Unified approach to fatigue crack growth

Salient features of the Unified Approach to fatigue crack growth developed by the authors are presented. It is recognized at the outset, which is being confirmed experimentally, that crack closure contributions are small and in many cases are negligible. Hence crack closure cannot explain many observed fatigue phenomena. The Unified Approach provides a more fundamental approach to account for many of the experimental observations. It is based on the concept that fatigue is fundamentally a two parametric problem. It requires two load parameters to define cyclic loads unambiguously. Correspondingly, there are two thresholds, Delta K*(th) and K*(max,th) that must be met for growth to occur. This concept is valid not only at threshold but also for all growth rates. Furthermore, consideration of these two parameters unifies several of the fatigue phenomena such as load-ratio effects, anomalous behavior of short cracks, overload and underload effects, environmental effects, high temperature effects, etc. Through the consideration of internal stresses that affect K-max, effects of residual stresses due to localized phase transformations, stresses due to welding, cold-work etc, are all can be accounted. Thus the approach brings into a unified frame work, all aspects of fatigue crack growth from nucleation, to short crack growth, to long crack growth and finally to failure.