CORR: 00181 - Elsevier...2.35.4 Stress Corrosion Test Methods 1540 2.35.4.1 Types of Test Cells 1540...

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2.35 Environmentally Assisted Cracking Test MethodsR. N. ParkinsDepartment of Metallurgy and Engineering Materials, Newcastle Univeristy, Newcastle-upon-Tyne NE1 7RU, UK

This article is a revision of the Third Edition article 8.10 by R. N. Parkins, volume 1, pp 8:215–8:242, � 2010 Elsevier B.V.

2.35.1 Introduction 1527

2.35.1.1 Stressing Systems 1527

2.35.1.2 Constant Total-Deflection Tests 1528

2.35.1.3 Constant-Load Tests 1530

2.35.1.4 Slow Strain-Rate Tests 1531

2.35.2 Testpiece 1534

2.35.2.1 Precracked Samples 1534

2.35.2.2 Comparison of the Results from Plain and Precracked Specimens 1536

2.35.2.3 Crack Velocity Measurements 1537

2.35.2.4 Effects of Surface Finish 1538

2.35.3 Choice of Environment 1538

2.35.4 Stress Corrosion Test Methods 1540

2.35.4.1 Types of Test Cells 1540

2.35.4.2 Initiation of Stress Corrosion Tests 1541

2.35.4.3 Hydrogen Embrittlement Tests 1541

2.35.4.4 Dynamic Tests 1542

2.35.4.5 Static Tests 1543

Appendix A Stresses in Bent Specimens 1545

References 1545

AbbreviationsASTM American Society for Testing and Materials

EAC Environmentally assisted cracking

UTS Ultimate tensile strength

SymbolsE Modulus of elasticity

Ecorr Corrosion potential

h Height

I Current density

KISCC Threshold stress intensity factor for the onset

of stress corrosion cracking

t Specimen thickness

tf Time to failure

V Potential

s Maximum tensile stress

sth Threshold stress for the onset of stress

corrosion cracking

sy Lower yield strength

s00052.35.1 Introduction

s00102.35.1.1 Stressing Systems

p0005Many different methods1–4 have been used for thestressing of specimens (or testpieces), from which itmay be reasonably assumed that there is no singlemethod that is markedly superior to all the others.Each method may have its peculiar advantages in agiven situation, but ideally, a test method should notbe so severe that it leads to the condemnation of amaterial that would prove adequate for service or sotrifling as to permit the use of materials in circum-stances where rapid failure ensues. Methods of stres-sing testpieces, whether initially plain, notched, orprecracked, can be conveniently grouped accordingto whether they involve:

1. a constant total strain or deflection;2. a constant load;3. an imposed strain or deflection rate.

Constant deflection tests usually have the attractionof employing simple and, therefore, often cheap

1527

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specimens and straining frames and of simulating thefabrication stresses that are most frequently asso-ciated with stress corrosion failure. Constant loadtests may simulate failure from applied or workingstresses more closely. Although tests involving theapplication of a constant deflection rate (strain rate)are commonly used, their relevance to service fail-ures continues to be debated.

s0015 2.35.1.2 Constant Total-Deflection Tests

p0010 Prismatic beams stressed by bending offer a simplemeans of testing sheet or plate material, typicalarrangements being shown in Figure 1(a)–1(e). Belowthe elastic limit, the stresses may be calculated1,5 or

determined from the response of strain gauges attachedto the surface at an appropriate position.

p0015Plastic bending of strip specimens to produce a‘U’-bend, Figure 1(d) and 1(e), will usually allow theuse of a lighter restraining system, although some ofthe effects of the plastic deformation, if not removedby subsequent heat treatment, may be to influencecracking response, and the stress obtained in theouter fibers of the specimen is usually less reproduc-ible than with more sophisticated specimens. Tubularmaterial may be tested in the form of ‘C’- or ‘O’-rings, the former being stressed by partial closing ofthe gap, Figure 1(c), and the latter by the forcedinsertion of a plug that is appropriately oversizedfor the bore. The circumferential stress at the outer

2-point bending(a)

H

‘C’-ring(c)

r

Constant-loadtensile test

(g)

Specimen

Compressionspring

‘U’-bend(d) (e) ‘U’-bend

Specimen

Constant-straintensile test

(f)

Jig

4-point bending(b)

L

A

Figure 1f0005 Stressing systems for stress corrosion test specimens; (a) to (f) constant strain, (g) constant load.

1528 Experimental Techniques for Evaluating Corrosion

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surface of a ‘C’-ring is maximal midway between thebolt holes, but for the ‘O’-ring, it is constant over theperiphery, the stresses being readily calculated interms of measured deflections.1,2

p0020 Constant-deflection tensile tests, Figures 1(f )and 1(g), are sometimes preferred to bend tests, butfor similar cross sections, require a more massive re-straining frame. In principle, this problem may be sur-mounted by the use of internally stressed specimenscontaining residual stresses as the result of inhomoge-neous deformation. The latter may be introducedby plastic bending, for example, by producing a bulgein sheet or plate material, or by welding, but such testscause problems in the systematic variation of the initialstress, which will usually be in the region of the yieldstress. Moreover, elastic spring-back, in introducingresidual stresses by bulging plate or partially flatten-ing tube, may introduce problems, and where weldingis involved, the structural modifications may raisedifficulties unless the test is simulative of a practicalsituation.

p0025 At least as important as the choice of methods ofstressing is the realization of the limitations of thevarious methods, these having been considered in areview of stress corrosion test methods.6 The stiffnessof the stressing frame in constant-deflection tests mayinfluence results because of relaxation in the speci-men during the initial loading stage and duringsubsequent crack propagation. Especially in testingductile materials, the initial elastic strain is convertedin part to plastic strain, even if the total deflectionremains constant during cracking. This is because, asthe crack propagates, the stress increases on theremaining uncracked portion of the specimen sectionbeyond the crack, eventually reaching the effectiveyield stress. Yielding will then occur, accompanied byyawning of the crack and frequently, with the propa-gation of a Luders band that results in a sharp loaddrop, which is sometimes mistaken as an indication ofthe crack having advanced by a burst of mechanicalfracture. Once load relaxation has been initiated, theextent to which it proceeds can vary from specimento specimen. Thus, Figure 2 shows load relaxationcurves for two specimens of the same maraging steelin the same stressing frame, which had a facility forload recording throughout the test. The specimensdiffered in the extent to which they showed loadrelaxation prior to sudden fracture, this differencebeing related to the number of cracks that developedin the specimens. Marked load relaxation was asso-ciated with the development of many cracks in thespecimen and little relaxation, with only a few cracks.

This can influence the time to failure, as is apparentfrom Figure 2, where the specimen stressed at theinitially higher load took longer to fail than that atthe initially lower one. This is because, when only asingle stress corrosion crack develops, it will not needto grow to large dimensions before sudden, finalfailure occurs, since the applied load remains high,whereas with the marked load relaxation associatedwith the multicracked specimen, one of the crackswill need to propagate much further before it reachesthe size for sudden fracture at the reduced load. Suchan explanation conforms to the observations7 that theload at fracture is related to the area of stresscorrosion cracking upon the final fracture surfaceand to the number of cracks initiated.

p0030This type of result will depend upon the nature ofthe stress corrosion system being studied, that is,upon such properties as the fracture toughness ofthe material and even upon the aggressiveness ofthe environment employed. It will also vary accord-ing to the stiffness of the restraining jig employed,since the stiffer the frame, the less the elastic strainthat is likely to remain in the specimen after thepropagation of a Luders band, so that a stress-corrosion crack may cease to propagate in some cir-cumstances, especially if the initial stress is in thevicinity of the threshold stress. This indicates some ofthe dangers inherent in comparing stress corrosionresistances in terms of times to failure at a giveninitial stress, an approach that is often practiced butcan be misleading. Figure 3 shows the results fromsome tests in which the time to failure of specimenspreviously cold worked in varying amounts is plottedagainst initial stress. Comparison of the effects ofdifferent amounts of cold work by tests at an initial

15

10

5

0100

Time (min)

Load

(kg

×10

−2)

Fracture

Fracture

200 300

Figure 2 f0010Load relaxation curves for a maraging steel

stress corroded in 0.6M NaCl at pH 2. Reproduced from

Parkins, R. N.; Haney, E. G. Trans. Metall. Soc., AIME 1968,242, 1943.

