Non Destructive Testing Experiment Report

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ITU MAK214E...






Instructor: Prof. Dr. Ahmet ARAN



Introduction 1. Definitions 2. Purposes of Non-Destructive Testing 3. Classification of NDT Methods a. Visual Inspection b. Liquid (Dye) Penetrant Method c. Magnetic Particle Method d. Eddy Current Testing e. Ultrasonic Inspection f. Radiography g. Acoustic Inspection


Experimental Procedure 1. Objective 2. Examination a. Eddy Current Inspection i. Equipments ii. Experiment b. Ultrasonic Inspection i. Equipments ii. Experiment c. Liquid Penetrant Method i. Equipments ii. Experiment d. Magnetic Particle Method i. Equipments ii. Experiment


Data Analysis 1. Eddy Current Inspection 2. Ultrasonic Inspection 3. Liquid Penetrant Method 4. Magnetic Particle Method


Results 1. Eddy Current Inspection Experiment 2. Ultrasonic Inspection Experiment 3. Liquid Penetrant Method Experiment 4. Magnetic Particle Method Experiment


Discussion 1. Advantages and Disadvantages of Each Method Performed








I. INTRODUCTION 1. DEFINITIONS Non-destructive testing, NDT (also called NDE, non-destructive evaluation, and NDI, non-destructive inspection) is the testing performed on materials without destroying the test objects and impairing its future usefulness. It is used for in-service inspection and for condition monitoring of operating plant. It is also used for measurement of components and spacing and for the measurement of physical properties such as hardness and internal stress. The essential feature of NDT is that the test process itself produces no deleterious effects on the material or structure under test; therefore it is vital for constructing and maintaining all types of components and structures. The subject of NDT has no clearly defined boundaries; it ranges from simple techniques such as visual examination of surfaces, through the well-established methods of radiography, ultrasonic testing, magnetic particle crack detection, to new and much specialised methods. NDT methods can be adapted to automated production processes as well as to the inspection of localised problem areas. 2. PURPOSES OF NON-DESTRUCTIVE TESTING “Since the 1920s, nondestructive testing has developed from a laboratory curiosity to an indispensable tool of production. No longer is visual examination the principal means of determining quality. Nondestructive tests in great variety are in worldwide use to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure the thickness of materials and coatings and to determine other characteristics of industrial products. Modern non-destructive tests are used by manufacturers to: - ensure product integrity, and in turn, reliability - avoid failures, prevent accidents and save human life - make a profit for the user - ensure customer satisfaction and maintain the manufacturer's reputation - aid in better product design - control manufacturing processes - lower manufacturing costs - maintain uniform quality level - ensure operational readiness.” (The American Society for NDT) 3. CLASSIFICATION OF NDT METHODS There is much kind of non-destructive test methods performed on various materials including metals, plastics, ceramics, composites, cermets, and coatings in order to detect flaws, cracks, internal voids, surface cavities, defective welds and other defects inside and on the surface of the materials. There are some principal factors used to characterize the methods:  energy source or medium used to probe the test object (such as X-rays, ultrasonic waves or thermal radiation)



 nature of the signals, image or signature resulting from interaction with the test object (attenuation of X-rays or reflection of ultrasound, for example)  means of detecting or sensing resulting signals (photo emulsion, piezoelectric crystal or inductance coil)  method of indicating or recording signals (meter deflection, oscilloscope trace or radiograph)  basis for interpreting the results (direct or indirect indication, qualitative or quantitative, and pertinent dependencies). The objective of each test method is to provide information about the following material parameters:  Discontinuities (such as cracks, voids, inclusions, delaminations)  Structure or microstructure (including crystalline structure, grain size, segregation, misalignment)  Dimensions and metrology (thickness, diameter, gap size, discontinuity size)  Physical and mechanical properties (reflectivity, conductivity, elastic modulus, sonic velocity)  Composition and chemical analysis (alloy identification, impurities, elemental distributions)  Stress and dynamic response (residual stress, crack growth, wear, vibration)  Signature analysis (image content, frequency spectrum, field configuration). (The American Society for NDT) Although very special tests have been developed for specific applications, following methods are universal NDT methods. a. VISIUAL INSPECTION:

