alternating current field measurement

Martin C. Lugg, TSC Inspection Systems, MiltonKeynes, United Kingdom

10C H A P T E R

Alternating CurrentField Measurement

PART 1. Introduction to Alternating Current FieldMeasurement

BackgroundAlternating current field measurement isan electromagnetic technique that usesinduced uniform currents and magneticflux density sensors to detect and sizesurface breaking discontinuities withoutcalibration. The term uniform means that,at least in the area under the probe,current lines (components of electric fieldintensity E) in the absence of adiscontinuity are parallel, unidirectionaland equally spaced (Ex = Ez = 0 andEy = constant ≠ 0).

In the 1980s, there were moves todevelop nondestructive test techniques fordetecting and sizing fatigue cracksunderwater in welded offshore structures.The development focused on two existingtechniques — eddy current testing fordetection and alternating currentpotential drop testing for sizing.

Conventional eddy current techniqueswere not particularly effective because ofthe scanning patterns needed across theweld and because of the signals receivedfrom the weld itself. The developmentstherefore concentrated on producing eddycurrent probes that could test the wholeweld while being scanned only parallel tothe weld. This was achieved by usingarrays of coils covering the weld. To assistdetection, because the probe was nolonger being scanned across the weld toe(and hence the discontinuity), coils ofmore than one orientation were used toobtain signals from the ends of adiscontinuity as well as the center. Boththe induction coils and sensing coils wererelatively large to give wide coveragerather than high sensitivity because therewas no need to detect discontinuities lessthan 1 mm (0.04 in.) deep in the types ofwelds found underwater. Thedevelopment emphasized detection ofdiscontinuities and suppression of liftoffeffects. Crack depth had to be sized withcalibration slots.1

Alternating CurrentPotential Drop TechniqueThe alternating current field measurementtechnique was developed out of work onthe potential drop techniques. Potentialdrop test applications in the 1980s tended

248 Electromagnetic Testing

to use direct current rather thanalternating current.2

Conventional potential drop testinginjects an alternating current at points oneither side of the crack (so that the testobject becomes part of the energizingcircuit) by using spot welded pins ormagnetically attached sprung pins andthen measures differences of electricpotential at the surface adjacent to andacross the crack. The current can,however, be induced into the test siteinstead of being injected. Current hasbeen induced where direct injection isdifficult (such as on threaded connectionsor on materials where magneticattachment does not work) or where theelectric field intensity would otherwise betoo low — for example, on lowconductivity metals such as aluminum.

An induced field can be achieved eitherby laying current carrying wires on thetest object surface across the line of thecrack or by building an inducing coil intothe voltage probe. This means ofinduction simplifies deploymentunderwater but in many cases requires thearea around the crack to be cleaned (andkept clean) to bright metal in order toallow voltage measurements to be made.When used with an induced field,alternating current potential dropbecomes an eddy current techniquealthough it is not usually recognized assuch.

The alternating current potential droptechnique has the advantage that, beinginitially uniform, the current flow past awide range of discontinuities can easily bemathematically modeled. Comparisonscan then be made between measured andpredicted voltages, allowing estimation ofcrack depth and shape withoutcalibration.3-8

The alternating current potential droptechnique struggled when deployedunderwater because of the need to makegood electrical contact with the metalsurface. For the potential drop techniqueto be usable underwater, means weredeveloped to obviate electrical contactwith the test object. Instead of measuringthe surface electric field with contactingpins, the magnetic flux density just abovethe surface was measured withnoncontacting coils while retaining theability of alternating current potential

drop to calculate crack depth withoutcalibration.

Magnetic FieldMeasurementWork in potential drop testing in the early1990s studied the surface electric fields todescribe the associated magnetic fields. Aswith alternating current potential droptesting, field measurement made itpossible to estimate crack depths withoutcalibration.9

As it turned out, making anoncontacting technique from alternatingcurrent potential drop meant that theprobe could be scanned along a weld veryeasily. Thus, although alternating currentfield measurement was developed fornoncontact sizing, it was useful also fordiscontinuity detection.

The modeling of alternating currentfield measurement does not require theinput field to be induced. Indeed, earlyexperiments with the technique oftenused injected currents to give better largescale uniformity. In this situation,therefore, alternating current fieldmeasurement does not use eddy currents,so the boundaries between alternatingcurrent potential drop, alternating currentfield measurement and conventional eddycurrent test techniques are blurred. Thereare other electromagnetic techniques ofthe same general type: uniform field eddycurrent, current perturbation,electromagnetic array and surfacemagnetic flux density measurementtechniques. Applications include offshoreoil platforms,10 petrochemicalequipment,11-13 threaded connections,14

cranes,15 bridges and rails.16

MOVIE.Testing ofthreads.

249Alternating Current Field Measurement

PART 2. Alternating Current Field MeasurementTechnique

FIGURE 1. Uniform magnetic field generated by horizontalsolenoid: (a) induction of magnetic field by alternatingcurrent in coil; (b) uniform field induced by alternatingcurrent in metal surface.

(a)

(b)

Resultant alternating magnetic field

Alternating current passed through coil

Alternating current induced in metal surface

Magnetic field

Area of approximatelyuniform field

Principle of OperationThe alternating current field measurementtechnique involves inducing a locallyuniform current into a test object andmeasuring the magnetic flux densityabove the test object surface. The presenceof a surface breaking discontinuityperturbs the induced current and themagnetic flux density. Relative, ratherthan absolute, amplitudes of componentsof the magnetic flux density are used tominimize variations caused by materialproperties, instrument calibration andother circumstances. These relativeamplitudes are compared with values insizing tables produced from amathematical model to estimatediscontinuity sizes without the need forcalibration using artificial discontinuitiessuch as slots. This feature of alternatingcurrent field measurement is usefulbecause calibration on slots is prone toerror for several reasons.

1. Calibration adds opportunities foroperator error (and one mistake on acalibration setting will affect allsubsequent sizing).

2. Slots behave differently, electrically,from real cracks. In particular, themagnetic fields inside the slot widthproduce extra induction effects.17

3. Slots in calibration blocks are often inmaterials with different properties (forexample, parent plate rather than heataffected zone or weld material).

4. Slots often have geometry differentfrom that of a real crack (for example,a rectangular shape rather than thesemielliptical shape more typical offatigue cracks).

5. The range of slots available in acalibration block is limited. Inparticular, they tend to be of the samelength, whereas the signal intensitycan be affected by crack length as wellas depth, particularly for short cracks.