Environmentally Assisted Cracking Test Methods 1529

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stress of 280 or 155MPa gives different orders ofsusceptibility, as shown inTable 1. It could be arguedthat neither of these results is correct because theprior cold work would result in different yieldstrengths being developed in the three different con-ditions and that the results should be rationalized bymaking the comparison a function of the respectiveyield strengths. Here again, however, the order of sus-ceptibility varies according to the rationalized stress atwhich the comparison is made, as the results inTable 1show. It is difficult to escape the conclusion that a moresatisfactory basis of comparison is the threshold, buteven the latter may not be a basis for comparison ofresults obtained using different restraining frames.

p0035 The simplicity of the rigs used in the constant-straintests is an advantage in the application of the corrosivesolution. Thus, in the case of two-point bending,Figure 1(a), several specimens may be strained in thesame rig, which can be constructed of plastic andimmersed in a tank containing the test solution.

s00202.35.1.3 Constant-Load Tests

p0040Dead-weight loading (with or without the assistanceof levers to reduce the load requirements) of tensilespecimens has the advantage of avoiding some of thedifficulties already discussed, not the least in allowingaccurate determination of the stress if the specimen isuniaxially loaded. The relatively massive machineryusually required for such tests upon specimens ofappreciable cross section is sometimes circumventedby the use of a compression spring, Figure 1(g),chosen with characteristics that ensure that it doesnot change significantly in length during testing,thereby approximating to a constant-load application.For immersion tests, the frame may be coated inpolymer and the specimen insulated from theshackles by plastic sleeves and washers to avoid bime-tallic effects; alternatively, the specimen may beenclosed in a glass cell containing the test solution.The alternative approach of minimizing the size ofthe loading system by reducing the cross sectionof the specimen to the dimensions of a wire is dan-gerous unless failure by stress corrosion cracking isconfirmed by, say, metallography. This is becausefailure may result from pitting and an attendantincrease in the effective stress to the ultimate tensilestrength (UTS) in some stress corrosion environments.Indeed, there is evidence for some systems that beforestress corrosion cracking proper can begin, a pit mustform wherein certain chemical or electrochemical con-ditions are established that permit cracks to be ini-tiated, and in such systems, the use of fine wires hasobvious pitfalls.

p0045The load relaxation that accompanies some, if notall, constant-deflection tests is replaced in constant-load tests by an increasing stress condition, since theeffective cross section of the test piece is reducedby crack propagation. This suggests that it will beless likely that cracks will cease to propagate onceinitiated, as may happen with constant deflectiontests at initial stresses in the region of the thresholdstress, and therefore, the threshold stresses are likelyto be lower when determined under constant-loadconditions than under conditions of constant deflec-tion. Some results attributed to Brenner and Gruhl8

for an aluminum alloy, Figure 4, confirm this exp-ectation. These results also show shorter times tofailure for the same initial stresswith constant load test-ing and, as already indicated for constant-deflectiontests, raise queries as to the significance of time tofailure, the parameter so frequently used in assessingcracking susceptibility.

Table 1t0005 Relative susceptibilities to cracking of a mild

steel in boiling 4M NH4NO3 after various amounts of cold

work

Initial stress Susceptibility of different cold-worked conditions

Most(%)

Intermediate(%)

Least(%)

280MPa 0 10 34155MPa 10 34 0

100% of yield stress 34 10 0

30% of yield stress 10 34 0

400

300

200

100

100 1000

10%

34%

0%

Lifetime (min)

Ap

plie

d s

tres

s (M

Pa)

0

Figure 3f0015 Effects of different amounts of prior cold work

(0, 10 and 34%) on the stress corrosion of a 0.07% C steelin boiling 4M NH4NO3.


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s0025 2.35.1.4 Slow Strain-Rate Tests

p0050 While this method of testing has been in use in somelaboratories for two decades or more and hasincreased in use considerably in very recent years,there remain some skepticism and unfamiliarity withthe method. In essence, it involves the application ofa relatively slow strain or deflection rate (�10�6 s�1)to a specimen9 subjected to appropriate electrochem-ical conditions. It should be emphasized that the strainrates employed are very much lower than thoseinvolved in straining electrode experiments wherethe object, the measurement of current transients, istotally different. In slow strain-rate corrosion tests,the object is to produce stress corrosion cracks thatare metallographically indistinguishable from thoseproduced in constant-load or constant-deflectionexperiments. The object in all these laboratory testsis normally to obtain data in a relatively shorterperiod of time, and this is frequently achieved byadopting an approach that increases the severity ofthe test. In stress corrosion testing, this usually takesthe form of increasing the aggressiveness of the envi-ronment by changing its composition, temperature, orpressure, stimulating the corrosion reactions (galva-nostatic or potentiostatic), increasing the susceptibil-ity of the alloy through changes in structure, orincreasing the severity of the stress by the introductionof a notch or precrack. The application of dynamicstraining to a stress corrosion test specimen alsocomes into this last category, and, like all of the otheraccelerating approaches, its justification will varyaccording to the circumstances in which it is used.

p0055Most stress corrosion crack velocities fall in therange from 10�3 to 10�6 mm s�1, which implies thatfailures in laboratory test specimens of usual dimen-sions occur in not more than a few days. This is foundto be so if the system is one in which stress corrosioncracks are readily initiated, but it is common experi-ence to find that some testpieces do not fail even afterextended periods of testing, which are then termi-nated at some arbitrarily selected time. The conse-quences are that considerable scatter may beassociated with replicate tests, and the arbitrary ter-mination of the test leaves an element of doubtconcerning what the outcome would have been if ithad been allowed to continue for a longer time. Justas the use of precracked specimens assists in stresscorrosion crack initiation, so does the application ofslow dynamic strain, which has the further advantagethat the test is not terminated after some arbitrarytime, since the conclusion is always achieved by thespecimen fracturing, and the criterion of crackingsusceptibility is then related to the mode of fracture.Thus, in the form in which it is normally employed,the slow strain-rate method will result in failure innot more than �2 days, either by ductile fracture orby stress corrosion cracking, according to the suscep-tibility towards the latter, and metallographic or otherparameters may then be assigned in assessing thecracking response. The fact that the test concludesin this positive manner in a relatively shorter periodof time constitutes one of its main attractions.

p0060Early use of the test was in providing datawhereby the effects of such variables as alloy

600

500

400

300

200

100

01 5

Tension

Yield strength

Bending(4-point load)

10 20Time to failure (h)

Tens

ile s

tres

s (M

Pa)

50 100 200 500 1000

Figure 4f0020 Comparison of test results from bend and tension tests upon Al–Zn–Mg alloy in 3% NaCl plus 0.1% H2O2.

