ASNT Level III Study Guide Electromagnetic [Yasser Tawfik].pdf

ASNT Level III Study Guide

ElectroDlag etic Testing second edition

The American Society for Nond estructive Testing

ASNT Level III Study Guide

ElectroDla Testing second edition

The American Society for Nondestructive Testing, Inc.

etic

Publi!>hed by The Ameriean Society for Nondestructive Testing. Inc. 17 11 Arlingale Lune PO Box 285 18 Columbu!>. OH 43228-05 18 No purt of this book may be reproduced or transmitted in any form, by means electronic or mechanical including photocopying , recording . or otherwise . without the expressed prior written pennission of the publisher. Copyright 10 2007 by The American Society for Nondestructive Testing. Inc. ASNT is not responsible for the authenticity or accuracy of infonnation herein. Products or services that are advertised or mentioned do not carry the endorsement or recommendation o f ASNT. IRRSP, NDT Handbook. The NDT Technician and www.asnt.org are trademarks of The American Society for Nondestructive Testing. Inc. ACep, ASNT, Level III Study Guide, Materials Evaluation . Nondestructive Testing Handbook, Research in Nondestructive Evalllation and RNDE and are registered trademarks of The American Society for Nondestructi ve Testing, Inc. ASNT Mission Statement: ASI\/'[ exists to create a safer world by promOTing the profession alld rechnologies afnondestructive resting.

ISBN-13: 978-1·57117-164-1 Printed in the United States of Amerie:l

09/07 first printing

II

Foreword ASNT methods committees, at the direction of the Technical and Education Council, have prepared Level III Study Guides that aTe intended to present the major a(('as in each nondestructive testing method. This Study Guide was updated and revised v..r:ith the assistance of the Electromagnetics Committee. The Lcvcllll candidate should use ASNT Level III Study GJ/ide: Electromagnetic Testillg Method only as a review, as it may not contain all of the informa tion necessary to pass a typical ASNT Level III cxaminCltion. The electromagnetic testing method has several subdisciplines. The general consensus at the time of this revision is that there are four specific field techniques: eddy current testing. flux leakage testing. remote field testing and alternating current field measurement. Each of these techniques may provide some information in specific material testing applications that the others may not be able to provide in the same test situation. The primary focus of this document wilt be eddy current testing. Some information is provided to define how the other electromagnetic testing techniques might be applied.. In using this Study Guide, the reader ,vill be given specific references, including page numbers, where additional detailed information can be obtained. Typical Level III question s are available at the end of each chapter to aid in detennining comprehension of the material. A typical use of this Study Guide might include the foUo·wing sequence: An individual should review the qucstions at the end of each chapter in the Study Guide to detcnnine if his or her comprehension of electromagnetic testing is adequate. The questions will serve as an indicator of the individual 's ability to pass a Level III examination. If the individual finds questions in a certain chapter of the Study Guide to be difficult, it is suggested that the individual carefully study the information presented in that chaptcr. This review of the information in the Study Guide will serve to refresh one's mt.'mory of theory and forgotten facts. If the individual encounters information that is new or not clearly understood, then it is important to note the specific references given tluoughout the Study Guide and carefully read this information. Referenccs are indicated by parentheses and the reference number: (N).

iii

Preface Early experimenters in the field of magnetism and electromagnetism established the basis fo r the principles of electromagnetic nondestructive testing used today. In 1820, Hans Christian Oersted discovered the magnetic field surrounding a conductor when current "w as passed through the conductor. In 1820, Andre-Marie Ampere discovered that equal currents flowing in opposite directions in adjacent conductors cancelled the magnetic effect. This discovery has led to development of modern coil arrangements and shielding techniques. In 1824, Dominique F. Arago discovered that the vibration of a m agnetic needle was rapidly damped when it was placed neM a n onmagnetic conducting disk. Michael Faraday discovered the principles of electromagnetic induction in 1831. James Clerk Maxwell integrated the results o f these and other works in a two-volume work published in 1873 and Max.vell's equations are still the basis for investigations of the magnetic and electromagnetic phenomena. The application of these laws and principles has led to the development of an industry whose purpose is to qualitatively and quantitatively investigate the properties and characteristics of electrically conductive materials using nondestructive electromagnetic techniques. As in any industry, controls and guidelines must be established to ensure consistent and reproducible products or services. This Study Guide is intended to provide ASNT Level III candidates w ith a concise reference with which to prepare for the ASNT Level III Examination.

iv

Acknowledgments A special thank you to the technical editor who coordinated this revision and updated major portions himself: Jim Cox, JECNDT, LLC A special thank you goes to the follow ing reviewers who helped with this publication: Claude Davis, Unified Testing & Engineering Services, Inc. Darrell Harris, Anchorage, Alaska Gary Heath, All T~ch Inspection, Inc. Michael J. Ruddy, Tuboscope NOV The Publications Review Committee includes: Chair, Joseph L. Mackin, Intemational Pipe Inspectors Association Stephen P. Black. Clermont, Florida Mark A. Randig, Team Industrial Services, Tnc.

