One of the problems in relation to predicting the rates of defect growth in structures displaying stress corrosion cracking (SCC) is that knowledge relating to realistic stress corrosion crack velocities is very limited. In monotonic slow strain rate tests (SSRTs) taken to total failure at usual strain rates and in tests on precracked 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 develop partly 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 SSRTs, the importance of considering the number of cracks is demonstrated, in which case it is possible to calculate (involving electrochemical data) the crack velocity-strain rate relationships in good agreement with the experimental data without recourse to empirical factors that ensure a good fit. However, another factor must be considered in relation to crack velocities that hitherto has received scant attention. Thus, new cracks continue to be nucleated with the passage of time; 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, in which its role can be critical; however, it is probable that the merging of small cracks may sustain crack growth under circumstances in which crack growth would otherwise cease.