The sizing tables have been producedby repeated running of the model forsemielliptical cracks in a wide range ofdifferent lengths and depths. The model iscalled the forward problem for which thediscontinuity size is known and thesignals are then predicted. For the inverseproblem, sizing an unknowndiscontinuity when the signal variationsare known, software is used to interpolate


between and within these tables. Themathematical model used in earlydevelopment work assumes that theincident current is uniform on a scalecomparable to the discontinuity. That is,the electric field lines were parallel andequally spaced. The model also assumesthat the standard depth of penetration issmall compared to the depth of thediscontinuity.

The method in its simplest form usesan instrument and a hand held probecontaining a uniform field induction

FIGURE 3. Effect of surface breaking discontinuity on magneticfield.

Clockwise flowgives Bz peak

Counterclockwiseflow givesBz trough

Am

plit

ude

(rel

ativ

e sc

ale)

Uniforminput

current

Bz

T

system and two magnetic field sensors.Software on an external personalcomputer is used to control theinstrument and to display and analyze thedata.

The required locally uniform magneticfield is induced using one or morehorizontal axis solenoids, with or withouta yoke (see Fig. 1). By convention, thedirection of this electric field E isdesignated as the Y axis and the directionof the associated uniform magnetic fluxdensity B (at right angles to the electricfield and parallel to the test surface) isdesignated as the X axis. The Z axis isthen the direction normal to the surface(Fig. 2).

With no discontinuity present and auniform current flowing in theY direction, the magnetic field is uniformin the X direction perpendicular to thecurrent flow. Thus, Bx, By and Bz are thethree orthogonal components (in tesla) ofmagnetic flux density B. Bx will have aconstant positive value whereas By and Bzwill both be zero.

Figure 3 shows the effect of a surfacebreaking discontinuity on the magneticfield. The presence of a discontinuitydiverts current away from the deepestparts and concentrates it near the ends ofa crack. The current distribution producesa broad dip in Bx along the discontinuitywith the minimum value coinciding withthe deepest point of the discontinuity.The amplitude of this dip is larger for adeeper discontinuity of a given length. Atthe same time, concentration of currentlines where it flows around thediscontinuity ends produces small peaksin Bx. The same circulation around thediscontinuity ends also produces anonzero Bz component. The flow isclockwise around one end, producing anegative value of Bz (pointing into thesurface) and counterclockwise around theother end producing a positive value of Bz(out of the surface). The locations of themaximum (positive and negative) values

FIGURE 2. Coordinates conventionally used in alternatingcurrent field measurement.

Electric field E

Mag

netic

flux

den

sity

B

X

Y

Z

of Bz are close to, but not coincident with,the ends of the physical discontinuity.

The By component also becomesnonzero in the presence of adiscontinuity, producing a peak and atrough at both ends of the discontinuitybut these are antisymmetric across theline of the discontinuity. Because a Bysensor scanning exactly along the line ofa discontinuity would see no response,the By component is not usually measuredin alternating current field measurement.

Measurements of Bx and Bz fromsensors in the probe are used withsoftware algorithms to determine thelength and depth of the discontinuity. Toaid interpretation, the Bx and Bzcomponents are often plotted againsteach other to produce a closed loopindication. Because of its shape, thedisplay is often called a butterfly plot(Fig. 4). This loop’s size is insensitive toprobe speed, so this display can help tointerpret data and evaluate indications.

The actual parameters used by thesoftware can vary but must include thefollowing.


Current linesfar apart givesBx trough

Current linesclose togethergives Bx peak

Am

plit

ude

(rel

ativ

e sc

ale)

Bx

LegendBx = magnetic flux component normal to electric field and parallel to test

surfaceBz = magnetic flux component normal to test surfaceT = time or scan distance (relative scale)

T

1. The perturbation amplitude is neededfor one component of the magneticflux density produced by thediscontinuity (usually Bx but Bz canalso be used).

2. The intensity of the input magneticflux density Bx is used to normalizethe perturbation. This background Bxvalue must therefore be measured inan area of properties similar to theperturbation value. This area isnormally next to the discontinuity butoutside its influence.

3. A measurement unit is needed toquantify the signal. This unit isusually the distance between the peakand trough in the Bz signal becausethese signals are sharply defined butthe distance between the peaks in Bxcan also be used.

Typical Probe DesignsFigure 5 shows components arranged in atypical alternating current fieldmeasurement test. The exact parametersused in a probe vary according to theapplication. The larger dimensions areused where possible because they give themost uniform field and allow the two


FIGURE 4. Data from longitudinaldiscontinuity: (a) chart recorder plot;(b) butterfly shaped plot.

(a)

(b)

Bx

0

0

Bz

Mag

netic

flu

x de

nsity

By

(rel

ativ

e sc

ale)

Magnetic flux density Bx

(relative scale)

Mag

netic

flu

x de

nsity

(rel

ativ

e sc

ale)

Time or scan distance(relative scale)

sensors to be wound concentrically, whichgives clear symmetric loops in thebutterfly plot. In probes designed for tightaccess applications or for highersensitivity, the smaller dimensions areused.

Uniform FieldThe alternating current field measurementtechnique uses a uniform input field toallow comparison of signal intensitieswith theoretical predictions. A uniformfield has advantages and disadvantagescompared with conventional eddycurrents. The main advantages are (1) theability to test through coatings severalmillimeters (one or two tenths of an inch)thick, (2) the ability to obtain depthinformation on cracks up to 25 mm(1 in.) deep and (3) easier testing atmaterial boundaries such as welds. Themain disadvantages are (1) lowersensitivity to small discontinuities,(2) signals obtained from nearby geometrychanges (such as plate edges) and(3) dependence of signals ondiscontinuity orientation relative toprobe. These advantages anddisadvantages are discussed below.

Advantages

Testing through Coatings. The primaryadvantage of using a uniform field is thatthe intensity of the input field decaysgradually with distance from the inducingcoil; the intensity of the field perturbedby a discontinuity also decays graduallywith distance above the surface. Theintensity of a uniform field performancedoes not drop off very rapidly with probeliftoff, so alternating current fieldmeasurement can be used to test throughthick nonconductive coatings. Thetechnique can be used on painted or rustysurfaces or on structures covered with

MOVIE.Testing throughcoatings.

FIGURE 5. Typical alternating current fieldmeasurement probe layout.

Probe coils, 1 to 5 mm (0.04 to 0.2 in.) indiameter

20 to 40 mm(0.8 to 1.6 in.)

Solenoid, 15 to 30 mm

(0.6 to 1.2 in.) long

Test object

protective or fire resistant coatings severalmillimeters (one or two tenths of an inch)thick.Depth Information. The second advantageis that the larger inducing coil forcescurrents to flow farther down the face of adeep crack. Currents from conventionaleddy current probes flow in circles a fewmillimeters (about an eighth of an inch)across. When a probe lies over a deepcrack, the current splits into two separatecircles, one on each side of the crack andconfined to the top few millimeters(about an eighth of an inch) of the crackface. Because essentially no current thenflows to the bottom of the crack, noinformation can be obtained about wherethe bottom is, so the depth of the crackcannot be measured.

