New perspective on slip band decohesion as unifying fracture event during fatigue crack growth in both small and long cracks

Fractographic observations obtained from the aluminium alloy 2024-T351 form in the basis for ii qualitative model of fatigue crack growth which seeks to provide a unified understanding of crack initiation, small crack growth, and long crack growth from threshold to well advanced stage II growth. During this range of crack growth, it is deduced that plastic deformation at the crack tip can be localised by a three-dimensional array of intense slip bands. Decohesion at these slip band interfaces is viewed as being the underlying fracture event that leads to crack growth. This concept explains the observed crack path, is consistent with several basic features of fatigue clack growth, and links crack growth with an accepted model for crack initiation. It identifies a unit increment of crack growth and provides a source of damage for the process zone ahead of the crack tip. The concept is not material dependent, but does depend upon the concentration of slip into persistent slip bands. An effect of environmental on crack growth is expected to follow from its effect on slip band decohesion. MST/4069.