Visual inspection is the one NDT method used extensively to evaluate the condition or quality of an item. It is easily carried out, inexpensive and usually doesn't require special equipment. It is widely used for inspections of macroscopic surface flaws; welding qualities, dimensional damages and changes, surface finish quality, delaminations, large cracks, cavities, and dents etc. The method requires good vision, good lighting and the knowledge of what to look for. Visual inspection can be enhanced by various methods ranging from low power magnifying glasses through to boroscopes. These devices can also be used with television camera systems. Surface preparation can range from wiping with a cloth to blast cleaning and treatment with chemicals to show the surface details. Visual inspection can sometimes identify where a failure is most likely to occur and identify when a failure has commenced. Visual inspection is often enhanced by other surface methods of inspection, which can identify flaws that are not easily seen by the eye. b. LIQUID (DYE) PENETRANT METHOD:

Dye penetrant inspection, also known as liquid penetrant examination, is a type of nondestructive testing used generally in the detection of surface breaking flaws in nonmagnetic materials for which magnetic-particle inspection is not possible. It can also be



used for the inspection of ferrous materials where magnetic-particle inspection is difficult to apply. In some cases it can be used on non-metallic materials, too. The ability of the liquid penetrant to be pulled into surface-breaking defects by capillary action is employed in this technique to locate cracks, porosity, and other defects that break the surface of a material and have enough volume to trap and hold the penetrant material. Liquid penetrant testing is used to inspect large areas very efficiently and works on most nonporous materials. c. MAGNETIC PARTICLE METHOD:

Magnetic particle inspection is an NDT method that can be used to find surface, near surface and layer flaws in ferromagnetic materials such as steel and iron by employing the principle that magnetic flux will be distorted by the presence of a flaw in a manner that will reveal its presence. The flaw (for example, a crack) is located from the "flux leakage" following the application of fine iron particles to the area under examination. There are variations in the way the magnetic field is applied, but they are all dependent on the above principle. The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible under a UV light. The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles. Surface irregularities and scratches can give misleading indications. Therefore it is necessary to ensure careful preparation of the surface before magnetic particle testing is undertaken. d. EDDY CURRENT TESTING:

Eddy current testing is an electromagnetic technique and can only be used on conductive materials. The technique is based on the principle that alternating electrical current passes through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are called eddy currents. Eddy currents produce their own magnetic field that can be measured and used to find flaws and characterize conductivity, permeability, and dimensional features. This method is used to detect surface and near-surface flaws in conductive materials, such as the metals. Eddy current inspection is also used to sort materials based on electrical conductivity and magnetic permeability, and measures the thickness of thin sheets of metal and nonconductive coatings such as paint. e. ULTRASONIC INSPECTION:

In this method high frequency sound waves are sent into a material by use of a transducer. Ultrasonic very short pulse-waves of frequencies ranging from 0.5-15 MHz and



occasionally up to 50 MHz are used. The sound waves travel through the material and are received by the same transducer or a second transducer. The amount of energy transmitted or received and the time the energy is received are analyzed to determine the presence of flaws. Changes in material thickness and material properties can also be measured. It is used to locate surface and subsurface defects in many materials including metals, plastics, and wood. Ultrasonic inspection is also used to measure the thickness of materials and otherwise characterize properties of material based on sound velocity and attenuation measurements. f.


Radiographic inspection is primarily used to find sub-surface flaws in materials. High voltage x-ray machines produce X-rays whereas gamma rays are produced from radioactive isotopes such as iridium 192. The chosen radiation source is placed close to the material to be inspected and the radiation passes through the material and is then captured either on film or digitally. The choice of which type of radiation is used (x-ray or gamma) largely depends on the thickness of the material to be tested and the ease of access to area of inspection. The sensitivity of the x-rays is nominally 2% of the materials thickness. Gamma sources have the advantage of portability, which makes them ideal for use in construction site working. High energy portable x-ray machines are available for special applications such as concrete structures. X-rays and gamma rays are very hazardous. Special precautions must be taken when performing radiography. Therefore the method is undertaken under controlled conditions, inside a protective enclosure or after assessment with appropriate barriers and warning systems to ensure that there are no hazards to personnel. This effect of the method makes it less popular. Advantages of this method are:    

Can be used to inspect virtually all materials. Detects surface and subsurface defects. Ability to inspect complex shapes and multi-layered structures without disassembly. Minimum part preparation is required.