The same feature occurs with analternating current field measurementprobe but, because the depth ofpenetration down the crack face is relatedto the size of the magnetic field inducingcoil, an alternating current fieldmeasurement probe can measure moredeeply, typically 15 to 30 mm (0.6 to1.2 in.), depending on probe type.

Greater depths could be achieved if adirectly injected current were used insteadof an induced one but direct injection isinfrequently used because it requires aclean metal surface and the currentdensity achieved (and hence the signalintensity) would be much less than withan induced field. In these circumstances,alternating current potential drop testingwould be more suitable.Material Boundaries. A third advantage ofa uniform field arises when testing at aweld or other boundary between twometals of different permeability orconductivity. In this case, if the probe isscanning for discontinuities parallel to theboundary, no probe motion is requiredacross the boundary and no signals arecaused by the change in materialproperty. Also, the currents are flowingperpendicularly across the boundary, sothe effect of this material change isreduced even when scanning up to it.

Disadvantages

Reduced Sensitivity. The maindisadvantage of using a uniform field isthat sensitivity is reduced. This reductionis of little consequence on welded orrough surfaces, where sensitivity would bereduced anyway. On smooth, cleansurfaces, however, alternating current fieldmeasurement is less sensitive to short orshallow discontinuities than conventionaleddy current techniques. The smallestdetectable discontinuity on a good surfacewith alternating current field

measurement is around 2 mm (0.08 in.)long or 0.25 mm (0.01 in.) deep.Geometry Changes. A seconddisadvantage of a uniform field is that,because the currents spread out farther,signals are obtained from local geometrychanges, such as plate edges and corners.Although these signals do not usuallyhave the same form as a signal from adiscontinuity, they can confuse theoperator. If many similar geometries arebeing tested, the operator can learn whatsignals are caused by the geometry aloneand then ignore these. Alternatively, scansfrom discontinuity free sites with thesame geometry can be stored anddisplayed for comparison or probes withdifferential sensors can be used toeliminate the large scale signals.Discontinuity Orientation. A thirddisadvantage is that the signals obtainedfrom a discontinuity depend on theorientation of the discontinuity. Theuniform field theoretical model wouldsuggest that no signal be produced whena probe scans across a transversediscontinuity, because the current flow isthen parallel to the discontinuity andwould not be perturbed. In fact, inpractice, there is a signal produced in thissituation (caused by magnetic flux linesjumping the discontinuity) but these donot conform to the signal expected from adiscontinuity. The operator is trained tolook for the signals caused by a transversediscontinuity in order to detect them.Additional scans must be made along theline of the discontinuity to size it.

Effect of Coating ThicknessOne of the main advantages of theuniform field used in alternating currentfield measurement is that it results in arelatively small reduction in signalintensity with probe liftoff. Consequently,alternating current field measurement candetect cracks through several millimeters(one or two tenths of an inch) ofnonconductive coating. Typical coatingsinclude paint, epoxy coatings, oxidelayers, fire protection layers and marinegrowth.

The magnetic field inducer is typicallya solenoid, either cylindrical or flat, withor without a steel core, with axis parallelto the surface being tested. The length ofthe solenoid is typically of the same orderas the distance above the metal surface. Atsuch distances, the magnetic flux densitydecays much slower than the 1·r –3 (wherer is coil radius) decay that occurs far fromthe solenoid on the axis of a circular coil.

The maximum coating thicknessthrough which a discontinuity can bedetected depends on the discontinuity


size, the probe type and the signal noise.Figure 6 shows rates measured at whichthe magnetic flux density Bx signalamplitude drops with coating thicknessfor a probe with a flat, 30 mm (1.2 in.)long solenoid 40 mm (1.6 in.) above thebase of the probe. The signal variationcaused by conditions such as surfaceroughness and material propertyvariations is usually less than 1 percent.


FIGURE 6. Effect of coating thickness on magnetic flux densityBx for 5 kHz, 30 mm (1.2 in.) long solenoid probe.

8

7

6

5

4

3

2

1

00 5 10 15 20

(0.2) (0.4) (0.6) (0.8)

Coating thickness, mm (in.)

Am

plit

ude

of m

agne

tic f

lux

dens

ity B

x

(per

cent

)

Legend= 20 × 1 mm (0.8 × 0.04 in.) slot= 20 × 2 mm (0.8 × 0.08 in.) slot= 50 × 5 mm (2 × 0.2 in.) slot

FIGURE 7. Coating thickness at which magnetic flux densityBx amplitude drops to 1 percent for solenoid probes of threesizes.

14 (0.56)

12 (0.48)

10 (0.40)

8 (0.32)

6 (0.24)

4 (0.16)

2 (0.08)

00 1 2 3 4 5 6

(0.04) (0.08) (0.12) (0.16) (0.20) (0.24)

Slot depth, mm (in.)

Coa

ting

thic

knes

s, m

m (

in.)

Legend= probe with 40 mm (1.6 in.) long solenoid= straight probe with 15 mm (0.6 in.) long solenoid= right angle probe with 15 mm (0.6 in.) long solenoid

The data show that, for example, a 5 mm(0.2 in.) deep discontinuity in a goodsurface should be detectable throughmore than 10 mm (0.4 in.) of coating.

The maximum coating thicknessthrough which a discontinuity should bedetectable depends on the size of theprobe solenoid. Figure 7 compares theperformances of different probe designs.

For sizing of discontinuities undercoatings, the sizing tables cover a range ofliftoff values to compensate for the factthat the amplitude is reduced. The coatingthickness needs to be known but only tothe nearest millimeter (about 0.04 in.)because the effect is small.

The limitation above applies tononconductive coatings. The alternatingcurrent field measurement technique canbe used to test through thin conductingcoatings (such as galvanizing, copperloaded grease, flame sprayed aluminum)but only if the coating thickness is smallcompared to the standard depth ofpenetration, about 1 mm (0.04 in.) at5 kHz in the cases described above.

Deep Crack LimitAny technique that uses induced currentsto interrogate surface breakingdiscontinuities will, for sufficiently deepdiscontinuities, face the problem that anyfurther increase in discontinuity depthhas no effect on the current distributionon the face of the discontinuity.Therefore, no information can be gainedabout where the bottom of the crack is.This limiting discontinuity depth dependson the probe design — in particular, onthe size of the inducing magnetic field.Figure 8 shows experimental results forthe rate of change in Bx signal amplitude

FIGURE 8. Rate of increase in magnetic flux density Bxminimum with increasing slot depth.

