A viewpoint on environmental contributions to fatigue crack growth

Fatigue crack growth behaviour is examined using the Unified Damage Approach developed by the authors since 1994. The approach is based on the understanding that a fatigue damage requires two loading parameters; a requirement manifesting in terms of two fracture mechanics parameters: maximum stress intensity factor K-max and a stress amplitude Delta K. K-max governs the breakage of crack tip bonds allowing for crack extension and Delta K controls the extent of cycle damage needed ahead of the crack tip. The overall changes in crack growth depend on several factors like microstructure, deformation mode and its properties and environment. The Unified Damage Approach describes these closely related changes in the damage mechanisms due to synergistic effects of mechanical and environmental driving forces. Crack growth trajectory maps developed using the Unified Damage Approach summarises these changes in mechanisms that occur with increasing crack growth rate or stress intensity.