Disadvantages of the radiographic testing are following: o o o o o o

Extensive operator training and skill required. Access to both sides of the structure is usually required. Orientation of the radiation beam to non-volumetric defects is critical. Field inspection of thick section can be time consuming. Relatively expensive equipment investment is required. Possible radiation hazard for personnel. g. ACOUSTIC METHOD:

Acoustic Emission is used as a type of nondestructive testing technology is in the ultrasonic regime, typically within the range between 100 kHz and 1 MHz. This range is not absolute, Acoustic Emissions can be monitored and detected in frequency ranges less than



1 kHz and have been reported at frequencies up to 100 MHz. This method is used for the detection of the subsurface flaws in the materials being good conductor o sound. A commonly accepted definition for AE is a transient elastic waves within a material due to localized stress release. Hence a source which generates one AE event is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. AE events can also come quite rapidly when materials begin to fail, in which case AE activity rates are studied as opposed to individual events. AE events that are commonly studied include the extension of a fatigue crack, or fiber breakage in a composite material among material failure processes. Examples of AE events generated from sources not involving material failure include leakage, cavitations and impact. Transducers are attached to the material in order to detect these waves. Most of these sensors are in the frequency range of 20 kHz to 650 kHz. Some geophysical studies with AE use much lower frequency sensors, while sensors in the MHz range are also available commercially. “AE analysis is used successfully in a wide range of applications including: detecting and locating faults in pressure vessels or leakage in storage tanks or pipe systems, monitoring welding applications, corrosion processes, partial discharges from components subjected to high voltage and the removal of protective coatings. Areas where research and development of AE applications is currently being pursued, among others, are process monitoring and global or local long-term monitoring of civil-engineering structures (e.g., bridges, pipelines, off-shore platforms, etc.). Another area where numerous AE applications have been published is fiber-reinforced polymer-matrix composites, in particular glass-fibre reinforced parts or structures. (e.g., fan blades). AE systems also have the capability of detecting acoustic signals created by leaks. The disadvantage of AE is that commercial AE systems can only estimate qualitatively how much damage is in the material and approximately how long the components will last. So, other NDE methods are still needed to do more thorough examinations and provide quantitative results. Moreover, service environments are generally very noisy, and the AE signals are usually very weak. Thus, signal discrimination and noise reduction are very difficult, yet extremely important for successful AE applications.” (“Nondestructive Testing Encyclopedia”)

II. EXPERIMENTAL PROCEDURE 1. OBJECTIVE The objective of the experiment is to examine the types, advantages, and applications of various non-destructive testing methods to gain the ability of choosing the best method for a given material. 2. EXAMINATION a. EDDY – CURRENT INSPECTION: i. Equipments: During this experiment, a Magnefest ED-51 0 type unit is used to supply alternating current, and a pencil type ferrite probe is used for the inspections of the part. The inspection is performed with a frequency of 2 MHz. (See the Figure II.2.1.)



Figure II.2.1 – Eddy Current Inspection Method ii. Experiment: As the first step applied during the experiment, the inspected area of the part is examined whether it is clean and free from any paint, dirt, grease or there is any visible damage or discontinuity on the surface. Then the probe is placed on the surface of the part with an angle of 90˚ and it is moved slowly on the surface paying attention not to change the angle and cut the contact between the probe and inspected area. (See the Figure II.2.1.)

b. ULTRASONIC INSPECTION: i. Equipments: For this experiment, a USM-2 type transducer (pulser/receiver), an oscilloscope for monitoring the changes, a probe supporting to work at frequency of 5MHz, and because the inspected surface of the is not flat a supporting fitting tool (shoe) is used. To transmit the ultrasonic waves from the transducer to the test part, glycerin is used as couplant. (See the Figure II.2.2.)



