Chapter 5 - Eddy Current Method Control and Test System Development

8/12/2019 Chapter 5 - Eddy Current Method Control and Test System Development

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CHAPTER - 5

EDDY CURRENT METHOD CONTROL AND TEST

SYSTEM DEVELOPMENT

GENERAL

Eddy current testing equipment requires method control for

maximum sensitivity. In some cases this can be very simple, such as a

specially designed jig, coil or probe. In other application it might be

handling and feeding equipment to control certain variables

Since you will use nondestructive testing to improve the structuralconfidence level, it is desirable to build reliability into the non-destructive

testing systems. his does not necessarily involve extreme complexity. In

many instances it can be !ept very simple. "e sure the entire system is

suited to the test to be performed. #ery little is accomplished, for example,

if the eddy current equipment used to test cylinder is speed sensitive and

the conveying or driving apparatus permits wide speed variations. he

basic elements of the eddy current testing system are the coil or sensing

unit, the generator, and the indicator. $ormally the generator and the

indicator may be considered a piece of standard equipment, which leaves

the coil as the element of primary interest. %oils are usually developed for

a particular function by selecting a material and coil parameters that will

match a particular generator which is to be used.

COIL PARAMETERS

1) GENERAL

he theory behind coil design is very complex, therefore, only a

limited coverage of this subject will be presented. &ctually, the design,

choice, and application is more of an art than a science since in many

cases, the $' specialist involved is not aware of problems caused by

certain variables until testing has actually begun. (urther, it is very difficult,

if not impossible, to predict what conditions are present in the test article

and how they will influence test system response.

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hrough building and testing of probes or coils, !nowledge of eddy

currents and their application may be acquired. "y observing the test

results, the $' specialist will be able to learn many of the characteristics

of properly and)or improperly designed probes or coils.

2) PROBE SIZE

*ne of the major faults in eddy current testing is the use of an over-

si+e probe. he probe coil should be no greater than twice the length of the

minimum discontinuity of interest.

3) FIELD SPREAD

It also must be reali+ed that the eddy current field in a probe will be

confined to an area approximately equal to the diameter of the probe coil.

his would mean that in eddy currents the field spread in negligible.

4) FILL FACTOR

hen designing an inside or encircling coil the fill factor /I' of coil

*' of article or vice versa0 is the primary consideration.

5) WINDING

In the design of the test probe or coil the actual winding should be as

close to the area of interest as possible so that the area will receive the

strongest eddy current field.

6) TYPE OF MOUNTING

he type of mounting material for the probe or coil does not have to

be elaborate. & well insulated stiff, nonconductor which will support or hold

the firmly against or around the article is sufficient.

) WIRE GUAGE

he wire gauge should be no smaller than number 12 /.3340. &

larger si+e is permissible.

177



!) NUMBER OF TURNS

he exact number of turns for the best results depends on the

application. Several examples are5

a. C"#$% (or an article 1)6 inch diameter or less, approximately 43 turns for

the coil wound to fit the article. (or article 1)6 to 7 inch diameter, about

14 turns for the coil. he coils may be wound /using the same number

of turns0 either with a short axial length, and large radial build up to

spread out the eddy currents, or the coils may be wound with a long

axial length and small redial build-up to test only a small area,

depending on the particular application.

b. P&"'(% 8eneral purpose probe5 panca!e 7)6 inch thic! /axial

dimension0 wound on a 7)6 inch diameter nonconductive rod with 793

turns of number 14 gauge enameled wire. his type can be used on

magnetic or non-magnetic metals.

c. I*#+( C"#$% :anca!e 7)6 inch thic! /axial dimension0 wound on a 3.393

inch diameter ferrite core 4)1; inch long with 7<3 turns of number 12

gauge enameled wire. (errite cores are used primarily to made a small

coil diameter possible.

,) EVALUATION

o determine that the proper number of coil wire turns were used

chec! the coil in the following manner5

a. :lace the object being tested fully within the coil, the meter indicator

pointer should read on-scale of any (requency control setting after

ma!ing "alance and =ift-off control adjustments at maximum

sensitivity.

b. If the conditions cannot be met, remove several turns and try again.

