Crack tip driving forces and crack growth representation under fatigue

Abstract Fatigue is considered as a two load parameter problem involving two crack tip driving forces. Conventional representation of crack growth rates, d a /d N , as a function of a single driving force, Δ K , is considered as inadequate. Mean stress effects are treated normally as R -ratio effects. These effects are due to a second crack tip driving force, K max . The role of K max has been generally ignored, even though it has more dominating influence than Δ K . A unified Approach developed by the authors considers interplay of these two governing forces, Δ K and K max , as fundamental to fatigue. Superimposed environmental effects and static load effects naturally follow in this Unified Approach, since K max is the governing parameter for these.