Near-threshold fatigue crack propagation: A perspective on the role of crack closure

In recent years, mechanistic and continuum studies on fatigue crack propagation, particularly at near-threshold levels, highlighted a dominant role of crack closure in influencing growth rate behavior. Various sources of closure induced by cyclic plasticity, corrosion deposits, irregular fracture morphologies, viscous fluids and metallurgical phase transformation are reviewed and modeled. It is shown that many of the commonly observed effects of mechanical factors, such as load ratio, microstructural factors, such as strength and grain size, and certain environmental conditions can be traced to the extrinsic influence of closure in modifying the effective driving force for crack extension. The implacations of such closure mechanisms are discussed in the light of constant and variable amplitude fatigue behavior, the existence of a threshold stress intensity for no fatigue crack growth and the validity of such threshold concepts for the case of short fatigue cracks.