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SYSTEM DEVELOPMENT

1) GENERAL

>any types of test system are available. &lthough most test

systems are basically impedance bridges of one form or another, they come

in a variety of si+es, shapes, and designs. Some systems are used only for

magnetic materials, others for non-magnetic materials. Some systems are

probe coils, others encircling or inside coils, while still others are designed

for use on a variety of materials and use both probe and encircling coils.

The question arises as to how the engineer or NDT specialist

chooses the proper test system. Perhaps the clearest way to

show the process involved in system development is to present

and then solve typical test problems.

2) THE SEPARATION OF IMPROPERLY UENCHEDMOLYDENUM-CHROMIUM-VANADIUM STEEL CYLINDERS

./ S0.0((0 " &"'$(

& number of ;4)1; inch diameter cylinder ruptured during dynamic

pressure testing. (ailure analysis disclosed that the failure was

metallurgical in nature and not mechanical being attributed to improper heat

treat with a resulting grain structure of ferrite, bainite and some martensite.

he latter causing the failure when the $#+(& was subjected to dynamic

pressure testing.

Since large volumes of these cylinders were in stoc! and no positive

means of identification as to heat, each would require some type of testing

to identify heat treat status. he solution required a rapid, economical

means of nondestructively screening the properly heated cylinders from

improperly heat treated.

179



'/ NDT A.$*#*

(1) The material in question was magnetic

(2) eat treatment will change the permeability and

conductivity properties in magnetic materials! the change

depends upon the stage o" heat treat.

(#) $ddy currents can separate and evaluate permeability

and conductivity changes.

(%) The e""ect o" improper quenching is a gradual change

and will not behave li&e a locali'ed discontinuity not

requiring 1* coverage o" the questionable articles.

/ NDT A.$*#*

/70 R((&(( S0.+.&+ Selected a properly quenched tube for the

reference standard /tube0 and verify metallographically for 733?

martensitic microstructure.

/;0 E++ C&&(0 S*0( S($(0#" 'ifferential encircling coil system

was selected with a test frequency of 23 cps or less for magnetic

material. he field strength was ;4 to @4 oersteds. his was

determined by obtaining normal induction curves from the articles

representing improper quench and properly quenched articles. Ifsuch curves are not available, a series of trail and error tests must be

conducted to resolve the difference between the articles by varying

the field strength.

+/ R(.+"0

Aeadout in this case could be on either a meter or an oscilloscope

/%A0. (igure 4-7 shows the %A wave pattern for properly quenched tube

/view &0 and improperly quenched tube /view "0. with a meter, an

improperly quenched tube indicated by a needle detection away from +ero

/(igure 4-;0.

180



(/ S.&

Several observations could be drawn from this problem. Since the

article was magnetic both permeability and conductivity would vary

according to the heat treat. &n improper quench would lower conductivityby increasing the permeability /opening of hysteresis loop0. his loss would

results in a different voltage being developed across the secondary of a coil

encircling the tubes with correct and incorrect quenching. he output from

the secondary of the test coil would be buc!ed against the secondary

voltage obtained from a reference coil containing a 733 percent martensitie.

F#7&( 5-1% CRT P&(*(0.0#"

F#7&( 5-2% M(0(& P&(*(0.0#"

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:roperly quenched tube and the difference between these two

voltages would be amplified and displayed in the readout sub-system. he

displayed voltage, therefore, could be related to the metallurgical difference

between the test tube and the reference standard.

3) SEPARATION OF IMPROPERLY WELDED TITANIUMPRESSURE VESSELS

In many cases the solution to a problem may not require designing a

special coil. *ften it is only necessary to prepare a representative

standard. Such was the case in the following problem5

a. Statement of problem

'uring the fabrication of a group of titanium alloy, ti-2<7-<#, pressure

vessels, an unauthori+ed change in welding procedure resulted in a number

of the vessels being welded with commercially pure titanium weld wire

which caused embrittled weldments. he records were not sufficient to

determine which vessels were properly welded and which were improperly

welded. herefore, it was necessary to develop nondestructive testing

procedure for sorting the good and bad vessels.

& study of the welding process revealed that at some point in the

fabrication of the pressure vessels, the approved i-2<7-<# welding wire

was replaced with a commericial pure titanium wire. Since these alloys

differ in conductively by a ratio of 157 /commercial pure titanium versus alloy

titanium0, an eddy current test was selected as the most promising $'

tool.