Figure II.2.2 – Ultrasonic Inspection Equipments ii. Experiments: As the first step of the experiment, the transducer is adjusted to a frequency of 5 MHz. Because the inspected surface is not flat, a prepared supporting fitting tool (shoe) is used between the probe and inspected surface. Then, both between the probe and tool, and between the inspected surface and tool glycerin is applied as couplant to transmit the ultrasonic waves and prevent any cavities that affect the transmission in a negative way. Then the probe is placed in the corresponding space in the tool with a specified angel, and then the tool is moved on the inspected surface to detect any discontinuity. (See the Figure II.2.2.)

c. LIQUID (DYE) PENETRANT METHOD: i. Equipments: Three types of sprays in different colors of cans are used during the experiment for different applications. The white can containing the cleaner/remover spray is applied to clean the surface of the part and to remove excess residues on the surface. The red can contains the liquid penetrant spray which infiltrates to the surface defects such as cracks and splits. The yellow can includes the developer spray that absorbs the infiltrated liquid penetrant and reacts with it. (See Figure II.2.3)

Cleaner/Remover Spray

Liquid Penetrant Spray

Developer Spray

Figure II.2.3 – Sprays Used in Liquid Penetrant Inspection ii. Experiment: Cleaning the surface from any dirt, dust etc. is the first step of the experiment, and this application is done using a special chemical ‘cleaner/remover’ (white spray can). The spray is applied to the inspected surface of the part and waited about 10 minutes for the infiltration of the liquid penetrant to the defects. Then the excess penetrant on the surface is



cleaned with a clean textile paying attention not to give some additional damage to the surface. As the last step the yellow can, developer spray, is applied to the surface. d. MAGNETIC PARTICLE INSPECTION: i. Equipments: For the experiment, a strong U shape magnet, a fluorescent magnetic spray, and UV light is used. The magnet provides the necessary magnetic field, and the spray and light is employed to higher the visibility of the flaws, cracks for the ease of the inspection. ii. Experiment: Because the surface of the part is examined, a clear surface is required for the experiment. Therefore, the surface of the part is applied cleaner/remover spray or wiped with a clean textile according to the surface quality of the part. The surface cleaned, fluorescent spray is applied on the inspected surface of the part to make the defects more visible under UV light. Then, according to the normal procedure of the experiment a magnetic field created by a strong magnet should be used; however, the magnet of the laboratory was lost and could not be found during experiment. Therefore, the experiment was continued without using a magnet. After waited for a while for fluorescent to cover the surface, the surface is examined under UV light.

III. DATA ANALYSIS 1. EDDY CURRENT INSPECTION As a result of the electromagnetic induction principle, some currents swirl and magnetic waves occur. The inspection of the discontinuities is done by observing the changes in the scale of the apparatus. When the probe comes to a discontinuity, such as a crack, magnetic field and thus the current reduces which can be observed on the scale. This procedure is applied all sections of the parts and the indications is recorded. 2. ULTRASONIC INSPECTION For data analysis of the experiment, the principle of the echo is employed to detect the flaws. When an ultrasonic sound energy is produced by the transducer, it propagates through the part in the form of sound waves until it reaches the opposing surface of the part or any discontinuity such as a crack. When the waves encounter a crack, it interrupts the propagation of the waves and reflects a portion of them. Therefore, the amount of the energy and the time spent for return of the waves gives information about the presence and location of defects. This operation is done by observing the monitor of the oscilloscope. 3. LIQUID PENETRANT INSPECTION

Kasırga, 10 For inspection, the ability of liquid penetrant to infiltrate into surface defects, capillary action is employed. This infiltrated penetrant is absorbed by developer with principle of the atomic attraction forces, and they reacts with each other. As a result of this reaction, there occurs some color differences on the different sections of the part. Two main colors are observed on the surface: pink/red areas indicate for the surface defects such as cracks and white areas indicates for the clear areas from any cracks. 3. MAGNETIC PARTICLE METHOD When the magnet is placed on the surface of the part, there occur magnetic fluxes around the part. If there is a discontinuity on the surface of the part, magnetic flux is broken and a new polarization exists between the edges of the discontinuity. As a result of this polarization, the edges of the discontinuity behaves like poles of a magnet and they attract some iron particles of which clusters make the discontinuity more visible than the actual one. The fluorescent magnetic spray and the UV light contribute to this inspection by improving this visibility.