Reproduced from Brenner, P.; Gruhl, W. Z. Metall. 1961, 52, 599.


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composition and structure or inhibitive additions tocracking environments could be compared, and alsofor promoting stress corrosion cracking in combina-tions of alloy and environment that could not becaused to fail in the laboratory under conditions ofconstant load or constant strain. Thus, they constitute arelatively severe type of test in the sense that theyfrequently promote stress corrosion failure in the labo-ratory where other modes of stressing plain specimensdo not promote cracking, and in this sense, they are in acategory similar to tests on precracked specimens. Inrecent years, an understanding of the implications ofdynamic strain testing has developed, and it nowappears that this type of test may have much morerelevance and significance than just that of an effectiveand rapid sorting test. It may, at first sight, be arguedthat laboratory tests involving the pulling of specimensto failure at a slow strain-rate show little relation to thereality of service failures. In point of fact, in constant-strain and constant-load tests, crack propagation alsooccurs under conditions of slow dynamic strain, to agreater or lesser extent depending upon the initial valueof stress, the point in time during the test at which astress corrosion crack is initiated, and various metallur-gical parameters that govern creep in the specimen.Moreover, there is an increasing amount of evidencefor some systems which suggests that the function ofstress in stress corrosion cracking is to promote a strainrate which, rather than stress per se, is the parameter thatreally governs crack initiation or propagation. In thesecases, the minimum creep rate for cracking is as muchan engineering design parameter as is the thresholdstress or stress–intensity factor obtained from constant-load tests on plain or precracked specimens.

p0065 The point may be illustrated by data for a ferriticsteel exposed to a carbonate–bicarbonate solution asfatigue precracked cantilever beams subjected to con-stant loads. Deformation in the plastic zone associatedwith the precrack is time dependent following loadapplications and can be measured, and the thresholdconditions for stress corrosion cracking defined, interms of a limiting average creep rate over a specifictime interval. That limiting creep rate may then beused in subsequent experiments to calculate thethreshold stress from creep data determined indepen-dently, these calculated threshold stresses then beingcompared with values determined experimentally.The creep properties of ferritic steels may be variedby prior strain aging, following different amountsof cold work, and Figure 5 shows the observed andcalculated threshold stresses from tests on specimenssubjected to various strain aging treatments. Clearly,

the general trend of the experimentally determinedcurve showing the effects of the amount of priordeformation is reflected in the calculated results.

p0070The equipment required for slow strain-rate test-ing is simply a device that permits a selection ofdeflection rates while being powerful enough to copewith the loads generated. Plain or precracked speci-mens in tension may be used, but if the cross sectionof these needs to be large or the loads high for anyreason, cantilever bend specimens with the beamdeflected at appropriate rates may be used. It is impor-tant to appreciate that the same deflection rate does notproduce the same response in all systems and that therate has to be chosen in relation to the particularsystem studied.

p0075The representation of the results from slow strain-rate tests may be through the usual ductility para-meters, such as reduction in area, the maximum loadachieved, the crack velocity, or even the time tofailure, although as with all tests, metallographic orfractographic examination, while not readily quanti-fiable, should also be involved. Since stress corrosionfailures are usually associated with relatively little

12

10

8

6

4

Def

orm

atio

n (%

)2

0150 200

Threshold stress (MPa)

250 300 350

400 �

150 �

CalculatedObserved

Figure 5 f0025Observed and calculated values of the

threshold net section stresses for stress corrosioncracking of a C–Mn steel after various prior deformations

and aging treatments.


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plastic deformation, the ductility of specimens will bevariable according to the extent to which stress cor-rosion contributes to the fracture process. This willalso influence the shape of the load–extension curvethat may be obtained by continuous monitoring of theresponse of a load cell incorporated in the system;Figure 6 shows the forms of curves obtained with andwithout attendant stress corrosion. It is apparent fromthese curves that not only is the extension to fracturedependent upon the presence or otherwise of stresscorrosion cracks, but so also is the maximum loadachieved. The latter may be used for expressingcracking susceptibility in some systems, as also maythe area bounded by the load–extension curve. How-ever, the variations in maximum load achieved inslow strain-rate tests in circumstances of varyingcracking severity are not always large enough forsignificant distinctions to be made. Even measure-ments of ductility, such as reduction in area, arereadily invariably not made, if only because thefinal fracture of the specimen does not always followa simple path, and the fitting of the two broken piecestogether is not easy. Probably, the easiest quantity tomeasure with reasonable accuracy is the time tofailure, which has as much significance in a slowstrain-rate test as it does in constant-load or constant-deflection tests. Indeed, the time to failure in slowstrain-rate tests is simply related to ductility para-meters, a not very surprising result when it is remem-bered that the less the intensity of stress corrosioncracking, the greater will be the ductility to fractureand therefore, the greater the time to failure for a givenstrain rate.

p0080Clearly, for slow strain-rate testing to have cre-dence, it should give results that are comparable withthose obtained by other methods. Figure 7 showssome results for tests upon low-alloy ferritic steelsin boiling 4M NH4NO3, the various alloying ele-ments producing a range of cracking susceptibilitiesas measured by the threshold stresses obtained fromconstant-strain tests. These results have been

500

400

300

200

100

0 5 10

Elongation (%)

Str

ess

(MP

a)

4M NaNO3

Oil

15 20 25

Figure 6f0030 Nominal stress–extension curves for mild steel in oil giving ductile failure, and in 4M NaNO3 producing

stress corrosion failure, at the same test temperature (104 �C).

0.6

0.5

0.4

0.3

0.2

0.1

00

t f, N

O3/

t f, o

il

1s th/sy

2

Figure 7 f0035Time to failure ratios from constant-deflection

rate tests and normalized threshold stresses sth/sy obtainedfrom constant-strain tests for a series of low-alloy ferriic

steels in boiling 4M NH4NO3.


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normalized by dividing the threshold stress sth by thelower yield strength sy for each steel, while the slowstrain test results have been normalized by dividingthe time to failure in the 4M NH4NO3 by the timeto failure in oil at the same temperature, so thatincreasing departure from unity indicates increasingcracking susceptibility. The general trend of theresults in Figure 7 is clear in indicating reasonableagreement between the two types of tests in placingthe steels in essentially the same order of merit.

p0085 Although slow strain-rate tests are most fre-quently taken to total failure in order to produce a‘go/no-go’ type of result in which threshold stressesare not defined, they can be conducted in a mannerthat allows such definition. Specimens are preloadedto various initial stresses in the absence of the crack-ing environment or at a potential that preventscracking, after which they are allowed to creep untilthe latter falls below the strain rate to be applied. Theapplied straining is continued for a sufficient timeonly to allow cracks to grow to a measurable size.During straining, the stress upon the specimen variesin a manner dependent upon the magnitude of theapplied strain-rate, hence the importance of restrict-ing the test time to no longer than that necessary toproduce measurable cracks. The cracks are probablymost conveniently measured by microscopy on lon-gitudinal sections of the gauge lengths, the length ofthe deepest detectable crack divided by the test timegiving an average crack velocity. Figure 8 showssome results from tests upon a cast nickel–aluminumbronze exposed to seawater10 and clearly it is possibleto define a threshold stress below which cracking isnot observed. However, that threshold stress dependsupon the strain-rate applied, as is to be expected.Another approach to defining threshold stresses inslow strain-rate tests that may sometimes be useful isto use tapered specimens, with the taper angle mini-mized to avoid complications by resolved compo-nents of the tensile load.11 Applied to the crackingof a-brass exposed to sodium nitrite solutions, asingle tapered specimen gave threshold stressesclose to those obtained by the use of a number ofplain specimens loaded at a given strain rate to vari-ous stress levels.