'/ NDT A.$*#*

7. weld material is nonmagnetic.

;. Impedance /surface probe0 method should be used.

1. here is considerable difference in electrical conductivity of the two fillerwires used.

<. Aepresentative standards of the two welds must be prepared.

/ NDT P&"(+&(

7. Standard welds were prepared, using each type of wire.

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;. & variable frequency eddy current instrument was selected for the test.

1. he standard probe, supplied with the instrument, and a frequency of

approximately 13 !cps were used.

<. he sensitivity and balance of the instrument were adjusted to give a

minimum on-scale reading when the probe was placed on the weldmade within the i-2&I-<#. when the probe /coil0 was then placed on

the weld made with the pure titanium wire, the instrument gave a full

scale deflection, (igure 4-1.

4. he suspect welds were than tested, with any large, up-scale deflection

being considered sufficient for pinpointing the discrepant welds.

F#7&( 5-3% T#0.# P&(**&( V(**($

183



+/ S.&

Several meter readings were ta!en at various location on each

vessels to eliminate the possibility that the coil may be detecting a crac!

and therefore giving a low scale deflection whereby the commercially pure

titanium welds could be mista!en for alloy weldments.

4) DETECTION OF GRINDING CRAC8S IN VALVEASSEMBLY

./ S0.0((0 " P&"'$(*

8rinding crac!s in a valve subassembly did not permit a seal to be

made. he material was special B-;33 steel with magnetic permeability of

729 in the annealed condition and an electrical resistivity of <3 micro-ohm-

cm. he part was a < cube with four 1)6 diameter holes extending ; into

the material /(igure 4-<0. he 1)6 diameter holes were finished with a

grinding operation which became the origin of the grinding crac!s. &ll

articles that contain grinding crac!s are questionable.

184



F#7&( 5-4% V.$9( A**('$

'/ NDT A.$*#*

:1) 8rinding crac!s are surface crac!s and in this particular problem will

only lend themselves to detection by eddy currents.

:2) he length of the expected crac!s will be .3334 inch ot .334 inch.:3) he material is ferromagnetic.

/ NDT P&"(+&(

:1) 'ue to the high resistivity of the material and the desired test depth a

relatively high frequency may be used. (or this problem, a

frequency of ;3 !cps was used.

:2) &n inside absolute type of coil must be used.

:3) Since the material had a high permeability, an encircling dc coil was

used to provide magnetic saturation. he dc coil design is not

criticalC however, the field must be strong enough the provide

saturation. he saturation point was ta!en from a " and D curve for

B-;33 Steel.

:4) he coil was wound with 43 turns of number 12 gauge magnet wire.

&fter winding the coil was chec!ed for compatibility with the test

instrument. his was performed by connecting the coil with the

instrument and by balance and lift-off, determines if the meter

remained on the scale.

:5) & reference standard was developed using a simulated sample with

!nown grinding crac!s.

:6) he part was tested by inserting the coil into the holes and sliding the

coil along the entire length of the hole while monitoring the meter for

a change in out-put indication. %hange in the output indication

indicated grinding crac!s and the article is questionable.

185



previously possible. he preceding examples of eddy current applications

indicate the diversity of this field.

he $' specialist will be concerned primarily with the testing of

hardware but in some cases may participate in the design and development

f highly complex systems.

he eddy current method should only be used where applicable and

not loo!ed upon as a solution to every $' problem. he $' specialist

who wishes to expand his !nowledge of eddy currents and their applicable

will find sample additional information presented in $' handboo!, journals

and reports.

187



CHAPTER ; 5

EDDY CURRENT METHOD CONTROL AND TEST SYSTEMDEVELOPMENT

LEVEL II - UESTIONNAIRE

1/ I 0<( &(*#*0.( # . 1 $"7 =#&( #* 2 "<* =<( #0 <.* >/1+#.(0(&? =<.0 =#$$ 0<( &(*#*0.( '( # . =#&( " 0<( *.( $(70<.+ .0(&#.$ '0 "$ >/>5 +#.(0(&@a0 7 ohmb0 ; ohmsc0 < ohmsd0 6 ohms

2/ W<( 0<( (++ &&(0 0(*0 **0( #* &(&(*(0(+ ' 0<(

0&.*"&(& 0<( *.$( . '( "*#+(&(+ 0<( *("+.& =#+#7=#0<a0 a single turnb0 73 turnsc0 +ero turnsd0 none of the above, it is not possible to determine