Probe with40 mm (1.6 in.)long solenoid

Probe with15 mm (0.6 in.)

long solenoid

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 5 10 15 20 25 30 35

(0.2) (0.4) (0.6) (0.8) (1.0) (1.2) (1.4)


Perc

enta

ge o

f ch

ange

in B

x

min

imum

per

1m

m (

0.04

in.)

FIGURE 10. Signals from 5 mm (0.2 in.) long, 0.2 mm(0.008 in.) deep slot using straight, pencil shaped probes of2.0 mm (0.08 in.) coil diameter: (a) at 5 kHz; (b) at 50 kHz.

(a)

le)

versus discontinuity depth. The points atwhich the curves fall below about0.2 percent per millimeter (5 percent perinch) are the deepest points that can bedetermined with each particular probetype.

Although this limiting depth is largerthan for standard eddy current probes,where the small input field usually gives amaximum distinguishable depth of about5 mm (0.2 in.), it is important to knowthe limitation during testing. If adiscontinuity is sized with a depth closeto the limit, it should be recognized thatthis depth is an estimate and that the truedepth may be larger.

Sensitivity to SmallDiscontinuitiesA larger input field than in a conventionaleddy current probe means that sensitivityto small discontinuities, particularly innonferrous metals, is reduced. Sensitivitycan be improved by using a higheroperating frequency and smaller sensorcoils but at the expense of noise. Ifuncorrected, the problems can give lessaccurate depth sizing. Using smallersensor coils allows the coils to bedeployed with centers closer to the metalsurface, which improves sensitivity toshallow discontinuities. Also, smallerdiameter coils give better detection of theends of short discontinuities becausewhen the coil is larger than about half thediscontinuity length, the positive andnegative Bz signals from the two endstend to cancel each other out.

FIGURE 9. Comparison of 5 kHz and 50 kHz, 2.0 mm(0.08 in.) diameter coil probes on slots in ferrous steel.

30

25

20

15

10

5

00 0.5 1.0 1.5 2.0 2.5

(0.02) (0.04) (0.06) (0.08) (0.10)


Sign

al a

mp

litud

e (p

erce

nt o

f B x

back

grou

nd)

Legend= 5 kHz, Bx amplitude= 5 kHz, Bz amplitude= 50 kHz, Bx amplitude= 50 kHz, Bz amplitude

The smallest discontinuity detectableby alternating current field measurementis a function of many parameters. Withsensitive probes on good surfaces,discontinuities as small as 2 mm (0.08 in.)long or 0.2 mm (0.008 in.) deep havebeen detected in ferritic steel. Innonferrous metals, the shallowestdetectable discontinuity is around 0.5 mm(0.02 in.) deep.

Experimental data showing signalamplitudes for slots with conventionaland high frequency probes are shown inFig. 9. Signals from a small discontinuityin steel at both frequencies are shown inFig. 10.

Plate EdgesCompared to a conventional eddy currentprobe, the larger size of the magnetic fieldinducer for an alternating current fieldmeasurement probe means that theinduced currents spread farther out fromthe center of the probe into the testobject. Nearby geometry changes canaffect the current flow and so producechanges in the measured magnetic fluxdensity. Features that can produce signalsin this way include plate edges, holes andsupport plates.


(b)

Am

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ude

(rel

ativ

e sc

a

Time or scan distance (relative scale)

Bx

Bz

Am

plit

ude

(rel

ativ

e sc

ale)

Time or scan distance (relative scale)

Bx

Bz

Figure 11 shows Bx signals from twoprobes scanning up to a plate edge inferritic steel. It can be seen that Bxdecreases as the probe approaches theedge before increasing rapidly to the in-airvalue as the sensor reaches the edge.Comparison of the two probes shows,however, that the effect on the probe withthe smaller inducer is restricted more tothe area near the edge. Another way toreduce the effect is to use a probe withtwo sets of sensors on a line parallel tothe edge, connected differentially. In thisway, the changing signal from the edge(seen equally by both sets of sensors)cancels out whereas a discontinuity signal(seen more strongly by the sensors passingover the discontinuity) still shows up. Thedrawback with using such a differentialprobe is that knowledge of thebackground value of Bx is lost, so it is notpossible to size the discontinuityaccurately.

To size a discontinuity that lies withinthe range of influence of a plate edge, thevalue of the background Bx magnetic fluxdensity must be estimated at the pointwhere the discontinuity is deepest. Itmust be estimated what value Bx wouldhave had if the discontinuity had notbeen present. This value is obtained eitherby drawing a curve joining the twosections of plot on either side of thediscontinuity (see Fig. 12) or by making asecond probe scan parallel to thediscontinuity but away from its influence.

Transverse DiscontinuitiesThe simple picture of currentperturbations producing the measuredsignals would suggest that discontinuities


FIGURE 11. Change in normalized magnetic flux density Bxreading, approaching edge of steel plate.

0 20 40 60 80 100

(0.8) (1.6) (2.4) (3.2) (4.0)

Distance of Bx sensor from edge, mm (in.)

125

120

115

110

105

100

95

90

Nor

mal

ized

Bx

valu

e



oriented transverse to the probe scandirection, thus being parallel to theuniform currents, would not be detected.

In fact, discontinuities in thisorientation in ferrous steel generallyproduce measurable signals that arisefrom flux leakage effects rather thancurrent perturbation. The signals arerelatively short (roughly the length of thesensor coils). The Bx signal consists of anupward peak (caused by the increased fluxdensity above the crack) whereas the Bzsignal is a close peak-to-trough pair(caused by the flux going up, out of andthen down into the metal on either sideof the crack). This combination results inan upward loop in a butterfly shaped plot(Fig. 13c), a loop that is distinct from thenormal longitudinal discontinuity signalbut may be confused with the signal froma seam weld. The differences between thesignals from a transverse discontinuityand a seam weld are that the transversediscontinuity gives shorter signals andthat the signal from a seam weld isconstant wherever the probe crosses it.The signals are strongest when crossingthe deepest, or widest, part of the crack —no strong signals are produced at thecrack ends.

Because the signal intensity is relatedas much to the crack opening as thedepth, signal intensity cannot be used tocalculate discontinuity depth. Also, nosuch signal is obtained in nonferrousmetals. For these reasons, to guaranteedetection of transverse discontinuities,test procedures should require theoperator to make two sets of scans withthe probe oriented in two orthogonaldirections (or to use an array probe thatcontinually switches between twoorthogonal current inputs).

FIGURE 12. Estimation of background magnetic field densityBx near plate edge.

Distance from edge (arbitrary unit)

Mag

netic

flu

x de

nsity

Bx

(arb

itrar

y un

it)

Legend1. Background value for calibration.2. Bx background with no discontinuity.3. Bx signal with discontinuity.

