Fatigue crack propagation modes: plastic deformation mode and damage accumulation mode

To define the fatigue crack propagation mode for the prediction and classification of the experimental results based on its evolution mechanism, successive mesoscopic observations of a fatigue process under pure cyclic mode II loading are performed. Thus, damage accumulation, which is considered to be a vacancy accumulation, is found to be a mechanism. The phenomena occur not at the crack tip but ahead of it. Thus, “plastic deformation mode” and “damage accumulation mode” are the terms proposed to represent the fatigue crack propagation modes instead of “tensile mode” and “shear mode,” respectively. Moreover, as a method to classify both the fatigue crack propagation modes using the experimental results, it is proposed to identify the plastic strain in the wake of the crack tip.