3/ G#9( . "#$ =#0< 5> "< &(*#*0.( .+ 5> #&"<(&#(*#+0.( .+ "(&.0(+ .0 5> H =<.0 #* 0<( "#$ #+0#9(&(.0.(@a0 3.< ohms

b0 7.2 ohmsc0 1.9 ohmsd0 74.@ ohms

4/ G#9( . "#$ =#0< 2 "<* &(*#*0.( .+ 2> H #+0.( .+"(&.0(+ .0 2> H? =<.0 #* 0<( "#$* #+0#9( &(.0.(@a0 7.49 ohmsb0 ;.47 ohmsc0 2.1 ohmsd0 73 ohms

5/ G#9( . "#$ =#0< 2> "<* &(*#*0.( .+ 6> #&"<(&#(*#+0.( # .#& .+ "(&.0(+ .0 5> H? =<( '&"7<0 (0 0" .#"($ *.$( 0<( &"'( #(+.( #* 2!/5 "<* .+ #(+.(<.*( #* 45? =<.0 #* 0<( &"'(* #+0#9( &(.0.(@a0 ;3.; ohmsb0 ;3.; microhenriesc0 <<.@ ohmsd0 not possible to determine with information given.

188



6/ G#9( . &"'( =#0< 5> "<* &(*#*0.( .+ 4>H #+0.(? =<("(&.0(+ (0 0" . "(& *.$( .0 2> H 0<( &"'( #(+.( #*55 "<* .+ #(+.( <.*( #* 4>? =<.0 #* 0<( #+0#9(&(.0.( " 0<( &"'( =<( "(&.0#7 " 0<( *.$(@a0 44 ohms

b0 <;.7 ohmsc0 14.1 ohmsd0 4 ohms

/ I 7#9( 0"0.$ #(+.( " . &"'( "(&.0#7 " . 0(*0 *.$( .+"=#7 0<( #(+.( <.*( .7$(? =<.0 (.0#" #* *(+ 0"+(0(&#( 0<( #+0#9( &(.0.( " 0<( &"'(@a0 Bp ;pif=b0 Bp Fp cos Gc0 Bp Fp tan Gd0 Bp Fp sin G

!/ V"$0.7( <.7(* .&"** 0<( &"'( +( 0" . +((0 # "*0 (++&&(0 #*(0#"* .&( " 0<( "&+(& "a0 7?b0 73?c0 733?d0 7333?

,/ W<( . *#$( '&#+7( .+( " 4 #(+.( .&*? 0<( 9"$0.7( #.+.(0 .&* " 0<( '&#+7( *0 '( (.$ #a0 amplitude

b0 phasec0 both a and bd0 no form

1>/W<( (&"&#7 . 0'#7 #*(0#" =#0< . 7((&.$ &"*( ECT#*0&(0 <.9#7 . #*0&(0 &(*"*( &(( " 3>> H?.# *.#7 *((+ #* .'"0 JJJ 0" +(0(0 .'&0 +((0*=#0<"0 +#*0"&0#"/a0 3.7 m)sb0 3.;4 m)sc0 7.3 m)sd0 ;.3 m)s

11/A0 <#7< "(&.0#7 &((#(*? 0<( ((0#9( "#$ +#.(0(& :*(*#7+#.(0(&) #* .&"#.0($ (.$ 0"a0 3.4 coil diametersb0 the actual coil diameterc0 ; coil diameterd0 the s!in depth

189



12/A &.0#.$ +(0< $##0 "& $.= +(0(0#" .+ $".0#" *#7 (++&&(0 0(*0 (0<"+* #* .'"0a0 7mmb0 2mm

c0 7;mmd0 76mm

13/W<#< " 0<( "$$"=#7 #* "0 . .+9.0.7( " 0<( (++ &&(0 0(*0(0<"+@a0 733? volumetric inspection is possible /within limits0b0 speedc0 clean smooth surfaces not requiredd0 no couplant required

14/E#&$#7 &"'(* :"& #0(&.$ &"'(*) .&( $#($ 0" '( &($.(+ '