Cynthia Meter Leeman Educational Materials Supervisor

v

Table of Contents Foreword ................ • ....•....•.... • .... •. .... Preface .................. . .. , . . .. .......... . . • . . . .•. Acknowledgments .......... . , .............. , .... . . .

. . . Ill . . . . 1V

. . . . . .. V

Chapter 1 - Principles of Eddy Current Testing · ....... ... . .... 1 · . . . . . .. . . ... . .. 1 Historical Background Generation of Eddy Currents ....... 2 Field Intensity ........ . ...... . . ..... .3 Current Density ....... . . .. .4 Phase ! Amplitude and Current Time Relationships . .5 Chapter 1 - Review Questions. , . . ..... . . . . .. 7 Chapter 2 - Test Coil Arrangements .... ... . . . •. . . . •. .. . •... ...... 9 Probe Coils ....... . ........ .•. . . .• . .. . • . . . . •. ...... .. 9 Encircling Coils ...........................•.. .. • ....... .. 9 Bobbin Coils .............. • .... • . . .. • .... • .... • .... . ... 10 Absolute Coils .............. ... . . . . . . . • .. . .. . .......10 Differential Coils . . .. . ...... . · •.• . .• ...... . .10 Hybrid Coils . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . .11 Additional Coil Characteristics . . . . . . . . . . . . . . . . . . .12 Chapter 2 - Review Questions. . . . . ....... . • . ...... . .. ... . . .13 Chapter 3 - Test Coil Design .... . .15 .. . .15 Resistance Inductance ..... . . .. 15 Inductive Reactance ...... . .... . .... . .... • . ... •. . .... .... 16 Impedance ............................. . .... ... .. ...... 17 Q or Figure of Merit . .. . . ... . ... . . ... . . .18 Permeability and Shielding Effects .... 18 Coil Fixtures ......... . .1 9 Chapter 3 - Review Questions. . . . . . . . . . . . .20 Chapter 4 - Effects of Test Object on Test Coil . . . . . .21 Electrical Conductivity .................... . . ... 21 Permeability .... ... ....... ... ..... ....... . .... . • .. ..... 22 Skin Effect ......... . • . . . . .• . . . . • . . . • •. .. . •. ..... .23 Edge Effect ... .. •.. .. •.. .. .. . . . .... . ....•...... .23 End Effect .......•. . ..•.... • .. ... .. 23 Lift Off ....... . . . • . .. .•. ..... .23 Fill Factor .......... . . . . . . . . .24 Discontinuities .. ... .•. . . . .25 Signal-to-Noise Ratio .25 Chapter 4 - Review Questions . . . . . • ..... . • . . •.•.... • . .•. ... ... 26

vii

Ch apter 5 - Selection of Test Frequency .. Frequency Selection .... . . . .... . Single Frequency Systems Multifreql1ency Systems Chapter 5 - Review Questions ...

. ..... 27 . .. 27 .... 27 . ..30 . .. • ... .. . . . ... .32

Chapter 6 - Instrument Systems . . . . . . . . . . . . .. 33 Impedance Testing . . . .. . .... . ......... . . . . . ... 34 Phase Analysis Testing . .. . ... ... . ... ...... .... ..34 Vector Point . . . ....... . .. .... . ... ... .. .. .. . . •. . . . . ..34 Ellipse .. . . . . ... .. . .. . . . . ... . . . ........ . . . ..34 Linear Tune Base ...... .. . . . . ...... . . . . . . . . . . .. 34 Impedance Plane Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 Mode of Operation ...... . . . . . . . .. . . . .. . . . . .•....... 36 Signal Compensation . . . . . . . . . . . . . . . . . . . .. 36 Test Coil Ex("; tation ..... .. .. . .. .. . . . . .. .36 Read Out Mechanisms . . . .. . . . . . . . . . . . . . . . . . . ..38 Indicator Lights .. . .. ........ .38 Audio Alarms .. .38 Meters .. . • .. . ... .39 Digital Displays .39 Cathode Ray Tubes ... 4() Recorders ... 4() Computers . . . . ..... .. . . . . .. .. ... . .. . . . .. Al Test Object Handling Equipment . . .... . . . . . . . . .. Al Probe Delivery Systems . . ... . . .. ...... . .42 Chapter 6 - Review Questions ..... .. ... . .. . . . . . .. 44 Chapter 7 - Eddy Current Applications ... ..A5 Discontinuity Detection . • ........ .45 Dimensiona l MeaslUements .. . . • .• ... .. . .47 Conductivity Measurements . . . . . .• . .•. • . ... • . . ..... 48 Hardness MeaslUements . . ..... . . . . ... 48 Alloy Sorting .. . . . . .... . ... .. ...... .. ..... . .. .. .. . .. . . .48 Chapter 7 - Review Question s .. . ............ .. .......... 50 Chapter 8 - Other Electromagnetic Techniq ues ...... . ............ .51 Alternating Current Field Measu rement .................. .51 Advantages Compared to Magnetic Particle and Dye Penetrant Inspection . ... ..... . . .. . . ....... . . . . .. . . . .. 51 Flux Leakage Testing .... .............. . .. . . . . . .52 Remote Field Testing .... . . . . . . ...53 Chapter 8 - Review Questions . . . . . . . . . . . . ....57 Chapter 9 - Eddy Current Procedures, Standards and Specifications .59 Ametican Society for Testing and Materials. . . . . . . , .59 Military Standard ....... ........... . . .. .60 American Society of Mechanical Engineers .......... . . .. . .60 Ss are usually constructed of high permeability powdered iron. Probe coils, for example, are wound on a form thai allows a powdered iron rod or slug to be placed in the center of the coil (4). It is common to increase the coil impedance by a factor of 10 by the addition of core materials. This increase in impedance withou t additional wind ing greatly enhances the Q of the coil. Some core materials are cylinder or cup shaped. A common term is Clip core (Figure 3.3). The coil i .. first "Wou nd and then p laced into the cup core. In the case of a probe coil in a cup core, not only is the impedance i"creased, but the benefit of sJri!!lrling is also gained. Shielding with a cup core prevents the electromagnetic field from spreading at the sides of the coil. This greatly reduces the signals producl,.'d by edge effect of adjacent members to the te~ t area, such as fasteners on an aircra ft wing. Shielding, while improving resolution, !lSI/ally sncrifices some amount of penetration into the part. Another technique of sh ielding uses high cond uctivity material, such as copper or aluminum, to suppress high frequency interference &om other sources and a1so to shape the electromagnetic field o f the test coil. A copper Clip would restrict the electromagnetic field in much the same manner as the powdered iron cup core. A disadvantage of high conductivity, low or no permeability shielding is that the coil's impedllllce is reduced when the

