Subcritical depinning describes interface propagation as an Arrhenius activation process, where the energy barrier diverges as a power law with decreasing driving force. This framework has been successfully applied in magnets and to fracture under creep loading but its naive extension to fatigue fails: in real materials, fatigue crack growth exhibits paradoxical behavior: the crack velocity is driven by the driving force (stress intensity factor) range so that the velocity increases when the minimum driving decreases. This is driven by processes occurring within the fracture process zone ahead of the crack front. To investigate this, we perform creep and fatigue experiments. By incorporating the history of the crack by an additional lengthscale into the observed crack front position, we reproduce the Paris--Erdogan-like fatigue behavior using subcritical depinning. We extend this approach to variable loading conditions and show how the approach can be used to explain the fatigue behavior in metals, the usual case.

Our results show in fracture the fundamental importance of the crack overcoming local barriers by activated avalanches. The crack tip local history may be encompassed in a lengthscale, that allows to apply a statistical fracture description to the time-dependent crack growth rate. We discuss the universal and material -dependent features of the developed crack growth theory.