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Environment assisted cracking assessment methods: the behaviour of shallow cracks

C. M. Holtam and D. P. Baxter
Structural Integrity Technology Group, TWI Ltd, Cambridge, UK.

Paper presented at ESIA9 - 9th International Conference on Engineering Structural Integrity Assessment - 15-19 October 2007, Beijing, China.

TWI Ltd has an ongoing research program aimed at validating and improving Fitness-for-Service assessment procedures for Environment Assisted Cracking. Initial studies have focused on the shallow crack phenomena and this paperreviews current assessment procedures, highlighting one area where further experimental work is required.

Introduction

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behaviour. Under staticloading conditions, shallow stress corrosion cracks may grow faster or slower than deeper cracks, depending on the material-environment system. There are several reasons why shallow cracks might behave differently to deep cracks. Forexample, a crack's size relative to microstructural features, environmental effects within the crack and the size of the crack tip plastic zone can all influence behaviour, Jones and Simonen. [5]

Review of EAC assessment procedures

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 highlight the importance of only using datarelevant to the actual environment and loading conditions. Methods for evaluating EAC within current integrity assessment procedures are usually based on avoiding the phenomena by limiting the stress ( σ< σ SCC ) for crack free components, or limiting the stress intensity factor (K<K ISCC ) where a crack or flaw already exists. For relatively deep cracks, K ISCC is an appropriate characterising parameter, as suggested in BS 7910, and standard test techniques can be used to determine material-environment specific data. Similarly, for defect free components, where theinitiation of cracks is the dominant factor on life, a threshold stress ( σ SCC ) is appropriate, and again test techniques exist for generating suitable data. Shallow cracks fall between these two extremes, and it is currently unclear how behaviour in this regime should be characterised.In some cases the extrapolation of 'deep-crack' test data to predict the performance of a component containing a shallow crack may be non-conservative, ie the threshold for avoiding cracking may be lower, or the crack growth rate may be higher. Only R6 makes specific reference to the limitations associated with the use of deep-crack test data, and only in reference to fatigueor corrosion fatigue behaviour.

Currently the European Fitness-for-Service Thematic Network's recently released final draft (FITNET Mk7) [4] is the only standard to contain a specific section that deals with EAC, although API RP 579 may incorporate something similar in the next revision. FITNET Mk7 highlights a number of weaknesses in current materials testing andintegrity assessment procedures, and specifically mentions difficulties associated with the assessment of shallow cracks ( [4] , Turnbull et al. [10] ). It also introduces the concept of a two-parameter approach to the assessment of EAC ( Figure 1). This diagram describes a transition between K-controlled and stress-controlled behavior as the crack size is reduced, and emphasizes the need for care in the shallow-crack regime as the critical stress may be lowerthan that obtained by extrapolating the deep crack data. Unfortunately, although the diagram in Figure 1 describes this aspect of the shallow-crack problem very well, it does not identify a clear methodology for the assessment of shallow flaws. The form of the curve in the shallow-crack regime has no experimentaljustification and further work in this area is needed before a robust procedure for a quantitative assessment of shallow flaws could be developed.

√m and σ SCC ≈ 220-440 MPa (Contreras et al. [3] , Pargeter et al. [8] , Sponseller [9] ), this transition is expected when the crack depth is approximately 3-6 mm. By contrast, for high strength aluminum alloys in seawater, where K ISCC ≈ 2-7 MPa √m and σ SCC ≈ 550 MPa, (Bayoumi [1] , Ohsaki [7] ), the critical regime is 4-50 µm. It is clear that specimen geometry and testing procedures for examining the anticipated transition in these two cases would differ considerably.

Conclusions

For applications where FFS assessments are based on NDT inspection limits, it is unlikely that flaws smaller than 1mm will be of practical interest. Therefore slow strain rate and/or constant load tests should be carried out toinvestigate the effect of crack depth on the measured value of K ISCC to examine the possibility that differences in crack tip environment have an influence on material behaviour. For other applications, where the design philosophy may be different, there may be an interest incharacterizing the behaviour of smaller flaws. Under these circumstances there may be a different transition from K-controlled to stress-controlled behaviour, and shallow crack data are needed to develop models for material behaviourin this regime.

Reference list

  1. Bayoumi M R, 1996: 'The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys', Engineering Fracture Mechanics 54, No. 6, pp879-889.
  2. BS 7910, 2005: 'Guide to methods for assessing the acceptability of flaws in metallic structures', British Standards Institution, London.
  3. Contreras A, Albiter A, Salazar M, Perez R, 2005: 'Slow strain rate corrosion and fracture characteristics of X-52 and X-70 pipeline steels', Materials Science and Engineering A407, pp45-52.
  4. FITNET, 2006, FITNET Fitness-for-Service Procedure Final Draft MK7, Prepared by European Fitness-for-Service Thematic Network FITNET.
  5. Jones R H and Simonen E P, 1994: 'Early stages in the development of stress corrosion cracks', Materials Science and Engineering, A176 211-218.
  6. Kitagawa H and Takahashi S, 1979: 'Applicability of fracture mechanics to very small cracks of the cracks in the early stage,' 2nd Int. Conf. on Mechanical Behaviour of Materials, pp627-631.
  7. Ohsaki S, Kobayashi K, Iino M, Sakamoto T, 1996 : 'Fracture toughness and stress corrosion cracking of aluminium-lithium alloys 2090 and 2091', Corrosion Science, 38, No. 5, pp793-802.
  8. Pargeter R J, Gooch TG and Bailey N, 1990 : 'The effect of environment on threshold hardness for hydrogen induced stress corrosion cracking of C-Mn steel welds', Conference Proceedings 'Advanced Technology in Welding, Materials, Processing and Evaluation', Japan Welding Soc, Tokyo, April 1990.
  9. Sponseller D L, 1992 : 'Interlaboratory testing of seven alloys for SSC resistance by the double cantilever beam method', Corrosion, 48, 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-service perspective', Proceedings of OMAE 2005, 24th International Conference on Offshore Mechanics and Arctic Engineering, OMAE2005-67566.