Equation 16

In coil design \t is often helpful to know also the included angle between the resistive component and impedance. A convenient method of notation is the polar form where Tan 9 = Xl. -:- R and e is the induded angle between resistance and impedance. In the previous example the im}>l>Cd near a test coil is to: A. reduce material permeability effects. B. produce possible magnetic sahlfation in the test materia 1. C. provide a balance source fOT the sensing coil D. both A and B.

Inductance might be referred to as being analogous to: A. force .

B. volume. C. inertia. O. velocity.

Q.3.3

Q.3.4

The unit of inductance is the: A. henry. B. m axv,'cl1. C. o hm. D. farad .

The inductance of a multilayer air core coil wi th the dimensions I = 0.2, r = 0.5, b = 0.1 and N = 20, is: A. 1.38 H . B. 13.8 ~ I-l . C. 13.8 ohms. D. 1.38 ohms.

Q.3.5

The inductive reactance of the coil in Q.3.4, opera ting at 400 kHz, would be: A. 1380 ohms. B. 5520 ohms. c. 34.66 ohms. D. 3466 ohms.

Q.3.6

The impedance of a lOO ll H coil with a res istance of 20 ohms operating at 100 kH z would be: A. 62.8 ohms. B. 4343.8 ohms. C. 628 ohms.

Q.3.1O The most important consideration when selecting a test coil is: A. sensitivity. B. resolution. C. stability. D. meeting established inspection criteria.

D. 65.9 ohms.

20

Chapter 4 Effects of Test Object on Test Coil conductor is a poor resistor. Conductance and resistance are direct reciprocals as s tated earlier. Conductivity and resistivity, however, have different origins and units; therefore, the conversion is not so direct. As previously discussed, conductivity is expressed on an arbitrary scale in percent lACS. Resistivity is expressed in absolute terms of micro-ohm-centimeters. To convert values on one scale to the other system of units a conversion factor of 172.41 is required. Once you know either the conductivity or the resistivity value for a material the other electrical property can be calculated.

As previously seen, the eddy current technique depends on the generation of induced currents within the test object. Disturbances in these small induced currents affect the test coil. The result is a variance of the test coil impedance due to test object variables. These variances are called operating or test ariablcs (15). The range of test variables encountered might include electrical conductivity, magnetic permeability, skin effect lift off, fill factor, end effecl, edge effect and signal-ta-noise ratio. Coil impedance was d iscussed at length in Chapter 3. In this chapter coil impedance changes

will be represented graphically to more effectively explain the intcmction of the operating variables.

