

8.9

Surface Crack Detection Using Sliding Probes

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Internship report on Non-Destructive Testing www.sajetc.com Many commercial aircraft applications involve the use of multiple fasteners to connect the multilayer skins. Because of the fatigue stress that is caused by the typical application of any commercial aircraft, fatigue cracks can be induced in the vicinity of the fastener holes. In order to inspect the fastener holes in an adequate amount of time, sliding probes are an efficient method of inspection. Sliding probes have been named so because they move over fasteners in a sliding motion. There are two types of sliding probes, fixed and adjustable, which are usually operated in the reflection mode. This means that the eddy currents are induced by the driver coil and detected by a separate receiving coil. Sliding probes are one of the fastest methods to inspect large numbers of fastener holes. They are capable of detecting surface and subsurface discontinuities, but they can only detect defects in one direction. The probes are marked with a detection line to indicate the direction of inspection. In order to make a complete inspection there must be two scans that are orthogonal (90 degrees) to each other.

8.10 Probe Types 8.10.1 Fixed Sliding Probes These probes are generally used for thinner material compared to the adjustable probes. Maximum penetration is about 1/8 inch. Fixed sliding probes are particularly well suited for finding longitudinal surface or subsurface cracks such as those found in lap joints. Typical frequency range is from 100 Hz to 100 kHz.

8.10.2 Adjustable Sliding Probes These probes are well suited for finding subsurface cracks in thick multi-layer structures, like wing skins. Maximum penetration is about 3/4 inch. The frequency range for adjustable sliding probes is from 100 Hz to 40 kHz.

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Internship report on Non-Destructive Testing www.sajetc.com Adjustable probes, as the name implies, are adjustable with the use of spacers, which will change the penetration capabilities. The spacer thickness between the coils is normally adjusted for the best detection. For tangential scans or 90 degree scanning with an offset from the center, a thinner spacer is often used. The spacer thickness range can vary from 0 (no spacer) for inspections close to the surface and small fastener heads to a maximum of about 0.3 inch for deep penetration with large heads in the bigger probe types. A wider spacer will give more tolerance to probe deviation as the sensitive area becomes wider but the instrument will require more gain. Sliding probes usually penetrate thicker materials compared to the donut probes.

8.11 Reference Standards Reference/calibration standards for setup of sliding probes typically consist of three or four aluminum plates that are fastened together within a lap joint type configuration. EDM notches or naturally/artificially- induced cracks are located in the second or third layer of the standard. Reference standards used should be manufactured from the same material type,

alloy,

material

thickness,

and

chemical composition that will be found on the aircraft component to be inspected. Sizes and tolerances of flaws introduced in the standards are usually regulated by inspection specifications.

8.12 Inspection Variables

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8.12.1

Liftoff signal Adjustment

Liftoff is normally adjusted to be relatively horizontal. The term "relatively horizontal" is used here because the liftoff signal often appears a curved line rather than a straight line. Sometimes liftoff can be a sharp curve and may need to be adjusted to run slightly upwards before moving downwards.

8.12.2 Scan Patterns A typical scan is centralized over the fastener head and moves along the axis of the fastener holes. This scan is generally used to detect cracks positioned along the axis of the fastener holes. For detecting cracks located transverse or 90 degrees from the axis of the fastener holes, a scan that is 90 degrees from the axis of the fastener holes is recommended.

8.12.3 Signal Interpretation When the probe moves over a fastener hole with a crack, the indication changes and typically will create a larger vertical movement. The vertical amplitude of the loop depends on the crack length, with longer cracks giving higher indications. If the crack is in the far side of the fastener, as the probe moves over it, the dot will follow the fastener line first but will move upwards (clockwise) as it goes over the crack. If the crack is in the near side, it will be found first and the dot will move along the crack level before coming down to the fastener level. If two cracks on opposite sides of the fastener hole are present, the dot will move upwards to the height by the first crack length and then come back to the fastener line and balance point.

8.12.4 Probe Scan Deviation 59 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com Most probes are designed to give a narrow indication for a good fastener hole so that the loops from the cracks are more noticeable. Some probes and structures can give wider indications and a similar result can be obtained if the probe is not straight when it approaches the fastener. It is important to keep the probe centralized over the fastener heads. Doing this will give you a maximum indication for the fastener and a crack. If the probe deviates from the center line, the crack indication will move along the loop that we saw in Figure A and is now present in Figure B. The crack indication is at "a" when the probe is centralized and moves toward "b" as it deviates in one direction, or "c" as it deviates in the opposite direction. Point "b" gives an important indication even if it loses a small amount of amplitude it has gained in phase, giving a better separation angle. This is because we deviated to the side where the crack is located.

a

b

c a

b

c

Fig. B

Fig. A

8.12.5 Crack Angle Deviation A reduction in the crack indication occurs when the crack is at an angle to the probe scan direction. This happens if the crack is not completely at 90 degrees to the normal probe scan or changes direction as it grows. Both the fixed and adjustable sliding probes are capable of detecting cracks up to about 30 degrees off angle.

8.12.6 Electrical Contact When inspecting fasteners that have just been installed or reference standards that have intimate contact with the aluminum skin plate, it is not unusual to obtain a smaller than normal indication. 60 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com In some extreme cases, the fastener indication may disappear almost completely. This is due to the good electrical contact between the fastener and the skin. This condition allows the eddy currents to circulate without encountering a boundary, and therefore, no obstacle or barrier. Because of this effect, it is recommended to paint the holes before fastener installation.

8.13 Tube Inspection By Eddy Current Eddy current inspection is often used to detect corrosion, erosion, cracking and other changes in tubing. Heat exchangers and steam generators, which are used in power plants, have thousands of tubes that must be prevented from leaking. This is especially important in nuclear power plants where reused, contaminated water must be prevented from mixing with fresh water that will be returned to the environment. The contaminated water flows on one side of the tube (inside or outside) and the fresh water flows on the other side. The heat is transferred from the contaminated water to the fresh water and the fresh water is then returned back to is source, which is usually a lake or river. It is very important to keep the two water sources from mixing, so power plants are periodically shutdown so the tubes and other equipment can be inspected and repaired. The eddy current test method and the related remote field testing method provide high-speed inspection techniques for these applications. A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat

exchanger.

With the

differential probe, no signal will be seen on the eddy current instrument as long as no metal thinning is present. When metal thinning is present, a loop will be seen on the impedance plane as one coil of the differential probe passes over the flawed area and a second loop will be produced when the second coil passes over 61 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag. The size of the indication provides an indication of the total extent of the corrosion damage. A tube inspection using a bobbin probe is simulated below. Click the "null" button and then drag either the absolute or the differential probe through the tube. Note the different signal responses provided by the two probes.

8.14 Thickness

Measurements

of

Thin

Material Eddy current techniques can be used to perform a number of dimensional measurements. The ability to make rapid measurements without the need for couplant or, in some cases even surface contact, makes eddy current

techniques

very

useful.

The

type

of

measurements that can be made include: 

thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate



cross-sectional dimensions of cylindrical tubes and rods



thickness of nonmetallic coatings on metallic substrates

8.15 Corrosion Thinning of Aircraft Skins One application where the eddy current technique is commonly used to measure material thickness is in the detection and characterization of corrosion damage on the skins of aircraft. Eddy current techniques can be used to do spot checks or scanners can be used to inspect small areas. Eddy current

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Internship report on Non-Destructive Testing www.sajetc.com inspection has an advantage over ultrasound in this application because no mechanical coupling is required to get the energy into the structure. Therefore, in multi-layered areas of the structure like lap splices, eddy current can often determine if corrosion thinning is present in buried layers. Eddy current inspection has an advantage over radiography for this application because only single sided access is required to perform the inspection. To get a piece of film on the back side of the aircraft skin might require removing interior furnishings, panels, and insulation which could be very costly. Advanced eddy current techniques are being developed that can determine thickness changes down to about three percent of the skin thickness.

