Fatigue fracture toughness and crack propagation rate

Abstract

Fatigue crack growth rate, da/dN, of two high strength steels were examined in a laboratory air at different stress ratios, covering almost the entire range of stress intensity, AK, from nearly threshold value, AKth, to final fracture. The fatigue fracture toughness, AKfc, corresponding to the final fracture in fatigue, was also determined. The lower the AKfc, the higher da/dN and reduced AKth are revealed.This correlation was analyzed quantitatively based on the four parameter Weibull function. And the stress ratio dependency of the fatigue crack propagation curve can be cleared in a successful manner.The fatigue characteristic stress intensities, Ke and Kv, are proposed to define the transition behaviour in fatigue crack growth curve, from so called region 1 to 2, and from region 2 to 3, respectively. Especially the Kv value can be specified to be the 0.63Kfc.