s0030 2.35.2 Testpiece

s0035 2.35.2.1 Precracked Samples

p0090 The literature contains many references to the use ofnotched, as opposed to precracked or plain specimens

in laboratory studies of stress corrosion, for reasons ofimproved reproducibility, inability to crack plain spe-cimens under otherwise identical conditions or ease ofmeasuring some parameters, such as crack growth rate,when the crack location is predetermined. However,the developments in fracture mechanics have resultedin a whole new field of stress corrosion testing in-volving the use of specimens containing a sharp pre-crack, usually produced from a notch by subjectingthe specimen to fatigue. The application of fracturemechanics to stress corrosion cracking is the subject ofan admirable review by Brown,12 and various aspectsof the method are considered in papers presented at anAdvisory Group for Aerospace Research and Devel-opment (AGARD) conference.13

p0095The problems associated with the choice of plainspecimens for assessing stress corrosion resistance may,at first sight, appear equally large in relation to pre-cracked specimens in the sense that in the relativelyshorter time during which such tests have been in use,a large number of specimen types have been used(Figure 9). However, the differing specimen geometriesare rationalized through the stress–intensity factor, withthe result that data from different testpieces are compa-rable, provided appropriate precautions are taken in

10−6

10−7

00 1 2 3

Stress (MPa)C

rack

vel

ocity

(mm

s−1

)

4 5 6 ×100

Figure 8 f0040Average stress corrosion crack velocity frommonotonic slow strain rate tests at 1.5 � 10�7 s�1

conducted over various restricted ranges of stress on a cast

Ni–Al bronze in seawater at 0.15V(SCE). The stress rangetraversed in each test is shown by the length of the bar.

Reproduced from Parkins, R. N.; Suzuki, Y. Corros. Sci.

1983, 23, 577.


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specimen preparation. The biggest single difficulty is inrelation to the large size of specimen that is necessaryfor highly ductile materials if the concepts of linearelastic analysis are to be applicable. Since it is probablethat most service stress corrosion failures occur inhighly ductile materials in relatively thin sections, it isclear that there are problems here, but the use of pre-cracked specimens that do not conform dimensionallyto the requirements for linear elastic analysis to bestrictly valid is still worthwhile in some instances, andin any case, developments in fracture mechanics overthe last decade or so allow alternative approaches thanthat of linear elastic analysis.

p0100 Precracked specimens are sometimes useful forother reasons than the analysis that they afford inrelation to stress–intensity factors. Such applications

may be associated with the simulation of servicesituations, the relative ease with which stress-corrosion cracks can be initiated at precracks, or theadvantages that sometimes accrue from the propaga-tion of a single crack. The claim that has sometimesbeen made of precracked specimen tests – that theycircumvent the initiation stage of cracking in plainspecimens, erroneously assumed invariably to berelated to the creation of a corrosion pit that providesa measure of stress concentration approaching thatachieved at the outset with a precrack – is rarelyentirely valid. Thus, the geometries of a pit, notch,or precrack are often as important for electrochemi-cal reasons as they are for any reason associated withtheir influences upon stress distribution. This isbecause a geometrical discontinuity may be necessary

Precracked specimen configurations for stress corrosion testing

Increasing stressintensity withcrack extension

Decreasing stressintensity withcrack extension

Constant stressintensity withcrack extension

Constantload

Constantdeflection

Constantload

Constantload

Remotetension

Single-edgecracked plate

Center-crackedplate

Doubleedge-crackedplate

Surface-crackedplate

Circumfer-entiallycrackedround bar


Cantileverbending

(W-a)dominated

(W-a)dominated (W-a)

indifferent

3-pointbending

4-pointbending

(W-a)indifferent

(W-a)indifferent

(W-a)indifferent

Remotebending

Double torsionsingle-edgecracked plate



Center-crackedplate

Taperedsingle-edgecracked plate

Remotebending

Cracklinebending

Cracklinebending

Cracklinebending

Cracklinetension

Figure 9f0045 Classification of precracked specimens for stress corrosion testing. Reproduced from Smith, H. R.; Piper, D. E.

Stress Corrosion Testing with Precracked Specimens, The Boeing Co, D6–24872, ARPA 878, June, 1970.


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to provide the localized electrochemical conditions,in terms of environment composition or electrodepotential, that are necessary for stress corrosioncrack propagation. The objections that have some-times been made against the use of precracked speci-mens, for example, to the validity of introducing atransgranular precrack into a specimen that suffersintergranular stress corrosion cracking, or of thenecessity of incurring a considerable expense to pro-duce a very sharp crack when the introduction of acorrosive environment may blunt the crack by thedissolution, miss the point that such sharp disconti-nuities do indeed exist in some real materials. Indeed,one of the major attractions of precracked specimentesting is that it can provide data that allow the desig-nation of the maximum allowable defect sizes instructures for the latter to remain in a safe condition.

p0105 In view of the significance of strain rate in stresscorrosion cracking, mentioned earlier, it is good toremember that its significance is as applicable toprecracked specimens as it is to initially plain speci-mens, in relevant systems. This has a number ofimplications, not the least of which is the possibleinfluence of time delay between loading precrackedspecimens and exposing them to the test environ-ment. Moreover, the limiting stress–intensity factorKISCC, above which cracks grow relatively rapidly(Figure 10) may well depend upon the conditionsunder which it is determined, and it should not beregarded as some property of the material equivalentto, say, a yield stress. There is now a considerablevolume of data that shows how relatively small fluc-tuating stresses may reduce the threshold stresses orstress-intensity factors for stress corrosion cracking,and some of these effects are probably related to cyclicloading sustaining creep-related effects. Crack-tip strainrates have consequently become a topic of interest, andexpressions are available for cyclically loaded pre-cracked specimens14,15 and also for multicracked speci-mens16 of the form that initially plain specimens takeduring slow strain-rate tests.

s0040 2.35.2.2 Comparison of the Results fromPlain and Precracked Specimens

p0110 It is clear that an initially plain specimen that devel-ops a stress corrosion crack may, if the geometry isappropriate, conform to the conditions obtaining inan initially precracked specimen. This raises a ques-tion, despite the opposing views of the protagonists ofthe two types of testpieces, as to the comparability ofthe result from each. Figure 11 shows the results17

10−4

10−6

10−8

Cra

ck v

eloc

ity (m

s–1)

10−100 20 40 60

Stress intensity (MPa m½)

Figure 10 f0050Effect of applied stress intensity upon

crack velocity for high-strength (1800MPa UTS) quenchedand tempered steel (AFC 77) in distilled water. Reproduced

form Spiedel, M. O.; In Conference on Hydrogen in Metals;

National Association of Corrosion Engineers, Houston,TX, 1975.