8.16 Thickness Measurement of Thin Conductive Sheet, Strip and Foil Eddy current techniques are used to measure the thickness of hot sheet, strip and foil in rolling mills, and to measure the amount of metal thinning that has occurred over time due to corrosion on fuselage skins of aircraft. On the impedance plane, thickness variations exhibit the same type of eddy current signal response as a subsurface defect, except that the signal represents a void of infinite size and depth. The phase rotation pattern is the same, but the signal amplitude is greater. In the applet, the lift-off curves for different areas of the taper wedge can be produced by nulling the probe in air and touching it to the surface at various locations of the tapered wedge. If a line is drawn between the end points of the lift-off curves, a comma shaped curve is produced. As illustrated in the second applet, this comma shaped curve is the path that is traced on the screen when the probe is scanned down the length of the tapered wedge so that the entire range of thickness values is measured. When making this measurement, it is important to keep in mind that the depth of penetration of the eddy currents must cover the entire range of thicknesses being measured. Typically, a frequency is selected that produces about one standard depth of penetration at the maximum thickness. Unfortunately, at lower frequencies, which are often needed to get the necessary penetration, the probe impedance is more sensitive to changes in electrical conductivity. Thus, the effects of electrical conductivity cannot be phased out and it is important to verify that any variations of conductivity over the region of interest are at a sufficiently low level.

8.17 Thickness Measurement of Thin Conductive Layers 63 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com It is also possible to measure the thickness of a thin layer of metal on a metallic substrate, provided the two metals have widely differing electrical conductivities (i.e. silver on lead where = 67 and 10 MS/m, respectively). A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself. The method has also been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals (i.e. chromium and nickel) on non-ferromagnetic metal bases. Depending on the required degree of penetration, measurements can be made using a single-coil probe or a transformer probe, preferably reflection type. Small-diameter probe coils are usually preferred since they can provide very high sensitivity and minimize effects related to property or thickness variations in the underlying base metal when used in combination with suitably high test frequencies. The goal is to confine the magnetizing field, and the resulting eddy current distribution, to just beyond the thin coating layer and to minimize the field within the base metals.

8.18 Pulsed Eddy Current Inspection Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a particular frequency to excite the probe. The pulsed eddy current technique uses a step function voltage to excite the probe. The advantage of using a step function voltage is that it contains a

continuum of frequencies. As a result, the electromagnetic response to several different frequencies can be measured with just a single step. Since the depth of penetration is dependent on the frequency of excitation, information from a range of depths can be obtained all at once. If measurements are made in the time domain (that is by looking at signal strength as a function of 64 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com time), indications produced by flaws or other features near the inspection coil will be seen first and more distant features will be seen later in time. To improve the strength and ease interpretation of the signal, a reference signal is usually collected, to which all other signals are compared (just like nulling the probe in conventional eddy current inspection). Flaws, conductivity, and dimensional changes produce a change in the signal and a difference between the reference signal and the measurement signal that is displayed. The distance of the flaw and other features relative to the probe will cause the signal to shift in time. Therefore, time gating techniques (like in ultrasonic inspection) can be used to gain information about the depth of a feature of interest.

8.19 EC Standards and Methods British Standards (BS) and American Standards (ASTM) relating to magnetic flux leakage and eddy current methods of testing are given below. National standards are currently being harmonized across the whole of Europe, and British Standards are no exception. Harmonized standards will eventually be identified by the initials BS EN; for example, BS 5411 has been revised and is now known as BS EN 2360. Harmonization is unlikely to be completed before 2001. The year of updating a British Standard is given in brackets. ASTM standards are published annually and updated when necessary. FLUX LEAKAGE METHODS (INCLUDING MAGNETIC PARTICLE INSPECTION) British Standards (BS) BS BS BS BS BS

6072:1981

(1986)

4489:1984

particle

Black

5044:1973 5138:1974

Magnetic

(1987) (1988)

3683

flaw

light

measurement

Contrast

Forged

and

(part

aid stamped

2):1985

BS 4069:1982 Inks and powders American Society for Testing and Materials (ASTM) 65 [email protected]

detection

paints crankshafts Glossary

Internship report on Non-Destructive Testing www.sajetc.com ASTM

E

ASTM

709

E

ASTM

Magnetic

125 E

particle

Indications 1316

inspection

practice

ferrous

castings

in Definition

of

terms

ASTM E 570 Flux leakage examination of ferromagnetic steel tubular products EDDY CURRENT METHODS British Standards (BS) BS

3683

(part

5):1965

(1989)

Eddy

current

flaw

detection

glossary

BS 3889 (part 2A): 1986 (1991) Automatic eddy current testing of wrought steel tubes BS

3889

(part

213):

1966

(1987)

Eddy

current

testing

of

nonferrous

tubes

BS 5411 (part 3):1984 Eddy current methods for measurement of coating thickness of nonconductive coatings on nonmagnetic base material. Withdrawn: now known as BS EN 2360 (2007).

9.

Radiography Testing (RT) 66 [email protected]

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9.1

History of Radiography

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (18451923) who was a Professor at Wuerzburg University in Germany. Working with a cathode-ray tube in his laboratory, Roentgen observed a fluorescent glow of crystals on a table near his tube. The tube that Roentgen was working with consisted of a glass envelope (bulb) with positive and negative electrodes encapsulated in it. The air in the tube was evacuated, and when a high voltage was applied, the tube produced a fluorescent glow. Roentgen shielded the tube with heavy black paper, and discovered a green colored fluorescent light generated by a material located a few feet away from the tube. Public fancy was caught by this invisible ray with the ability to pass through solid matter, and, in conjunction with a photographic plate, provide a picture of bones and interior body parts. Scientific fancy was captured by the demonstration of a wavelength shorter than light. This generated new possibilities in physics, and for investigating the structure of matter. Much enthusiasm was generated about potential applications of rays as an aid in medicine and surgery. Within a month after the announcement of the discovery, several medical radiographs had been made in Europe and the United States, which were used by surgeons to guide them in their work. In June 1896, only 6 months after Roentgen announced his discovery, X-rays were being used by battlefield physicians to locate bullets in wounded soldiers. In 1922, industrial radiography took another step forward with the advent of the 200,000-volt X-ray tube that allowed radiographs of thick steel parts to be produced in a reasonable amount of time. In 1931, General Electric Company developed 1,000,000 volt X-ray generators, providing an effective tool for industrial radiography. That same year, the American Society of Mechanical Engineers (ASME) permitted X-ray approval of fusion welded pressure vessels that further opened the door to industrial acceptance and use. 67 [email protected]

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9.2

A Second Source of Radiation

Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896, French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Henri Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. Becquerel was researching the principles of fluorescence, wherein certain minerals glow (fluoresce) when exposed to sunlight. He utilized photographic plates to record this fluorescence. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with his photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light. In addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. Bacquerel's discovery was, unlike that of the X-rays, virtually unnoticed by laymen and scientists alike. Relatively few scientists were interested in Becquerel's findings. It was not until the discovery of radium by the Curies two years later that interest in radioactivity became widespread. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During World War II, industrial radiography grew tremendously as part of the Navy's shipbuilding program. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive. The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography.

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9.3

Health Concerns

The science of radiation protection, or "health physics" as it is more properly called, grew out of the parallel discoveries of X-rays and radioactivity in the closing years of the 19th century. Experimenters, physicians, laymen, and physicists alike set up X-ray generating apparatuses and proceeded about their labors with a lack of concern regarding potential dangers. Such a lack of concern is quite understandable, for there was nothing in previous experience to suggest that Xrays would in any way be hazardous. Indeed, the opposite was the case, for who would suspect that a ray similar to light but unseen, unfelt, or otherwise undetectable by the senses would be damaging to a person? More likely, or so it seemed to some, X-rays could be beneficial for the Today, it can be said that radiation ranks among the most thoroughly investigated causes of disease. Although much still remains to be learned, more is known about the mechanisms of radiation damage on the molecular, cellular, and organ system than is known for most other health stressing agents. Indeed, it is precisely this vast accumulation of quantitative doseresponse data that enables health physicists to specify radiation levels so that medical, scientific, and industrial uses of radiation may continue at levels of risk no greater than, and frequently less than, the levels of risk associated with any other technology. X-rays and Gamma rays are electromagnetic radiation of exactly the same nature as light, but of much shorter wavelength. Wavelength of visible light is on the order of 6000 angstroms while the wavelength of x-rays is in the range of one angstrom and that of gamma rays is 0.0001 angstrom. This very short wavelength is what gives x-rays and gamma rays their power to penetrate materials that light cannot. These electromagnetic waves are of a high energy level and can break chemical bonds in materials they penetrate. If the irradiated matter is living tissue, the breaking of chemical bonds may result in altered structure or a change in the function of cells. Early exposures to radiation resulted in the loss of limbs and even lives. Men and women researchers collected and documented information on the interaction of radiation and the human body. This early information helped science understand how electromagnetic radiation interacts with living tissue. Unfortunately, much of this information was collected at great personal expense.

