Dislocation processes that affect kinetics of fatigue crack growth

Fatigue damage involves an irreversible plastic flow, and requires two load parameters for its unique description. We have examined the dislocation processes at the crack tip due to cyclic loads and showed that the two-parameter requirement naturally, follows front the analysis. Hence. there are two fatigue thresholds, K-max,K-th and DeltaK(th), instead of just DeltaK(th), as is normally assumed. Examination of dislocation behaviour reveals that the K-max threshold can be related to the stress necessary to nucleate additional slip needed for crack growth. Similarly, DeltaK(th), can be related to the stress to overcome reversible slip. The value of K-max,K-th is always greater than or equal to DeltaK(th). The two-parameter requirement can be related to the presence of monotonic and cyclic plastic zones characteristic of a fatigue crack. The size of monotonic plastic zone is always larger than that of the cyclic plastic zone. The former moves the crack tip forward while the latter re-sharpens it. The analysis also shows that the effect of crack wake plasticity oil the crack tip driving force is limited and becomes increasingly negligible with increasing crack lenght. Dislocation concepts and continuum concepts are brought together to analyse the two-parameter requirement of fatigue crack growth.