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FROM MICROSTRUCTURAL ASSESSMENT TO MONITORINGCOMPONENT PERFORMANCE – A REVIEW RELATINGDIFFERENT NON-DESTRUCTIVE STUDIESFROMMICROSTRUCTURAL ASSESSMENT TO MONITORINGCOMPONENT PERFORMANCE – A REVIEW RELATINGDIFFERENT NON-DESTRUCTIVE STUDIESFROMMICROSTRUCTURAL ASSESSMENT TO MONITORINGCOMPONENT PERFORMANCE – A REVIEW RELATINGDIFFERENT NON-DESTRUCTIVE STUDIESFROMMICROSTRUCTURAL ASSESSMENT TO MONITORINGCOMPONENT PERFORMANCE – A REVIEW RELATINGDIFFERENT NON-DESTRUCTIVE STUDIESA. J. ALLEN a & D. J. BUTILE aa National NDT Centre, B521, Harwell Laboratory, AEA Technology , Oxon, OXII ORA, UKPublished online: 28 Aug 2009.

To cite this article: A. J. ALLEN & D. J. BUTILE (1992) FROM MICROSTRUCTURAL ASSESSMENT TO MONITORING COMPONENTPERFORMANCE – A REVIEW RELATING DIFFERENT NON-DESTRUCTIVE STUDIESFROM MICROSTRUCTURAL ASSESSMENTTO MONITORING COMPONENT PERFORMANCE – A REVIEW RELATING DIFFERENT NON-DESTRUCTIVE STUDIESFROMMICROSTRUCTURAL ASSESSMENT TO MONITORING COMPONENT PERFORMANCE – A REVIEW RELATING DIFFERENTNON-DESTRUCTIVE STUDIESFROM MICROSTRUCTURAL ASSESSMENT TO MONITORING COMPONENT PERFORMANCE – AREVIEW RELATING DIFFERENT NON-DESTRUCTIVE STUDIES, Nondestructive Testing and Evaluation, 7:1-6, 9-30, DOI:10.1080/10589759208952985

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Nondestr. Test. Eva/., Vol. 7, pp. 9-30Reprints available directly from the publisherPhotocopying permilled by license only

© 1992 Gordon and Breach S.A.Printed in the United Kingdom

FROM MICROSTRUCTURAL ASSESSMENT TOMONITORING COMPONENT PERFORMANCE - A

REVIEW RELATING DIFFERENTNON-DESTRUCTIVE STUDIES

A. J. ALLEN' and D. J. BUTILENational NDT Centre, B521, Harwell Laboratory, AEA Technology, Oxon OXll

ORA, UK

There is an increasing number of industrial applications where the performance of either existing or newspecialised materials, often in hostile operating conditions, is critical to safe operation. There isfrequently a need for both rigorous quality control in production and careful inspection throughoutservice-life. Unfortunately. changes in microstructure during fabrication or in service can haveunpredictably deleterious effects on overall material or component performance. It is therefore essentialto have non-destructive measurement techniques to relate microstructural effects to bulk properties. Inthis paper. a number of selected microstructural and NDE techniques (some novel) are reviewed, andtheir results are interrelated to show where such combinations of diagnostic methods make possiblemore complete assessments of component material microstructure and performance during service life.

KEY WORDS: Systems approach, microstructural NDE, radiation damage, plastic damage, residualstress. surface finishing

INTRODUCfION

The performance of industrial component materials presents a number of problemsto both materials scientists and engineers. While NDE methods for crack detectionand for monitoring other gross material failure are well developed, this stage isoften too late for useful decisions to be made concerning residual service-life.Furthermore, safety issues may dictate that considerable forewarning even ofmodest failure is required. Hence, large safety margins are a feature of most designcodes [1], even when machining processes such as shot-peening or case-hardeninghave been employed to enhance component performance.

A number of emergent NDE techniques can be used to assess radiation damage,residual stress and other incipient material degradation phenomena in componentsprior to gross failure. Frequently only partial data is obtainable from anyonetechnique, which can be difficult to interpret unambiguously, especially if there areuncertainties concerning the microstructure. However, considerable improvementsin the prediction of residual service life and component performance may bepossible if the new NDE methods are used together and the results are interrelated.To be effective, such a multisensor approach requires that the NDE techniques are

• Formally at this address.

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10 A. J. ALLEN AND D. J. BUITLE

fullycalibrated and validated against other diagnostic methods, which although lab­based, can give a comprehensive account of the degradation and deformationprocesses and of the underlying material microstructures.

