Mechanisms of Fatigue and Environmentally Assisted Fatigue

Fatigue cracks often initiate either by formation and development of coarse slip steps or by extensive deformation in localized soft bands producing intrusions and extrusions. Microstructural changes induced by fatigue, for example, formation of fine subgrain structures within soft bands, are probably necessary for intrusion, extrusion, and for subsequent Stage I crack growth. Stage II crack growth involves plastic-blunting/alternate-slip processes at crack tips. Increments of crack growth per cycle, for a given effective crack-opening displacement, depend mainly on the distribution of slip around crack tips during loading; this distribution is determined by both the microstructure and the environment. The effects of aggressive environments on crack growth can be explained, in many cases, on the basis that chemisorption of environmental species influences interatomic bonds and thereby facilitates nucleation of dislocations at crack tips. Striations ('ductile' and 'brittle' types) on fracture surfaces are produced because unloading deforms part of the fracture surface produced during loading.