Evolution of the plastic anisotropy with straining and its implication on formability for sheet metals

The plastic anisotropy r-value is an important material parameter in sheet metal forming. The length and width strains are measured conventionally in the uniaxial tensile test using two extensometers and the r-value is fitted within a certain strain range by linear regression according to the international standard. In this study, the physical character of the plastic anisotropy is analyzed for several forming steels and an aluminium alloy. In principle, the plastic anisotropy r-value is not a material constant. It is a fitted parameter within a given strain range and its value is dependent on the strain range chosen. More accurate approximation of the current state of anisotropy of a material is given by the incremental r-value that is defined in this study. Furthermore, this parameter can be predicted well by the VPSC code for materials in the as-received and deformed state. Contrary to previous studies elsewhere, the evolution of this incremental r-value does not correlate with anisotropic work hardening and the magnitude of the r-value. It is however closely related to texture evolution during plastic deformation. Texture evolution can have remarkable effect on the plastic anisotropy r-value, the yield locus and ultimately formability.