Anisotropic ductile failure in titanium alloy at quasi-static and high strain rates

High strain rate dynamic tensile tests have been conducted on a titanium alloy Ti-6Vn4 in both the longitudinal and transverse directions with respect to the axis of the bar material. For unnotched specimens a high degree of anisotropy of fracture behaviour was observed, with the mean fracture strain in the longitudinal direction being 60% greater than in the transverse direction. For notched specimens, the degree of strain fracture anisotropy was much reduced and became statistically insignificant (p > 0.05) for notch profile radii of 1 mm or smaller. For all specimen geometries and directions of loading, dynamic failure strains were much higher than the corresponding quasi-static results. The paper links these results with numerical predictions of the local stress, strain and strain rate conditions in the specimens carried out using a modified Armstrong-Zerilli constitutive model for the titanium alloy. The most important result is that the fracture anisotropy observed at low values of stress triaxiality virtually disappears at higher values of triaxial tension.