Mi L- Hdbk-728-2 Eddy Current Testi̇

8/9/2019 Mi L- Hdbk-728-2 Eddy Current Test

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MI L- HDBK-728/2 -

le December 1986

MILITARY HANDBOOK

EDDY CURRENT TESTING

NO DELIVERABLE DATA REQUIRED BY Ttil S DOCUMENT

AMSC NIA I AREA NDTI \


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Section2.02.12.22.2.12.2.22.2.32.2.42.2.52.2.62.2.72.2.82.2.92.2.102.2.112.32.3.12.3.2.?.3.32.3.42.3.52.42.4.12.4.22.4.32.52.5.12.5.22.62.7

2.7.12.7.22.7.32.7.42.7.52.7.62.82.92.102.11

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TABLE OF cONTENTS - CHAPTER 2

Safety Notice . . . . . . . .Introduction . . . . . . . . .Basic Principles . . . . . . .

Electrical Currents . . .Electromagnets . . . . .Electromagnetic InductionMutual Induction . . . .Self Induction . . . . .Vector Diagrams . . . . .Current-Plane Diagrams .Impedance-Plane Diagrams.Eddy Currents . . . . . .Eddy Current Variables. .Eddy Current Limitations.Equipment and Methods . .Absolute and Differential

Test Coilg. . . . . . . .Basic Bridge Circuits . .Data Displays . . . . . .

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. . . . . .. . . . . .. . . . . .Instruments

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Newest Equipment and Methods . . . . . . . . . . . . . .Basic Procedures and Techniques . . . . . . . . . . . . .Lift-Off Compengatiou . . . . . . . . . . . . . . . . . .Thickness Measurements . . . . . . . . . . . . . . . . .Conductivity Measurements . . . . . . . . . . . . . . . .Stand side . . . . . . . . . . . . . . . . . . . . . . . .General Requirements.. . . . . . . . . . . . . . . . .Specific Requirements. . . . . . . . . . . . . . . . . .Applications . . . . . . . . . . . . . . . . . . . . . . .Guidelines forSpecific Disciplines . . . . . . . . . . .

Administrator . . . . . . . . . . . . . . . . . . . . .Designers . . . . . . . . . . . . . . . . . . . . . . . .Production Engineers.. . . . . . . . . . . . . . . . .Quality Assurance Personnel . . . . . . . . . . . . . . .NDT Engineers . . . . . . . . . . . . . . . . . . . . . .NDTTechnic ian8 . . . . . . . . . . . . . . . . . . . . .

Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . .Cloaaary . . . . . . . . . . . . . . . . . . . . . . . . . . .Bibliogr~phy . . . . . . . . . . . . . . . . . . . . . . . . .Index . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.2-12.2-12.2-22.2-42.2-42.2-52.2-52.2-72.2-92.2-9

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2.3-22.3-72.3-72.3-82.4-12 .4-12.4-62.4-82.5-12.5-12.5-12.6-12.7-1

2.7-12.7-12.7-12.7-12.7-22.7-22.8-12.9-1

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?.0 SAFETY NOTICE

Eddy current testing uses high-frequency electrical circuits. Therefore,standard safety practices associated with electrically operated devices should

he ohse rved . Eddy current testin& does not present any other mown uniquesafety hazards to personnel. See Section 2.8 for additional comments onanfety.

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2.1 INTRODUCTION

Eddy current te8ting is one of the standard nondestructive testing methods. Ituses electromagnetic fialds and is very aenaitive to many material and geomet-ric variables. Because it is an electromagnetic test, its si~nals move withthe speed of light, and its measurements can be made very quickly. Being elec-trical in nature, eddy current testing can easily be autom~ted and its signalscan be used to control other electrical devices. Because the sensing devicesare coupled to the test specimens through electromagnetic fields, the devicesdo not have to contact tha test specimen. Eddy current testing can even beused in a vacuum. For these and other reaaons, eddy current testing is exten-sively used throughout the industrial world.

This chapter provides the fundamental principles and guides associated withs , . . . . . . . . .

eaay currenz zeszlng. lT lncluaaa tne thOOry or operation, the type of equip-ment, the adv~ntages and disadvantages of the method, various application andstandards, and guides for specific disciplines. Tha information contained inChapter 1 should be included with this chBpter for general guidelines to the

emplo~ent of all NM methods and for a more complete understanding of eddycurrent testing as it compares with other basic methods.

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2.2 BASIC PRINCIPLES

Eddy current testing is the process of electromagnetically inducing small

electrical currents into an electrically conductive specimen and observing thechang$s in the effects caused by these currents on the electromagnetic fields.Every variable that affects the inducLiOn Of eddy currents. their flOW in thematerial, and the reactions to their electromagnetic field is capable of beingutilized as a test variable.

A knowledi?e of electricity and ma.qnecism Qnd the electrical properties ofmaterials is required to understand tile theoretical operationtesting. In this chapter these subjects are reviewed as theycurrent testing.

2.2.1 ELEcTRICAL cURRENTS

Figure 2.2(1) shows a simple electrical circuit consisting of

of eddy currencapply to eddy

a voltage sourceand a conductor of electricity that provides a closed-path circuit throughwhich current will flow. 11 the voltage source ia removed, the currenc willcease flowing. lf the circuit is broken, or otherwise incerrupced at anypoint, che current will not flow.