A short review cannot give a comprehensive account of all the interrelatedstudies which could be made. Frequently, even where multisensor studies areunderway, interrelatable data have yet to be modelled and fully interpreted. In thesections which follow, we can only illustrate the power of the multisensor approachby describing some pertinent examples of work in the National NDT Centre atAEA Technology's Harwell Laboratory:

(i) the assessment of embrittling precipate microstructures and associated radia­tion damage in reactor pressure vessel steels using Barkhausen emission (BE),magnetoacoustic emission (MAE), positron annihilation (PA) and small angleneutron scattering (SANS);

(ii) the calibration and validation of field-applicable NDE measurements ofresidual stress, using MAE, stress-induced magnetic anisotropy (SMA) and theultrasonic velocity combinations method (UV), against high resolution neutrondiffraction (ND);

(iii) the comparison of plastic damage spatial variation, as measured by PA orultrasonic attenuation (UA), with residual stress measurements or with in-situacoustic emission monitoring (AE);

(iv) the quality control of surface treatments using PA, UA and MAE.

Where relevant in the following discussions, reference is made to other microstruc­tural methods which can be employed to further enhance the power of each study.

RADIAnON DAMAGE IN REACfOR PRESSURE VESSEL STEELS

In reactor pressure vessel components, a major factor limiting service-life can beirradiation-induced embrittlement of the steel [2-4]. The radiation embrittlementproblem is summarised in Figure 1 for the effects 'of 290°C neutron irradiation(2.4 x 1023 neutrons m-2

) and subsequent isochronal annealing on the room temper­ature hardness of a model Fe-O.2%Cu steel [5,6]. The figure demonstrates thatradiation causes a marked increase in hardening, due to the production ofirradiation-induced point defect clusters and precipitates, and that their annealingout or overageing in the temperature range 550° to 650°C causes isochronalrecovery. (Note that in this alloy the copper content is too low for precipitation tooccur by thermal annealing alone, but this is not so when the copper content ishigher.) Since transmission electron microscopy (TEM) and atom probe field ionmicroscopy (AP/FlM) reveal that the problem indeed results from irradiation­induced coarsening of copper-rich precipitates within the alloy microstructure [2­4], it is desirable to monitor such coarsening throughout the service life, and tomeasure the accumulated radiation dose and damage.

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MICROSTRUcrURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 11

• Control Specrmens

• Irradiated Specimens

150

140

130I

120 I

I

0 110 I

> I:r I~

100 J..Ic

~ Je:r I

JI

IIL., ....

50550

SolutionValue TemperatureJoC

Figure 1 Effects of 29O"C neutron irradiation and isochronal annealing on room temperature hardnessof Fe-O.2%Cu alloy.

TEM and AP/FIM can provide the basic qualitative information concerning thesize and shape of embrittling precipitates on irradiation damage. SANS has provedparticularly powerful in providing quantitative volume fraction and size distributiondata for the coarsening precipitate population, whether such coarsening be due toirradiation or thermal annealing effects [2-4]. For such studies, small sampleremoval techniques have advanced sufficiently in recent years [7] to permit theextraction of small amounts of surface material from components at intervalsduring their service-life. For in-service assessments of overall pressure vesselembrittlement, rapid in-situ NDE methods will be increasingly required in futureplant life extension (PLEX) programmes. Two NDE techniques show considerablepromise: PA for direct measurements of radiation damage accumulation, and BEfor mean size and volume fraction of the embrittling precipitate population.

SANS and TEM Studies of Cu Precipitate Coarsening

While TEM and other microscopical techniques (described elsewhere [8]) givedetailed qualitative information on the shapes and sizes of individual embrittlingprecipitates, and other comprehensive local information including the effects ofirradiation damage, the microstructural statistics are poor. ln contrast, SANS givesa good macroscopic average of the microstructural parameters (here the quantita­tive volume fractions and size distributions of the embrittling preipitates), and

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12 A. J. ALLEN AND D. J. BUTfLE

shows well the thermal- or irradiation-induced coarsening of the precipitates. Thequalitative information on the nature of the precipitates, gained from the TEM, isneeded to model the SANS data. The technique is based on the fact that when aneutron beam passes through condensed matter, a small fraction of the incidentbeam is scattered out of the incident direction by small imhomogeneities orprecipitates [2] (see Figure 2). The angular width of the scattered component isinversely proportional to the precipitate size, while the intensity of the scatteredcomponent is proportional to both size and volume fraction. Figure 3 compares thesize distributions obtained by TEM and SANS for the embrittling precipitate phasein a thermally aged Fe- 1.3%Cu steel, as a function of thermal ageing time. Notethat the particle statistics for SANS is many orders of magnitude greater than forTEM (typically up to _10 ' 5 precipitates over 100 mrrr' of sample volume, com­pared to _102_103 over somewhat less than 0.1 ,urn). Also many of the smallestparticles do not have sufficient contrast to show up in the TEM images. Theabsolute SANS measurements of precipitate number density and size distributioncan be calibrated against thermal ageing time or temperature, or against accumu­lated radiation dose. These SANS and TEM data themselves provide calibration ofthe portable NDE techniques below.