1 2

3

For cracks in ferrous steel orientedsomewhere between the purelylongitudinal and the purely transverse,the signals lie between the two extremes(Figs. 4 and 13, respectively). Fordiscontinuities within about 30 degrees ofthe longitudinal direction, the signalsappear similar to a longitudinaldiscontinuity except that the amplitude of

FIGURE 13. Magnetic flux density signals from transversediscontinuity compared to parallel discontinuity and seamweld: (a) chart recorder plot of Bx measurements; (b) chartrecorder plot of Bz measurements; (c) butterfly shaped plotof magnetic flux density.

(a)

(b)

(c)

170 (1.7)

160 (1.6)

150 (1.5)

140 (1.4)

130 (1.3)

120 (1.2)

110 (1.1)Mag

netic

fie

ld d

ensi

ty B

x, m

T (k

G)

Time (relative scale)


25 (250)

20 (200)

15 (150)

10 (100)

5 (50)

0

–5 (–50)

Mag

netic

fie

ld d

ensi

ty B

z, m

T (G

)

170 (1.7)

160 (1.6)

150 (1.5)

140 (1.4)

130 (1.3)

120 (1.2)

Mag

netic

fie

ld d

ensi

ty B

x, m

T (k

G)

1

2

3

1

2

3

1

2

3

Legend1. Transverse discontinuity.2. Parallel discontinuity.3. Seam weld.

–5 0 5 10 15 20 25

(–50) (50) (100) (150) (200) (250)

Magnetic field density Bz, mT (G)

the Bx trough is reduced and that the Bzsignal becomes asymmetric — the peak(or trough) at the leading end of the crackis larger than the corresponding trough(or peak) at the trailing end.

For cracks oriented within 30 degreesof the transverse direction, the signalslook like those from a transversediscontinuity, except that the Bz signal isstrongly asymmetric.

For cracks oriented at about 45 degrees,the Bx signal can practically disappear butBz signals are obtained from both thecenter and the ends of the discontinuity(Fig. 14).

Restrictions in TheoreticalModelThe theoretical model used to produce thesizing tables is based on a number ofassumptions. One assumption is that theinput current is unidirectional and ofuniform intensity. It is also assumed that


FIGURE 14. Magnetic flux density from cracks oriented atdifferent angles to scan direction: (a) Bx measurement;(b) Bz measurement.

(a)

(b)

165 (1.65)

160 (1.60)

155 (1.55)

150 (1.50)

145 (1.45)

140 (1.40)

135 (1.35)

130 (1.30)

125 (1.25)

120 (1.20)Mag

netic

fie

ld d

ensi

ty B

x, m

T (k

G)

25 (250)

20 (200)

15 (150)

10 (100)

5 (50)

0

–5 (–50)Mag

netic

fie

ld d

ensi

ty B

z, m

T (G

)

1 2

3

4

1

2

3

4

Legend1. 0 degrees.2. 30 degrees.3. 45 degrees.4. 90 degrees.



the standard depth of penetration is smallcompared to the dimensions of thediscontinuity and that the discontinuityhas a semielliptical shape with a length atleast twice as large as the depth.9

There are also restrictions in theparameter space covered by the sizingtables for practical reasons (time neededto generate each datum, memory requiredfor storage and other software functions).Consequently, there are limits to theminimum and maximum length ofdiscontinuity that can be sized and to themaximum liftoff that can becompensated.

As stated above, the assumption of auniform input field is required to simplifythe modeling of the interaction betweenthe current and a planar discontinuity.Practical alternating current fieldmeasurement probes are designed to havea uniform field but there is inevitablysome nonuniformity caused by the finitesize of the inducer, particularly for thesmaller probes. The effect of thisnonuniformity, together with any directinduction between the induction solenoidand the sensors, is compensated forduring manufacturing setup. Also, themodels have been extended to cover theeffects of nonuniformity in a real probe18

and to improve accuracy.The restriction to a thin standard depth

of penetration means that the problembecomes two-dimensional where the testobject surface and the crack face can beconsidered as one continuoustwo-dimensional surface. This assumptionsimplifies the problem but means that theresults from the model cannot be used tosize discontinuities in nonferrous, lowconductivity metals such as stainless steel,titanium and nickel alloys. Even in highconductivity metals such as aluminumand copper, the standard depth ofpenetration is often comparable to thediscontinuity depth. In these materials,estimating discontinuity depth requirescalibration (although each probe iscalibrated once, at the manufacturingstage).

A further consequence of assuming asmall standard depth of penetration isthat the inclination of the crack plane tothe surface has no effect on the results.Therefore, no information on crackinclination can be obtained in practice.The depth values obtained are thedistances measured down the crack face,which for an inclined crack will be greaterthan the through-thickness penetration ofthe crack. If the standard depth ofpenetration is not small compared to thediscontinuity, there is likely to be someasymmetry in the signal from a scanmade across the discontinuity.Measurement of this asymmetry couldgive information on the inclination of the


discontinuity to the surface (as has beendone with voltage measurements in thealternating current potential droptechnique19). The asymmetry inalternating current field measurement,however, is much smaller.

Finally, a small standard depth ofpenetration means that no signalperturbation is produced by adiscontinuity that does not break thesurface. In thick skin materials, it ispossible to detect subsurfacediscontinuities but theory does not allowthe submerged depth or size of subsurfacediscontinuities to be calculated fromalternating current field measurements.

The restriction to semielliptical crackshapes is again a practical restriction.Sizing tables can be produced for othershapes (such as circular arc or rectangular)if required but semielliptical shapes arechosen because they best fit the real shapeof fatigue cracks. The restriction that thecrack must be shallower than semicircularis a limit of the transformation used for asemielliptical coordinate system. However,in practice, it is unusual for cracks to growdeeper than this. Also, for a semicircularcrack, the currents already flowpredominantly around the ends of thecrack rather than underneath. Any furtherincrease in depth for the same surfacelength then has very little effect on thecurrent distribution, so it is not possibleto accurately measure the depth of suchdiscontinuities — any estimate obtainedwill be less than the true depth.

The shortest crack length measurablewith the technique is determined by thephysical size of the Bz sensor coil because,when the crack length is less than abouttwice the coil diameter, the distancebetween the peak and trough in the Bzsignal is related to the coil diameter ratherthan the crack length. For this reason,sizing tables in the 1990s were restrictedto lengths above about 5 mm (0.2 in.).

As crack length gets long compared tothe size of the probe, the effect of lengthon the Bx signal amplitude (and hence onthe calculated depth) is reduced. Above acertain limit, the current density at themiddle of a long discontinuity will beindependent of exactly where the ends ofthe crack are. Therefore, the sizing tablesare truncated at an upper length limit,usually around 300 mm (12 in.).

The signal intensity reduces withheight above the discontinuity (liftoff) sothe liftoff tables also need to cover a rangeof liftoff to give accurate sizing. Forreasons of space, the tables are truncatedat an upper limit of about 5 mm (0.2 in.).