172.42

% [ACS= ~~~~~~~---­

Electrical Conductivity

Resistivity (in rnicro-ohm-cm)

In electron theory the atom consists of a positive nucle us surrounded by orbiting negative electrons. Materials that allow these electrons to be easily moved out of orbit around the nucleus are classified as condllctors. In conductors electrons are moved by applyi ng an outside electrical force. The ease with which the electrons are made to move through the conductor is called cOlldl/Ctallce. A unit of conductance is the siell1e11s (mho). The siemens is the reciprocal of the ohm or conductance G = l / R where G is conductance in siemens and R is resistance in ohms. In eddy current testing, instead of describing conductance in absolute terms, an arbitrary unit has been aSSigned. Since the relative conductivity of metals and alloys varies over a wide range, the need for a conductivity benchmark is of prime importance. The International Electrochemical Commission established in 1913 a convenient technique of comparing one material to another. The commission established that a specific grade of high purity copper, 1 m in length, with a uniform cross section o( 1 mm 2, measuring 0.017241 ohms at 20 °C would be arbitrarily considered to be 100% conductive. The symbol for conductivity is (} (sigma) and the unit is percent lACS or percent of the International Annealed Copper Standard. T electrical conductivity by redistributing elements in the materiaL Dependent on materials and degree of heat treatment, conductivity can either increase or decrease as a result of heat treatment. Stresses in a materia l due to cold working produces lattice distortion or dislocation (2). This

mechanical process changes the grain structure and hardness of the material, changing its electrical conductivity. Hardness in age hardellnble aluminum alloys changes the electrical conductivity of the alloy. The electrical conductivity decreases as hardness in creases. As an example, a Brinell hardness of 60 is represented by a conductivity of 23 and a Brinell hardness of 100 of the same alloy would have a conductivity of 19.

Fig ure 4.1: Conductivity curve

IN--.! I I tr o(air) % I r--. Co~d";IiV;~ I '< I

Permeability Permeability of any material is a measure of the ease with v·:hich its magnetic domains can be aligned or the ease with which it can establish lines of force (2). Materials are rated on a comparative basis. Air is assigned a permeability of l. Ferromagnetic metals and alloys induding nickel. iron and cobalt tend to concentrate magnetic flux lines (15). A5 discussed in Chapter 3, some ferromagne tic materials or sifltered ionic compollnds are also useful in concentrating magnetic flux (4). Magnetic permeability is not constant for a given material. The permeability in a test samp le depends on the magnetic field acting on it. As an example, consider a magnetic steel bar placed in an encircling coil. As the coil current is increased, the magnetic

"

t

\

2%

15-

,%

'\

10% .....

I I

100% lACS ......

Resistance -

Table 4.1: Electrical resistivity and conductivity of several metals and alloys Material

Resistivity micro-ohm-cm Utncm)

6.90 2.65 4.10 5.30

Admiralty Brass Aluminum (99.9)

6061-T6 7075-T-6 2024-T4

5.70 12.00

Aluminum Bronze Copper Copper Nickel 90-10 Copper Nickel 70-30 Gold Corrosive Resistant Nickel Alloy High Temperature Nickel Chromiu-m Alloy Lead Magnesium (99%) Stainless Steel 304 Stainless Steel 316 TItanium 99% Tung sten Zirconium

1.72 18.95 37.00 2.35 130.00

100.00 20.77 4.45 72.00 74.00

48.60 5.65 40.00

22

Conductivity % lACS

25.00 64.94 42.00 32.00 30.00 14.00 100.00

9.10 4.60 75.00 1.30

1.72 8.30 38.60 2.39

2.33 3.50

30.00 4.30

field of the coil wi ll increase. The magnetic flux within the steel will increase rapidly at first and then will tend to level off as the s teel approaches magnetic saturation. This phenomenon is called the

Figure 4.2: Edge effect

Good coupling

Bllrkha/lsell effect (4). When increases in the magnetizing fo rce produce little or no change on the flu x w ithin the steel bar, the bar is magnetically satu rated. When ferromagnetic materials are satu rated, permeability becomes constant. With magnetic permeability constant, ferromagnetic materials may be inspected using the eddy current method. Without magnetic saturation, ferromagnetic materials exhibit such a wide range of permeability variation that signals produced by discontinuities or condu ctivity variations are masked by the permeability signal (15).

(

--•

-•

.-

)

~

Skin Effect

Decreased coupling

(

r-

OO

Electromagnetic l'esLs in many applications are most sensitive to test object va riables nearest the test coil because of skin effect. Skill effect is a result of mutua l interaction of eddy currents, operating frequency, test object conductivity and permeability. The skin effect, the concentration of eddy currents it' the tes t object nearest the test coil, becomes more evident as test frequency, tes t object conductivity and permeability are increased (4). For current density or eddy current distribution in the test object, refer to Figure l.8 in Chapter 1.

E"d effect follows the same logic as edge effect. End effect is the signal observed when the end of a product approaches the test coil. Response to end effect can be reduced by coil shielding or reducing coil width in outside diameter encircling or inside diameter bobbin coils. End effect is a term most applicable to the inspection of bar or tubular products.

Edge Effect

Lift Off

The electromagnetic field produced by an excited test coil extends in all directions from the coil. The coil's field p recedes the coil by some distance (2) deh'!rmined by coil par,mleters, operating frequency ""d lest object characteristics. As the coil approaches the edge of a test object, eddy current flow in the test sample becomes distorted by the edge. This is known as edge effect. Edge effect can create a change in the coil's impedance that is similar to a discontinuity (Figure4.2). The response would move back along the conductivity curve toward the air point. The coil is responding to a slightly less conductive situation (.li r) at the leading edge of the coil's field of view. It is therefore essen tial that edge effect be eliminated as a variable during a surface sca nning test. Response to the edges of test objects can be reduced by: incorporating magnetic s hields around the test coil, increasing the test frequency, reducing the test coil diameter or by changing the scanning pattern used. Edge effect is a term most applicable to the inspection of sheets or plates with a probe coil.