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9.4

Present State of Radiography

In many ways, radiography has changed little from the early days of its use. We still capture a shadow image on film using similar procedures and processes technicians were using in the late 1800's. Today, however, we are able to generate images of higher quality and greater sensitivity through the use of higher quality films with a larger variety of film grain sizes. Film processing has evolved to an automated state, producing more consistent film quality by removing manual processing variables. Electronics and computers allow technicians to now capture images digitally. The use of "filmless radiography" provides a means of capturing an image, digitally enhancing, sending the image anywhere in the world, and archiving an image that will not deteriorate with time. Technological advances have provided industry with smaller, lighter, and very portable equipment that produce high quality X-rays. The use of linear accelerators provide a means of generating extremely short wavelength, highly penetrating radiation, a concept dreamed of only a few short years ago. While the process has changed little, technology has evolved allowing radiography to be widely used in numerous areas of inspection. Radiography has seen expanded usage in industry to inspect not only welds and castings, but to radiographically inspect items such as airbags and canned food products. Radiography has found use in metallurgical material identification and security systems at airports and other facilities. Gamma ray inspection has also changed considerably since the Curies' discovery of radium. Man-made isotopes of today are far stronger and offer the technician a wide range of energy levels and half-lives. The technician can select Co-60 which will effectively penetrate very thick materials, or select a lower energy isotope, such as Tm-170, which can be used to inspect plastics and very thin or low density materials. Today gamma rays find wide application in industries such as petrochemical, casting, welding, and aerospace.

9.5

Future Direction of Radiographic Education

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Internship report on Non-Destructive Testing www.sajetc.com Although many of the methods and techniques developed over a century ago remain in use, computers are slowly becoming a part of radiographic inspection. The future of radiography will likely see many changes. As noted earlier, companies are performing many inspections without the aid of film. Radiographers of the future will capture images in digitized form and e-mail them to the customer when the inspection has been completed. Film evaluation will likely be left to computers. Inspectors may capture a digitized image, feed them into a computer and wait for a printout of the image with an accept/reject report. Systems will be able to scan a part and present a three-dimensional image to the radiographer, helping him or her to locate the defect within the part. Inspectors in the future will be able to peal away layer after layer of a part to evaluate the material in much greater detail. Color images, much like computer generated ultrasonic C-scans of today, will make interpretation of indications much more reliable and less time consuming. Educational techniques and materials will need to be revised and updated to keep pace with technology and meet the requirements of industry. These needs may well be met with computers. Computer programs can simulate radiographic inspections using a computer aided design (CAD) model of a part to produce physically accurate simulated x-ray radiographic images. Programs allow the operator to select different parts to inspect, adjust the placement and orientation of the part to obtain the proper equipment/part relationships, and adjust all the usual x-ray generator settings to arrive at the desired radiographic film exposure.

9.6

Properties of X-Rays and Gamma Rays 71 [email protected]

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They are not detected by human senses (cannot be seen, heard, felt, etc.).

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They travel in straight lines at the speed of light.

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Their paths cannot be changed by electrical or magnetic fields.

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They can be diffracted to a small degree at interfaces between two different materials.

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They pass through matter until they have a chance encounter with an atomic particle.

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Their degree of penetration depends on their energy and the matter they are traveling through.

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They have enough energy to ionize matter and can damage or destroy living cells.

9.6.1 

X-Radiation

X-rays are just like any other kind of electromagnetic radiation. They can be produced in parcels of energy called photons, just like light. There are two different atomic processes that can produce X-ray photons. One is called Bremsstrahlung and is a German term meaning "braking radiation." The other is called K-shell emission. They can both occur in the heavy atoms of tungsten. Tungsten is often the material chosen for the target or anode of the x-ray tube.



Both ways of making X-rays involve a change in the state of electrons. However, Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of the electron shot at the tungsten changes. The negatively charged electron slows down after swinging around the nucleus of a positively charged tungsten atom. This energy loss produces X-radiation. Electrons are scattered elastically and inelastically by the positively charged nucleus. The inelastically scattered electron loses energy, which appears as Bremsstrahlung. Elastically scattered electrons (which include backscattered electrons) are generally scattered through larger angles. In the interaction, many photons of different wavelengths are produced, but none of the photons have more energy than the electron had to begin with. After emitting the spectrum of X-ray radiation, the original electron is slowed down or stopped.

9.6.2

Bremsstrahlung Radiation 72 [email protected]

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X-ray tubes produce x-ray photons by accelerating a stream of electrons to energies of several hundred kilovolts with velocities of several hundred kilometers per hour and colliding them into a heavy target material. The abrupt acceleration of the charged particles

(electrons)

produces

Bremsstrahlung photons. X-ray radiation with a continuous spectrum of energies is produced with a range from a few keV to a maximum of the energy of the electron beam. Target materials for industrial tubes are typically tungsten, which means that the wave functions of the bound tungsten electrons are required. The inherent filtration of an X-ray tube must be computed, which is controlled by the amount that the electron penetrates into the surface of the target and by the type of vacuum window present. 

The bremsstrahlung photons generated within the target material are attenuated as they pass through typically 50 microns of target material. The beam is further attenuated by the aluminum or beryllium vacuum window. The results are an elimination of the low energy photons, 1 keV through l5 keV, and a significant reduction in the portion of the spectrum from 15 keV through 50 keV. The spectrum from an x-ray tube is further modified by the filtration caused by the selection of filters used in the setup.



The applet below allows the user to visualize an electron accelerating and interacting with a heavy target material. The graph keeps a record of the bremsstrahlung photons numbers as a function of energy. After a few events, the "building up" of the graph may be accomplished by pressing the "automate" button.

9.6.3 Gamma Radiation 73 [email protected]

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Gamma radiation is one of the three types of natural radioactivity. Gamma rays are electromagnetic radiation, like X-rays. The other two types of natural radioactivity are alpha and beta radiation, which are in the form of particles. Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nanometer.



Gamma radiation is the product of radioactive atoms. Depending upon the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable. When the binding energy is not strong enough to hold the nucleus of an atom together, the atom is said to be unstable. Atoms with unstable nuclei are constantly changing as a result of the imbalance of energy within the nucleus. Over time, the nuclei of unstable isotopes spontaneously disintegrate, or transform, in a process known as radioactive decay. Various types of penetrating radiation may be emitted from the nucleus and/or its surrounding electrons. Nuclides which undergo radioactive decay are called radionuclides. Any material which contains measurable amounts of one or more radionuclides is a radioactive material.

9.7

Types Radiation Produced by Radioactive Decay

When an atom undergoes radioactive decay, it emits one or more forms of radiation with sufficient energy to ionize the atoms with which it interacts. Ionizing radiation can consist of high speed subatomic particles ejected from the nucleus or electromagnetic radiation (gammarays) emitted by either the nucleus or orbital electrons.

