Crack tip driving forces and crack growth representation under fatigue

Fatigue is considered as a two load parameter problem involving two crack tip driving forces. Conventional representation of crack growth rates, da/dN, as a function of a single driving force, ΔK, 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, Kmax. The role of Kmax has been generally ignored, even though it has more dominating influence than ΔK. A unified Approach developed by the authors considers interplay of these two governing forces, ΔK and Kmax, as fundamental to fatigue. Superimposed environmental effects and static load effects naturally follow in this Unified Approach, since Kmax is the governing parameter for these.