Incident beam

Sample ---t~

Unscallered beam

Scallering vector a = 4~ sin (-/21

Figure 2 Schematic of a typical SANS experiment geometry.

PA Studies of Irradiation-Induced Vacancy Defect Damage

PA is a newly emerging NDE technique [9] in which a small 22Na or '"'Ge positronsource is held in close proximity to an alloy component surface (see Figure 4).Positrons emitted, which enter the component material, annihilate there withvalence or conduction electrons. The annihilation gamma ray energy of 511 keY ismodified slightly for each event by the kinetic energy of the annihilating electron.This Doppler broadening of the annihilation gamma ray line is greater for thelocalised valence electrons than for the conduction electrons. Because the latter are

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MICROSTRUCTURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NOT 13

0·5Fe/Cu' 550°C 10

23

TEM 2hrs-- Fe/Cu. 550°C

0·4 10hrs ------ 'E SANS 2hrs-c lOhrs -.•--

M

>- Eu >.Iii -'iii::J cCT GIIII a.. rr1.IL ....

1021 jGI GI

.D>0·2 E:;::

E ::JZ

GI GIa: [;

0·'~.~..a.

10190 30 0

Figure 3 Copper precipitate size distributions in Fe-I.,3%Cu alloy after 2 and 10 hours anealing: TEMparticle count compared with size distribution determined from SANS.

unlocalised, any vacancy defects in the material are generally negatively chargedand are therefore capable of trapping positrons. Furthermore, annihilation at suchsites is exclusively with conduction electrons. The resultant narrowing of theannihilation line can be detected in a high resolution intrinsic germanium gammaray detector by parameterising the lineshape. This lineshape parameter, S, is theratio between the counts in the gamma ray energy spectrum within an electronicallygated central window to those in the whole annihilation peak. Thus line-narrowingresults in an increase in S. An increase in S is therefore a direct measure ofirradiation-induced (or mechanical) vacancy-defect damage. Although the tech­nique is being developed to detect such incipient damage and could map thedamage distribution over the component surface [9,10], presently, the mainirradiation work is done using low energy positron beams to study the near-surfaceeffects of ion implantation [11] (see Figure 5). Future NDE applications could playa significant role in irradiation damage assessment.

BE and MAE Studies of Cu Precipitate Coarsening

The related micromagnetic NDE methods of BE and MAE can be used to studydislocation density [12,13], precipitate coarsening [14,15,16] and other microstruc­lural parameters (e.g. solute aggregation [17]) in ferromagnetic alloys includingcomplex steels. The techniques are sensitive to the presence of microstructuralfeatures which impede domain wall movement during rnagnetisation processes. Asan applied magnetic field is increased so that magnetic domains closely aligned to

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14 A. J. ALLEN AND D. J. BUTILE

Gamma raydetector.

0.5 mm Pb backing.

-J----Flexible support.

Sealed positron source, - 1mm x 1mm.

MCA

Positronfromsource

Detector

Pulseprocessingelectronics

Energy

Sample Cryostat

Figure 4 Schematic of positron lineshape analysis equipment.

the applied field direction grow at the expense of other domains, the domain wallsrepeatedly become pinned and 'break free' from their pinning sites. BE is based onthe detection, in a search coil placed close to the specimen, of sharp transientvoltage pulses, associated with this discontinuous and thermodynamically irrevers­ible jumping of the domain walls. The domain wall motion can also causegeneration of elastic waves known as magnetoacoustic emission (MAE). MAE

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MICROSTRUCTURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NOT 15

0·48D

Mean Depth (A)

2000 3000

0·47

....<II-<IIE0....0

c,

Vl 0·46

0·45

o 2000 4000 6000

Bias (V)

8000

Figure 5 Low energy positron beam result (controlled by bias voltage) for lOOkeV Kr " ion implantedtitanium: (a) pure titanium; (b) after implantation (1.5 x 101Om-'); (c) after annealing at 800'C.

arises from the abrupt changes in magnetostrictive strain which occur if the domainwalls are of the non-180°C type, but it can also arise from domain wall creation andannihilation processes such as occur at high magnetisation fields. The sensitivitiesand characteristics of BE and MAE are quite different. BE signals arise only fromnear-surface material at depths less than O.3mm, but the penetration of MAE istypically lOmm, and can be controlled. Typical BE and MAE profile measurementsas a function of magnetic field are shown in Figure 6 for a quenched and temperedmild steel; correspondence between features in the profiles emphasises the comple­mentary nature of the two techniques. For precipitate coarsening, the BE signalincreases with precipitate size until their diameter is close to the domain wall width