CURRENT FLOW, I

BA17ERY

-n

RESISTO l.EPRESENTING

OR POWER + ~R THE TOTAL CIRCUIT

%%:, ~RESISTANCE,R

Figure 2.2(1). Simple electrical circuit.

As long as there is a continuous circuit of an electrically conductive materialwith a voltage applied to the circuit , current will flow. The amount of cur-rent flow can be determined from the mathematical expression known as OhmsLaw. Ohms law states that che electromotive force (E) i]]volts across thecircuit is equal to the current (1) in amperes flowing through the circuitmultiplied by the total resis Lauce (N) of the circuit in ohms. Equation 1shows this relationship.

E=SR (1)

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This equation can be transposed to find the current value or the resistancevalue in terms of the other two parameters: I = E/R, R = E/I.

2.2.2 ELECTROMAGNETS

When an electrical current flows through a wire a mBgnetic field exists a=oundthe wire. The magnetic field can be represented by magnetic force lines. Thedirection of the magnetic field or lines of force around the wire depends onthe direction of current flow through the wire. This directional relationshipmy be determined by the right-hand rule as illustrated in Figure 2.2(2).

MAGNETICFIELODIRECTION\

RIGHTHANn

WIRE

CURRENT DIRECTIONICURRENT DIRECTIONISOPPOSITETO ELECTRON FLOWI

RIGHT-HAND RuLE. IFTHE WIRE ISGRASPEO INTHERIGHT HPJNDWITH THE THUMB POINTINGINTHE DIREC.TIONOF CURRENT FLOW. THE FINGERSWILL POINT INTHE DIRECTIONOF THE MAGNETIC FIELO.

Figure 2.2(2). Magnetic field around a straight conductor.

If the straight wire is wound into a coil (many 100P9 of wire), the magneticlines of force encircling the wire form a nap,netic field inside and outsidethe loops as illustrated in Figure 2.2(3). The field created is similar tothe field of a bar magnet which has opposite magnetic poles at each end. Thestrength of the magnetic field is dependent upon the number of turns in thecoil and the ma.gnirude of the current. Increasing ei ther one increases thestrength of the magnetic field. The strength of the magnetic field is alsodependent upon geometric factors - airily the number of turns per unit lengthor the epacing between the tume of the coil, the coil area and, for eachmeasuring point, its distance end direction from the coil.


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Figure 2.2(3). Magnetic field of a coil.

The direction of the magnetic field depends on the direction of the currentflow through the coil as shown on Figure 2.2(8). If the direction of thecurrent through the coil is reversed, the direction of the magnetic field isreversed. The end of a coil can be identified as a North or a South moledepending on whether it is attracted by the North or the South magnetic pole

..

of the earth. By curving ad pointing the fingers of the right hand in thedirection of the current flow arod the coi], the thumb will identify the endof the coil that will be attracted to the earth, ~ .

Sou

North magnetic pole.

ANORTH

,..CURRENT N4

CURRENT OUT

FiHure 2.2(6>. Direction of a magnetic field in a coil.

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current in the seconda?y coil does occur, the changing magnetic field of thesecondary coil induces a current in the primav coil that opposes thedirection of flow of tha original current in the primary circuit. This0pp03ing current is the result of the magnetic field of the secondary coilcutting across the windings of the primary coil . The inducement of current in

one coil by the changing magnetic flux of another coil is called mutualinduction.

2.2.5 SELF INDUCTION

The magnetic field created by each turn of wire in a coil induces a current inall of the other turns in the same coil. This effect is called self inductionend it, too, opposes the original current.

Figure 2. 2( 6) shows an alternating current source connected to a single coil.The coii shown in this figure represents a real coil, which, would have finiteresistance as well as inductance. A voltmeter ia provided to measure thevoltage applied to the coil and an ammeter is provided to measure the currentthrough the coil. If the instantaneous values of voltage and current areplotted on .a graph, the current is found to lag in time behind the voltage asehown in the lower portion of the figure. The highest current does not occurat the same time as the highest voltage. Thus, self-inductance affects thephase relationship between the voltage and the current. This electrical pro-perty of the coil is called inductive reactance and is designated by thelettCrS XL. It has been determinc6 that in an ac circuit containing onlyinductive reactance (no measurable resistance) , the current will lag behind asine voltage by exactly 90 degrees. The inductive reacr.ante of a coil is..a.,l-unction of the frequency of the alternating current. Since the higherfrequencies cause the magnetic field to change more rapidly, the inductivereactance increases as the f requency increases , The increase in the in-duc :ive reactance due to zhe increase in frequency cauaes the currenc throughthe coil to be reduced, thereby reducing the strength of the magneric field ofthe coil.

2.2.6 VECTOR DIAGRAMS

Some resistance is present in all circuits. The total resistance in a circuitincludes the resistance of the wiring as well as the resistance of the coil.In finSC circuit containing only resi.seance, the resistance simply limits thennount of currect thet flaws througk the circuit. It does not change the phaserel.tionshi? between th? volt +en snd the cur%.~t. The c.rrre,cts exactly inphase with the voltage.

