Abstract
This work is focused on experimental study of micromechanisms of mode II and mode III fatigue cracks in metallic materials in the near-threshold regime. The resistance to fatigue crack growth can be divided to an intrinsic component (ahead of the crack tip) and an extrinsic component (shielding, closure), which is significantly higher than the intrinsic one. Fracture surfaces from the Ti6Al4V alloy and pure zirconium were observed in three dimensions. Experiments were conducted using a special device for simultaneous crack loading in modes II and III. Additionally, pure mode II and pure mode III experiments were done using CTS and torsion specimens, respectively. At the beginning of all experiments, crack closure was eliminated due to precracks generated under cyclic compressive loading. A common mechanism of local mode II advances was observed in both modes II and III. The results were similar to those of pure titanium. The hcp metals exhibit a transition behaviour between materials with coplanar shear-mode crack propagation and materials with a high tendency to deflect to the opening mode I.
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