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 resu lting in the development of crack-tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many as pects 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 threshol d. Recently, however, the significance of crack closure has been quest ioned 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 ma ximum 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.