Resistance, however, is not the only variable that affects the flow of cur-rent. Impedance (designated by the letter Z) is the name given to the combi-

nation of all those electrical properties that restrict or limit tbe flow ofcurrent through the ci~cuit. For eddy current testing, this includes resist-ance (R) tilldindlJCtiVe reaCtafiCe (XL). (Capacitive r=saccance is aotgcrna l:::,::c:nccm in *it:; cur:ur.t :sstir.);. :ir.ce t}lese t,.,ofac:ur.s :::.us%results that, for sine waves, occur Y(Jde


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MAX 0 . 270 360 90 0

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Figure 2.2(6). ac voltage and current plot .,,e.

Figure 2.2(6) shows the relationship between the voltage and the current thatresulted from che impedance of the circuit. In this case the current is shownto lag behind the applied voltage by 45 degrees, which means that the resist-ance and inductive reactance were equal. The same relationship can be shownby means of a vector diagram (a vector whose length represents a value andwhose direction represents a phase relationship). A vector diagram of thevolta~e and currenc show in Figure 2.2(6) is shown in Figure 2.2( 7).

Iz - MAXIMUM

ROTATIONY

CUR14ENT

z@-

45LAG

o V - MAXIMUMVOLTAGE

Figure 2.2(7). Vector diagram.

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The vector lengthe represent the maximum values of voltage and current. The

actual or instantaneous values are represented by the magnitude of their hori-zontal projections, which vary as the vectors rotate. If the vectors are con-

sidered to be rotating in a clockvise direction, tl]e maximum current willoccur 45 degrees in rotation after the maximum voltage occurs.

2.2.7 CURRENT-PLANE DIAGRAMS

When the current lags the voltage, as shown in Figure 2.2(7), the currentvector representation can be broken up into two components, one in phase withthe voltage and the other 90 de~rees behind the voltage. These two components ,shown in Figure 2.2(8), are defined as the resistive currenc (1~) and thereactive current (IX ). The original vector is defined as the Impedance

L

current (12). As shown in Figure 2.2(8), a vertical line drawn from theimpedance value to the zero degree (in phase) Iine gives the value of the

resist ive current ( IR) while a horizontal line drawn from the impedancevalue to the 90 degree line gives the value of the reactive current (IX ).L

If the values of lR and IX are known, these values can be plottedL

on the zero and 90-degree lines of a current impedance diagram, a reccangleconstructed, and the diagonal drawn will represent the value of 12.

I

Kx z

a - 45/

0 I_

k

Iz . IMPEDANCE CURRENT

IxL. REACTIVECURRENT

Ii? - RESISTIVECURnENT

a . PHASE ANGLE

0(APOINTOF MAXIMUM APPL EL ,3,

Figure 2.2(8). Vector diagram showing values of IR and 1x.

L

FiK,,re 2.2(9) shows the vector diagrams of two circuits having different cur-

renls d,,P tn different values for resistance and inductive reactance.A,

1 Vievthe resistive current is relatively low while the reactive current is

relat ively l ,iRh. Note Lhe elrect on che phase angle (a) as compared to thephase angle shown cm View B where the resistive current is relatively high andthe reactive current is relatively low. Varying values of resistance andinductive reeccance therefore affects the phase angle as well as the magni-tude of the currents.

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90 90

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I . CURRENTXL - z R - RESISTANCE

I L- REACTANcE

1IxL

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z

laJ ao~

%0 0 0

A B

Figure 2.2(9). Current vector diagrams.

As indicated in the previous paragraphs, vector diagrams mav

- IMPEDANCE

- PHASE ANGLE

be used to showthe impedance curt-et , the reactive current, and the resistjve current. Thesame type of diagrams may be used to show applied and induced voltages and to

show actual impedance, inductive reactance, and resistance values,

On the electrical circuit 5hown schematically in Figure 2.2(10); separateelectrical symbols are used to indicate the inductive reactance (XL) and theresistance (R) in the circuit. When an ac voltage is applied co this circuit,the current flows through both the inductive reactance and through the resist-

ance. This current is common to all elements in the circuit.

..RcE ~-q]cOILFigure 2.2(10). Inductive reactance and resistance in an ac circuit.

When a sine wave current flows through the inductive reactance a voltage willexist across the inductive reactance. This voltage is identified as V2 onFigure 2.2(10). The same principle applies to the resistance and this volt-age is identified as V3. The maximum value of either voltage is the product

of the maximum current ( 1) ad the induct ive reactance (XL) or the resistance(R). Ths V2=lx XLa,}d Vq= IXR.

II

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The maximum voltage (V2) across the inductive reactance is 9cI degrees out ofpha Be with the maximum voltage (V3) across the resistance. The8e two Volt-

ages can be represented on a graph as shown on Figure 2.2(11)- Since voltagesare shown, the graph is cal led a Voltage-Plane diagram.

J2

txx L

L

so

o V3I.R Id_L z I01ORvOLTAGE Pu+4E IMPEDANCE PLANE

Figure 2.2(11). Voltage-plane and impedance-plane diagrams.

2.2.8 IMPEDANCE-PLMJE DIAGRAMS

Since the current through both the inductive reactance and the resistance isthe same, the voltage values on the voltage-plane diagram may be divided bythe current value to give the values of inductive reactance and resistance inthe circuit. The result is also plotted on Figure 2.2(11). The graph is

called the impedance-Plane diagram.