9.7.1 Alpha Particles

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Internship report on Non-Destructive Testing www.sajetc.com Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha particles. These alpha particles are tightly bound units of two neutrons and two protons each (He4 nucleus) and have a positive charge. Emission of an alpha particle from the nucleus results in a decrease of two units of atomic number (Z) and four units of mass number (A). Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate. All alpha particles from a particular radionuclide transformation will have identical energies. 9.7.2 Beta Particles A nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high speed electron called a beta particle. This results in a net change of one unit of atomic number (Z). Beta particles have a negative charge and the beta particles emitted by a specific radionuclide will range in energy from near zero up to a maximum value, which is characteristic of the particular transformation. 9.7.3 Gamma-rays A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

9.8

Filters in Radiography

At x-ray energies, filters consist of material placed in the useful beam to absorb, preferentially, radiation based on energy level or to modify the spatial distribution of the beam. Filtration is required to absorb the lower-energy x-ray photons emitted by the tube before they reach the

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Internship report on Non-Destructive Testing www.sajetc.com target. The use of filters produce a cleaner image by absorbing the lower energy x-ray photons that tend to scatter more. The total filtration of the beam includes the inherent filtration (composed of part of the x-ray tube and tube housing) and the added filtration (thin sheets of a metal inserted in the x-ray beam). Filters are typically placed at or near the x-ray port in the direct path of the x-ray beam. Placing a thin sheet of copper between the part and the film cassette has also proven an effective method of filtration. For industrial radiography, the filters added to the x-ray beam are most often constructed of high atomic number materials such as lead, copper, or brass. Filters for medical radiography are usually made of aluminum (Al). The amount of both the inherent and the added filtration are stated in mm of Al or mm of Al equivalent. The amount of filtration of the x-ray beam is specified by and based on the voltage potential (keV) used to produce the beam. The thickness of filter materials is dependent on atomic numbers, kilovoltage settings, and the desired filtration factor.

9.9

Radiation Safety

Ionizing radiation is an extremely important NDT tool but it can pose a hazard to human health. For this reason, special precautions must be observed when using and working around ionizing radiation. The possession of radioactive materials and use of radiation producing devices in the United States is governed by strict regulatory controls. The primary regulatory authority for most types and uses of radioactive materials is the federal Nuclear Regulatory Commission (NRC). However, more than half of the states in the US have entered into "agreement" with the NRC to assume regulatory control of radioactive material use within their borders. As part of the agreement process, the states must adopt and enforce regulations comparable to those found in Title 10 of the Code of Federal Regulations. Regulations for control of radioactive material used in Iowa are found in Chapter 136C of the Iowa Code.

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Internship report on Non-Destructive Testing www.sajetc.com For most situations, the types and maximum quantities of radioactive materials possessed, the manner in which they may be used, and the individuals authorized to use radioactive materials are stipulated in the form of a "specific" license from the appropriate regulatory authority. In Iowa, this authority is the Iowa Department of Public Health. However, for certain institutions which routinely use large quantities of numerous types of radioactive materials, the exact quantities of materials and details of use may not be specified in the license. Instead, the license grants the institution the authority and responsibility for setting the specific requirements for radioactive material use within its facilities. These licensees are termed "broadscope" and require a Radiation Safety Committee and usually a full-time Radiation Safety Officer. Complicating matters further is the fact that Gamma and X-ray radiation are not detectable by the human body. However, the risks can be minimized when the radiation is handled and managed properly. The law requires that individuals receive training in the safe handling and use of radioactive materials and radiation producing devices. Some of the topics this training should cover include: 

Health concerns associated with exposure to radioactive materials or radiation.

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Precautions or procedures to minimize exposure to radiation.

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Purposes and functions of protective devices employed.

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The permit conditions and the applicable portions of the Radiation Safety Manual.

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Worker‘s responsibility to promptly report any condition that may lead to or cause a violation of the regulations or cause an unnecessary exposure.

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Actions to take in the event of an emergency.

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Radiation exposure reports that workers have a right to receive.

9.10 Radiographic Film X-ray films for general radiography consist of an emulsion-gelatin containing radiation sensitive silver halide crystal, such as silver

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Internship report on Non-Destructive Testing www.sajetc.com bromide or silver chloride, and a flexible, transparent, blue-tinted base. The emulsion is different from those used in other types of photography films to account for the distinct characteristics of gamma rays and x-rays, but X-ray films are sensitive to light. Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inch thick. Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide, and thus increases the film speed. The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a reasonable time. A few of the films used for radiography only have emulsion on one side which produces the greatest detail in the image. When x-rays, gamma rays, or light strike the grains of the sensitive silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden) image." However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer), and the reaction results in the formation of black, metallic silver. It is this silver, suspended in the gelatin on both sides of the base, that creates an image. See the page on film processing for additional information.

9.10.1

Film Selection

The selection of a film when radiographing any particular component depends on a number of different factors. Listed below are some of the factors that must be considered when selecting a film and developing a radiographic technique. 1. Composition, shape, and size of the part being examined and, in some cases, its weight and location. 2. Type of radiation used, whether x-rays from an x-ray generator or gamma rays from a radioactive source. 3. Kilovoltages available with the x-ray equipment or the intensity of the gamma radiation. 4. Relative importance of high radiographic detail or quick and economical results. 78 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com Selecting the proper film and developing the optimal radiographic technique usually involves arriving at a balance between a number of opposing factors. For example, if high resolution and contrast sensitivity is of overall importance, a slower and finer grained film should be used in place of a faster film.

9.10.2 Film Packaging Radiographic film can be purchased in a number of different packaging options. The most basic form is as individual sheets in a box. In preparation for use, each sheet must be loaded into a cassette or film holder in the darkroom to protect it from exposure to light. The sheets are available in a variety of sizes and can be purchased with or without interleaving paper. Interleaved packages have a layer of paper that separates each piece of film. The interleaving paper is removed before the film is loaded into the film holder. Many users find the interleaving paper useful in separating the sheets of film and offer some protection against scratches and dirt during handling. Industrial x-ray films are also available in a form in which each sheet is enclosed in a light-tight envelope. The film can be exposed from either side without removing it from the protective packaging. A rip strip makes it easy to remove the film in the darkroom for processing. This form of packaging has the advantage of eliminating the process of loading the film holders in the darkroom. The film is completely protected from finger marks and dirt until the time the film is removed from the envelope for processing. Packaged film is also available in rolls, which allows the radiographer to cut the film to any length. The ends of the packaging are sealed with electrical tape in the darkroom. In applications such as the radiography of circumferential welds and the examination of long joints on an aircraft fuselage, long lengths of film offer great economic advantage. The film is wrapped

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Internship report on Non-Destructive Testing www.sajetc.com around the outside of a structure and the radiation source is positioned on axis inside, allowing for examination of a large area with a single exposure. Envelope packaged film can be purchased with the film sandwiched between two lead oxide screens. The screens function to reduce scatter radiation at energy levels below 150keV and as intensification screens above 150 keV.

9.10.3 Film Handling X-ray film should always be handled carefully to avoid physical strains, such as pressure, creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible holders and external clamping devices are used, care should be taken to be sure pressure is uniform. If a film holder bears against a few high spots, such as on an un-ground weld, the pressure may be great enough to produce desensitized areas in the radiograph. This precaution is particularly important when using envelope-packed films. Another important precaution is to avoid drawing film rapidly from cartons, exposure holders, or cassettes. Such care will help to eliminate circular or treelike black markings in the radiograph that sometimes result due to static electric discharges.

9.10.4 Film Processing As mentioned previously, radiographic film consists of a transparent, blue-tinted base coated on both sides with an emulsion. The emulsion consists of gelatin containing microscopic, radiation sensitive silver halide crystals, such as silver bromide and silver chloride. When x-rays, gamma rays or light rays strike the the crystals or grains, some of the Br ions are liberated and captured by the Ag+ ions. In this condition, the radiograph is said to contain a latent (hidden) image because the change

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Internship report on Non-Destructive Testing www.sajetc.com in the grains is virtually undetectable, but the exposed grains are now more sensitive to reaction with the developer. When the film is processed, it is exposed to several different chemicals solutions for controlled periods of time. Processing film basically involves the following five steps. 

Development - The developing agent gives up electrons to convert the silver halide grains to metallic silver. Grains that have been exposed to the radiation develop more rapidly, but given enough time the developer will convert all the silver ions into silver metal. Proper temperature control is needed to convert exposed grains to pure silver while keeping unexposed grains as silver halide crystals.

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Stopping the development - The stop bath simply stops the development process by diluting and washing the developer away with water.

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Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind.

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Washing - The film is washed with water to remove all the processing chemicals.

