Cracki Assessment Methods_ Shallow Cracks (October 2007)

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Env i r onm ent ass isted c rack ing assessm ent

m e thods : t he behav iou r o f shal l ow c racks

C. M. Ho l t am * , D. P. Bax t e r * *Structural Integrity Technology Group, TWI Ltd, Granta Park,Great Abington, Cambridge, CB21 6AL, UK.

Paper presented at ESIA9 - 9th International Conference onEngineering Structural Integrity Assessment - 15-19 October2007, Beijing, China.

TWI Ltd has an ongoing research program aimed at validating

and improving Fitness-for-Service assessment procedures forEnvironment Assisted Cracking. Initial studies have focused onthe shallow crack phenomena and this paper reviews currentassessment procedures, highlighting one area where furtherexperimental work is required.

I n t r o d u c t i o n

Setting conditions for the avoidance of in-service crack growth in

aggressive corroding environments has long been a majorchallenge due to the number of variables that have a significanteffect on material behaviour. Under static loading conditions,shallow stress corrosion cracks may grow faster or slower thandeeper cracks, depending on the material-environment system.There are several reasons why shallow cracks might behavedifferently to deep cracks. For example, a crack's size relative tomicrostructural features, environmental effects within the crack


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and the size of the crack tip plastic zone can all influence

behaviour, Jones and Simonen. [5]

Review o f EAC assessm ent p r ocedur es

It is fair to say that none of the established Fitness-for-Service(FFS) standards contain comprehensive assessment procedures

for environment assisted cracking (EAC), although all highlightthe importance of only using data relevant to the actualenvironment and loading conditions. Methods for evaluating EACwithin current integrity assessment procedures are usually based

on avoiding the phenomena by limiting the stress ( < SCC )

for crack free components, or limiting the stress intensity factor(K


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transition between K-controlled and stress-controlled behavior asthe crack size is reduced, and emphasizes the need for care inthe shallow-crack regime as the critical stress may be lower thanthat obtained by extrapolating the deep crack data.Unfortunately, although the diagram in Figure 1 describes thisaspect of the shallow-crack problem very well, it does not

identify a clear methodology for the assessment of shallowflaws. The form of the curve in the shallow-crack regime has noexperimental justification and further work in this area is neededbefore a robust procedure for a quantitative assessment ofshallow flaws could be developed.

Fig.1 .S c h e ma t i c

d i a g r a m o f

t w o - p a r a m e t e r

a p p r o ach t o

s t ress

cor ros ion

c rack ing

( FI TNET 2 0 0 5p 9 - 3 )

Reg im es o f behav iou r

There are a number of ways that the early stages of EAC may be

modelled, not covered in this paper. The diagram in Figure 1describes one aspect of the shallow crack problem, in that the

appropriate characterising parameter changes from K to asthe crack size decreases. In the transitional regime, modelsdeveloped to explain the nature of this transition will rely on theavailability of suitable shallow crack test data, and work iscurrently under way at TWI to generate data of this kind for anumber of material-environment systems. In Figure 1 the


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shallow crack regime is essentially defined by the relative

magnitude of K ISCC and SCC . However, crack size

dependency associated with differences in crack tip chemistrymay occur over a different scale and the potential influence thatthis has on observed thresholds or crack growth rates should notbe ignored. With respect to material response to static loading,

there is a need to define the extent over which deep-crack K ISCCor da/dt data can be used to assess real, potentially shallower,flaws.

In defining the applicable limits of deep-crack data ( ie K ISCCand/or da/dt), it can initially be assumed that the stress statecan still be characterised in terms of an applied stress intensityfactor (K), and that we are considering the possibility that cracksmay be 'chemically small' due to differences in crack tipchemistry. If flaws are still large with respect to the extent ofcrack tip plasticity or microstructure, K remains an appropriatecharacterising parameter. Under these conditions, conventionalassessments (using K ISCC and/or da/dt data) may be carried

out, so long as the input data are conservative, ie test data areassociated with specimens containing flaws of a comparable sizeto those being assessed. In many instances this will be anadequate approach, as the minimum crack size that can be

reliably detected by NDT, may be significantly larger than thatneeded for the stress field to be characterised by K. Under thesecircumstances there is no need to consider the behavior ofsmaller ('mechanically small' or 'microstructurally small') flawsas any FFS assessment will be limited to a minimum assumedflaw size. Only if such flaws are 'chemically small' is there adanger of non-conservatism. It is likely that conventionalfracture mechanics test techniques can be adequately modified

for exploring material behaviour in this regime.

