Localization of plastic deformation in commercially pure titanium in a complex stress state under high-speed tension

In this work, the effect of a triaxiality stress state on the mechanical behavior and fracture of commercially pure titanium VT1-0 (Grade 2) in the range of strain rates from $0.1$ to $1000$ s$^{-1}$ is studied. Tensile tests are carried out using a servo-hydraulic testing machine Instron VHS 40 / 5020 on flat specimens with a constant cross-sectional area and on flat specimens with a notch. To study the effect of the complex stress state on the ultimate deformation before fracture, the samples with the notch of various radii ($10$, $5$, $2.5$ mm) are used in the experiments. Phantom V711 is employed for high-speed video registration of specimen's deformation. Deformation fields in a working part of the sample are investigated by the digital image correlation method. It is shown that the effect of the strain rate on the ultimate deformations before fracture has a nonmonotonic behavior. An analysis of strain fields in the working part of the samples shows that the degree of uniform deformation of the working part decreases with an increase in the strain rate. At strain rates above $1000$ s$^{-1}$, the shear bands occur at the onset of a plastic flow. Commercially pure titanium undergoes fracture due to the nucleation, growth, and coalescence of damages in the bands of localized plastic deformation oriented along the maximum shear stresses. The results confirm that the fracture of commercially pure titanium exhibits ductile behavior at strain rates varying from $0.1$ to $1000$ s$^{-1}$, at a triaxiality stress parameter in the range of $0.333 \leqslant \eta <0.467$, and at a temperature close to $295$ K.