Using the strip-yield mechanics to model fatigue crack growth by damage accumulation ahead of the crack tip

Elber found in the early 70s that fatigue cracks can close under tensile loads, and assumed that fatigue crack growth (FCG) would be controlled by ΔKeff=Kmax−Kop, where Kmax and Kop are the maximum and opening values of the stress intensity factor. This hypothesis can rationalize many transient effects observed under service loads, but it cannot explain many other effects like FCG retardation or arrest after overloads under high R=Kmin/Kmax, when Kmin>Kop; FCG at constant rates under highly variable ΔKeff; cracks arrested at a given R that can reinitiate to grow at a lower R without changing their ΔKeff; or the R-insensitivity of FCG in inert environments. Nevertheless, strip-yield models (SYM) based on ΔKeff ideas are more used for FCG life predictions than alternative models based on any other principles. To verify whether SYMs are indeed intrinsically better, their mechanics is used to predict FCG rates based both on Elber’s ideas and on the alternative view that FCG is instead due to damage accumulation ahead of the crack tip, which does not need the ΔKeff hypothesis or arbitrary data-fitting parameters. Despite based on conflicting principles, both models can reproduce quite well FCG data obtained under quasi-constant ΔK loading, a somewhat surprising result that deserves to be carefully analyzed.