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16 A. J. ALLEN AND D. J. BUTfLE

[15]. For coarser particles, the BE signal begins to decrease. MAE is not sensitiveto very small precipitates less than 50nm in diameter, but the signal is increased bythe presence of dislocations [13], large (more than 100nm diameter) precipitatesand, to a lesser extent, dislocation loops. Thus, in principle BE can be used toindicate the mean size and volume fraction of embrittling precipitates, while MAEcan give a check on precipitate coarsening and detect concentration of dislocationloops.

18.-----------------------.18

>E

0cOl

8 l/l

Wa:i

6

4

2

02·01·0-1- 0-2·0

O'----'----~OL---------'-------'--------J

>E

oc:Ol

l/l

W

«:::E

Figure 6 Examples of MAE and BE profile measurements as a function of applied magnetic field. I. Cand F correspond to initial peak, low field and final peak parameters,

Figure 7 shows the variation in MAE and BE signals resulting from irradiationand subsequent isochronal annealing for the Fe-0.2%Cu steel characterised inFigure 1. Both the MAE and BE signals increase on irradiation with the formationof dislocation loops, and, for BE, the formation and growth of copper-richprecipitates. On isochronal annealing, the recovery of dislocations in the 350 to500°C range results in the fall in the MAE signal observed. For BE, this effect is

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MICROSTRUcrURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 17

masked by the growth and eventual overageing of the copper precipitates; thesignal grows until at 550°C the associated dislocations are annealed out, and there isa drop. The consequent rise in BE signal for higher annealing temperatures resultsfrom precipitates coarsening, until overageing occurs, reducing the BE signal oncemore at 650°C. (The MAE signal meanwhile increases slightly through thecoarsening and overageing regime.) With sufficient calibration and validation usingthe techniques described above, it is considered that the BE method could give themean size and volume fraction of the embrittling precipitates, and is potentially themost promising NDE check on their coarsening.

18 ,.50

~ >lr E0 UJ-l 16 /fil 0!!! =>u.. / !:{ 1-0 t-

==/ 15 :J

« <t: n,0 lr :E-l lr <t:0- 14 UJ ~::.:: !«

0<t: -l

UJ B /1UJ

n, <t: u.lr / I 0·5-l lr / I

==~1·2 z/ 0

t- l I • -lZ I UJ

I I COUJ I I<t::E I I

1·0 0400 450 500 550 600 650

ANNEALING TEMPERATURE, (OCI

Figure 7 Effects of 290"C neutron irradiation and isochronal annealing on low field amplitude BE andMAE ratio for Fe-O.2%Cu alloy.

Potential of the Multisensor Approach

For an effective multisensor approach to the irradiation-induced embrittlementproblem, it is necessary to compare and relate: TEM and SANS microstructuralresults using small sample removal techniques, BE and PA results using NDE oncomponents, and independent assessments of radiation dose and of hardness.However, with these conditions satisfied, the potential of the multisensor approachto improve estimates of residual service life could prove invaluable for nuclearPLEX assessments.

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18 A. J. ALLEN AND D. J. BUlTLE

CALIBRATION AND VALIDATION OF RESIDUAL STRESSES

Knowledge of the stresses within engineering components is essential for theaccurate prediction of defect growth and structural integrity. Hole-drilling andsectioning have become well established destructive or partially destructive tech­niques for determining residual stresses in industrial components. When used withfinite element analysis, much progress has been made in solving some of themost serious generic residual stress problems in industry. Unfortunately, uncertain­ties in formulating the precise boundary conditions for a given case, together withuncertainties in stress/thermal history and microstructure, cause problems with theaccurate prediction of in-depth stress distributions in real materials. Thus, there isstill a need to determine the actual stresses present in critical industrial componentsboth during production and during service, and welds are the critical regions ofmany structures where defects may arise as the material can be of poorer toughness(18).

There exist few NDE methods to determine in-depth residual stress in the field,the MAE, SMA and UV methods being the most promising. Each of thesepotentially powerful techniques gives part of the spatial and tensor information forthe residual strain distribution from which residual stresses may be inferred.However, these methods must be calibrated and validated not only against finiteelement modelling and the destructive stress measurement techniques, but alsoagainst other methods which characterise microstructure, texture. plastic damageor give more comprehensive stress information. One major application, describedbelow, lies in determining the in-depth stress distribution in and around welds, aswelded joints are frequently subject to further load or fatigue during service-life.