Electromagnetic coupling between test coil and test object is of prime importance when conducting an eddy current examination. The coupling between test coil and test object varies with spacing between the test coil and test object. This spacing is called lift off (4). The effect on the coil impedance is called lift

End Effect

off effect. Figure 4.3 shows the relationship between air, conductive materials and lift off. The electromagnetic field, as previously discussed, is strongest near the coil and dissipates w id1 distance from the coil. This fact causes a p ronounced lift off effect for small variations in coil to object spacing. As an example, a spacing change from contact to 0.0254 mm (0.001 in.) will produce a lift off effect many times greater than a spacing change of 0.254mm (0.010 in.) to 0.2794 mm (0.011 in.) (15). Lift off effect is generally an undesired effect causing increased noise and reduced coupling resulting in poor measuring ability (12). In some instances, equipment haVing phase d iscrimination capability can readily separa te lift off from cond uctivity o r other variables. Lift off can be

23

used to advantage when measuring nonconductive coatings on conductive bases. A nonconductive coating such as p aint or plastic causes a space betv/een the coil and conducting base, allowing lift off to represent the coating thickness. Lift off is also useful in profilometry and proximity applications. Lift off is a term m ost applicable to testing objects with a smfilce or probe coil.

equation resulting in the d ivision of effective coil and part area,>. Because the term rr. / 4 appear.:; in both the numerator and the denominator of this fractional equation the term rr. / 4 cancels out, leaving the ratio of the diamctf::!rs squared:

d'

- , = 11 = Fill Factor D-

Fill Factor

Equation 21

Fill factor will always be a number less than 1 and efficif::!nt fill factors approach L A fill factor of 0.99 is more desirable than a fill factor of 0.75. The effect of fill factor on the test system is that poor fUl factors do not allow the coi! to be sufficiently coupled to the test object. This is analogous to the effect of drawing a bow only slightly and releasing an arrow. The result is, with the bow slightly drawn and released, little effect is produced to propel the arrow. in electrical terms it is said that the coil is loaded by the test object. How much the coil is loaded. by thf::! test object due to fill factor can be calculated. in relative terms. A test system with constant current capabilities being affected by a conductive nonmagnetic bar placed into an encircling coil can be used to d emonstra te this f::!ffect. For this example, the system parameters are as follows: (a) Unloaded coil voltage equals 10 V. (b) Tes t object effective permeability equals 0.3. (c) Test coil ins ide diameter equals 25.4 mm (1 in.) (d) Test object outside diameter equals 22.9 mOl (0.9 in.)

Fill facfor is a term used to describe how well a test object will be electromagnetically coupled. to a test coil that surround s or is in serted into the test object. fill fac tor then pertains to inspections using bobbin or cllCi rclin g coils. Like lift off, electromagnetic coupling between test coil and test object is most efficient when the coil is neaTCst the surface of the pilIt. The area of a circle (A) is determined using the equation:

Eq uation 20

Fill factor can be described as the ratio of test object diameter to coil diameter sq uared (Figure4.4). The d iameters squared is a simplified Figure 4.3 : Lift off/conductivity relationships

90" 0% lACS

(09)'

Fi II Faclor q~ - ;-

~ 0.81

Equation 22

An equation demonstrating coil loading is given by: Angle A

I

olr---L--~

__-1-_-1-_ --'-_ --'-_--"-_

100% lACS ---"' 0'

where: Eo

Resistance - --

E lift off: The change in coil impedance due to a changing (air) gap

between the coil and the material being tested.

'I .uiff =

24

coil voltage with coil affected by air coil voltage with coil affected by test object fill factor effective permeability

establishes a standard depth of penetration at the midpoint of the rube wall. Th is condition would allow a Encircling Rod 10 relative current coillD OD - - density of about 20% on the far surface of the tube. With this condition, identical near and far surface discontinuities \·..,ould have greatly different responses. Due to current magnirude alone, the near surface discontinuity response would be nearly five times that of the far surface discontinuity. Discontinuity orientation has a dramatic effect on response. As seen earlier, discontinuity response is maximum when eddy currents and discontinuities are at 90 degrees or perpendicular. D iscontinuities parallel to the eddy current flow produce little or no response. The easiest technique to ensure detectability of discontinuities is to use a reference standard or model that provides a consistent means of adjusting instrumentation (12).

Figure 4.4: Fill factor ratios OR Compare either: I

,,

,,, ,,

Tube Bobbin 10 ID - - - coil 00

,,,

\

,,

When a nonferromagnetic test object is inserted into the test coil, the coil's voltage wil l decrease.