8

9

10

11

180170160150

KIS

CC (M

Pa

m½

)

s th (MPa)

Figure 11 f0055Threshold stress intensities KISCC from

precracked specimen tests, and threshold stresses sth fromplain specimen tests, for a Mg–7Al alloy in various structural

conditions tested in chromate–chloride solution.Reproduced from Wearmouth, W. R.; Dean, G. P.; Parkins,

R. N. Corrosion 1973, 29, 251.


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obtained from stress corrosion tests upon a Mg–7Alalloy exposed to a chromate–chloride solution, thecracking susceptibility of the alloy being varied bydifferent heat treatments.The implication ofFigure 11is that the threshold stress sth, determined upon initi-ally plain specimens of small cross section, is relatedto the threshold stress intensity KISCC obtained fromprecracked specimens of relatively large section. SinceKISCC represents the stress intensity below which anexisting crack does not propagate, it would appearthat the threshold stress given by tests on plain speci-mens corresponds to values belowwhich cracks do notpropagate to give total failure, that is, the thresholdstress for plain specimens is not necessarily the stressbelow which cracks do not form. Examination ofplain specimens stressed below the threshold stressrevealed the presence of small stress corrosion cracksthat had ceased to propagate and moreover, the maxi-mum sizes of the cracks that did not propagate to totalfailure were quadratically related to the thresholdstress, as would be expected if the concepts of fracturemechanics were applicable to these initially plainspecimens. Nonpropagating cracks have also beenobserved at stresses below the threshold stress inother systems, such as low-alloy ferritic steels exposedto various environments, and would therefore, sup-port the suggestion that what are being measured intests upon plain and precracked specimens are not asdifferent as has sometimes been suggested.

s0045 2.35.2.3 Crack Velocity Measurements

p0115 In mechanistic studies of stress corrosion and also inthe collection of data for remaining-life predictionsfor plants there is need for stress corrosion crackvelocity measurements to be made. In the simplestway, these can be made by microscopic measurementsat the conclusion of tests, the assumption being thatthe velocity is constant throughout the period ofexposure, or, if the crack is visible during the test,in situ measurements may be made by visual observa-tion, the difficulty then being that it is assumed thatthe crack visible at a surface is representative of thebehavior below the surface. Indirect measurementsmust frequently be resorted to, and these have in-volved observation of the elongation of the specimen,crack-opening displacement, changes in the electricalresistivity of the specimen, and acoustic emissions thatsometimes accompany crack extension.

p0120 Measurement of the elongation of the specimen isprobably the least satisfactory of all these, eventhough it is often the simplest, requiring a transducer

that responds to dimensional change. Multiple crack-ing of initially plain specimens raises problems ininterpreting the data in terms of crack velocities,and so the technique has been frequently used forattempting to determine the point in time whencracking was initiated, the preceding time duringwhich the transducer shows no response beingequated to an incubation period for cracking. How-ever, such results can be completely misleadingbecause the sensitivity of most transducers is suchthat they will only detect change when the specimenundergoes some plastic deformation, resulting fromthe propagation of a crack to the size where theremaining uncracked portion of the section beyondthe crack is raised to its yield strength. Consequently,crack propagation can occur during the (so-called)incubation period when the stress is insufficient tocause the propagation of a deformation band. Thelatter frequently occurs suddenly, producing a sharpresponse by the transducer, which has sometimesbeen interpreted as evidence of a burst of fast mechan-ical fracture but which may, in fact, be nothing ofthe sort. It is much more satisfactory to use a crack-opening displacement gauge18 located across a pre-crack. These gauges usually take the form of two thincantilever beams to which strain gauges are attached,the beams being located at opposite sides of theextremity of the precrack. As crack extension occursand the sides of the crack undergo relative displace-ment, the strain gauges respond to the unbending ofthe beams.

p0125Changes in the electrical resistivity of a specimencontaining a propagating crack19 depend upon apply-ing a high constant direct current at each end of thespecimen and measuring the potential differenceacross electrical leads situated at the opposite sidesof the crack. The potential field in the region ofthe crack is disturbed by the presence of the latterand as the crack extends, the potential differencebetween the leads on opposite sides increases, thetotal current remains constant. This requires a reli-able constant current source, and the technique isdependent in some degree upon the exact positioningof the leads and gives less reproducible results ifcrack branching occurs. The initial thought that theapplication of DC to the specimen may influence theelectrochemistry of the stress corrosion reactions isnot sustained in practice, and the technique can pro-vide reliable data.

p0130High-frequency stress waves are generated whenstress corrosion cracks propagate in some materials,especially the high-strength steels when these undergo


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hydrogen-induced cracking. The detection of theseacoustic signals, which are filtered from lower ampli-tude background noise, affords a means of studyingcrack propagation.20 While the technique involves theuse of sophisticated and relatively costly equipment ifit is to be correctly practiced, it has been suggested thatit may also offer a means of distinguishing betweenactive paths and hydrogen-embrittlement mechan-isms of cracking.21 However, that is not universallyaccepted, and the data from acoustic signals needtreating with caution.22

s0050 2.35.2.4 Effects of Surface Finish

p0135 It is hardly surprising that the preparation of surfaces ofplain specimens for stress corrosion tests can some-times exert a marked influence upon the results. Heattreatments carried out on specimens after their prepa-ration is otherwise completed can produce barely per-ceptible changes in surface composition, for example,decarburization of steels or dezincification of brasses,that promote quite dramatic changes in stress corrosionresistance. Similarly, oxide films, especially if formed athigh temperatures during heat treatment or working,may influence results, especially through their effectsupon the corrosion potential.

p0140 However, quite apart from these chemical changesat surfaces occasioned by the method of specimenpreparation, physical effects may be important. Paxtonand Procter23 have prepared a review of what littleis known about the effects of machining and grindingupon stress corrosion susceptibility, the most obviouseffects being related to surface topography and theintroduction of residual stresses into the surfacelayers. The former is more likely to be important inthe higher strength notch-sensitive materials, whilesurface compressive stresses are likely to have thegeneral effect of delaying or preventing failure.

s0055 2.35.3 Choice of Environment

p0145 Although the list of environments reported as pro-moting stress corrosion cracking in any alloy con-tinues to grow with time, the concept of solutionspecificity remains in that not all corrosive environ-ments will initiate or sustain stress corrosion crackingin all alloys. While it is inevitable that the environ-ment will always remain as one of the variables thatmay need to be assessed by stress corrosion tests,nevertheless certain solutions, by their widespreaduse over many years, have tended to become standardtest solutions for certain types of alloys. Thus, boiling

MgCl2 solution for stainless steels, boiling nitratesolutions for carbon steels, and 3.5% NaCl for alumi-num alloys, to mention but a few, have been exten-sively used, for example, in comparing the effects ofmetallurgical variables upon cracking propensities.Such approaches raise two questions: the firstconcerned with the extent to which ‘standard’ solu-tions prepared in different laboratories may beregarded as identical, and the second with the extentto which degrees of susceptibility of a range of alloysto cracking in one environment are related to crackingin a different environment.

p0150While the relatively small differences that may beexpected to occur between laboratories preparing asolution to the same specification often will not influ-ence stress corrosion test results, there are situationswhere relatively small changes in environment canpromote marked changes in cracking response. Thus,Streicher and Casale24 have pointed to the potentialproblems associated with the use of nominally 42%boiling MgCl2 in testing stainless steels. Since thehydrate of MgCl2 is hygroscopic, solution prepara-tion by weighing may lead to appreciable differencesin boiling point and hence, times to failure in a stresscorrosion test, so that it is preferable to prepare thesolution by adding water to the hydrate to achieve aparticular boiling point.

p0155Similarly, pH variations resulting from either theinitial preparation or from changes during a stresscorrosion test may exert a marked influence uponresults in some systems. Thus, the cracking of carbonsteels in nitrates is markedly pH sensitive and, depend-ing upon the volume of solution and the surface area ofthe specimen exposed, as well as upon the timeinvolved in making the test, significant pH rises canoccur, and cracking can cease as a result. Moreover, iftests are carried out with anodic stimulation, theseeffects may be aggravated, especially if the counterelectrode is immersed in the test cell. In other cases,for example, the medium and higher strength steels,the initiation and maintenance of cracking frequentlyrequires localized pH changes within the confines ofthe crack, and these can only occur if the initial condi-tions of exposure are appropriate.

p0160The oxygen concentration of the solution, as inmany instances of corrosion, can also be critical instress corrosion cracking tests. Instances are availablein the literature that show very markedly different testresults according to the oxygen concentration in sys-tems as widely different as austenitic steels immersedin chloride-containing phosphate-treated boiler water25

and aluminum alloys26 immersed in 3% NaCl.