Calibration of Residual Stresses using ND

The ND technique [19,20) for stress measurement is similar in principle to thewell-known X-ray diffraction method for near-surface stress measurement.However, ND is unique in that it nondestructively gives complete strain tensorinformation for a discrete and spatially defined gauge sampling volume of cubedimension down to 3mm at up to 25mm depth in steel. Nearer the surface, a 0.5mmcube gauge volume is possible. Unfortunately, the technique is not portable, andthe sample must be brought to a nuclear reactor or pulsed neutron source facility.However, components or sections of up to 500mm dimension can usually beaccommodated, and the method satisfies most of the requirements for a nondes­tructive comprehensive stress calibration tool. The basic principle lies in detectingthe small changes in scattering angle, tp, of Bragg diffraction peaks, associated withstrain-induced changes in the crystal lattice spacing, d. Each diffraction peaksatisfies the Bragg condition: 2dsin(tp/2)=)., and hence for given Miller indices(hkl):

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MICROSTRUcrURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 19

The direction of strain measurement is that of the scattering vector, Q,(Q =(4.1l/J.)sin(Ij>/2)), which bisects the incident and scattered beams. If equation(1) shows that greatest strain sensitivity is given for back-scattering, best spatialdefinition results from 90° scattering because the gauge volume is determined bythe intersection of the incident and scattered beams.

The sample must be rotated about the gauge volume to give 6 or more non­equivalent orientations of Q if the strain tensor if required. Zero strain must bedefined from measurements on reference unstrained material, or by inferring itfrom the mechanical boundary conditions for the stress variation across the sample.The stress tensor itself must be determined from the strain tensor measurements byapplication of appropriate polycrystalline elastic constants for the (hkl) crystalliteorientation associated with the Bragg peak. This differentiation between differentsets of crystallites for different diffraction peaks permits crystalline anisotropyeffects to be probed. Indeed. ND is the only method for determining the stresses inthe separate component phases of composite materials below the surface.

Figure 8 shows the stress distribution for a section from a mild steel double-veetest weldrnent, as measured by neutron diffraction and through the use of straingauges. The observed yield-point tension at the edges of the weld, and compressionin the centre are as predicted from the thermal cycling during the welding process,and agreement between the techniques is generally excellent. These data can formthe basis for a calibration of the portable NDE methods, especially where these canonly measure outside the Heat Affected Zone (HAZ). By monitoring the widthand intensity variations of the diffraction peaks, 'it is also possible to characterisetexture variations within the sample [21], as well as infer information on intergranu­lar microstrain effects [22], as an aid to interpretation of the portable NDEmethods.

UV Measurements of Residual Stress

The ultrasonic birefringence and velocity combinations techniques are described indetail elsewhere [23,24]. The former is applicable to elastically isotropic materials,and is based on the fact that stress results in anisotropic ultrasonic velocities. Thelatter has been devised for elastically anisotropic materials where texture effectsand variations in microstrucure must be considered. It is based on the fact thatwhile lattice strain. texture and microstructural variations all affect ultrasonicvelocities, combinations of the velocities for different ultrasonic modes and propa­gation directions can be used to suppress the influence of texture and of microstruc­tural fluctuations (as well as cancelling out any errors due to uncertain transit pathlengths). The techniques have proved capable of determining path-average residualstresses in non-heterogeneous alloy materials in which elastic anisotropy is relati­vely mild. For other cases, such as in ferritic steel components, there can beproblems due to uncertainties in texture and in heterogeneity of microstructure. Itis particularly important in these cases either to measure the texture (which can bedone by measuring ND [21] or XRD peak intensity over all orientations of the

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20 A. J. ALLEN AND D. J. RUTILE

• $ Neutron results.

Range of straingauge resuIts.

/....--J~ /y//~'r . --__ .,

"" '" /1,,/ ,,// --~-..Jf-/ --..//I -r-_ X / /"" - . / /lc, - __ / /

---- r-« /-----.1/

.......--(Oy-OZI Oy----I

•23-7 -20 -10 0 10

Position through weldment (Zl (mm)

16·3

Figure 8 Residual stress variation across a mild steel double-Vee weld section. as measured by neutrondiffraction and by use of strain gauges.

scattering vector), or to infer its effect on the UV measurements from knowledge ofthe boundary conditions [24]. For example, fibre and rolling textures can becorrected for in principle, and measurement of stresses in and around welds hassometimes been successful. However, correcting for uncertain textures and micros­tructures remains a serious problem for stress measurement in many industrialcomponents, and the magnetic techniques described below may be more promisingfor future NDE measurements, at least in ferritic steels. Certain specialised (UV)developments remain of future interest, for example, in-situ continuous monitoringto detect changes in residual stress.

