In-plane anisotropy of strain-rate sensitivity in a mild steel sheet

Plastic flow of sheet metals is typically anisotropic because of their deformation history and the resulting texture, yet anisotropy of strain-rate effects, embodied in the parameter strain-rate sensitivity, has hardly been studied before. In the present study, we perform uniaxial tensile tests in three loading directions (in rolling direction, 45° to the rolling direction, and in transverse direction) to systematically investigate the effect of different strain rates (from 10-3 s-1 to 3 s-1) on the mechanical behavior of a mild steel sheet. From the resulting true stress-true strain data, we determine strain-rate sensitivity values as a function of loading direction. We use Barlat’s Yld2000-2D yield stress function to determine yield loci that are characteristic for the three different applied strain-rates, and to model strain-rate sensitivity as a continuous function of the in-plane loading direction. Based on symmetry considerations, we further propose a simple model based on a Fourier series approach. Our experimental results as well as distinct differences in the yield loci associated with different strain rates clearly demonstrate that strain-rate sensitivity exhibits an in-plane anisotropy. We discuss this key finding in the light of generic continuum mechanics modeling approaches, highlighting that explicitly modeling an anisotropy of strain-rate effects may further improve the accuracy of forming simulations.