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p0165 The assumption that the relative crackingresponses of a series of alloys will be the same irre-spective of the environment to which they areexposed can be extremely dangerous. Many examplescould be quoted of the dangers of drawing conclu-sions from tests in a given environment and applyingthese to a different situation, but some results byLifka and Sprowls27 will suffice. The results for therelative cracking susceptibilities of three aluminumalloys subjected to different exposure conditions areshown in Figure 12, which indicate that an intermit-tent spray test using acidified 5%NaCl solution givesthe same order of susceptibility for the three alloys aswas observed in outdoor exposure tests at three dif-ferent locations. On the other hand, an alternateimmersion test in 3.5% NaCl, widely used for testingaluminum alloys, places the alloys in a completelydifferent order of susceptibility. This single examplewill suffice to indicate the necessity for simulatingservice conditions as closely as possible where labo-ratory data are to be used for selection or design inrelation to industrial equipment.

p0170 While the dangers inherent in using standardizedenvironments in relation to environment-sensitivefracture are readily indicated by many examples thatcan be quoted, there remains a problem in relation toalloy development where possible service environ-ments may not always be identifiable at the time ofthe development program. In such circumstances, itappears desirable that an alloy should be assessed in arange of environments, but even then, it is necessaryfor realism to be injected into the program if anexcessively large number of test environments are

not to be involved. The potential dependence ofcracking, with its implications for dissolution and film-ing reactions or the discharge of hydrogen, suggeststhat the solution pH is also likely to exert significantinfluence upon cracking. Plots of cracking domains onpotential–pH diagrams sometimes indicate correla-tions with certain reactions, and this may be useful inguiding a testing program,28 that is, involving solutionsof different pH values and exploring the crackingpropensity as a function of potential.

p0175The importance of potential cannot be overemphasized, and some aspects of this part of stresscorrosion testing may be conveniently discussed inthe context of Figure 13. This shows the differentpotential ranges for the cracking of ferritic steelaccording to the environment in tests involvingpotentiostatic control. Also shown are the free corro-sion potentials for that steel in the different solutions.These indicate that while failure would occur in thenitrate at the free corrosion potentials, this would notbe so in the hydroxide or carbonate–bicarbonatesolution. This does not mean that carbon steels willnever fail by stress corrosion in these two environ-ments at the free corrosion potential, since the latteris, of course, dependent upon the composition ofthe steel, its surface condition, the composition of theenvironment, and other factors. It is possible, there-fore, that as the result of, say, small additions to theenvironments, added intentionally or present as impu-rities, the corrosion potential can be caused to bewithin the cracking range, or that as a result of smalladditions to a steel, the corrosion potential may falloutside the cracking range. Quite small changes in

100

0New Kensington

industrialatmosphere(791 days)

7178

–T6

5170

79–T

651

X70

06–T

651

Pt. Judithsea coast

atmosphere(764 days)

Pt. Comfortsea coast

atmosphere(738 days)

Acid 5%NaClintermittent

spray(28 days)

35% NaClalternate

immersion(84 days)

200

300

Low

est

sust

aine

d t

ensi

on s

tres

ste

sted

tha

t ca

used

failu

re(M

Pa)

Figure 12f0060 Relative resistance to stress corrosion cracking of three aluminum alloys subjected to different environments.The stress levels employed corresponded to 75, 50, and 25% of the respective transverse yield strengths. Reproduced from

Lifka, B. W.; Sprowls, D. O. Stress Corrosion Testing, ASTM STP No. 425, 1966; p. 342.


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potential, often only a few tens of millivolts, can there-fore produce dramatic changes in cracking responseand point to the necessity, especially in laboratory testsattempting to simulate a service failure, of reprodu-cing the potential with precision.

s0060 2.35.4 Stress Corrosion TestMethods

s0065 2.35.4.1 Types of Test Cells

p0180 The cells that contain the specimen and environmentfor stress corrosion tests often need to be more than a

vessel made in some substance, usually glass, that isinert to the environment and which produces noelectrical response upon the test specimen. Wherecracking is initiated at surfaces through which heattransfer occurs, it may be necessary to design a cell inwhich such an effect is incorporated, since the con-centration of substances in solution that may occurat an interface through which heat passes, may playa significant role in promoting cracking, especiallyif surface deposits allow concentration by evapora-tion while preventing mixing with the bulk or theenvironment. The cracking of riveted mild-steel boi-lers and the concentration of carbonate–bicarbonate

Eco

rr O

H

Eco

rr C

O3

Eco

rr N

O3

–1.2 –0.6 0 +0.6 +1.2

Potential V (SCE)

0.01

0.1

1.0

10

100

1000

1

I(mA

cm

–2)

CO3OH NO3

t f so

ln/t f

oil

Figure 13f0065 Current density differences between fast and slow sweep rate polarisation curves and stress corrosioncracking susceptibility as a function of potential for C–Mn steel in nitrate, hydroxide, and carbonate–bicarbonate solutions.


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solutions under pipeline coatings to produce crack-ing in high-pressure gas transmissions lines are sig-nificant examples. Dana29 has developed a methodfor simulating the conditions for cracking of stainlesssteels in contact with thermal insulating materials,while concentration in leaking boiler seams issimulated in the ‘embrittlement detector’ developedby Schroeder and Berk.30

p0185 Such test cells involve, among other things, acrevice, the essence of which is that the volume ofsolution that it contains is relatively small comparedwith the area of exposed metal, a ratio that mayinfluence stress corrosion test results determined inmore conventional cells where crevices do not exist.The experiments of Pugh et al.,31 on the stress corro-sion of 70–30 brass in ammoniacal solutions of vari-ous volumes are particularly instructive in indicatinghow this ratio may influence results, the time tofailure varying by about an order of magnitude for asimilar change in solution volume. Changes in thesurface area of exposed specimens, apart from theeffects already implied, may influence the crackingresponse for other reasons, as shown by the results ofFarmery and Evans32 for an Al–7Mg alloy immersedin a chloride solution. They found that couplingunstressed to stressed specimens of varying arearatio influences failure times, relatively shortertimes being obtained when the area of unstressed tostressed specimen was large.

s0070 2.35.4.2 Initiation of Stress Corrosion Tests

p0190 It may be felt that the initiation of a stress corrosiontest involves no more than bringing the environmentinto contact with the specimen in which a stress isgenerated, but the order in which these steps arecarried out may influence the results obtained, asmay certain other actions at the start of the test.Thus, in outdoor exposure tests, the time of the yearat which the test is initiated can have a marked effectupon the time to failure,26 as can the orientation of thespecimen, that is, according to whether the tensionsurface in bend specimens is horizontal upwards ordownwards or at some other angle. However, even inlaboratory tests, the time at which the stress is appliedin relation to the time at which the specimen isexposed to the environment may influence results.Figure 14 shows the effects of exposure for 3 h at theapplied stress before the solution was introduced to thecell, upon the failure of a magnesium alloy immersedin a chromate–chloride solution. Clearly, such priorcreep extends the lifetime of specimens and raises the

threshold stress very considerably, and since othermetals are known to be strain-rate sensitive in theircracking response, it is likely that the type of resultapparent in Figure 14 is more widely applicable.