E E E

E

=

10[(1 - 0.81) + (0.81) (0.3)] 10[0.19 + 0.243J 10[0.433] 4.3 V

This allows 10 - 4.3 or 5.7 V available to respond to test object changes caused by discontinuities o r

decreases in effective conductivity of the test object. It is suggested that the reader calculate the resultant loaded voltage developed by a 12.7 mm (0.5 in.) bar of the same material and observe the relative sensitivity difference.

Signal-Io-Noise Ralio

Discontinuities

Signal-fa-liaise ratio is the ratio of signals of interest to unwanted signals (4). Common noise sources are test object variations of surface roughness, geometry and homogeneity. Other electrical noises can be due to such external sources as welding machines, electric motors and generators. Mechanical vibrations can increase test system noise by physical movement of test coil or test object. In other words, anything that interferes with a test system' s ability to define a measurement is considered lIoise. Sigllal-to-noise ratios can be improved by several techniques. If a part is dirty or scaly, signal-to-noise ratio can be improved by cleaning the part. Electrical interference can be shielded or isolated. Phase discrimination and filtering can improve signal-to-noise ratio. It is common practice in nondestructive testing to require a minimum signal-to-noise ratio of 3 to 1. This means a signal of interest must have a response at least three times that of the noise at that point.

Any d iscontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage and corrosion, generally cause the effective conductivity of the test object to be reduced. Discontinuities open to the surface are more easily detected than subsurface discontinuities (15). Discontinuities open to the surface can be detected with a wide range of frequencies; subsurface investigations require a more careful frequency selection . Discontinuity detection at depths greater than 12.7 mm (0.5 in.) in stainless steel is very difficult. This is in part due to the sparse distribution of magnetic flux lines at the low frequency required for such deep penetrations. Figure 1.8 is again useful to illustrate discontinuity response because of current distribution. As an example, consider testing a nonferromagnetic tube at a frequency that

25

Chapter 4 Review Questions Q.4.1

Materials that hold their electrons loosely are classified as: A. resistors. 8. conductors. C. semiconductors. O. insulators.

QA.6

Diamagnetic materials have: A. a permeability greater than air. B. a permeability less than air. e. a permeability greater than ferromagnetic materials. D. no permeability.

Q.4.2

100'%, lACS is based on a specified copper bar h aVing a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms.

Q.4.7

Edge effect can be reduced by: A. shielding. B. selecting a lower frequency. C. u sing a smaller coil. D. both A and C.

Q.4.8

Q.4.3

A resistivity of 13llohrn em is equivalent to a conductivity in percent lACS of: A. 11.032. 6. 0.0625. C. 1652. D. 13.26.

Ca lculate the effect of fill factor when a conducting bar 12.7 mm (0.5 in.) in diameter with an effective pi!I meability of 0.4 is placed into a 25.4 mm (1 in.) diameter coil with an un l oa d ~d voltage of 10V. The loaded voltage is: A. 2V. B. 4.6V. C. 8.5V

Q.4.4

D. 3.2V.

A prime factor affecting conductivity is: A. temperature.

B. hardness. QA.9

C. heat treatment. D. a ll of the .0

.b

Zplw,) 0.9

0"'------';;_ _ _ _ __ R, Resistance R (relative scale)

(b)

§:

0.8

'-I

0.7

~

'

. 1.2 1.4 -'''''~=t--+-~ 0.6 >.6

kr = r J(W\.l(f) '" 2.

Solid CYlindrJI bar

~

0.6

2.'

28'_t-_ 3.0 ·/' 1

05

,. ,.,,"il---I--/,- .

0.4

Er...._f

'.0

-/-+-_ 4-+ >.2

0.'

I

Or-Nl, 0.2

0.'

c

o

1'.4 n

H)()

'~=---.L2.0 o

0.'

0.2

," 0.3

'6[-1-1 0.4

0.5

0.6

Normalized resistance H{UlLo)-1 (0) B

k = v(t~.J(f) = electromagnetic wave propagation constant for conducting material r = radius of conducting cylinder (m) J1 = magnetiC permeability of bar (4n x 10-7 H'm- I if bar is nonmagnetic) o = electrical conductivity of bar (S'm- I ) (J) angular freQuency = 2n'where , freQuency (Hz) v(UlLoG) = equivalent of v(~o) for simplified electrical circuits, where G = conductance (8) and inductance in air (H)

o Resistance R (relative scale)

=

B. C, 0, E, F ",loci for selected values of Zp G = secolldary conductance Zp = primary impedance w = angular frequency = 'lit, where' = frequency (Hz) UlLS = secondary reactance

=

Lo '"

29

Figure 5.3 also ill ustrates a sensitivity ratio for surface and subsurface d iscontinuities. Notice with an fllg ratio of 50, a relatively high frequency, the response to subsurface discontinuities is not very pronounced. Figure 5.4 shows responses to the same discontinuities with an flig ratio of 15. This lower frequency allows better detection of subsurface d iscontinuities as shown in Figure 5.4.