The vector addition of the values of inductive reactance and resistance,plotted 90 degrees apart, will indicate the impedance value (Z) and the same

phase angle (ff) as the current vectors showed in Figure 2.2(8).

2.2.9 EDDY CURRENTS

Eddy currents ca easily be explained by taking a transformer as shown inFigure 2.2(5), replacing the dc battery with an ac source as shown in Figure2.2(6), and then replacing the secondary coil with an electrically conductivetest material as shown in Figure 2.2(12). The electrically conductive test

material can be viewed as a large number of closed or shorted turns of asecondary coil. The induced current in the secondary coil becomes inducedcurrent within the test material itself. These currents are called eddy

current s. As the5e currents flow, they cause an electromagnetic inductionthat opposes the induction of the primary coil, and the effect will be indi-

cated as changes in the voltage and current values, andlor in their phase re-lationships, in the primary circuit, or in any other test circuit th~t islocated in their direct vicinity.

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Figure 2.2(12

2.2.10 EDDY CURRENT VARIABLE:

/

ELECTRICALLYCONDUCTIVETESTMATERIAL

Induction of eddy currents.

The variables in the eddy current test circuit can be quickly identified. Inthe primary (test ) circuit , the frequency and magnitude of the source voltageand the resistance ad self-inductance of the test circuit are all basic input

variables. These variab les, along with the number of turns in the coil andthe shape and size of the coil, determine the strength and shape of the elec-

tromagnetic field that expands and collapses around the test coil. The ori-entation and location of this coil with respect to the material under teat is

also an important parameter. The closer the coil is to the material undercent, the greater the mutual inductance that is established. These variables

all help to determine the magnetic forces induced within the test specimen.

The eddy currents that result from the induced magnetic forces will be a

function of certain characteristics of tbe test specimen. The thickness andother geometric characteristics of the test specimen can affect the amount of

current induced into the specimen by the test circuit. As the materialbecomes very thin or edges are approached, for example, the mutual inductanceis decreased which can be considered as a reduction in the number of turns inthe eecondary coil. The electrical conductivity of the material will also

affect the amount of current irtduced, essentially fixing the resistance ofeach shorted coil. If any cracks or other material defects are present, which

would have the effect of breaking any of these closed current loops, reducedcurrent F1OW will OCCr. As these eddy currents flow, first in one directionand then in the opposite direction as the electromagnetic field first expands

and then collapses, they set up their own electromagnetic fields.

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The sensing @f the cd6y currefit elechrcmaglletic fields, or change8 In thesefields, completes the test. Norma] Iy, the test coil is also used as the sens-ing COL1, and the s~me orientation and proximity relationships required to in-duce the eddy currents are el.go used to establish the ret,urn i nduction. Thisreturn induction affects the normal voltage-amperage relationship in the pri -

ma~ circuit. This change can be senged and displayed on one or mare ampli tudeor phase meters, or on an oscilloscope where displays similiar to some of thephase diagrams discussed in Z.Z.5 can be observed.

If the test material exhibits any magnetic permeability effects, the enti reinteraction will be greatly affected, both with respect to the effect of theinitial induction from the test coil, and any subsequent return induction fromthe eddy current flow. If the magnetic permeability of the material interfereswith the eddy current testing (that is, it i~ not the variable being easured) ,its effects can be removed by using a strong, fixed, external magnetic fieldthat holds the test material at magnetic saturation.

2.2.11 EDDY CURRENT LIMITATIONS

The electromagnetic induction of the eddy currents opposes the electromagneticinduction of the test coil. This opposition has the effect of restricting eddycurrents to the surface or near surface region of the test specimen. Any eddycurrent present at the surface of the test material will reduce the inductj,oneffect of the tegt coil in the deeper regions of the materiel, and eddy cur-rents therefore fall off at an exponential rate. Therefore, eddy current test-ing must normally be limited to locating those variables that are at or nearthe surface of the ~ngpected. p~rts.

2.2-11

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2.3 EQUIFSS.SNTND METHODS

A variety of co~ercie~ eddy Current test inSErumenCS ~re available. They areoften designed to measure, detect, or gage specific parameters of the testmaterial such as conductivity, flaws, and thickness. Specific aystema are notdiecus=-d, but some of che ranges of choices are indicated.

2.3.1 ABSOLUTZ AND D1FFERENT2AL INSTRUMENTS

Basically, as shown in Figure 2. 3( 1), there .sre two types of instruments:differen Ciil inetrument,q and absolute instruments. The number of test probesassociated with an instrument~cative of its type. For example, a dif-ferential instrument has CUD identical test probes (coils) that are elect~Cl l~Y opposed. The absolute test instrument has only one test coil or probeassociated with it.

ABSOLUTE TECHN1OUE DIFPEREN71ALTECHNIOUEMfASUnEk4ENTSMADE BY A SINGLEcOIL

F4D1NGIL

A COMPARISON OF IMPEDANcEVARIATIONSBETWEEN AOJAtENT SECTIONSOF THE SAMETE.s7PART,oflw17H A sELEc7A0LE STANOARO,

&,o,yDING~wsg7ES7 P4RT

READOUT METHODS:VARIOUS TYPEs OF READol-lTDEVICES AE AVAIL&BLEINCLUOINGMETER. SCOPE,PEN R.5cORDER,FLASHINGLIGHTs.AUDIBLE AL ARMS,COUNTERS. AUTOMATICMARKING ANO SOHTING. USE ANY ONE OR ALL AS BESTSU$TSTHE PARTICULAR APPLICATION.