Nonuniform Field EffectsThe original model assumed a uniforminput field and probes are designed to

FIGURE 15. Amplitude of magnetic flux density Bx obtainedfrom long slots with 15 mm (0.6 in.) long solenoid probe.

0 5 10 15 20 25 30

(0.2) (0.4) (0.6) (0.8) (1.0) (1.2)


25

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0B xam

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provide a uniform input. However, auniform field would mean that no signalswould be obtained from a long crack ofuniform depth, making such a crackundetectable by alternating current fieldmeasurement. In fact, such cracks arereadily detected by a strong dip in the Bxreading as a probe crosses the crack. Thesize of this dip depends on the crackdepth (Fig. 15).

To quantify this effect, some modelingwork was carried out on the effects ofnonuniformity in the magnetic fluxdensities actually generated by finite sizedsolenoids in real probes.18 This model wasable to show the change in Bx signalamplitude with crack depth and alsoaccounts for the direct induction betweenthe solenoid and the sensor coils,induction required for accurate sizingwhen the probe liftoff is high.

The nonuniform model requires moreparameters (the size, shape and turndistribution of the solenoid) than doesthe uniform field model. Because theseparameters are specified, any set of resultsis specific to a particular design of probe.


PART 3. Alternating Current Field MeasurementAccuracy

Legend= magnetic particle testing (95 percent confidence level)= alternating current field measurement (95 percent confidence level)= magnetic particle testing, experimental probability of detection= alternating current field measurement, experimental probability

of detection

FIGURE 16. Probability of detection for underwater alternatingcurrent field measurement and magnetic particle testingfrom 1991 trials: (a) versus length; (b) versus depth.

100908070605040302010

0Prob

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Probability of Detectionand Probability of SizingAs with any nondestructive testtechnique, it is necessary to understandthe capabilities and reliability ofalternating current field measurement inorder to properly use the information itprovides. Reliability can be determinedonly through extensive trials carried outon realistic discontinuities in realistic testobjects. The results of such trials are thenusually expressed in terms of probabilityof detection, probability of sizing orreceiver operating characteristic.Equipment using the alternating currentfield measurement technique hasundergone a number of such trials, bothseparately and with other techniques.

Trials with alternating current fieldmeasurement equipment were carried outduring technique development.20-22 Alibrary of welded tubular nodes (K, T, Xand Y shaped joints) was produced andwere fatigued to produce real fatiguecracks of varying length and depth. About200 fatigue cracks located in variousgeometries were produced and were testedusing underwater equipment by thealternating current field measurementtechnique together with other techniques,for a comparison of performance.Probability of detection curves wereproduced for all of the techniques.Underwater alternating current fieldmeasurement proved to have detectioncapabilities similar to those of underwatermagnetic particle testing, both whencalculated against length (Fig. 16a) anddepth (Fig. 16b) but alternating currentfield measurement had fewer false calls(10 compared to 39 for magnetic particletesting, out of 120 real discontinuities).

It should be noted that the limitingprobability of detection of 90 percentshown in Fig. 16a is a lower boundestimate resulting from the finite numberof discontinuities in the trial. Thediscontinuities were arranged in order ofcharacterized length and then assigned tofour groups of 29 discontinuities. To beconservative, each group was assigned tothe length of the longest crack in thegroup. Binomial statistics dictate that if all29 discontinuities in a group are detected,there is a 95 percent confidence level that,of all discontinuities of the same length,


the test technique would detect90 percent. In reality, neither techniquemissed any discontinuities longer than20 mm (0.8 in.) In other words,experimental probability of detection was100 percent for discontinuities longerthan 20 mm (0.8 in.).

Another independent evaluation of thereliability of the technique was carried outfor an array probe system deployed on aremotely operated vehicle. The remotelyoperated vehicle test system was subjectedto blind trials where a series of cracked

FIGURE 17. Experimental probability of detection foralternating current field measurement by divers on tubularjoints underwater in tanks: (a) versus length; (b) versusdepth.

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Legend= tank trial with most discontinuities detected= tank trial with fewest discontinuities detected= sea trial

and uncracked plates were tested.23 Alldata were transferred via the remotelyoperated vehicle’s umbilical to the surfacewhere interpretation was carried out.Although there were insufficientdiscontinuities for a meaningfulprobability of detection measure, the trialincluded 47 discontinuities ranging in sizefrom 15 to 200 mm (0.6 to 8 in.) longwith depth from 2 to 10 mm (0.08 to0.4 in.). A detection percentage of 98 wasachieved and two false calls wererecorded. The false calls were both calledas discontinuities smaller than the targetsize of 15 mm × 2 mm (0.6 × 0.08 in.)whereas the one discontinuity missed wasclose to this limit. The performance of thealternating current field measurementsystem deployed by the remotely operatedvehicle was comparable with that ofmanual alternating current fieldmeasurement but with a lower false callrate.

Further tests were carried out blind onreal fatigue cracks in realistic geometries.The project included a wider range of testtechniques (including some deployed byremotely operated vehicles), a wider rangeof test sites (including one in seawaternear shore) and a wider range of testobjects. The large number of test objects(almost 200) and discontinuities (morethan 300) also allowed the project tostudy operator variability for varioustechniques.24,25

Some experimental results from thesetrials are shown in Fig. 17. Figure 17ashows the range of experimentalprobability of detection versus cracklength obtained in diver deployed tanktrials on tubular welded joints. The totalnumber of fatigue cracks included in thetank trials was 89. Also shown are resultsfrom a more limited sea trial offshore.Figure 17b shows the same results plottedagainst crack depth.

Although it is important for anondestructive test technique to have ahigh probability of detection, it is alsoimportant that it does not produce toomany false calls. As well as measuringprobability of detection, the trials alsocounted false calls and combined theresults as a receiver operatingcharacteristic.

As part of a program to obtainapprovals for alternating current fieldmeasurement in the United Kingdom railindustry, a comparative blind trial wascarried out under normal workshopconditions on 15 railroad car axles.16 Allof the axles had discontinuities present(fatigue cracks or corrosion pits) producedduring service, which previously wouldhave caused them to be scrapped by theoverhauler using magnetic particle andultrasonic testing. Alternating currentfield measurement before cleaning

produced an experimental probability ofdetection of 84 percent on discontinuitiesmore than the target size of 0.5 mm(0.020 in.) deep. This result compared to44 percent for the same discontinuitieswith magnetic particle testing, eventhough the magnetic particle testing wascarried out after cleaning. The alternatingcurrent field measurement system,deployed on a lathe, was able to detectdiscontinuities down to 0.2 mm(0.008 in.) in depth.


