True stress-strain identification accounting for anisotropy of sheet metals

Abstract

Sheet metals for the automotive industry are subjected to continuous research efforts aiming at ever increasing mechanical performance. A remarkable feature of modern high strength sheet metals is their anisotropy, intrinsic in the technologic process of their production. When the effect of anisotropy on the mechanical response of a material cannot be neglected, specimens along different directions are usually tested, possibly under different stress states, to assess the flow curves and the deforming ratios for each direction and each loading mode. Such data are then used to calibrate many possible plastic anisotropy models available in the literature. In this work, the experimental procedures for determining the stress-strain curve and the anisotropic straining ratio are studied in detail, referring to representative tensile tests along the rolling direction of two anisotropic sheet metals, respectively PHS-1800 steel and 6181 aluminium alloy. Both alloys are ductile and exhibit remarkably long post-necking phases in tension, revealing that, in such cases, the standard procedures for the experimental derivation of the hardening curves and of the anisotropic strain ratios are limited to the very early phases of the material life and miss to cover the major part of the strain range up to failure. Different alternative procedures for the derivation of experimental data and for their postprocessing are considered and compared to each other, identifying a set of guidelines for achieving a good engineering accuracy up to failure in deriving both the stress-strain curves and the anisotropic strains ratios. The above analyses are made on the results of tensile tests at static, intermediate and high strain rate, confirming the generality of the identified procedure.