Some Considerations of the Influence of Sub-Critical Cleavage Growth during Fatigue-Crack Propagation in Steels

Abstract

The results are interpreted in terms of a 3-stage relationship between log growth-rate and log δK, and show that, for intermediate and high growth-rates, effects of mean stress are caused directly by the occurrence of ‘static’ modes. The final acceleration in rate near instability is considered to be associated with the critical linkage of such modes. During this stage it is inappropriate to relate the crack growth-rate to the alternating stress intensity, δK, because the maximum value of stress intensity, Kmax, is critical in determining how much of the total crack front is composed of transgranular cleavage facets and hence how fast the crack is growing. Discussion is also included on the difference between the terminal stress intensity during fatigue and the K _Ic value (fracture toughness) under monotonic loading. A possible explanation for such differences is proposed in terms of plasticity-induced heating at the crack tip.