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MICROSTRUcrURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 21

MAE and SMA Measurement of Residual Stress in a Simple Weld

There are a number of magnetic techniques currently being developed for NDEstress measurement, including hysteresis, permeability [25-28], BE [29-31], MAE[32,33] and SMA [34]. MAE and SMA provide a potentially powerful combination.The sensitivity of these magnetic techniques to stress derives from the magnetos­triction effect. Stress results in partial alignment of the magnetic domains alongprincipal stress axes so as to minimise the stored elastic energy in the material. Thisresults in an increase in the permeability along the most tensile direction and areduction in the orthogonal direction. In the SMA technique, the permeabilityanisotropy is measured, and the stress anisotropy is then inferred by calibration(see [34]). The alignment of domains also results in a significant reduction in MAE,even for small stress levels, because the population and size of non-180° domainwalls are reduced. The MAE signal typically decreases by 50-70% for appliedstresses of either sign up to 300MPa. In the case of biaxial stress, the MAE signal issensitive to the sum of both principal stress components. In contrast, SMAmeasures the sign of the stress, but, for a biaxial stress state, is sensitive to thedifference in principal stress components. Thus the two techniques can be usedtogether to investigate biaxial stress states (in the plane of the surface), includingmeasurement of the principal components and their directions. The accuracy ofthese two techniques with calibration is typically ± 20MPa for stress level measure­ment. For principal stress directions in normalised mild steel plate, the accuraciesare: ± 15° and ±5° for MAE and SMA respectively [30].

Examples of SMA and MAE measurements, made across a weldment of A533Bsteel, are shown in Figures 9 and 10 respectively. As the weld-line is approached,the SMA increases in a negative sense, indicating that the stress parallel to theweld-line is more compressive than the perpendicular component. Over the welditself, the SMA signal is positive, indicating a change in sign of the stress difference,so that the stress parallel to the weld is more tensile. The MAE results show agradual increase in the sum of the stress components as the weld-line isapproached. Thus, on comparing the results, it can be deduced that, away from theweld, compressive stresses exist parallel to the weld-line, while, in the weld itself,both stress components are tensile. These general trends are as expected, and areconsistent with the ND data discussed above for the double-Vee weld. Future workwill quantify the results further, and this study demonstrates the power ofcombining complementary methods in this way.

Status of Residual Stress NDE

No portable NDE technique exists which will measure complete strain tensorcomponents below the surface in a discretely defined gauge volume. The NDmethod comes close if the component can be brought to the neutron source.Portable neutron sources could in principle be developed, but are still likely to beexpensive, cumbersome and require specialised operation. XRD provides a com­prehensive surface measurement, but also remains a comparatively expensive NDE

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22 A. J. ALLEN AND D. J. BUlTLE

800

Figure 9 Schematic of the weld specimen showing the X-scan direction, the definition of the axes andthe SMA data obtained.

Field along X3·0.--------------,

Field along Z3·0r-------;--------,

w~

:::E

w~::E:

Distance

Figure 10 Variation of the MAE initial peak to low field ratio across the weld specimen.

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MtCROSTRUCfURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 23

method. The genuine NDE methods available are still under development, but arelikely to give useful partial information, as they become more widely used and arefurther refined. Use of several NDE methods together, combined with goodgeneric calibration/validation studies would seem the most profitable way forward,particularly if other information is obtainable concerning microstructure, residualstress and plastic damage.

PLASTIC DAMAGE ASSESSMENT

Plastic damage, due to static deformation, creep or high cycle fatigue, can usuallybe associated with the stress or thermal history of a component. Thus, itsmeasurement provides important complementary data to that obtainable forresidual stresses, as well as providing a direct measure of the accumulated incipientstructural degradation, which will ultimately lead to component failure. A numberof NDE methods are applicable, or are being developed. These include both theUA [35] and UV [36] techniques for the assessment of creep damage. PA forfatigue damage [9,10,37], NO and XRD peak widths for the detection of largemicrostrains [22], BE and MAE for the detection of high dislocation densities, andeddy currents or similar methods for incipient microstructural changes [37]. Twoexamples are given below: high cycle fatigue damage assessment using PA, andplastic deformation damage assessment using BE and MAE.

