Some critical aspects of low rate fatigue crack propagation in metallic materials

This paper proposes an overview of some critical aspects of fatigue crack propagation in metallic materials mainly concentrated on the near-threshold domain. The role of predominant influencing parameters including intrinsic parameters as alloy composition and microstructure, or extrinsic factors as loading condition, crack geometry, crack closure, atmosphere environment is particularly discussed. The first section of the paper is devoted to basic mechanisms of fatigue crack propagation after correction for crack closure, with a modeling framework including the intrinsic propagation stages in high vacuum considered as a reference inert environment, and the environmentally-assisted crack propagation regimes identified in gaseous atmosphere. The second section of the paper is dedicated to application of this basic framework to analyze three critical examples of crack propagation including small cracks, propagation at low temperature and ultra-slow crack propagation at ultrasonic frequency.