*&.( &"'(* "& 0'#7 =#0< . +#.(0(& 7&(.0(& 0<. 5>/ T<(&(.*" "& 0<#* #*a0 encircling probes cannot be made biggerb0 fill factor becomes too difficult to regulate for large encircling probesc0 higher defect sensitivity can be achieved using surface probesd0 both b and c

15/T" #&(.*( *(*#0#9#0 0" (.& *&.( +((0* *#7 . '"''# *0$(&"'( "#$ $(70< .+ 0<#(** .&( &(+(+/ T<#* <"=(9(& &(*$0*#a0 reduced frequency range

b0 increased probe-cable capacitancec0 decreasing sensitivity to the far surface defectsd0 bobbin brea!down

16/S(.&.0#" " +#.(0(& .+ "+0#9#0 ((0* #* '(00(& .&&#(+"0 .0 &(( &.0#"* 7&(.0(& 0<. 4 '(.*(a0 greater penetration afforded permits better determination of bul!propertiesb0 the angle between diameter and conductivity locii is greaterc0 the angle between diameter and conductivity locii is 93Hd0 none of the above, frequency ratio should be less than < for suchwor!

1/I $.0( 0(*0#7? 0" ###( ((0* " $#0-" 9.&#.0#"* " ="$+a0 increase coil to part spacingb0 increase coil diameterc0 decrease coil to part spacingd0 decrease coil diameter

190



1!/S(*#0#9#0 " "+0#9#0 (.*&((0 =#0< 0<( &"'( "#$ #*a0 proportional to the coils geometric field gradientb0 a function of the specimen thic!nessc0 a function of the effective coil distanced0 all of the above

1,/I . *<((0 =.* ""*(+ " 2 (0.$$# $.(&* =#0< 0<#(**(* D1.+ D2 .+ "+0#9#0#(* K1 .+ K2? =<.0 ="$+ 0<( (#9.$(0&"+0 '( =<( 0(*0(+ ' 0<&"7< 0&.*#**#"@a0 J' J7'7 K J;';b0 J' J7J; K '7';c0 J' /J7'70 /J;';0d0 J' /J7'70L K /J;';0L

2>/D(0(&##7 $.0#7 0<#(** " . "+0#7 ".7(0#.0(&#.$ " ."0<(& "+0#7 ".7(0# .0(&#.$ &(#&(*

a0 a difference in conductivities between the two materialsb0 use of a lift-off compensating probec0 a resonance circuit be usedd0 all of the above

21/T" +#*(& 9(& *<.$$"= &.* *#7 . *&.( "#$ " ="$+ *(. &($.0#9($ <#7< &((-"+0#9#0 &"+0 :K)/ W<#< " 0<("$$"=#7 ="$+ 0<( '( 0&(@a0 angle between crac! direction and lift-off effect increasesb0 magnitude of crac! effect decreasesc0 lift-off effect increases

d0 all of the above

22/W<( *#7 *&.( "#$* "& &. +(0(0#"? *<.$$"= &.* .+$#0-" ."0 '( *(.&.0(+ $(**a0 lift-off compensating probes are usedb0 frequency is high enoughc0 frequency is low enoughd0 both a and b

23/W<( #*(0#7 *<(&(* =#0< . (#&$#7 "#$? =<.0 #* 0<((#9.$(0 ((0 " #&(.*#7 0<( "#$ $(70<@a0 a frequency increaseb0 a frequency decreasec0 an increase in material conductivityd0 a decrease in fill factor

191



24/I 0<( M## +(0(0.'$( +#*"0##0 #* 1? &"'( "#$ +#.*<"$+ "0 '( 7&(.0(& 0<.a0 3.4mmb0 7mmc0 ;mm

d0 1mm

25/M. 7.7( " " 0<( =#&( *(+ . '(a0 12b0 1@c0 <3d0 <;

192



CHAPTER ; 5

EDDY CURRENT METHOD CONTROL AND TEST SYSTEMDEVELOPMENT

LEVEL II ; ANSWER

/NO/ ANS7 '; &1 '< "4 &2 %@ '6 &9 %

73 "77 "7; "71 %7< %74 %72 "7@ "

76 '79 &;3 &;7 ';; ';1 ';< %

;4 & /=arge si+e, smaller gage no is

permissible0

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Chapter 5 - Eddy Current Method Control and Test System Development

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Transcript of Chapter 5 - Eddy Current Method Control and Test System Development