In cases where there is a need to model the behaviour ofmechanically or microstructurally small cracks, the conventionalcriterion for avoiding subcritical crack growth (K


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and in the shallow crack regime some transitional behaviourmight be expected. Leaving aside the precise details of thistransitional regime, a dual criterion for avoiding stress corrosioncracking could be envisaged as follows:

/ SCC< 1 and K/ K I SCC< 1 (1)

as described by the two dashed lines in Figure 1. Of course, forstatic loading conditions, the conventional way of describing theinteraction between two failure mechanisms (one K-controlledand one stress-controlled) is a failure assessment diagram(FAD). Indeed for the simplest type of FAD ( ie BS 7910 level 1)[2]the criterion for avoiding failure can be expressed as:

S r = re f / f < 0.8 and K r = K l / Kmat < 0.707. (2)

Safety factors aside, the similarities between the two sets ofcriteria are apparent. Exploring the validity of either the straightline construction in Figure 1, or some form of postulatedtransitional behaviour requires the availability of shallow cracktest data. In this case test data are required over a wide range ofcrack sizes, although some estimate of the crack size of interest

can be determined by examining the values of K ISCC and SCCfor the particular material-environment system of interest. The

two straight lines in Figure 1 will intersect at a pointcorresponding to:

(3)

where Y is a geometry dependent term. It is clear that the ratio K

ISCC / SCC is critical in determining the crack size over which

the anticipated transition from K-controlled to stress-controlledbehaviour occurs and this will depend on the material-environment system of interest. For example, for pipeline steels

in a sour environment, where K ISCC 30 MPa m and SCC

220-440 MPa (Contreras et al. [3], Pargeter et al. [8], Sponseller[9]), this transition is expected when the crack depth isapproximately 3-6 mm. By contrast, for high strength aluminum

alloys in seawater, where K ISCC 2-7 MPa m and SCC


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550 MPa, (Bayoumi [1], Ohsaki [7]), the critical regime is 4-50m. It is clear that specimen geometry and testing proceduresfor examining the anticipated transition in these two cases woulddiffer considerably.

Conclus ions

For applications where FFS assessments are based on NDTinspection limits, it is unlikely that flaws smaller than 1mm willbe of practical interest. Therefore slow strain rate and/orconstant load tests should be carried out to investigate the effectof crack depth on the measured value of K ISCC to examine the

possibility that differences in crack tip environment have aninfluence on material behaviour. For other applications, wherethe design philosophy may be different, there may be an interest

in characterizing the behaviour of smaller flaws. Under thesecircumstances there may be a different transition from K-controlled to stress-controlled behaviour, and shallow crack dataare needed to develop models for material behaviour in thisregime.

Refer ence l is t

1. Bayoumi M R, 1996: 'The mechanics and mechanisms offracture in stress corrosion cracking of aluminium alloys',Engineering Fracture Mechanics5 4 , No. 6, pp879-889.

2. BS 7910, 2005: 'Guide to methods for assessing theacceptability of flaws in metallic structures', British StandardsInstitution, London.

3. Contreras A, Albiter A, Salazar M, Perez R, 2005: 'Slow strainrate corrosion and fracture characteristics of X-52 and X-70pipeline steels', Materials Science and EngineeringA 4 0 7 ,pp45-52.

4. FITNET, 2006, FITNET Fitness-for-Service Procedure FinalDraft MK7, Prepared by European Fitness-for-ServiceThematic Network FITNET.

5. Jones R H and Simonen E P, 1994: 'Early stages in thedevelopment of stress corrosion cracks', Materials Science andEngineering, A 1 7 6 211-218.

6. Kitagawa H and Takahashi S, 1979: 'Applicability of fracture


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mechanics to very small cracks of the cracks in the earlystage,' 2nd Int. Conf. on Mechanical Behaviour of Materials,pp627-631.

7. Ohsaki S, Kobayashi K, Iino M, Sakamoto T, 1996 : 'Fracturetoughness and stress corrosion cracking of aluminium-lithiumalloys 2090 and 2091', Corrosion Science, 3 8 , No. 5, pp793-

802.8. Pargeter R J, Gooch TG and Bailey N, 1990 : 'The effect of

environment on threshold hardness for hydrogen inducedstress corrosion cracking of C-Mn steel welds', ConferenceProceedings 'Advanced Technology in Welding, Materials,Processing and Evaluation', Japan Welding Soc, Tokyo, April1990.

9. Sponseller D L, 1992 : 'Interlaboratory testing of seven alloys

for SSC resistance by the double cantilever beam method',Corrosion, 4 8 , No. 2, pp.159-171.

10. Turnbull A, Koers R W J, Gutierrez-Solana F and Alvarez J A,2005: 'Environment induced cracking - A fitness-for-serviceperspective', Proceedings of OMAE 2005, 24th InternationalConference on Offshore Mechanics and Arctic Engineering,OMAE2005-67566.

Last Reviewed 2007 / Copyright 2007 TWI Ltd

Cracki Assessment Methods_ Shallow Cracks (October 2007)

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