A Mechanism for Fatigue Crack Initiation in Pure Metal

In this paper, a model for fatigue crack initiation is proposed, based on the experimental observation that there is always a net irreversible slip in one direction of the active slip in a metallic material. Such irreversible slip leads to a local shear stress which encourages the dislocations with a Burgers vector of opposite sign to slip out of the surface minimize the net irreversible slip effect. Continuous slipping out of the surface by dislocations from two adjacent positive and negative sources could lead to crack nucleation on the surface. This net irreversible slip model appears to be also able to explain other observed fatigue surface damage such as formation of macrobands, extrusions and intrusions in Al, Cu, Cu-Al alloy and Mg single crystals. It may form the basis to build up a quantitative model for fatigue crack initiation in metallic materials.