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Drying - The film is dried for viewing.

Processing film is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require a high degree of consistency and quality control.

9.10.4.1

Manual Processing & Darkrooms

Manual processing begins with the darkroom. The darkroom should be located in a central location, adjacent to the reading room and a reasonable distance from the exposure area. For portability, darkrooms are often mounted on pickups or trailers. Film should be located in a light, tight compartment, which is most often a metal bin that is used to store and protect the film. An area next to the film bin that is dry and free of dust and dirt should be used to load and unload the film. Another area, the wet side, should be used to process the film. This method protects the film from any water or chemicals that may be located on the surface of the wet side. 81 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com Each of step in the film processing must be excited properly to develop the image, wash out residual processing chemicals, and to provide adequate shelf life of the radiograph. The objective of processing is two fold: first, to produce a radiograph adequate for viewing, and second, to prepare the radiograph for archival storage. Radiographs are often stored for 20 years or more as a record of the inspection.

9.10.4.2

Automatic Processor Evaluation

The automatic processor is the essential piece of equipment in every x-ray department. The automatic processor will reduce film processing time when compared to manual development by a factor of four. To monitor the performance of a processor, apart from optimum temperature and mechanical checks, chemical and sensitometric checks should be performed for developer and fixer. Chemical checks involve measuring the pH values of the developer and fixer as well as both replenishers. Also, the specific gravity and fixer silver levels must be measured. Ideally, pH should be measured daily and it is important to record these measurements, as regular logging provides very useful information. The daily measurements of pH values for the developer and fixer can then be plotted to observe the trend of variations in these values compared to the normal pH operating levels to identify problems. Sensitometric checks may be carried out to evaluate if the performance of films in the automatic processors is being maximized. These checks involve measurement of basic fog level, speed and average gradient made at 1° C intervals of temperature. The range of temperature measurement depends on the type of chemistry in use, whether cold or hot developer. These three measurements: fog level, speed, and average gradient, should then be plotted against temperature and compared with the manufacturer's supplied figures.

9.11 Radiograph Interpretation - Welds In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps: (1)

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Internship report on Non-Destructive Testing www.sajetc.com detection, (2) interpretation, and (3) evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.

9.12

Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specifications used to invoke and control an inspection, are referred to as defects.

9.13 Welding Discontinuities The following discontinuities are typical of all types of welding.

9.13.1 Cold Lap Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding.

Fig. : Cold Lap in Welding 83 [email protected]

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9.13.2

Porosity

It is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will have a higher radiographic density than the surrounding area.

. Fig. : Porosity in Welding

9.13.3 Cluster porosity Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into a gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

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Fig. : Cluster porosity in Welding

9.13.4 Slag inclusions Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Fig. : Slag Inclusions

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9.13.5 IP and LOP Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Fig. : Incomplete penetration (IP) or lack of penetration (LOP) in Welding

9.13.6 Incomplete fusion It is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

Fig. : Incomplete fusion in Welding

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9.13.7

Internal concavity or suck back

Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

Fig. : Internal concavity or suck back in Welding

9.13.8 Internal or root undercut It is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

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Fig. : Internal or root undercut in Welding 9.13.9

External or crown undercut

External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Fig. : External or crown undercut in Welding

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Offset or mismatch

Offset or mismatch is terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Fig. : Offset or mismatch in Welding

9.13.11

Inadequate weld reinforcement

Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be higher (darker) than the image density of the surrounding base material.

Fig. : Inadequate weld reinforcement In Welding

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9.13.12

Excess weld reinforcement

It is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Fig. : Excess weld reinforcement

9.13.13

Cracks

Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Fig. : Cracks in Welding 90 [email protected]

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9.13.14

Discontinuities in TIG welds

The following discontinuities are unique to the TIG welding process. These discontinuities occur in most metals welded by the process, including aluminum and stainless steels. The TIG method of welding produces a clean homogeneous weld which when radiographed is easily interpreted.

9.13.15

Tungsten inclusions

Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is more dense than aluminum or steel, therefore it shows up as a lighter area with a distinct outline on the radiograph.

Fig. : Tungsten inclusions

9.13.16

Oxide inclusions

Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding material and, therefore, appear as dark irregularly shaped discontinuities in the radiograph.

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Fig. : Oxide inclusions in Welding

9.13.17

Discontinuities in Gas Metal Arc Welds (GMAW)

The following discontinuities are most commonly found in GMAW welds. Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld or contained within the weld. On a radiograph they appear as light, "wire like" indications.

9.13.18

Burn-Through

Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld, creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On a radiograph, burn-through appears as dark spots, which are often surrounded by light globular areas (icicles).

Fig. : Burn-Through 92 [email protected]

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9.14 Real-time Radiography Real-time

radiography

(RTR),

or

real-time

radioscopy, is a nondestructive test (NDT) method whereby an image is produced electronically, rather than on film, so that very little lag time occurs between the item being exposed to radiation and the resulting image. In most instances, the electronic image that is viewed results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs. The fluorescent elements of the screen form the image much as the grains of silver form the image in film radiography. The image formed is a "positive image" since brighter areas on the image indicate where higher levels of transmitted radiation reached the screen. This image is the opposite of the negative image produced in film radiography

9.15 Advantages of Radiography  Information is presented pictorially.  A permanent record is provided which may be viewed at a time and place distant from the test.  Useful for thin sections.  Sensitivity declared on each film suitable for any material.  Suitable for any material.

9.16 Disadvantages of Radiography  Generally an inability to cope with thick sections.  Possible health hazard.  Need to direct the beam accurately for two-dimensional defects.  Film processing and viewing facilities are necessary, as is an exposure compound.  Not suitable for automation, unless the system incorporates fluoroscopy with an image intensifier or other electronic aids  Not suitable for surface defects. 93 [email protected]

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10. Ultrasonic Testing 10.1 Introduction of Ultrasonic Testing This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials. The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals with little attenuation. The velocity at which these waves propagate is related to the Young‘s Modulus for the material and is characteristic of that material. For example the velocity in steel is 5900 metres per second, and in water 1400 metres per second. Ultrasonic energy is considerably attenuated in air, and a beam propagated through solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall) between that material and air reflect a considerable amount of energy in the direction equal to the angle of incidence. For contact testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid (referred to as ‗couplant‘), usually oil, water or grease, is applied to the surface. As mentioned previously, the crystal does not oscillate continuously but in short pulses, between each of which it is quiescent. Piezo electric materials not only convert electrical pulses to mechanical oscillations, but will also transducer mechanical oscillations into electrical pulses; thus we have not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe

10.2 Basic Principles of Ultrasonic Testing Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional

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Internship report on Non-Destructive Testing www.sajetc.com measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used. A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained. Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include: 

It is sensitive to both surface and subsurface discontinuities.



The depth of penetration for flaw detection or measurement is superior to other NDT methods.



Only single-sided access is needed when the pulse-echo technique is used.



It is highly accurate in determining reflector position and estimating size and shape.



Minimal part preparation is required.



Electronic equipment provides instantaneous results.



Detailed images can be produced with automated systems.



It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include: 

Surface must be accessible to transmit ultrasound.



Skill and training is more extensive than with some other methods.

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It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.



Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.



Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.



Linear defects oriented parallel to the sound beam may go undetected.



Reference standards are required for both equipment calibration and the characterization of flaws.

The above introduction provides a simplified introduction to the NDT method of ultrasonic testing. However, to effectively perform an inspection using ultrasonics, much more about the method needs to be known. The following pages present information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information.

10.3 History of Ultrasonics Prior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sokolov studied the use of ultrasonic waves in detecting metal objects. Mulhauser, in 1931, obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. Firestone (1940) and Simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique. Shortly after the close of World War II, researchers in Japan began to explore the medical diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation with a two dimensional, gray scale image. Japan's work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Researchers then presented their findings on the use of ultrasound to detect gallstones,

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Internship report on Non-Destructive Testing www.sajetc.com breast masses, and tumors to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation. Ultrasound pioneers working in the United States contributed

many

innovations

and

important

discoveries to the field during the following decades. Researchers learned to use ultrasound to detect potential cancer and to visualize tumors in living subjects and in excised tissue. Real-time imaging, another significant diagnostic tool for physicians, presented ultrasound images directly on the system's CRT screen at the time of scanning. The introduction of spectral Doppler and later color Doppler depicted blood flow in various colors to indicate the speed and direction of the flow.. The United States also produced the earliest hand held "contact" scanner for clinical use, the second generation of B-mode equipment, and the prototype for the first articulated-arm hand held scanner, with 2-D images.