s00752.35.4.3 Hydrogen Embrittlement Tests

p0195The absorption of hydrogen by various materials,including high-strength steels, results in loss of duc-tility, which in turn can result in cracking and frac-ture when the metal is subjected to a sustained tensilestress. Hydrogen may be introduced into these vari-ous alloys from the gas phase (during manufacture orwelding), or from aqueous solution during surfacetreatment (pickling, plating, and phosphating) orfrom the environment during a spontaneous corro-sion process in which the development of aciditywithin the crack results in hydrogen evolution andabsorption. Various test methods33 may be used toevaluate the effect of hydrogen on the properties ofalloys, including some ad hoc tests that were specifi-cally developed for high strength steels.

p0200Although similar constant-load test rigs are usedfor both active-path corrosion and hydrogen stresscracking, there is one fundamental difference in thetest procedure. In the case of active-path corrosiontesting, it is always carried out in the presence of thecorrosive environment, but in the case of hydrogen-related cracking, testing may be carried out afterhydrogen has been introduced into the alloy eitherdeliberately by gaseous or cathodic charging, or fol-lowing processes such as welding, pickling, or elec-troplating. However, with precharged specimens, loss

1 10010tf (min)

220

200

180

160

Str

ess

(MP

a)

3-h creep0-h creep

Figure 14 f0070Effect of delay period between application of

load and introduction of solution to test cell in the failure of aMg–7Al alloy exposed to a chromate–chloride solution.

Reproduced from Wearmouth, W. R.; Dean, G. P.; Parkins,

R. N. Corrosion 1973, 29, 251.


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of hydrogen may occur34 when they are removedfrom the environment, resulting in the entry of thatsubstance and so, sustained-load tests are also carriedout in the presence of an environment (gaseous oraqueous) so that hydrogen is introduced into the testpiece during the application of the tensile stress.

s0080 2.35.4.4 Dynamic Tests

p0205 All of the properties evaluated by the conventionaltensile test (yield strength, tensile strength, elonga-tion, and reduction in area) are affected by the pres-ence of hydrogen, but in the case of the tensilestrength and yield strength, the effect is significantonly when the steel has a very high tensile strengthand has been severely embrittled. On the other hand,the reduction in area, and, to a lesser extent, theelongation may be used for detecting embrittlement.Hobson and Sykes35 found that with low-carbonsteels there was an almost linear relationship betweenreduction in area and hydrogen content of the steel.Slow strain-rate tests are sometimes employed intesting materials (and not only steels) after preexpo-sure to a source of hydrogen. The strain rate may becritical in that not only can it be too high but also, if itis too low the hydrogen may diffuse out of the speci-men before cracking occurs.

p0210 Various types of bend tests have been used toevaluate embrittlement. Beck et al. 36 used thin stripspecimens and determined the decrease in height, Dh,at fracture when the specimen was bent by compres-sing it at a constant rate in a tensile testing machine(Figure 15). The decrease in height, Dh, gives a mea-sure of the embrittlement, the maximum elongation ofthe outside fiber of the specimen being calculatedfrom the radius of curvature at maximum bending.In general, the ductility is found to increase with therate of straining, and for this reason, high-strain-ratetests, such as impact tests, are insensitive to hydrogenembrittlement. Where the material is available only inthe form of tubing, semicircular specimens may beused in place of flat strips in the compression bendtest. The total cross-head travel from the unstressedheight along the diameter to the point of fracture givesa measure of embrittlement, which may be comparedwith that obtained from an unembrittled specimen ofthe same steel.

p0215 A constant-rate bend-test machine, which providesan effective method for testing highly embrittled steelwires of high-tensile strength, was designed by Zapffeand Haslem37 (Figure 16). The motor A pulls a chordattached to the traveling arm D that rotates about a

pivot pin. The wire specimen G (1.6 � 100mm) isinserted in D and is supported by the fixed arm F, thearrangement being so designed that tensile or torsionalstresses are avoided. The specimen is thus bent aroundthe pivot pan E (radius 1.6mm) at a constant rate, theangle of bend to cause fracture giving a measure of itsductility. Since ductility increases with rate of strain-ing, the bending rate must be slow and (4%) is

Δh

Specimen

Figure 15 f0075Bend test using a tensile testing machine.Reproduced from Beck, W.; Klier, E. P.; Sachs, G. Trans.

AIME 1956, 206, 1263.

90

180

E

G

0

D

C

F

B

A

Figure 16 f0080Constant-rate bend-test for determining

hydrogen embrittlement of wires. A, drive unit; B, pulley; C,

semicircular base;D, traveling arm; E, axial pin; F, fixed arm;

G, wire specimen. Reproduced from Zapffe, C. A.; Haslem,M. E. Trans. AIME 1946, 167, 281.


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considered to be suitable for detecting embrittlement.A similar machine has been used for studying theembrittlement of spring steel strip after hydrogen hasbeen introduced by cadmium plating.

p0220 An alternative procedure is the reverse-bend testin which the specimen in the form of wire or stripis bent repeatedly backward and forward over a man-drel until it fractures, the number of bends

indicating the degree of embrittlement. However,this method is considered to be less sensitive thanthe single-bend test.

s00852.35.4.5 Static Tests

p0225Whereas ductile materials, such as iron and mildsteel, are often considered not to crack when chargedwith hydrogen and subjected to a tensile stress belowthe yield stress, the position is different with high-strength ferrous alloys where, depending on thestrength of the steel and the hydrogen content, failuremay occur well below the yield stress. However,the fracture process is not instantaneous, and thereis a time delay before cracks are initiated; for thisreason, the phenomenon is sometimes referred to as‘delayed failure.’

p0230In the majority of cases, the tests are conductedusing a dead-weight lever-arm stress-rupture rig withan electric timer to determine the moment of frac-ture, but a variety of test rigs similar to those shown inFigure 1(g) are also used. The evaluation of embrit-tlement may be based on a delayed-failure diagramin which the applied nominal stress versus time to

10310210110Fracture time (h)

Incubation time

Fracture time

Lower critical stress

Upper critical stress

Ap

plie

d s

tres

s

Figure 17f0085 Schematic representation of delayed failure

characteristics of a hydrogenated high-strength steel.

(a)

Specimen withthreaded ends

Upper head oftesting machine

Spherical bearing

Spherical bearing

Upper head oftesting machine

Splitsocket

Solidclampingring

Specimen withshouldered ends

(b)

Figure 18f0090 Methods of gripping specimens in order to avoid bending stresses: (a) Device for threaded end specimens, and

(b) device for shouldered-end specimens. Reproduced from Stress Corrosion Testing, ASTM STP No. 425, 1967.


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failure is plotted, Figure 17, or the specimen maybe stressed to a predetermined value (say 75% ofthe ultimate notched tensile strength) and is consid-ered not to be embrittled if it shows no evidenceof cracking within a predetermined time (say 500 h).Troiano38 considers that the nature of delayed frac-ture failure can be described by four parameters (seeFigure 17):

1. the upper critical stress corresponding to the frac-ture stress of the unembrittled notched specimen;

2. the lower critical stress, which is the applied stressbelow which failure does not occur;

3. the incubation period or the time required for theformation of the first crack;

4. the failure time or the time for specimen failure ata given applied stress; in the intermediate stressrange, this includes a period of relatively slowcrack growth.