PA Assessment of High Cycle Fatigue Damage

The basis for the sensitivity of PA to plastic damage is similar to that for radiationdamage. Plastic damage, like radiation damage, causes vacancy defects to begenerated in the alloy matrix. These are negatively charged, due to the presence ofconduction electrons in the absence of a balancing positive crystal site metal ion.Thus, positrons can be trapped, and the PA gamma-ray line narrows as the overallprobability of annihilation with conduction electrons, as opposed to valenceelectrons, increases. The site vacancy concentration range for which the techniqueis sensitive is _10-7 to 10-4 • For greater concentrations, all positrons get trappedand annihilate with conduction electrons, and thus no further line-narrowing isobserved. PA is sensitive to the concentration of vacancies, microvoids anddislocations in decreasing order. In the context of high cycle fatigue, this meansthat the technique is suitable for the study of the early to intermediate stages of thefatigue life.

Figure 11 shows the spatial distribution of incipient plastic damage around andahead of the crack mapped out with about lrnrn spatial resolution over the surfaceof a fatigue test specimen. The concentration of damage close to the crack tip is asexpected, but note the rather extended area of damage, and also the second area ofdamage towards the far side of the sample, opposite the crack, where the bendingmoment has been greatest. The stress distribution, as measured by NO (not shown

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24 A. J. ALLEN AND D. J. BUITLE

here), shows the predicted oscillatory stress state ahead of the crack, not whollycoordinated with the plastic damage variation - a hallmark of high cycle fatigue,since this is virtually the only mechanism which can lead to uncoordinatedoscillatory stress and damage distributions. The combined use of residual stress andplastic damage measurement using PA could lead to future improved assessmentsof fatigue damage accumulation and residual service life [38]. Although theexamples quoted involve steels, the PA technique is applicable to most metals andalloys.

IL

.loS "olu'S• 0 008o 0006

I

~:;

L,I

I 90_

Figure II The spatial distribution of PA lineshape parameter over the surface of a cracked fatigue testspecimen, mapped oUIwith lmm spatial resolution (High cycle fatigue to 8mm crack length).

Plastic Damage Assessment Using the BE and MAE Techniques

As has been disussed in the section on radiation damage, both MAE and BE aresensitive to the effects of dislocations on domain wall motion, and the theoreticalunderstanding of the interaction of domain walls with dislocations is well estab­lished [39]. The strongest interaction is with 180° walls, which contributes to the BEsignal, but 90° walls are also affected, giving MAE some sensitivity. Figure 12shows the comparative sensitivity of the BE and MAE signals to gross plasticdeformation in iron, and its subsequent removal by successive annealing heattreatments [13]. Pure iron specimens were plastically strained by 5% tensileloading. The BE and MAE signals were then measured before and after successive1 hour isochronal heat treatments between 250 and 750°C in 100 degree intervals.The data show that the high dislocation densities, present after the plasticdeformation, decrease somewhat after the 550°C anneal, and substantially after the650°C anneal. At higher annealing temperatures, the BE and MAE signals declineto 2% and 32% of their unannealed values respectively. Microscopy [40] predictsthat, after 550°C annealing, the dislocation density should decrease by 10-20%,whereas, after 650°C annealing, almost all the dislocations should have been

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MICROSTRUCTURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NOT 25

20

>Eell'0:J-0-E«'0ell

U.

~0-I

5 wCD

o

c

7,--------==----=--------...,

w«~

..><oQIll.

>E

650550450350250Beforeanneal

oL_..L-_--...L_-----l.__L-_..l...-_.....!:===~O

750

Annealing Temperature (OC)

Figure 12 MAE and BE after 5% plastic strain and subsequent isochronal annealing of pure iron.

removed. Thus, the results demonstrate the ability of the two techniques tomonitor plastic damage, although BE would be expected to be approximtely twentytimes more sensitive than MAE.

Value of the Multisensor Approach for Plastic Damage Assessment

Residual stress measurements give an indication of where greatest residual loadexists and high microstrains can indicate the presence of plastic damage; PA cangive an early indication of incipient damage based on the onset of vacancies andmicrovoids in the alloy matrix; BE (and MAE) can indicate the onset of largedislocation densities, associated with more extensive incipient plastic damage inferritic steels. Combined use of these techniques could playa significant role infuture monitoring strategies.

QUALITY CONTROL OF FINISHING TREATMENTS

To protect industrial components against some of the problems associated withresidual stresses and structural degradation mechanisms, certain surface-modifyingtreatments, applied during fabrication, can be effective during service-life.Principally, these are shot-peening and case-hardening. Quality assurance of theseprocesses is desirable because these treatments are comparatively expensive andpoor treatment quality can render such treatment useless. Shot-peening introduces

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26 A. J. ALLEN AND D. J. BUTILE

work-hardening damage into the surface layers (-O.lmm depth), and is thereforeamenable to study by PA and BE. The residual stress field, assocated with thepeening, extends to greater depths and can therefore also be studied by some of theother methods described above. In case-hardening, heat treatment is used toincrease the hardness. Thus, microstructural changes other than plastic damage areintroduced, to greater depths than in the peening process. The UA and MAEtechniques are therefore more appropriate. Work on fully calibrated peenedsurfaces is at a very preliminary stage, but results for a combined UA and MAEstudy of cases-hardened steel plates are given below.