10.4 Present State of Ultrasonics Ultrasonic testing (UT) has been practiced for many decades. Initial rapid developments in instrumentation spurred by the technological advances from the 1950's continue today. Through the 1980's and continuing through the present, computers have provided technicians with smaller and more rugged instruments with greater capabilities. Thickness gauging is an example application

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Internship report on Non-Destructive Testing www.sajetc.com where instruments have been refined make data collection easier and better. Built-in data logging capabilities allow thousands of measurements to be recorded and eliminate the need for a "scribe." Some instruments have the capability to capture waveforms as well as thickness readings. The waveform option allows an operator to view or review the A-scan signal of thickness measurement long after the completion of an inspection. Also, some instruments are capable of modifying the measurement based on the surface conditions of the material. For example, the signal from a pitted or eroded inner surface of a pipe would be treated differently than a smooth surface. This has led to more accurate and repeatable field measurements. Many ultrasonic flaw detectors have a trigonometric function that allows for fast and accurate location determination of flaws when performing shear wave inspections. Cathode ray tubes, for the most part, have been replaced with LED or LCD screens. These screens, in most cases, are extremely easy to view in a wide range of ambient lighting. Bright or low light working conditions encountered by technicians have little effect on the technician's ability to view the screen. Screens can be adjusted for brightness, contrast, and on some instruments even the color of the screen and signal can be selected. Transducers can be programmed with predetermined instrument settings. The operator only has to connect the transducer and the instrument will set variables such as frequency and probe drive. Along with computers, motion control and robotics have contributed to the advancement of ultrasonic inspections. Early on, the advantage of a stationary platform was recognized and used in industry. Computers can be programmed to inspect large, complex shaped components, with one or multiple transducers collecting information. Automated systems typically consisted of an immersion tank, scanning system, and recording system for a printout of the scan. The immersion tank can be replaced with a squirter systems, which allows the sound to be transmitted through a water column. The resultant C-scan provides a plan or top view of the component. Scanning of components is considerably faster than contact hand scanning, the coupling is much more consistent. The scan information is collected by a computer for evaluation, transmission to a customer, and archiving.

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Internship report on Non-Destructive Testing www.sajetc.com Today, quantitative theories have been developed to describe the interaction of the interrogating fields with flaws. Models incorporating the results have been integrated with solid model descriptions of real-part geometries to simulate practical inspections. Related tools allow NDE to be considered during the design process on an equal footing with other failure-related engineering disciplines. Quantitative descriptions of NDE performance, such as the probability of detection (POD), have become an integral part of statistical risk assessment. Measurement procedures initially developed for metals have been extended to engineered materials such as composites, where anisotropy and inhomogeneity have become important issues. The rapid advances in digitization and computing capabilities have totally changed the faces of many instruments and the type of algorithms that are used in processing the resulting data. Highresolution imaging systems and multiple measurement modalities for characterizing a flaw have emerged. Interest is increasing not only in detecting, characterizing, and sizing defects, but also in characterizing the materials. Goals range from the determination of fundamental microstructural characteristics such as grain size, porosity, and texture (preferred grain orientation), to material properties related to such failure mechanisms as fatigue, creep, and fracture toughness. As technology continues to advance, applications of ultrasound also advance. The high-resolution imaging systems in the laboratory today will be tools of the technician tomorrow.

10.5 Future Direction of Ultrasonic Inspection Looking to the future, those in the field of NDE see an exciting new set of opportunities. The defense and nuclear power industries have played a major role in the emergence of NDE. Increasing global competition has led to dramatic changes in product development and business cycles. At the same time, aging infrastructure, from roads to buildings and aircraft, present a new set of measurement and monitoring challenges for engineers as well as technicians.

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Internship report on Non-Destructive Testing www.sajetc.com Among the new applications of NDE spawned by these changes is the increased emphasis on the use of NDE to improve the productivity of manufacturing processes. Quantitative nondestructive evaluation (QNDE) both increases the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control. The phrase, "you cannot inspect in quality, you must build it in," exemplifies the industry's focus on avoiding the formation of flaws. Nevertheless, manufacturing flaws will never be completely eliminated and material damage will continue to occur in-service so continual development of flaw detection and characterization techniques is necessary. Advanced simulation tools that are designed for inspectability and their integration into quantitative strategies for life management will contribute to increase the number and types of engineering applications of NDE. With growth in engineering applications for NDE, there will be a need to expand the knowledge base of technicians performing the evaluations. Advanced simulation tools used in the design for inspectability may be used to provide technical students with a greater understanding of sound behavior in materials. UTSIM, developed at Iowa State University, provides a glimpse into what may be used in the technical classroom as an interactive laboratory tool. As globalization continues, companies will seek to develop, with ever increasing frequency, uniform international practices. In the area of NDE, this trend will drive the emphasis on standards, enhanced educational offerings, and simulations that can be communicated electronically. The coming years will be exciting as NDE will continue to emerge as a fullfledged engineering discipline.

10.6 Wavelength and Defect Detection In ultrasonic testing, the inspector must make a decision about the frequency of the transducer that will be used. As we learned on the previous page, changing the frequency when the sound velocity is fixed will result in a change in the wavelength of the sound. The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general

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Internship report on Non-Destructive Testing www.sajetc.com rule of thumb is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected. Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases. The wave frequency can also affect the capability of an inspection in adverse ways. Therefore, selecting the optimal inspection frequency often involves maintaining a balance between the favorable and unfavorable results of the selection. Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered. As frequency increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and other sound scatters that require lower frequencies to be used for evaluations of these products. Wrought and forged products with directional and refined grain structure can usually be inspected with higher frequency transducers. Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetrating power (or the maximum depth in a material that flaws can be located) is also reduced. Frequency also has an effect on the shape of the ultrasonic beam. Beam spread, or the divergence of the beam from the center axis of the transducer, and how it is affected by frequency will be discussed later. It should be mentioned, so as not to be misleading, that a number of other variables will also affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument. These are discussed in more detail in the material on signalto-noise ratio.

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10.7 Sound Propagation in Elastic Materials In the previous pages, it was pointed out that sound waves propagate due to the vibrations or oscillatory motions of particles within a material. An ultrasonic wave may be visualized as an infinite number of oscillating masses or particles connected by means of elastic springs. Each individual particle is influenced by the motion of its nearest neighbor and both inertial and elastic restoring forces act upon each particle. A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m. The spring constant is the restoring force of a spring per unit of length. Within the elastic limit of any material, there is a linear relationship between the displacement of a particle and the force attempting to restore the particle to its equilibrium position. This linear dependency is described by Hooke's Law. In terms of the spring model, Hooke's Law says that the restoring force due to a spring is proportional to the length that the spring is stretched, and acts in the opposite direction. Mathematically, Hooke's Law is written as F =-kx, where F is the force, k is the spring constant, and x is the amount of particle displacement. Hooke's law is represented graphically it the right. Please note that the spring is applying a force to the particle that is equal and opposite to the force pulling down on the particle.

10.8 Speed of Sound Hooke's Law, when used along with Newton's Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the properties of the material and is independent of the amplitude of the sound wave. Newton's Second Law says that the force applied to a particle will be balanced by the particle's mass and the acceleration of the the particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's Law then says that this force will be balanced by a force in the opposite direction that is dependent on the

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10.9 Properties of material affect its speed of sound Of course, sound does travel at different speeds in different materials. This is because the mass of the atomic particles and the spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

Where V is the speed of sound, C is the elastic constant, and p is the material density. This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. The typical elastic constants of a materials include: 

Young's Modulus, E: a proportionality constant between uniaxial stress and strain.