FACTORS WHICH AFFECT THE EleCtriCalCHARACTERISTICSOF THE PICKUP COIL:

TEST PART: TEST SYSTEM:1. CONDLItTIVITY 1. FREOUENCY2, PERMEABILITY 2. COILSLZE& SHAPE:3.UA33 ITHICKNE221 3,CURRENT4. HoMOGENEITY 4. SPACING

(COUPLING)

Figure .2. 1 . Absolute and differential coils.

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In the differential instruments the signal displayed is the difference betweenvariables simultaneously affecting two coils, while in the abeolute instrumentit ig the change in the signal of the eingle coil from one position to anotherthat must be attributed to difference in the variables being tested. Bothinstnunenta therefore produce similnr information regarding the properties ofthe test item, but their ~perating characteristics tirequite different. Thisdifference in operati~n playg a 18rt?e role eddy currenttest.

Figure 2.3(2) illustrates tha three basic types of test coils that are used ineddy current testing: the surface coil, the encircling coil, and the inter-nal, ar babbin-type, coil. Because the p>sitianing af the sl,~face c>il rela-tive ts the material differs from that of the encircling and internal coils,the eddy cu~?ent ~emetry differe since eddy currents alweys have e de?initesJls:.1.qlelati o.ns.~iph.ith the magnetic fjr:ldsprDtiL ciFItp.em. lJnder3tandiI>gt.:is:i.1 :s :?dy ,:.,>rer,t~e:i~l rel.t::r.s?.p i: v ,.-.:i.r]73r:*l:lt: i].?n1>;kinr::,, Jr :e~3[::.iz~.:IP hfir,rtssr~:~n~. ii:;c:,r::i;);~iliau.

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SURF ACE COIL EXTERNAL COIL

INTERNAL COIL

Figure 2.3 2 . Basic test coil types.

Eddy currents produced by rest coils essentially flow parallel to the plane ofthe coil producing them. Eddy currents are etrongest near the surface of chematerial and their strength decreases with depth. A discontinuity whose majoraxis lies parallel to the eddy current flow will not have as great an effect

on the eddy currents as one whose major axis cucs across the flow of current ;

and a deep lying discontinuity will not have as great an effect as one lyingnear the surface.

Figure 2.3(3) shows a surface probe as it is placed near the surface of a testarticle. The alternating magnetic field of the coil is essentially perpendic-ular co the face of the coil. The induced eddy current flows at right angleeto the magnetic fields. Therefore, the induced eddy current6 flow parallel to

the surface when the surface probe is held perpendicular to the ~urface.

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Figure 2.3 3 . Eddy currents produced by a surface coil.

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shows an encirclin~ coil. installed ~round m rod. Here, eddyin the same direction aB the current flow in the coil; i.e.,

around the circumference of the rod. Thus the encircling coil is especially

adapted to locating those d iscontinuities that are parallel to the length ofthe rod.

Figure 2.3 4

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Figure 2.3(4). Eddy currents produced by an encircling coil.

The eddy currents induced by an internal coil will be the same as an encir-cling coil, except the eddy currents will be concentrated on the inner surfaceand not on the outer surface.

Lift-off and fill factor are terms that define the sDace that exists betweenthe article under test and the inspection coil as sh&.n on Figure 2.3(5).Each has an identical effect on the eddy currents. Lift-off and fill factorare essentially the same thing; lift-off is the term applied to surface coilsand fill factor is applied to encircling and internal coils.

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MATERIAL

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a. Lift-off. When a surface coil is energized and held in air above aconductor, the impedance of the coil has a certain value. As thecoil is moved closer to the conductor the initial impedance value ofthe coil will change uhcn the field of the coil begins to intercept

the conduc tor. Because the field of the coil is strongest close tothe coil , the imped,lnce val~,e will contin$w to change until the coilis directly on the conductor. Conversely, once the coil is on the

conductor, any small variation in the separation of coil and conduc-tor will change the impedance of the coil. The lift-off effect is BOpronounced that small variations in spacing can mask many indications.

b. Fill Factor. In an encircling coil, or an internal coil, fill factoris a measure of how well the test specimen (conductor) fits the coil.It is mathematically the square of the ratio of the specimens diam-

eter to the coils diameter. The closer the fill factor is co 1 themore sensitive and prec ise the test.

It is necessary to maintain a constant relatianehip between the diameter of

the coil and the diameter of che test specimen. Again, small changes in the~iameter of the test specimen can cause changes in the impedance of the coil.This effect can be sefl in detecting changes in the diameter of the testspecimen but it can also mask other indications.

Because of the necessity to rnaintai lift-off effects at a constant value(assuming no change in the diameter of the rod or tube under teet) it isnecessary to provide the means to guide the specimen through the center of thecoil as shown in Figure 2.3(6).