frequency arc called I1Il1ltifreqllellcy or mll/tiparalllt>fer systems. It is conunon for a test coil to be driven with three or more frequencies. Although several frequenci es may be applied Simultaneously or sequentially to a test coil, each of the individual frequencies follows rules established by single frequency tI...'C hniques. Signals generated at the various frequencies are often combined or mixed in electron ic circuits that algebraically add or subtract signals to obtain a desired result . One multurequency approach is to apply a broadband signal, with many frequency components, to the test coil (4). The information transmitted by this signal is p roportional to its bandwidth and the logarithm of 1 plus the signal-to-noise po'wer ratio, Thi s relationship is stated by the eq'llation:

Multifrequency Systems It becomes obvious that the technician must have a good working knowledge of current density and phase relationships to make intell igent frequency choices. The frequency chOice discussed to da te deals with coil systems driven by only one frequency. Test systems driven by more than one

Figure 5.3: Impedance variations caused by surface and subsurface cracks

Figure 5.4: Impedance variations caused by surface and subsurface cracks

0.14

0.1

-g" -" og~'I:: 2 0 '·

0.08

ciJ&

~"',g

0

6£'0

0.06

Distance of crack from surface in %

0.04

of diameter

0.02 0 6R 6R

0 .02

0 .04 0,06 wLo

"'Lo

Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders. at a freque ncy ratio flfg = 50.

Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders, at a frequency ratio flfg '" 15,

30

First, a frequency is selected to give optimum phase and amplitude information about the tube wall. This is ca lled the prime frequency. At the prime frequency, the response to the tube support and to a calibration through wall hole are about equal in amplitude. They may also ha\'e about the same phase angle. A second freque ncy called the sub/rador frequency is selected on the basis of the phase angle of the tube support response. Because the tube support surrounds the outside diameter of the tube, a lower frequency is selected . At the subtractor frequency the tube support signal response is about 10 times greater than the calibration through wall hole. The phase difference between the support signal and the through wall hole in this lower frequency will be about 90 degrees. Parameter separation limitations are greatest for those parameters producing nearly similar Signals, such as dents. If the prime and subtractor channels have been selected properly then Signal subtraction algorithms should be able to suppress the tube support signal leaving only slightly attenuated prime data (discontinuity) information. For suppression of inside or near surface Signals, a higher subtractor frequency would be chosen. A combination of prime, low and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. Bandwidth of the coil is of prime importance when operation over a wide frequency range is required in multifrequency I multiparameter testing. Optimization of a test frequency (or frequencies) will therefore depend on the desired measurement or parameter(s) of interest (11, 12, 4).

Equation 29 where:

C

=

B

=

S/N =

rate of information transmitted in bits per second bandwidth of the signal signal-ta-noise power ratio

This is known as the Shannon-Hartley theorem. Another approach to multiparameter techniques is to use a multiplexing process (12). The multiplexing process places one frequency at a time on the test coil. This results in zero crosstalk between freque ncies and eliminates the need for channel specific bandpass filters. The major advantages of a m ultiplex system, in addition to the crosstalk reduction issues, are lower cost and greater flexibility in frequency selection. If the multiplex switching rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of ed dy current signals by their phase angle and ampliru de is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data (12). Two frequencies are required to remove each unwanted variable. Practica l multipara meter freq uency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin-wall non ferromagnetic heat exchanger tubing. Tubing is structu rally supported by ferromagnetic tube supports at several locations. It is desired to remove the tu be su pport response signal from tube wall data. Solution: Apply a ffiultiparameter technique to supprt.'Ss the tube support signal response.

31

Chapter 5 Review Questions Q.'s.l

WheConds per sample. When crack detection is required the part is normally rotated with one or more coils positioned near the surface of the specimen. This type of inspection ensures 100% inspection of critica l areas in one test. The eddy current technique can often demon.ting inspections are: A. usually higher than thoS€ used in conventiona l eddy current tests. B. usually lower than those uS€d in conventional eddy current tests. C. identical to those used in conventional eddy current tests. D. about one half of those used in conventional ed dy current tests.

alternating current coil excitution process? A. alternating (\lTrent field measurement

B. eddy current testing flux leakage testing D. remote field testing

C.

Q.S.2

Which of the following electromagnetic testing techniques ShOll Id provide the best discontinuity depth and length sizing capabi lity for cracks in ferromagnetic weldments? A. alternating current fi eld measurement

B. eddy current testing C. flux leakage testing

O. remote field testing Q.8.3

Which of the following techniques should perform best in nonferromagnetic materials? A. alternating current field measurement

B. eddy current testing C. flux leakage testing D. remote field testing Q.S.4

Q.8.5

A generally accepted definition oi remote field testing is: A. electromagnetic testing done at remote locations. B. the electromagnetic field which has been transmitted through the test object and is observable beyond the direct coupling o f the exciter. C. through transmission eddy currents, detected. on the far side of a material or object under test by a remote receiver coil. D. the opposite of direct field. When a nonferromagnetic tube is inspected w ith a self-comparison differential encircling coil arrangement a nondetection could occur when a discontinuity is:

Q.8.10 The amplitude or voltage of the detected response from a discontinu ity is most often rel ated to: A. the width of the d iscontinuity. B. the location of the discontinuity. C. the depth of the discontinuity. D. the volume of the discontinuity.

