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.