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A test coil must therefore be chosen that wil~ have an acceptable fill fsctorand the necessary adjustable guides or controls that will maintain a reasonablyconstant position and orientation with respect to the article tested.

When a test coil approaches an edge or the ed of a part, a8 show in Figure2.3 7 , the eddy currents become distorted, producing a False indication knownas edg? effect . Since, to the test circuit, the edge of a part looks lihe avery large crack or hole, this strong response masks 811 other variables

that may be present , and therefore 1 imi ts the usefulness of the eddy currenttest near these areas. These limits are affected by the size of the coils.

The smaller the coil and its magnetic field, the closer the edge can be ap-proached without encountering edge effect s.

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Figure 2.3 7 . Distortion of eddy currents due to edge effect.

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2.3.3 BASIC BRIDGE clkcl lITS

Since the signals being measured

sensitive bridge type circuit isin Figure 7.3 fj .

in eddy current Lesting are otten small, a

normally empioyed. A typical example is shown

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Eddy current data can be displayed bY amplitude meters, combined amplitude and

phase meters, or by an oscilloscope. bata signals can also be used to turnwarning llght S off c,r o, ~~und alarm5, or operate other controls. Data sig-nals can be recorded on x-Y plotters, strip chart recorders, magnetic tape, orother modern recording devices. oscilloscope displays that simultaneously show

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2.4 BASIC PROCEDURES AND TECHNIQUE..

Eddy cu=rent testing is aensizive to many variables. The ultimate utility ofeddy current testing will therefore be dependent upon the ability of the opera-tor to separate out or identify the effects of the desired variable or vari-ables from the effects of all other non-critical or unimportant variables.

Theuse of eddy current testing becomes limited when discrimination between theeffects cannot be accomplished. In this section, several basic procedures andtechniques are presented in detail. kn understanding of these exampleo willmake it possible to apply eddy current techniques to most situations. Thesetechniques will include lift-off compensation, thic mese memurements, and con-ductivity measurements. Basically, thesethe effects of one variable from those ofeffects is accomplished and verified, thealways be in doubt.

2.4.1 LIFT-OFF COMPENSATION

technique involve the separation ofanother. Until .geparation ofreliability of the testing will

A8 was mentioned (in section 2.3.2 on coil lift-off effects and in section

2.2.10 on digtance effects), changes in the apace or distance between a coiland the test material can cause large changes in the signal, or meterreadings. lf it is deeired to detect a material property thst does not relateto lift-off, then meter reading changeg due to lift-off effects should besuppressed. Where probes are hand held or moved over a eurfece that ig notperfectly .maooth, some lift-off effects, due to slight tilting or otherreasons, will almost alwayB be present.

.

If an instrument uses a bridge circuit as shown in Figure 2. 3(8), where thereactance and resistance can be manually adjusted, the meter can be zeroed, or minimized, for any existing test condition. When a zere point is estab-lished, the reactance and resistance of the dials can be read, recorded, andplotted on an impedance-plane diagram (see Section 2.2.8) aa a point repre-

sentative of that particular teet condition. Figure 2.4(1 ) shows an imped-ance plane diagram with one point plotted, representing a resistance dialsetting of 15 ohms and a reactance dial setting of 5 ~~~.

Figure 2.4(1). Impedance plane diagrem.

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if resistance and induct ivc ~e.get.gnce reading= are taken on a series of tnate-

riols that vary only in Lileir conductivity, and theme readings are plotted onan impedance-planr diagram, the res%ilt is a curve that represent the effectof changes in cond,,ctivity [or that particular test setup. Then if a series

of resistance and inductive reactance readings are take as the test coil iElifted off the material that is to be tested ana tlmse reaiings are plot Led onthe same rnpedace-plane diagram, the result is a second curve that representsthe effect of changes i lift-off for that particular msterial in the sametest setup. Figure 2.11(2) shows how lift-off effects, in general, vary tromconductivity effects for a particular test setup. When the conductivity and

lift-off points plot at different positions o the impedance-plane diagram,

then the effect of B change in lift-off can be separated from the effect of achange in conductivity by selecting particular test points from which the

bridge circuic operates.

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Frequency is also a consideration for conductivity measurements. Figure2.4(10) chows the shift in relative positions on the impedance-plene of theconductivity for various materials. As frequency is changed this shift inconductivity occurs end the angle between the lift-off curve and theconductivity curve changes. As higher frequencies are used, this anglebecomes larger, and will allow for better separation between lift-offvariables and conductivity variables. This advflntage, however, must alwaysbe considered along with any disadvantages that may occur because the use ofa higher frequency will limit the test to near surface measurements of theconductivity and will not allow deeper penetration.

These specific examples of test methods will prove us.s.flfor most eddy cr-rent testing. If an oscilloscope display unit is being used, where both resis-tive and reactive effects are continuously displayed, then the test setup wouldbe much easier and quicker. Plotting of individual points can be bypassed,and immediate separation between variables is usually obvious.


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Success in eddy current testing will oftenand in their proper use. Both general andfollow.

