An overview of the mechanics of fracture and fatigue in the presence of residual stress

Abstract

Many engineering components contain sharp crack-like defects, or they may develop such defects early in their lifetime: welded components and those subjected to fretting or aggressive chemical or thermal environments are obvious examples. When components containing such defects are subjected to cyclic (fatigue) loading the cracks may extend during each loading cycle until they reach a critical length, causing failure of the component. The presence of residual stresses may dramatically alter the fatigue lifetime. In this paper the basic concepts of Linear Elastic Failure Mechanics, fatigue crack growth and residual stress are reviewed in order to demonstrate that the amalgamation of these concepts is logical and consistent. The importance of both geometry and loading in the determination of stress intensity factor ( K) is indicated, as is the validity of the super-position technique as applied to cracks in residual-stress fields. In the general area of fatigue-crack growth, it is noted that the correction of K to account for residual stresses is important for the determination of both stress intensity range and R (the ratio of the minimum to the maximum value of K during a loading cycle). Such information may be employed in predicting the form of standard S— N curves for welded details containing residual stress. The requirement for a test specimen in the investigation of fatigue-crack growth is noted, and suggestions made for a point loaded, ring-type standard test specimen.