Two-parameter fatigue crack growth driving force: Successive blocking of the monotonic and cyclic plastic zones at microstructural barriers

A micromechanical description of small fatigue crack growth based on the successive blocking of both, the monotonic plastic zone (MPZ) and cyclic plastic zone (CPZ) of a crack at microstructural barriers such as grain boundaries and phase limits is presented. The model establishes two different thresholds to crack growth. The first is associated with whether or not the MPZ is able to overcome the barriers. We call this the Maximum (stress)-controlled fatigue threshold. The second threshold imposes the same condition, but for the CPZ and, therefore we call it the Range (of stress)-controlled fatigue threshold. The implications of this micromechanical model and its applicability depending on the loading conditions are discussed. It leads in a natural manner to the use of two-parameters crack driving force (ΔK and Kmax) to characterize the fatigue crack growth in the long crack regime.