Poisson's Ratio, n: the ratio of radial strain to axial strain

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Bulk modulus, K: a measure of the incompressibility of a body subjected to hydrostatic pressure.



Shear Modulus, G: also called rigidity, a measure of a substance's resistance to shear.



Lame's Constants, l and m: material constants that are derived from Young's Modulus and Poisson's Ratio.

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson's Ratio are commonly used. When calculating the velocity of a shear wave, the shear modulus is used. It is often most convenient to make the calculations using Lame's Constants, which are derived from Young's Modulus and Poisson's Ratio. It must also be mentioned that the subscript ij attached to C in the above equation is used to indicate the directionality of the elastic constants with respect to the wave type and direction of wave travel. In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one direction and compressed in the others and the elastic constants for the longitudinal direction are different than those for the transverse or short transverse directions. Examples of approximate compressional sound velocities in materials are: 

Aluminum - 0.632 cm/microsecond



1020 steel - 0.589 cm/microsecond



Cast iron - 0.480 cm/microsecond.

Examples of approximate shear sound velocities in materials are: 

Aluminum - 0.313 cm/microsecond



1020 steel - 0.324 cm/microsecond



Cast iron - 0.240 cm/microsecond.

When comparing compressional and shear velocities, it can be noted that shear velocity is approximately one half that of compressional velocity. The sound velocities for a variety of 104 [email protected]

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10.10 Piezoelectric Transducers The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and

Piezoelectric Transducer

vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

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Internship report on Non-Destructive Testing www.sajetc.com Additional information on why certain materials produce this effect can be found in the linked presentation material, which was produced by the Valpey Fisher Corporation. The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. A large piezoelectric ceramic element can be seen in the image of a sectioned low frequency transducer. Preceding the advent of piezoelectric ceramics in the early 1950's, piezoelectric crystals made from quartz crystals and magnetostrictive materials were primarily used. The active element is still sometimes referred to as the crystal by old timers in the NDT field. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. They also operate at low voltage and are usable up to about 300oC. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. New materials such as piezopolymers and composites are also being used in some applications.

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Internship report on Non-Destructive Testing www.sajetc.com The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric

crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency contact transducers are not produced is because the element is very thin and too fragile.

10.11 Characteristics of Piezoelectric Transducers The transducer is a very important part of the ultrasonic instrumentation system. As discussed on the previous page, the transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.

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Internship report on Non-Destructive Testing www.sajetc.com A cut away of a typical contact transducer is shown above. It was previously learned that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength. This keeps waves that were reflected within the matching layer in phase when they exit the layer (as illustrated in the image to the right). For contact transducers, the matching layer is made from a material that has an acoustical impedance between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance between the active element and water. Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching. The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

10.12 Radiated Fields of Ultrasonic Transducers The sound that emanates from a piezoelectric transducer does not originate from a point, but instead originates from most of the surface of the piezoelectric element. Round transducers are often referred to as piston source transducers because the sound field resembles a cylindrical mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown below. The intensity of the sound is indicated by color, with lighter colors indicating higher intensity.

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Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference as discussed in a previous page on wave interference. These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area. The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the beam spreads out in a pattern originating from the center of the transducer. The transition between the near field and the far field occurs at a distance, N, and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer. The near/far field distance, N, is significant because amplitude variations that characterize the near field change to a smoothly declining amplitude at this point. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area.

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For a piston source transducer of radius (a), frequency (f), and velocity (V) in a liquid or solid medium, the applet below allows the calculation of the near/far field transition point.

10.13 Transducer Types Ultrasonic transducers are manufactured for a variety of applications and can be custom fabricated when necessary. Careful attention must

be paid to selecting the proper

transducer for the application. A previous section on Acoustic Wavelength and Defect Detection gave a brief overview of factors that affect defect detectability. From this material, we know that it is important to choose transducers that have the desired frequency, bandwidth, and 110 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com focusing to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system. Transducers are classified into groups according to the application.

10.13.1 Contact transducers are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component being inspected.

10.13.2 Immersion transducers These transducers do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a planer, cylindrically focused or spherically focused lens. A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.

10.13.3

More on Contact Transducers

Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducers described below might be used.

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10.13.4

Dual element transducer

It contains two independently operated elements in a single housing. One of the elements transmits and the other receives the ultrasonic signal. Active elements can be chosen for their sending and receiving capabilities to provide a transducer with a cleaner signal, and transducers for special applications, such as the inspection of course grained material. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience (when single-element transducers are operating in pulse echo mode, the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function). Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material. 10.13.5 These

Delay line transducers provide

versatility

with

a

variety

of

replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts its "listening" function so that near surface resolution is improved. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite

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10.13.6

Angle beam transducers

These are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incidence and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the backwall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

10.13.7 Normal incidence shear wave transducers These transducers are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB.

10.13.7

Paint brush transducers

It is used to scan wide areas. These long and narrow transducers are made up of an array of small crystals that are carefully matched to minimize variations in performance and maintain uniform sensitivity over the entire area of the transducer. Paint brush transducers make it possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive transducers are often then required to further define the details of a discontinuity.

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10.14 Couplant A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface. When scanning over the part or making precise measurements, an immersion technique is often used. In immersion ultrasonic testing both the transducer and the part are immersed in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

10.15 Pulser-Receivers Ultrasonic pulser-receivers are well suited to general purpose

ultrasonic

testing.

Along

with

appropriate

transducers and an oscilloscope, they can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a unique, low-cost ultrasonic measurement capability.

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Internship report on Non-Destructive Testing www.sajetc.com The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer. Most pulser sections have very low impedance outputs to better drive transducers. Control functions associated with the pulser circuit include: 

Pulse length or damping (The amount of time the pulse is applied to the transducer.)



Pulse energy (The voltage applied to the transducer. Typical pulser circuits will apply from 100 volts to 800 volts to a transducer.)

In the receiver section the voltage signals produced by the transducer, which represent the received ultrasonic pulses, are amplified. The amplified radio frequency (RF) signal is available as an output for display or capture for signal processing. Control functions associated with the receiver circuit include 

Signal rectification (The RF signal can be viewed as positive half wave, negative half wave or full wave.)



Filtering to shape and smooth return signals



Gain, or signal amplification



Reject control

The pulser-receiver is also used in material characterization work involving sound velocity or attenuation measurements, which can be correlated to material properties such as elastic modulus. In conjunction with a stepless gate and a spectrum analyzer, pulser-receivers are also used to study frequency dependent material properties or to characterize the performance of ultrasonic transducers.

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10.16

Angle Beams I

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.

10.17 Angle Beams II Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

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10.18 Calibration Methods

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, 117 [email protected]

Internship report on Non-Destructive Testing www.sajetc.com to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually only used when an absolute value is measured and in many cases, the standards are traceable back to standards at the National Institute for Standards and Technology. In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in measurements and to help interpret and quantify the information contained in the received signal. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size. This section will discuss some of the more common calibration and reference specimen that are used in ultrasonic inspection. Some of these specimens are shown in the figure above. Be aware that there are other standards available and that specially designed standards may be required for many applications. The information provided here is intended to serve a general introduction to the standards and not to be instruction on the proper use of the standards.

10.19

Weldments (Welded Joints)

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of sidewall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks. With the exception of single gas pores all the defects listed are usually well detectable by ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding applications. This safe, accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology.

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Internship report on Non-Destructive Testing www.sajetc.com Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principles of refraction and mode conversion to produce refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are performed using refracted shear waves. However, material having a large grain structure, such as stainless steel may require refracted longitudinal waves for successful inspections.] This inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam.

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To determine the proper scanning area for the weld, the inspector must first calculate the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the crown, sidewall, and root of the weld.

10.20

Advantages of Ultrasonic Flaw Detection

 Thickness and lengths up to 30 ft can be tested.  Position, size and type of defect can be determined.  Instant test results.  Portable.  Extremely sensitive if required.  Capable of being fully automated.  Access to only one side necessary.  No consumables.

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10.21

Disadvantages of Ultrasonic Flaw Detection

 No permanent record available unless one of the more sophisticated test results and data collection systems is used.  The operator can decide whether the test piece is defective or not whilst the test is in progress.  Indications require interpretation (except for digital wall thickness gauges).  Considerable degree of skill necessary to obtain the fullest information from the test.

