TWI Industrial Member Report Summary 888/2007
By C Holtam and D Baxter
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. It is therefore imperative that service conditions are properly characterised and the mechanism of environmental damage fully understood. Only then can a reasoned judgement be made regarding the manner in which the environmental damage influences structural integrity.
Methods for evaluating environment assisted cracking (EAC) within 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<KISCC) where a crack or flaw already exists. Under cyclic loading a threshold stress range (??TH) or stress intensity range (?KTH) can likewise be determined.
In cases where EAC cannot be avoided entirely, rates of crack growth can be determined, and calculations carried out to determine whether the extent of crack growth is tolerable for a given design life or inspection interval. Under these circumstances the crack growth rate, with respect to time for static loading (da/dt), or with respect to the number of cycles for fatigue loading (da/dN) needs to be determined.
For relatively deep cracks, KISCC is an appropriate characterising parameter for static loading, as suggested in BS 7910, and standard test techniques can be used to determine material and environment specific data (ie BS 7539 Parts 6 and 9). Similarly, for defect free components, where the initiation of cracks is the dominant factor on life, a threshold stress (?SCC) is appropriate, and again test techniques exist for generating suitable data (ie BS 7539 Parts 2-5 and 7). 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 true threshold for avoiding crack extension for the shallow crack may be lower, or the crack growth rate may be higher. This report sets out to describe how different assessment procedures tackle this problem, and to review recent research in this area.
- Review how current assessment procedures deal with EAC, in particular focusing on what they say about predicting the behaviour of components containing shallow cracks.
- Review recent research regarding the modelling of EAC in the shallow-crack regime.
- Review and propose experimental test methods for examining different aspects of material behaviour in this regime.