Influences on SizingAccuracyThe accuracy of length sizing is expectedto be good for alternating current fieldmeasurement because the physicallocations of the Bz peak and trough areclosely related to the discontinuity ends.However, there are instances where thecrack length measured is shorter than thatmeasured by magnetic particle or liquidpenetrant testing. After sectioning ofsome discontinuities in the underwatertrials mentioned above,20-22 it was noticedthat there were instances where thediscontinuity had wing shaped ends tooshallow to be picked up by alternatingcurrent field measurement. Instead, the Bzsignal was responding to the points wherethese wings ended and the crack depthsuddenly increased.

The accuracy of depth sizing, on theother hand, can be affected by a numberof factors including crack inclination,crack shape and morphology, geometryeffects and material property changes.

Crack InclinationAs mentioned above, alternating currentfield measurement testing measures (asdoes alternating current potential droptesting) the crack depth down the crackface. If the crack is inclined to the surface,this distance will be greater than thethrough-thickness penetration of thecrack (the important parameter forcalculating the remaining mechanicalstrength). The test technique will alsooverestimate discontinuity depth if thecrack branches under the surface.

MorphologyOn the other hand, some discontinuitiescan be discontinuous under the surface.In this case, alternating current fieldmeasurement will only measure the depthof the discontinuity connected to thesurface and so will underestimate thedepth of the deepest, unconnected, partof the discontinuity. All these factors needto be kept in mind when depths fromalternating current field measurement arecompared with depths from ultrasonicmeasurements that locate the crack tiprelative to the surface.

A situation where simple interpretationof alternating current field measurementsignals can incorrectly size discontinuitiesis undergoing testing for fatigue cracks inrailroad rail heads. The stress conditionsin the head of a rail mean that thesediscontinuities tend to grow sideways asthey propagate, making the length underthe surface greater than the surfacebreaking length. With this shape, there is


a tendency for more of the current to flowaround the ends of the discontinuity onthe surface than would normally flowaround a semielliptical fatigue crack of thesame length. In this case, however, thediscontinuities tend to grow in welldefined patterns, so there is a closerelationship between discontinuity shapeand depth. Crack depth can then bemeasured accurately by calibration.

Geometry EffectsGeometric effects need to be taken intoaccount when sizing discontinuities. Theeffect of geometry on current flow andhow to compensate for it are describedabove (in the discussion of plate edges). Inother situations, such as discontinuities atplate ends or in grooves, it is best tomeasure the background Bx signal at thesame place on a similar geometry ratherthan immediately outside thediscontinuity.

Another situation where adiscontinuity is not semielliptical is whena crack grows to a plate edge. The crackmay start from a corner, for example, ormay run the full width of a plate. In thesesituations, where there are not twodiscontinuity ends to measure between,the normal sizing procedure cannot befollowed.

A crack growing from a corner is aproblem. If the crack is symmetric aroundthe edges, the current perturbation willalso be symmetric, so using twice thedistance from the crack end to the platecorner as the crack length should give thecorrect answer.

For cracks that are highlynonsymmetric, most of the current flowsaround the short part of the crack, so thesignal perturbation is independent of thelength of the long part of the crack.

Sizing a full width crack requires thenonuniform current model mentionedabove. Otherwise, use of plate width ascrack length results in a reasonableestimate of discontinuity depth.

Sensor Coverage andLateral DisplacementA large, uniform input field in alternatingcurrent field measurement means that thecurrent perturbation from a discontinuityextends some distance away from the lineof a discontinuity. However, there is alimit beyond which a probe will no longerbe able to detect a given discontinuity.This limiting distance is larger for deeper(and, to a lesser extent, longer)discontinuities and determines the testwidth covered by a probe in one scan.This width coverage in turn determinesthe number of passes needed to inspect a

FIGURE 19. Decrease in predicted depth with lateraldisplacement: (a) weld probe with 40 mm (1.6 in.) longsolenoid; (b) probe with 20 mm (0.08 in.) diameter coil.

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given weld cap, for example, or theoptimum spacing between sensors in anarray probe.

Detectability of a discontinuity is itselfdependent on surface roughness,background signal variations and otherfactors but it is reasonable to expect that adiscontinuity will be detected if the Bxsignal amplitude is 1 percent or more. Onthis basis, Fig. 18 shows the lateraldisplacement at which discontinuities atthree depths can still be detected with avariety of probe types. The discontinuitiesall have a length around ten times thedepth, typical for fatigue cracks at welds,but the results are relatively insensitive todiscontinuity length. The plot shows thatdifferent pencil shaped probes performsimilarly but that, because of its largesolenoid coil measuring 40 mm (1.6 in.)in length, the weld probe offers bettercoverage for the deeper discontinuities.For example, a 5 mm (0.2 in.) deepdiscontinuity could be detected from adistance of 18 mm (0.7 in.) by using thisweld probe.

The minimum discontinuity sizereliably detected by alternating currentfield measurement in blind trials at weldsis usually found to be around 1 mm(0.04 in.) deep. Figure 18 shows that thissize discontinuity could be detected in thetrial from 5 to 9 mm (0.2 to 0.4 in.) away.Because the detection range is symmetricaround the center line of the sensor, thisdetectability implies that a probeadequately tests a band between 10 and18 mm (0.4 and 0.7 in.) wide. There willbe some variation with results fromdifferent probes and from discontinuitiesin different geometries. A coverage of15 mm (0.6 in.) is typical. If the test isrequired to find only deeper

FIGURE 18. Lateral displacement at which amplitude ofmagnetic flux density Bx drops to 1 percent.

20 (0.80)18 (0.72)16 (0.64)14 (0.56)12 (0.48)10 (0.40)8 (0.32)6 (0.24)4 (0.16)2 (0.08)0D

isp

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Legend= weld probe with 40 mm (1.6 in.) long solenoid= 5 kHz pencil shaped probe with 5 mm (0.2 in.) diameter coil= 5 kHz pencil shaped probe with 2 mm (0.08 in.) diameter coil= 50 kHz probe with 2 mm (0.08 in.) diameter coil

discontinuities, this coverage will bewider.

Because the Bx amplitude decreaseswith lateral displacement, discontinuitydepths will be underestimated if thelateral displacement is higher than thevalue assumed in the theoretical sizingtables. This value is zero for pencil shapedprobes (expected to be scanned directlyalong the line of the discontinuity) and2.5 mm (0.1 in.) for weld probes with40 mm (1.6 in.) long solenoids (where itis assumed that the discontinuity is at theweld toe whereas the sensors are set backfrom the front of the probe).