A Case Hardening Study using VA and MAE

Ten lOmm thick plates of EN3B steel were case-hardened by carbo-nitriding toproduce hardened martensitic layers with depths of zero to 2mm. The case-depthswere measured by optical microscopy to where the martensitic case-hardenedsurface layer meets the parent microstructure. Unfortuntely, upon metallographicexamination, some of the plates with thicker case-depths were found to possess amartensiticlbainitic or purely martensitic core, which affects some of the UAresults.

The UA technique is based on the fact that ultrasound is scattered andattenuated by microstructural features such as grain size, planar defects, cracks anddislocation structures [41,42). Ultrasonic pulses can also be reflected or transmittedby thin layers with different acoustic impedance to the matrix. Using a broadband50MHz focussed transducer, the attenuation and peak frequency of the first back­wall echo have been measured as a function of case-depth in the EN3B steel plates.Figure 13 shows the results. Given that higher frequencies are attenuated morestrongly than lower ones, it is not surprising that the curves for attenuation andpeak frequency show a negative correspondence with each other. Since themartensitic region is likely to contain smaller grains than the parent matrix, it mightbe expected that the attenuation will decrease with increasing case-depth. This isindeed observed until the case-depth becomes greater than the mean wavelength ofthe back-wall echo. There is then little dependence until the attenuation risessuddenly at -lmm case-depth. The falling attenuation with case-depth, when thisdepth is small, may instead be associated with thin film reflection effects and furtherwork is needed to explore the effect. Nevertheless, the technique appears to give areproducible drop in attenuation with case-depth when this is less than -O.3mm,and which could ultimately form the basis for a sensitive quality control method forsmall case-depths.

The more complicated effects at large case-depths are probably due to changes inthe parent microstructures resulting from the different heat-treatments used, andemphasise the need to develop ultrasonic attenuation inspection methods whichprobe only the near-surface region. Such measurements have proved possible withthe MAE technique.

For each specimen, MAE measurements have been made using 10 magnetizationfrequencies from 10 to 124 Hz, thus varying the penetration of the measurements

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MICROSTRucrURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 27

ee..::><T..~

u,

(e) i Typicel error

2·0'·5,,0

Case Depth Imm)

05OL.......--__-=""=:-- ~---____,:'_=_---____='_=_-

o

------1Typicol error

'ee'""0

c:o

e::>c:.. 0·2

<

2·01· 51·0Cose Depth lmm)

0·5°0L.......----~=-----~-------,'"-=------::I-=--

Figure 13 UA measurements of attenuation and frequency of the 1st back-wail echo for case-hardenedmild steel plates as a function of case-depth.

through the influence of the eddy-current screening effect on the magnetic field. Asthe frequency is reduced, the effective penetration of the flux is increased until itpenetrates into the parent ferritic matrix, and more MAE is generated. Thus thefrequency dependence of the MAE is monotonically related to the case-depth. Thearea under the MAE profile is linearly related to the square root of the magnetisa­tion frequency with a gradient characteristic of the case thickness [43]. Figure 14shows the gradient values for each case-depth, and demonstrates that there is goodsensitivity to case-depths less than O.5mm. Greater sensitivity still would result for apurer martensitic layer and a better defined parent microstructure in all thesamples.

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28 A. J. ALLEN AND D. J. BUITLE

20r------------------------,

.:f'N:z:>E

'0L-_L-----J_----'_----'-_---.L_---L_---L_-..L_--l...._...l-_...J....----I

1-"4 1-6

Measured Case Depth I mml

Figure 14 MAE 'gradient' values for ease-hardened mild steel plates as a function of case-depth.

CONCLUSION

These few examples serve to illustrate the multisensor approach, which can beapplied to support both the increased use of new technological materials, and theenhanced use of existing ones in hostile environments. For many applications, itmay be argued that the stochastics of failure due to microstructural degradation aresuch that the safety margins in design codes have to be so large in any case that thecost of multisensor inspection is not justified. However, as new materials aredeveloped for critical applications and the microstructures used become bellercontrolled in fabrication, the economic benefit of the multisensor approach willbecome increasingly obvious.

Acknowledgements

The work described in this paper has been carried as part of the HarwellUnderlying and AEA Technology Corporate Research programmes of theUKAEA.

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MICROSTRUCfURAL ASSESSMENT TO COMPONENT PERFORMANCE BY NDT 29

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