Fatigue crack initiation and growth behavior of Ti–6Al–4V under non-proportional multiaxial loading

This paper deals with multiaxial low cycle fatigue crack behavior of Ti–6Al–4V under non-proportional loading. Strain controlled fatigue tests under proportional loading and non-proportional loading with 90° out-of-phase difference between the axial and shear strains ε and γ were carried out on tubular specimens at room temperature. As a result, Mises strain based fatigue lives under non-proportional loading were approximately 1/10 of those under proportional loading. The specimen surfaces were observed to evaluate life reduction. These results showed that non-proportional loading caused 10 times more cracks than those of proportional loading. Non-proportional loading changes directions of the principal stress and strain axes, and high shear stresses acting on many planes stimulate more slip planes. Consequently, non-proportional loading resulted in many cracks. In addition, the high shear stresses also resulted in larger misorientation under non-proportional loading. The crack initiation life defined as the length 2a of 30 μm did not differ significantly between proportional loading and non-proportional loading. However, the fatigue cracks under non-proportional loading propagated faster those under proportional loading. This is because the fatigue cracks under non-proportional loading are subjected to more severe strain field than those under proportional loading, even though the crack size and applied strain are the same. Thus, the significant reduction in fatigue life under non-proportional loading is caused by the accelerated crack growth due to a higher strain intensity factor.