2.5.1 GENERAL REQUIREMENTS

depend upon the standards availablespecific comments on standards

The variables found in eddy current testing are almost always non-linear.There are some Situations where the variable~ are not even monotonic, Figure2.4( 5), as an example, chows that over certain thickness ranges, the thicknessimpedance curve reverses directions even though changea in the thicknesscontinued in the same direction. Actually, there are test points nn allimpedance-plane diagrams where signal reverses will occur. Therefore,intermediate variable standards will often be required in addition tostandards that cover the end point~ of the tested variables. In conductivitytests, minimum standards that are used for calibration are often provided withthe equipment. For greater accuracy, however, additional etandarda that coverthe specific range and material being tested are recommended.

Alloy segregation, heat treat testing, hardness determination, and thicknesgmeasurement must have standards that properly match all the changes in thevariables that might exist in the test. Standardization and setup is ex-tremely critical to inspection for defects . Inspection choices might includedifferent fraquenciea, different probes, different orientations, or differentprocedures in order to obtain sufficient information required.

Many standards ara commercially available. Some atandardizatinn information1S provided by the National Bureau of Standards.

2.5.2 SPECIFIC REQUIREMENTS

2.5.2.1 Sorting Standarde. In sorting using the absolute (encircling) coilmethod, a known acceptable calibration standard and a known unacceptablestandard are requi red. When using the comparative (encircling) coil method,usually two known acceptable specimens of the piece tested and one knownunacceptable specimen are required. For a three-way sort it is best tn havethree calibration standards, two of which represent the high and low limits ofacceptability for one grnup or one each of the two unacceptable grnups. Thethird standard represents the acceptable lot of matarial.

2.5.2.2 Coating Thickness Measurements Standarda. Calibration Standards forthickneaa can be foils of known thicknesses laid on a proper substrate oractual coatinga on a prepared substrate.

2.5.2.3 Conductivity Standards. Primary standards are standards which have avalue assigned through direct comparison with a standard calibrated byNational Bureau of Standards or have been calibrated by an agency which has

access to such standards. The primary standards are usually kept in alaboratory environment end are used only to calibrate oecondary standards.

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Secondary standards are those standards supplied with the inutrumentntion orstandards constructed by the user for a epecific test. These standards areused to calibrate the instrumentation during most testing of materials.

2.5.2 .4 Standards for Tubular Products. The standard used to adjust thesensi tivity of che apparatus shall be free of interfering diacontinuities antishall be of the same nominal alloy, heat treatment, and dimensions as thetubular products co be examined. It shall be of sufficient length to permitthe spacing of artificial discontinuities to provide good signal resolutionand be mechanically stable while in the examining position in the apparatus.Artificial discontinuities placed in the tube shall be one or more of thefollowing typea:

a. Notches - Notches may be produced by Electric Discharge Machining (EDM) ,mill- other means. Longitudinal, transverse notcheg, or both mav beused. Orientation, dimenaion~, configuration and poeition of the notc~eeaffecr the regponse of the eddy current oyatem.

b. Holes - Drilled holes may be used. They are usually drilled completely

through the wall. Care should be taken during drilling to avoid distortion ofthe test pieca and hole.

2.5-.2


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2.6 APPLICATIONS

Bddy current testing applicetiorm are extensive. They are usedmanufacturing plants to inspect tubing, wire, pipe, bolt holes,and large parts. There are autometic inspection stations wherespection is limited only by the test frequency used in the teat

in raw m8terialfor small partsthe rate of in-circuits. Eddy

current testing is an excellent ethod to measure paint and coating thick-nesses, to geparate alloys, to measure electrical conductivity values, indeedalmost any electromagnetic property or geometric variable can be considered.The main limitation to the choice of eddy current testing are that the mate-rial must include at least one element that is electrically conductive; thateddy current is limited in its depth of penetration, alwaye being moreconcentrated at the eurface than below the eurface; and that it ie oftenaffected by more variables than desired for meaeuremente. Eddy currenttesting will be greatly limited when separation of these variables cannot bereasonably achieved. It will rileo be limited when standards orstandardization methode are not available.

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2.7 GUIDELINES FOR SPECIFIC DISCIPLIN12S

Administrators, designers, production engineers, quality assurance personnel(QA), N~ engineers, and technicians should study gection 1,5 (in Chapter 1).Each should study the areas that apply tn others as well as their own identi-

fied areas.

2.7.1 ADMINISTRATORS

Administrators must recognize that eddy current testing, as with other NDTmethods, must have .etandards. The standards required usually vary with each.9pecific task. Knowing that an eddy current facility exists or eddy currentequipment is on hand does not ean that it can be directly applied to a newtack. Therefore, administrators must ensure adequate lead time and fundingfor the ordering or production of prnper stnndards and confirming theiradequecy.

2.7.2 DESICNERS

When designers are aware that eddy current inspections will be required, theyshould consider the difficulties of ueing eddy current where edge effec te arepresent, where more than one variable may be present, or where coatingthicknesses must be meaeured and both the coati~ and substrate ereconductive. Also, designerg can improve eddy current successes by consideringimplants that can act as standards. If a component has e set of partiallydrilled holes, reqUITe one or two be drilled to a depth that an eddy currentreeponse will occur from the opposite side. If inspections for cracks ust beaccomplished, present an interface that has a controlled gap that willduplicate an acceptable or unacceptable condition. Certainly, theseimplants cannot always be considered, but often they can be added at almostno additional cost or loss in component efficiency.

