Why Shear Cracks Grow

Fracture mechanics are widely used to predict the fatigue crack growth of flawed structures. Conventionally tensile crack growth is assumed to be dominant and Mode I fatigue limits are adopted as a primer design criteria. However, the paradigm of focusing primarily on Mode I crack growth does not fit perfectly all situations: in many practical problems material is subjected to a complex pattern multi-axial loading when shear loading can trig cracks initiation at stress intensities much lower than conventional threshold ones, show higher sensitivity to a inclusion size, or even accelerate integral crack growth leading to an unexpected failures. This work presents extensive, State-of-the-Art characterization for fatigue behavior in steel, along with sound experimental procedures for these characterizations. It explores near threshold fatigue crack propagation and fatigue strength under pure Mode III and Multi-axial loading. The work discusses mechanisms that govern fatigue propagation in the near-threshold zone and describes a novel technique for estimating the effects of micro flaws on fatigue strength under mixed-mode out-of-phase loading.