IV. RESULTS 1. EDDY CURRENT INSPECTION EXPERIMENT Some cracks have been detected on the surface of the part and more of these cracks were concentrated around the projections on the part, and sections close to the edges of the part. 2. ULTRASONIC INSPECTION EXPERIMENT Some cracks inspected in the part. One of them can be seen in the following figure. (Figure IV.2.1)


Kasırga, 11

Figure IV.2.1 – A Crack Detected During the Experiment 3. LIQUID PENETRANT EXPERIMENT Pink areas indicate for the defects, cracks and when the part examined it is seen that they are concentrated around the holes and canal on the part. (See the Figure IV.3.1) Crack

Figure IV.3.1 – Result of Liquid Penetrant Experiment

4. MAGNETIC PARTICLE EXPERIMENT When the surface of the part is examined it is seen there is some big cracks concentrated around the bolt holes and the edges of the part. This is because the possibility of a crack is higher around the holes and edges in a formed part due to the high inner stresses and stress concentrations around these areas. (See Figure IV.4.1) Crack

Kasırga, 12

Figure IV.4.1 – Magnetic Particle Inspection Method



o o o o

Detects surface and near surface defects. Test probe does not need to contact the part. Method can be used for more than flaw detection. Minimum part preparation is required.

ii. Disadvantages:

      

Only conductive materials can be inspected. Ferromagnetic materials require special treatment to address magnetic permeability. Depth of penetration is limited. Flaws that lie parallel to the inspection probe coil winding direction can go undetected. Skill and training required is more extensive than other techniques. Surface finish and roughness may interfere. Reference standards are needed for setup.


i. Advantages: o o o o o

Depth of penetration for flaw detection or measurement is superior to other methods. Only single sided access is required. Provides distance information. Minimum part preparation is required. Method can be used for much more than just flaw detection.

ii. Disadvantages:

   

Surface must be accessible to probe and couplant. Skill and training required is more extensive than other technique. Surface finish and roughness can interfere with inspection. Thin parts may be difficult to inspect.

Kasırga, 13  

Linear defects oriented parallel to the sound beam can go undetected. Reference standards are often needed.


i. Advantages: o Large surface areas or large volumes of parts/materials can be inspected rapidly and at low cost. o Parts with complex geometry are routinely inspected. o Indications are produced directly on surface of the part providing a visual image of the discontinuity. o Equipment investment is minimal. ii. Disadvantages:

     

Detects only surface breaking defects. Surface preparation is critical as contaminants can mask defects. Requires a relatively smooth and nonporous surface. Post cleaning is necessary to remove chemicals. Requires multiple operations under controlled conditions. Chemical handling precautions are necessary (toxicity, fire, waste). d. MAGNETIC PARTICLE INSPECTION

i. Advantages: Large surface areas of complex parts can be inspected rapidly. Can detect surface and subsurface flaws. Surface preparation is less critical than it is in penetrant inspection. Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity. o Equipment costs are relatively low. o o o o

ii. Disadvantages:

     

Only ferromagnetic materials can be inspected. Proper alignment of magnetic field and defect is critical. Large currents are needed for very large parts. Requires relatively smooth surface. Paint or other nonmagnetic coverings adversely affect sensitivity. Demagnetization and post cleaning is usually necessary.


Kasırga, 14

Modern nondestructive tests contains useful methods used by manufacturers for the purposes to ensure product integrity, and in turn, reliability; to avoid failures, prevent accidents and save human life; to make a profit for the user; to ensure customer satisfaction and maintain the manufacturer's reputation; to aid in better product design; to control manufacturing processes; to lower manufacturing costs; to maintain uniform quality level; and to ensure operational readiness.

VI. REFERANCES [1] American Society for Non-Destructive Testing, retrieved from, on October 31, 2006. [2] “Non-destructive Testing Encyclopaedia”, by R. Diederichs and E. Ginzel [3] The British Institute of Non-Destructive Testing, retrieved from, on November 1, 2006. [4] Canadian Institute for NDT, retrieved from, on November 1, 2006. [5] WIKIPEDIA, retrieved from, on November 1, 2006. [6] Material Measurements Ltd, retrieved from, on November 3, 2006. [7] CD International Technology, retrieved from, on November 4, 2006. [8] NDT Resource Centre, retrieved from, on November 4, 2006. [9] “Material Testing Laboratory Manual”, ITU, 2006.

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