Near-threshold fatigue: a review

Interest in extending the high cycle fatigue life of structures has increased in the last decade of the 20th century. Since a long fatigue life depends on keeping cyclic loads near or below the fatigue threshold, understanding 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 relation to the 'fatigue limit'. The fatigue threshold for crack propagation is not a single number. And, when it appears expressed in crack propagation equations as Delta K-th, it is usually involved with crack closure. To explore what it is, the published information on near-threshold fatigue crack growth is reviewed here with emphasis on current trends. Linear elastic fracture mechanics (LEFM) is the formalism used, but effects such as overload are difficult to explain using a purely elastic model. A single threshold stress intensity range is inadequate to explain the effects of the stress ratio. Concepts of crack closure, shielding and closure-induced shielding have been introduced, leading to the emergence of dual-parameter threshold models. Interpretation of these models has revived debate about the physical significance of crack closure because contributions to fatigue crack growth have been observed from portions of the loading cycle taking place while the crack is closed. In seeking alternatives to closure, fatigue mechanisms based on dislocation generation, residual stresses and microstructural stress singularities have recently been elucidated to supplement work advancing the understanding of what is termed partial closure. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.