A. filled , .... ith water. B. deep but very narrow. C. long " 'ith slowly v"'ying depth. D. short and wide. 57

Chapter 9 Eddy Current Procedures, Standards and Specifications Procedures, specifications and standards are produced to provide a means of controlling product or service quality. Written instructions that guide a company or individual to a de5ired end result and are acceptable to industry, are the basis of procedures, specifications and standards. Many publica tions are available to guide or instruct us. Some of the most frequently u sed references are the American Society for Testing and \Iatcnals (ASTh1), American Society o f Mechanical Engineers (ASME), American National Standards Institute (ANSI) and Military Standards ('.1lL -STD-)()()()(). These publications arc labariou!>])' p roduced by committees made up o f scien tific and technical people. Usually after a committee produces a draft document, it is submitted to industry and the scientific community for comment and subsequent reVision. In certain cases, standards combine to assist each other. As an example, ASME Section V Article 8 - Appendix IV uses ASTM £1316 to provide Sta ndard Terminology for Nondestructive Testillg . The military standard, M1L-STD-1537C Electrical

terms specific to the equipmen t or examination covered by the standard. SiglIifica w:e alld Use is a more detailed discussion of test TeSt1lt~ and probable causes of indications expected during the examination. The Basis of Application section identifies items which are subject to contractual agreement between the parties u sing or referencing the standard such as persOlUlel qualification, qualification of nondestructive testing agencies, procedures and techniques, sUIface preparation, timing of eXamination, extent of examination, reporting criteria / acceptance criteria, reexamination of repaired / reworked items. Apparatus describes the general requirements for the inspection system includ ing instrumentation, coils, position ing and driving mechanisms. The fabrication requirements for artificial discontinuity standards used for standardization are discussed under reference standards. A discussion of the reference specimen and the geometrical requirements of the artificial discontinuities in it is usually included. Standardization provides instructions for adjustment of the apparatus used for the examination. The response to known discontinuities in the reference standard is usually described in thi s section . Detailed in structions to process the inspection appears under procedure. These instructions may include acceptance limits and the handling of components that arc not acceptable. ASTM publishes severa l standards pertaining to the eddy current method. These standurds arc numbered; for example, E 571-98 . "E 571" refers to the standard and "98" refers to the vear of revision. Some ASTM standards that pert;in to the eddy current method are: £ 215 Standard Pmctice for Standardizing Equipment

Conductivity Test for Veri/icatiol! of Heat Trelltmf.'llt of Alumillum Alloys, Eddy Current Method, references _-\STM B193 Resistivity of Electrical COllductor Jlla terials and ASTM E18 Rockwell Hard/less a/1d Rockwell SIIpe1ficiai Hardness of Metallic Materials.

American Society for Testing and Materials American Society for Testing and Materials (ASTM) standards (practices or guides) usually include in the ·w ritten instructions headings such as scope, referenced documents, terminology, Significance and use, basis of application, apparatus, reference standards, standardization, procedure and keywords. Scope makes a general statement about the document's applicability and intent. Referenced Documents refers to other publications used as references within the standard . The termillology section usually may contain definitions of unique

for Electromagnetic Examination of Seamless Aluminum-Alloy Tube E 24,'J Standard Practice for Electromagnetic (EddyCurrent) E.mmillatiOIl of Copper and Copper-Alloy Tubes E 426 Electromagnetic (Eddy-Cllrrel1t)Jesting of Seamless mid Welded Tubulal" Products, Austenitic Stainless Steel and Similar Alloys

59

American Society of Mechanical Engineers

E 571 Stal/dard Practice for Electromaglletic (EddyCllrrCllt) hamillation of Nickel and Nickel Alloy Tubular Products E 690 Standard Practice for III Situ Electromagnetic (Edd y-Cllrrmt) Examination of Nonmagnetic Heat ExcJ lUlJger Tubes E 1316 Standard Terminology for Nondestrllctil'e Testing

In 1911 the American Society of Mechanical Engineers (ASME) set up a committee to establish rules o f safety for design, fabrication and inspection of boilers and pressure vessels. These rules ha v(' become known throughout industry as the ASME code. The ASME Boiler and Pressure Vessel Committee is a large group from industry and the scientific community. The Committee has many subcommittees, subgroups and working groups. Each subcommittee, subgroup and working group combines as a unit for a specific area of interest. For example, the Subcommittee on Pressure Vessels (SC Vlll) has {'wo working groups and five s ubgroups reporting to it. The purpose of these groups is to interface with industry to keep pace with changing requirements and needs of indus try and public safety. The ASME Boiler and Pressure Vessel Code is divided into 11 sections. ASME Section V, NOlldestrnctive Examination, is divided into two subsections, A and B. Subsection A deals with Nondestructive MetllOds of Examination. Article 8 is

Military Standard The United States Military uses the Military Standard document to control testing and materials. Standard procedures are provided by a series of MIL-STO-XXXXX document