Role of anisotropy on strain localization at high-strain rates in Ti6Al4V

A systematic experimental investigation of the plastic anisotropy at high strain-rates of Ti6Al4V was carried out using a Split Hopkinson bar system. Dynamic tensile tests were conducted on flat specimens cut at seven orientations with respect to the rolling direction of the sheet. To quantify the effect of anisotropy on both the global response prior to necking as well as on the inclination of the strain localization bands that developed in the neck, both surface strains and local displacements were measured using high speed imaging in conjunction with digital image correlation. It was found that the material has a weak anisotropy in uniaxial yield stresses while it displays a strong anisotropy in Lankford coefficients; the inclinations of the bands in the neck depend on the loading orientation. Moreover, only for the rolling direction specimen, the neck contains two localization bands equally inclined to the loading axis. These observations confirm the correlations established in the theoretical study of Cazacu and Rodríguez-Martínez (2019) between the necking band angles and the variation in the uniaxial flow stresses with the loading direction. A constitutive model with yielding described with the orthotropic criterion of Cazacu and Barlat (2001) and a hardening law accounting for the strain rate effects was used to model the observed behavior. Comparison between finite element simulations, analytical calculations, and test data show that the experimental trends are captured.