Abstract
One of the problems in relation to predicting rates of defect growth in structures displaying stress corrosion cracking is that knowledge relating to realistic stress corrosion crack velocities is very limited. In monotonic slow strain rate tests taken to total failure at usual strain rates and in tests on pre-cracked specimens at stress intensity factors in excess of KIscc stress corrosion crack velocities may be appreciably higher than are likely to occur in service, except in the very late stages of crack growth before a service failure. These differences probably arise in part because of differences in the effective crack tip strain rate and consideration is given to various expressions for calculating the latter from the applied strain rate. For multiple cracked specimens, as in slow strain rate tests, the importance of taking into account the number of cracks is shown, in which case it is possible to calculate, involving electrochemical data, crack velocity-strain rate relationships in good agreement with experimental data without recourse to empirical factors that ensure good fit. However, there is another factor that needs to be taken into consideration in relation to crack velocities and which hitherto has received scant attention. Thus, new cracks continue to be nucleated with the passage of time and these together with cracks previously initiated and grown may coalesce or merge to produce larger cracks. Crack coalescence is considered particularly in relation to the later stages of crack growth, where its role can be critical, but it is probable that the merging of small cracks may sustain crack growth in circumstances where otherwise crack growth would cease.