 Very thin sections can prove difficult.

11

Applications of Non-Destructive Testing

11.1

Aerospace Industry

Testing components including aero-engine, Landing gear and air frame parts during production 11.2

Aircraft Overhaul

Testing components during overhaul including aero-engine and landing gear components 11.3

Automotive Industry

Testing Brakes-Steering and engine safety critical components for flaws introduced during manufacture. Iron castings – material quality. Testing of diesel engine pistons up to marine engine size. 11.4

Petrochemical & Gas Industries

Pipe-Line and tank internal corrosion measurement from outside. Weld testing on new work. Automotive LPG tank testing 11.5

Railway Industry

Testing locomotive and rolling stock axles for fatigue cracks. Testing rail for heat induced cracking. Diesel locomotive engines and structures. 11.6

Mining Industry

Testing of pit head equipment and underground transport safety critical components. 11.7

Agricultural Engineering

Testing of all fabricated, forged and cast components in agricultural equipment including those in tractor engines.

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Power Generation

Boiler and pressure vessel testing for weld and plate defects both during manufacturing and in subsequent service. Boiler pipe work thickness measurement and turbine alternator component testing. 11.9

Iron Foundry

Testing ductile iron castings for metal strength on 100% quality control basis. 11.10 Shipbuilding Industry Structural and welding testing. Hull and bulkhead thickness measurement. Engine components testing. 11.11 Steel Industry Testing of rolled and re-rolled products including billets, plate sheet and structural sections. 11.12 Pipe & Tube Manufacturing Industry Raw plate and strip testing. Automatic ERW tube testing. Oil line pipe spiral weld testing

12.

Conclusion

In Bangladesh NDT is a new technology and system for inspection and testing. But many developed countries use it because of its huge benefits. Modern NDT methods are becoming ever more quantitative and non-intrusive. This is valid for NDT of new construction and for maintenance inspections. For NDT of new construction this implies that, the more one knows about the material properties and operational conditions, the better the acceptance criteria for weld defects can be based on the required weld integrity and fine-tuned to a specific application. In pipeline industry, this is already going to happen. In plant maintenance, the availability of quantitative and non-invasive screening NDT methods will reduce the time needed for shutdowns and increase the intervals between them. Modern NDT methods will become just as important a tool for Risk Based Inspection approaches and maintenance planning as operational parameters and degradation mechanisms already are.

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13.

Recommendation

The Non-Destructive Testing (NDT) is a new technology in Bangladesh. Saj Engineering & Trading Company is the first Company that bring this technology in Bangladesh. The NDT equipments are very expensive. Although recently some others company come in market to give this service. For a developing country like Bangladesh, NDT is very needed for industrial development. The company should make so more focus about this technology in industrial level. Al the engineers should know about this technology, especially in industrial level. The Bangladesh Government has the only one institution that only offers the NDT courses. That is the Bangladesh Atomic Energy Commission – NDT Division. Another Institution, Bangladesh Boiler Association also use this technology for the boiler testing and inspection. All companies should have the NDT division with NDT practitioner.

14.

References 1. ASNDT- American Society of Non-Destructive Testing. 2. Bangladesh Atomic Energy Commission- NDT Division. 3. Bangladesh Atomic Energy Commission – NDT Division ( NDT fundamental course Handbook) 4. American Airlines. Nondestructive training manual: qualifications programDrury CG, Watson J. (2000). Human factors good practices in borescope inspection. FAA/Office of Aviation Medicine, Washington, D.C.. @ URL: http://hfskyway.faa.gov. Oct 2002. 16 Drury CG. (1999). 5. Human factors good practices in fluorescent penetrant inspection. Human factors in aviation maintenance - phase nine, progress report,FAA/Human Factors in Aviation Maintenance. @ URL: http://hfskyway.faa.gov.Oct 2002.

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Internship report on Non-Destructive Testing www.sajetc.com 6. Adams LK, Czepiel EJ, Krulee GK, Watson J. (1999). Job task analysis of the aviation maintenance technician. FAA/Office of Aviation Medicine, Washington,D.C. @ URL: http://hfskyway.faa.gov. Oct 2002. 7. Allen D. (1970). Phase III Report: A national study of the aviation mechanics occupation. FAA, Washington, DC. [Cited by Adams et al. (1999). Job task analysis of the aviation maintenance technician. FAA/Office of Aviation Medicine, Washington, D.C. @ URL: http://hfskyway.faa.gov. Oct 2002.] 8. Bray, Don E. and Don McBride: ―Nondestructive Testing Techniques,‖ John Wiley & Sons, Inc., 1992. 9. McMaster, Robert C.: ―Nondestructive Testing Handbook,‖ Volume II, The Ronald Press Company, New York 1963. 10. Metals Handbook, Volume 17: ―Nondestructive Inspection and Quality Control,‖ pp. 89128, ASM International, Metals Park, OH, 1989. 11. ASTM E 1444-93: ―Standard Practice for Magnetic Particle Examination,‖ American Society for Testing and Materials, 1916 Race St. Philadelphia, PA 18103. MSFC-STD1249: ―Standard NDE Guidelines and Requirements for Fracture Control Programs,‖ Marshall Space Flight Center, AL 35812, September 1985. 12. R.A. Quinn and C.C. Sigl ―Radiography in Modern Industry,‖ 4th Edition, , Eastman Kodak Company, 1980. 13. ―Industrial Radiography Radiation Safety Personnel,‖ ASNT Practice No. ASNT-CPIRRSP-1A, 2001 Edition, American Society for Nondestructive Testing. 14. ASNT Level III Study Guide and Supplement on Visual and Optical Testing, American Society for Nondestructive Testing, Columbus, OH, 2005. 15. Reliability of Visual Inspection for Highway Bridges, Publication Nos. FHWA-RD-01-020 and FHWA-RD-01-021, June 2001. 16. Reliability of Visual Inspection for Highway Bridges, Publication Nos. FHWA-RD-01-020 and FHWA-RD-01-021, June 2001. 17. F.A. Iddings, Visual Inspection, Materials Evaluation, Vol. 62,No. 5, May 2004, pp. 500501.

18. ASTM International, ASTM Volume 03.03 Nondestructive Testing 19. ASNT, Nondestructive Testing Handbook

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Internship report on Non-Destructive Testing www.sajetc.com 20. Bray, D.E. and R.K. Stanley, 1997, Nondestructive Evaluation: A Tool for Design, Manufacturing and Service; CRC Press, 1996. 21. Hellier, C., Handbook of Nondestructive Evaluation, McGraw-Hill Professional; 2001 22. Shull, P.J., Nondestructive Evaluation: Theory, Techniques, and Applications, Marcel Dekker Inc., 2002. 23. Albert S. Birks, Robert E. Green, Jr., technical editors ; Paul McIntire, editor. Ultrasonic testing, 2nd ed. Columbus, OH : American Society for Nondestructive Testing, 1991. ISBN 0931403049. 24. Josef Krautkrämer, Herbert Krautkrämer. Ultrasonic testing of materials, 4th fully rev. ed. Berlin; New York: Springer-Verlag, 1990. ISBN 3540512314. 25. J.C. Drury. Ultrasonic Flaw Detection for Technicians, 3rd ed., UK: Silverwing Ltd. 2004. (See Chapter 1 online (PDF, 61 kB)). 26. Nondestructive Testing Handbook, Third ed.: Volume 7, Ultrasonic Testing. Columbus, OH: American Society for Nondestructive Testing. 27. Detection and location of defects in electronic devices by means of scanning ultrasonic microscopy and the wavelet transform measurement, Volume 31, Issue 2, March 2002, Pages 77–91, L. Angrisani, L. Bechou, D. Dallet, P. Daponte, Y. Ousten 28. Cartz, Louis (1995). Nondestructive Testing. A S M International. 29. Blitz, Jack; G. Simpson (1991). Ultrasonic Methods of Non-Destructive Testing. Springer-Verlag New York, LLC. 30. www.asndt.com- (NDT Course Material)

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