2.7.3 PRODUCTION ENGINEERS

It ig true that eddy current signals travel with the speed of light, and fewnondestructive test methods have quicker response times between testing andresults. However, almost all eddy current devices require meter needles tomove, or gwitches to operate, or other ections to occur that do place limitson inspection rates. Also, the basic frequency of the eddy current signalmust also place a finite limit on the inspection rate. Therefore, productionengineers must consider proper limits on sensor head velocity rates end/orspecimen motions in eddy current inspection aysteme.

2.7.4 QUALITY ASSURANCE PERSONNEL

Proper calibration of eddy current devices will be a concern to QualityAssurance personnel. Proper calibration will often require temperaturecontrols and frequency controls that are not normelly established . Althnugheddy current testing is often simple, beceuse it ie potentially affected by emultitude of electromagnetic and geometrical variebles, it can in manysituations be one of the more difficult methods to epply.

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2.7.5 NDT ENCINEERS

To maximize successful eddy current operations, the two most important ele-ments, other than personnel and their training, are the use of adequateequipment and adequate standards. All those involved in eddy current testingshould understand the potential complexity. of eddy current testing. Standardsthat represent 311 the variables that are present should be used.

It is vital to recognize that separation of all variables is ot always po9si -ble. This does not mean that eddy current testing is of no value under thesecircumstances. A partial reparation may be sufficient when combined withother inspection procedures.

Because of the large number of veriables involved, experience with everyvariable helps in their recognition or identification if they unexpectedlyoccur in a test. When edge effect, lift-off, magnetic permeability, con-ductivity (heat treat, alloy, work hardening, temperature, and all otherparameters that affect conductivity) are all to be considered along with amultitude of test coil geometries and circuit perameterg, an inexperienced

operator will often miss observations that are important to the adequateinterpretation of the results of the test. Only careful attention to detailsand procedures, with impedance-plane variables well established by an adequatenumber of standards, will assure that acceptable results will be ettained. .

2.7.6 NDT TECHNICIANS

One source of difficulty for technicians will always be the unexpectedappearance Of En unknown variable while meaeuring another variable. One ofthese unexpected variables can be a change in temperature. Temperaturechanges can be due to nomnal environmental changes, those seen within each day,or due to the effects of equipment warm-up or overheating, or due to heating orcooling sources introduced by surrounding equipment.

Temperature changes can have a multitude of effects. Sometimes the signalsource of the coil is not compensated for temperature changea resulting infrequency and amplitude variatiomi. Figure 2.4(10) shows that a change infrequency can duplicate what is thought to be a change in conductivity. Be-sides these artificial changes, the actual property of the standards and/orgpecimens may also change with temperature. Therefore, standards and specimensused for setup and test should always be at the same temperature. When test-]ng for very small changes, -md when specimen and standards are not at ambienttemperature, great care must be exercised by the technician in how he picks upand holds these parts. ManY times just the temperature change caused by handcantac t is sufficient to introduce an error. It is wise to develop the habitof not directly touching the test standards or parts during a teat.

Sometimes, especially when measuring thicknesses of parts, a test being con-ducted with the parts lying on a bench with a conductive surface will resultin misleadin~ readings. Work stations with nonconductive surfaces should beused to preclude such influences.

The beginning and end of every eddy current test should include routine checksof the procedure with standards. Such checkg should algo be conducted at rea-sonable time intervals during the testing.

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2..9 SAFETY

Personnel safety in eddy current testing involves standard safety practicesfound in almost all industrial settings. Most eddy current devices require anex t emal electrical power source. Therefore, all safety procedures relatingto the handling of power cords, their maintenance and their use, must be ob-served. Damaged insulation, positioning cords in placee where they can bestepped on, or where they can trip pessing personnel, or cauae equipment to bepulled off of table tops are all safety points that can be monitored.

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2.9 GLOSSARY

Reference ASTM E-268 .StandrirdDefinitions of Terms Relating toElectromagnetic Tegting.

2.9-1

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2.11 I ~DEX

Subject

Absolute Instruments

AdIuinistratersApplicmtionaBridge CircuitcoilsConductivity MeasurementCurrent-Plane DiagramDesignersDifferential InstrumentsEddy CurrentgEddy Current Test LimitationsEddy Current VariablesEdge EffectsElectric CurrentElectromagnetic FieldsElect romagnetsEquipmentExternal CoilsFaraday a LawFill FactorGuidelines For Specific DisciplinesImpedance-Plane DiagramsInductionInstrumentsInternal CoilsMutual InductionLift-offNDT Engineers

NDT TechniciansOhms LawPhase AnglePr@ciple9Production EngineersQuality Aseurance PersonnelRight-hand RuleSafetySelf InductionStandard Depth of PenetrationStandardsSurface CoilsTechniquesTest CoilsThicknese MeasurementsVector Diagrama

Section

2,3.1

2.7.12.62.3.32.3.22.4.32.2.72.7.22.3.12.2.92.2.112.2.102.3.22.2.12,2.22.2.22.32.3.22.2.32.3.22.72.2.82.2.32.32,3.22,2.42.3.2/2.4.12.7.5

2,7.62.2.12.2.6/2.2.72.22.7.32.7.42.2.22.0/2.82.2.52.4.22.52.3.22.42.3.22.4.22.2.6

2.11-1


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