During the constant-load test, it is essential that onlyaxial tensile stresses are applied, as any bendingstresses that are introduced will result in a highertrue stress than that calculated. For this reason, theends of the specimens and the grips must be designedto avoid bending stresses. The ASTM Standard E8(Standard Test Methods for Tension Testing ofMetallic Materials) specifies that in the case of speci-mens with threaded ends the grips should be attachedto the heads of the testing machine through properlylubricated spherical-seated bearings and that the dis-tance between the bearings should be as great as isfeasible (Figure 18).

p0235 In order to simplify the test procedure, a numberof investigators have designed test rigs in which thebulky lever arm is replaced by a loading nut, thestress in the specimen being determined by meansof strain gauges; these rigs are similar in principle tothose shown in Figure 1(g). Figure 19 shows aspring-loaded rig that was used by Cavett and vanNess39 to study the embrittlement produced byhydrogen gas at high pressures, in which the tensileload is applied by compressing a heavy-duty spring.

p0240 Raring and Rinebold40 have devised a method inwhich the specimen is supported along the diameterof a steel loading ring, Figure 20, and the stress isapplied by tightening the bottom nut until the diam-eter corresponds with the required load. The suddenrelease of elastic energy stored within the ring whenthe specimen fractures results in displacement of thetightening nut, and this is used to actuate a micro-switch and timer. Williams et al. 41 have used notched‘C’-rings, the stress being applied by tightening the

nut of a calibrated loading bolt that passes through thediameter of the ‘C’ ring, Figure 21. The strain gaugesattached to the bolt form two arms of a Wheatstonebridge circuit and to compensate for temperature

Figure 20 f0100Loading-ring method of stressing a specimen.

Reproduced from Raring, R. H.; Rinebold, J. A. ASTMBulletin No. 213, 1956.

Hydrogeninlet1

Loading spring

Bolt bearing

Stopping block

Steel ring

Brass ring

Specimen

Teflon gasket

Specimen gripwith bolt joint

Figure 19 f0095Spring-loaded rig for sustained load testing of

a steel specimen in gaseous hydrogen at high pressure.Reproduced from Cavett, R. H.; van Ness, H. C. Welding J.

(research supplement) 1967, 42, 317.


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changes, the other two arms consist of two identicalstrain gauges attached to a similar unstrained bolt.

Appendix A: Stresses in BentSpecimens

In each of the following equations: s¼maximumtensile stress, E=modulus of elasticity, and t¼ speci-men thickness.

A.1 Two-point bending, Figure 1(a):

L ¼ k:t :E

ssin�1 H :s

k:t :E

� �

where L= specimen length, k= constant (1.280), andH=holder span.

A.2 Four-point bending, Figure 1(b):

s ¼ 12E:t :y

3L2 � 4A2

where y=maximum deflection, L=distance betweenouter supports, and A=distance between inner andouter supports.

A.3 ‘U’-bends, Figures 1(d) and 1(e):

Applied strain e= t/2r, when t< r, where r= radius ofcurvature at section of interest.

A.4 ‘C’-rings, Figure 1(c):

s ¼ 4E:t :Z:DpD2

where D=ODf � ODi, ODf = final outside diameterof stressed ‘C’-ring, ODi = initial outside diameterof unstressed ‘C’-ring, D=mean diameter, that is,(OD � t), and Z=a correction factor, related to D/t asindicated in Figure 22.

References

1. Ailor, W. H. Ed.; Handbook on Corrosion Testing andEvaluation; Wiley: New York, 1971.

2. Stress Corrosion Testing, ASTM STP No. 425, 1967.3. BS 6980, Stress corrosion testing, Parts 1–7, (ISO 7539–1/7)

BSI, Milton Keynes.4. Logan, H. L. The Stress Corrosion of Metals; Wiley: New

York, 1966; p 273.5. Haaijer, G.; Loginow, A. W. Corrosion 1965, 21, 105.6. Parkins, R. N.; Mazza, F.; Royuela, J. J.; Scully, J. C. Br.

Corr. J. 1972, 7, 154.7. Parkins, R. N.; Haney, E. G. Trans. Metall. Soc., AIME

1968, 242, 1943.8. Brenner, P.; Gruhl, W. Z. Metall. 1961, 52, 599.9. Ugianski, G. M.; Payer, J. H. Eds., Stress Corrosion

Cracking – The Slow Strain Rate Technique, ASTM STP665; ASTM: Philadelphia, 1975.

10. Parkins, R. N.; Suzuki, Y. Corros. Sci. 1983, 23, 577.11. Yu, J.; Holroyd, N. J. H.; Parkins, R. N. In Environment-

Sensitive Fracture: Evaluation and Comparison of TestMethods, ASTM STP 821; Dean, S. W., Pugh, E. N.,Ugianski, G. M., Eds.; ASTM: Philadelphia, 1984; p 288.

12. Brown, B. F. Met. Rev. 1968, 13, 17.13. Specialists Meeting on Stress Corrosion Testing Methods,

AGARD Conference Proceedings, No. 98 NATO, 1972.

Two external strain gauges

Steel bolt(heat treated to 859 MNm–2)

6.35 mm 28 NF

1.58-mm dia hole

6.35 mm 28 NF4.49 mm

1.9 mm

50.29 mm31.75 mm

5.69-mmhole

0.076 – 0.013-mm

60�

60�

5.69-mmdiahole

Internalstrain gauge

2024 aluminium alloy bolt

Notch radius(Kt–4)Straingauges

2.9m

m

Inside surfacemasked

Both sidesmasked

Washer

Masked area

Figure 21f0105 Notched ‘C’-ring specimen with attachedstrain gauges.

100

0.98

0.96

0.94

0.92

0.90

0.88

0.86

0 10 20 30 40 50 60 70 80 90 100D/t

Cor

rect

ion

fact

or

Figure 22f0110 Correction factor Z for calculating stress in

‘C’-ring tests.


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14. Lidbury, D. P. G. Embrittlement by the Localized CrackEnvironment; Gangloff, R. P., Ed.; AIME: New York, 1983;p 149.

15. Parkins, R. N. Corrosion 1987, 43, 130.16. Congleton, J.; Shoji, T.; Parkins, R. N. Corros. Sci. 1985,

25, 633.17. Wearmouth, W. R.; Dean, G. P.; Parkins, R. N. Corrosion

1973, 29, 251.18. Fisher, D. M.; Bubsey, R. T.; Srawley, J. E. Design and Use

of a Displacement Gauge for Crack ExtensionMeasurements, NASA IN-D 3724, 1966.

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CORR: 00181

Non-Print Items

Abstract:Environmentally assisted cracking (EAC) results from the interactions, in a critically interdependentmanner, between a metal and its response to the application of an appropriate stress in an environ-ment. Recognition of these conjoint requirements has frequently led to the use of tests that attempt tosimulate a practical situation, especially with regard to the structure and composition of the material,but less frequently in the manner in which the stress is generated in the test piece and in achievingrepresentative environmental conditions. This article, which outlines some of the main test methodsand procedures used to study EAC, was originally written by Redvers Parkins for the 3rd edition ofShreir. It retains its relevance till date.

Author and Co-author Contact Information:

R N ParkinsDepartment of Metallurgy and Engineering MaterialsNewcastle UniversityNewcastle-upon-Tyne NE1 7RUUK

CORR: 00181

CORR: 00181 - Elsevier...2.35.4 Stress Corrosion Test Methods 1540 2.35.4.1 Types of Test Cells 1540...

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