Near-threshold fatigue: a review

Interest in extending the high cycle fatigue life of structures has increas ed in the last decade of the 20th century. Since a long fatigue life depend s on keeping cyclic loads near or below the fatigue threshold, understandin g near-threshold behavior is now all the more important. This paper reviews the status of the 'threshold of fatigue' and to a lesser extent its relati on to the 'fatigue limit'. The fatigue threshold for crack propagation is n ot a single number. And, when it appears expressed in crack propagation equ ations as Delta K-th, it is usually involved with crack closure. To explore what it is, the published information on near-threshold fatigue crack grow th is reviewed here with emphasis on current trends. Linear elastic fractur e mechanics (LEFM) is the formalism used, but effects such as overload are difficult to explain using a purely elastic model. A single threshold stres s intensity range is inadequate to explain the effects of the stress ratio. Concepts of crack closure, shielding and closure-induced shielding have be en introduced, leading to the emergence of dual-parameter threshold models. Interpretation of these models has revived debate about the physical signi ficance of crack closure because contributions to fatigue crack growth have been observed from portions of the loading cycle taking place while the cr ack is closed. In seeking alternatives to closure, fatigue mechanisms based on dislocation generation, residual stresses and microstructural stress si ngularities have recently been elucidated to supplement work advancing the understanding of what is termed partial closure. (C) 1999 Published by Else vier Science Ltd. All rights reserved.