Near-threshold fatigue crack propagation in an ECAP-processed ultrafine-grained aluminium alloy

Abstract

In the present work, the near-threshold fatigue crack propagation (FCP) at different load ratios is studied for an aluminium alloy processed by equal-channel angular pressing (ECAP). The conditions under investigation represent different stages of microstructural refinement as well as a ductility-optimized condition with superior crack growth properties, obtained by a combination of ECAP and aging. The results show a strong dependency of the threshold and its load ratio sensitivity on the grain size and grain size distribution. These observations can be rationalized on the basis of crack path tortuosity and the contribution of (roughness-induced) crack closure. Moreover, the experimental data is evaluated using the two-parametric concept of Vasudevan and Sadananda, which employs two necessary minimum conditions for crack growth, namely a critical cyclic ΔK* th, and a critical maximum stress intensity K* max. The application of this concept shows a strong interaction of both parameters for all ECAP-processed conditions, where the ductility-optimized condition reveals superior FCP properties compared to the "as-processed" conditions.