Crack Closure and the Fatigue‐crack Propagation Threshold as a Function of Load Ratio

The phenomenon of crack closure, which involves the premature closing of fatigue cracks during the unloading portion of a fatigue cycle resulting in the development of crack‐tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many aspects of the behaviour of fatigue cracks in metallic materials; these include effects of load ratio, variable‐amplitude loading, crack size, microstructure, environment and the magnitude of the fatigue threshold. Recently, however, the significance of crack closure has been questioned and alternative suggestions made for many of these phenomena, e.g. the effect of the load ratio (i.e. the ratio R of the minimum to maximum loads) on threshold behaviour. In the light of this, the present paper provides evidence to rebut the assertion that crack closure is an insignificant process. Particular attention is given to the effect of crack closure on the threshold level as a function of load ratio.