Modeling of Strength Differential Effects in HCP Sheet Materials

A simple approach is proposed to employ symmetric yield functions for modeling the tension-compression asymmetry commonly observed in hcp materials. In this work, the strength differential (SD) effect is modeled by choosing separate symmetric plane stress Barlat Yld 2000 yield functions for the tension i.e., in the first quadrant of principal stress space, and compression i.e., third quadrant of principal stress space. In the second and fourth quadrants, the yield locus is constructed by adopting Bézier interpolating functions between uniaxial tensile and compressive stress states. The main advantage of this proposed approach is that the yield locus parameters are deterministic and relatively easy to identify when compared to the Cazacu-Plunkett-Barlat (CPB) family of yield functions commonly used for modeling the SD effect observed in hcp materials. The proposed yield function is implemented as a user material subroutine (UMAT) within the commercial finite element software, LS-DYNA. The predictions of the developed material model are compared with the measured load-displacement and strain distributions from a three-point bend experiment on AZ31B sheet.