Figure 19 shows the experimental effectof lateral displacement on depth sizingaccuracy. It can be seen that adiscontinuity will be sized around70 percent of the true depth at a lateral


(b)120

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Legend= 50 × 5 mm (2.0 × 0.2 in.) notch= 20 × 2 mm (0.8 × 0.08 in.) notch= 20 × 1 mm (0.8 × 0.04 in.) notch

displacement of 5 mm (0.2 in.). The weldprobe overestimates discontinuity depthat zero displacement because of theassumption in the sizing model that thefront edge of the probe, not the sensors,runs along the weld toe.

False CallsThe false call rate for alternating currentfield measurement is generally low unlessfrequency is turned too high in an effortto detect very small discontinuities.Nevertheless, large signals can beconfused with discontinuities. Suchsituations occur mainly when there is amaterial property change transverse to thescanning direction. This situation can becaused by a seam weld perpendicular tothe weld being tested, especially where itis ground off and so not visible to theoperator. A seam weld will usually give apeak in Bx (as in Fig. 13a) but can give atrough shaped indication if the basematerial provides an opposite propertydifference. A similar effect can also arisewhere a discontinuity has previously beenground out and refilled with weld metal.These signals can usually be differentiatedfrom discontinuity indications by takingparallel scans some distance away: adiscontinuity indication will drop rapidlyin amplitude whereas a weld seam signalwill not.

One material where particular care hasto be used to avoid false calls is duplexsteel. This is a mixture of ferritic andnonferritic steels in which thepermeability can vary across the surfaceand where shallow localized grinding ofthe surface greatly changes the localpermeability.26


PART 4. Alternating Current Field MeasurementIndications

Cracks

Fatigue CracksAlternating current field measurementwas designed for the detection and sizingof fatigue cracks. Three considerationsmake the technique well suited forfinding such discontinuities.

1. Fatigue cracks are generally surfacebreaking discontinuities.

2. Fatigue cracks tend to grow at definedstress concentrations well suited forthe linear scanning path of alternatingcurrent field measurement probes.

3. Fatigue cracks tend to grow in asemielliptical shape and at right anglesto the surface, as assumed in thetheoretical model used for sizing.

There are situations where fatiguecracks are not semielliptical, however.One such situation occurs on large tubularwelded intersections where cracks ofteninitiate at multiple sites. The curved shapeof the weld in this case means that, as theseparate cracks grow, they are notcoplanar. This means that the ends ofneighboring cracks often grow past eachother, resulting in crack overlaps orbridges of metal between the cracks.While the cracks remain separated, theycan be treated as two separatesemielliptical discontinuities. Eventually,however, the bridge of metal between thecracks breaks and the cracks connect. Atthis stage, the crack has a W shape. Thealternating current field measurementsignals from such a discontinuity aredistinctive but accuracy of depth sizing isreduced. As the crack grows deeper, itrapidly becomes semielliptical again.

Stress Corrosion CrackingStress corrosion cracking can take theform of a series of parallel cracks acting asa colony. In other cases, it can be presentas crazed cracking. The orientation of thecracking and the proximity of individualcracks can lead to problems ininterpretation of alternating current fieldmeasurement signals. The large scaleinput field means that the signal fromone discontinuity is superimposed onsignals from neighboring discontinuities.It is difficult to isolate discontinuities

closer together than the distance overwhich each indication extends. Whenthere are many discontinuities, it is alsodifficult to match the two discontinuityend signals together correctly.

Some work has been carried out onquantifying these effects.27 In general, ithas been found that detection of clustersof stress corrosion cracking is reliable andthat depth values obtained by treatingisolated clusters as single discontinuitiesagrees reasonably well with the typicaldiscontinuity depth.15

Hydrogen Induced CrackingThe alternating current field measurementtechnique has also been used to detectsulfide stress concentration cracking,hydrogen induced cracking, hydrogensulfide cracking and stress orientatedhydrogen induced cracking in the basemetal adjacent to the heat affected zone.28

Hydrogen cracks are different fromfatigue cracks: they are not mechanicallyinduced but result from a combination ofinternal or external chemical reactions,usually resulting in the production ofhydrogen. If these pockets of hydrogenare beside inclusions or very hard areas,cracking will occur.

The cracks tend to have similar featuresin that they are parallel to the surface andcan occur at the sites of inclusion clusters,especially elongated inclusions and inareas of hard metallurgical structures suchas martensite or bainite found in heataffected zones. Sulfide stressconcentration cracks normally occur inclusters; the other types of hydrogencracks are lenticular, occurring in parallelbands, and may be shallow. They do nothave the normal elliptical shape of fatiguecracks because they are metallurgicalrather than mechanically associateddiscontinuities and thus can be affectedby the metallographic structure. Althoughalternating current field measurement candetect these cracks, their complicatedsubsurface structure (branching andsplitting) makes depth sizing difficult.

Fatigue Cracks in Rail HeadsNonsemielliptical discontinuities occuralso in railroad rails. Head checking (alsocalled gage corner cracking) is cracking thatinitially grows into the top surface of arail at a highly inclined angle (typically


25 degrees to the surface). As they growbelow a certain depth, they turn to asteeper angle but also start to growsideways, so their length is greaterbeneath the surface.29 Such complicatedshapes are difficult to model, so depthsizing relies on empirical curves or oncalibration with reference standards.

Corrosion PittingThe unidirectional currents used inalternating current field measurement aremost strongly perturbed by planardiscontinuities. However, surfacecorrosion pitting also perturbs currentflow to some extent and can also bedetected. The degree of currentperturbation is much lower than for acrack of the same depth and length, so onan initial scan, a corrosion pit looks like ashallow crack. However, thedistinguishing feature of a pit is that,unlike a crack, it will produce the samesignal regardless of the orientation of theinterrogating current. Systems designed todistinguish cracks and pits therefore usetwo orthogonal current inducing coils,usually with an array of sensors to speedup tests.30

Some modeling work has also beencarried out on the perturbation ofuniform currents by hemispherical pits.31


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12. Raine, G.A. and N. Smith. “NDT of Onand Offshore Oil and Gas InstallationsUsing the Alternating Current FieldMeasurement (ACFM) Technique.”Materials Evaluation. Vol. 54, No. 4.Columbus, OH: American Society forNondestructive Testing (April 1996):p 461, 462, 464, 465.

13. LeTessier, R., R.W. Coade andB. Geneve. “Sizing of Cracks Using theAlternating Current FieldMeasurement Technique.” InternationalJournal of Pressure Vessels and Piping.Vol. 79. Amsterdam, Netherlands:Elsevier Science (2002): p 549-554.

14. Gaynor, T.M., D.L. Roberts, E. Holmanand W.D. Dover. “Reduction in FatigueFailures through Crack Detection byAlternating Current FieldMeasurements.” Paper IADC/SPE35033. Presented at IADC/SPE DrillingConference [New Orleans, LA,March 1996]. Houston, TX:International Association of DrillingContractors (1996).


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