Analysis of overload effects and related phenomena

Overloads and underloads perturb steady state fatigue crack growth conditio ns and affect the growth rates by retarding or accelerating the growth. Cle ar understanding of these transient effects is important for the reliable l ife prediction of a component subjected to random loads. The overload effec ts have predominately been attributed to either plasticity induced crack cl osure behind the crack tip, residual stresses ahead of the crack tip, or a combination of both. These effects are critically examined in the context o f the Unified Approach proposed by the authors. Recent experimental and ana lytical evaluation of crack closure has confirmed its negligible contributi on to crack growth and has demonstrated that changes in the stresses ahead of the crack tip are more important than closure behind the crack tip. It i s shown that the overload effects and other transient effects arise due to perturbation of the stresses ahead of the crack tip, and these can be accou nted for by the two parametric approach emphasized in the unified theory. I t is shown that related phenomena including the role of K-max the existence of propagation threshold K-pr, and effects of overloads on K-pr and K-max etc, are all accounted for by the Unified Approach. (C) 1999 Published by E lsevier Science Ltd. All rights reserved.