Material modeling of AZ31 Mg sheet considering variation of r-values and asymmetry of the yield locus

A commercial AZ31 magnesium alloy sheet was tested under uniaxial tension/compression loading and proportional biaxial tensile loading using cruciform specimens. Plastic work contours and directions of the plastic strain rates were determined over a range of equivalent plastic strain, 0.001≤ε0p≤0.008, to quantitatively determine the elastic–plastic deformation behavior. The shape of the work contours changed with plastic strain and the r-values increased with plastic strain. The measured work contours and directions of plastic strain rates were compared with those predicted using selected yield functions in the first quadrant of the stress space. A recently proposed yield function, CPB2006 [O. Cazacu, B. Plunkett, F. Barlat, Int. J. Plast. 22 (2006) 1171–1194] was also evaluated to account for the asymmetry of the yield locus with consideration of the uniaxial compression test data. The standard deviation between the measured and calculated data for the work contours and the directions of plastic strain rates were calculated, allowing the validity of these yield functions to be evaluated. The CPB2006 yield criterion with an exponent of 3 provided a fairly accurate description of the anisotropy and asymmetry of the work contours for the AZ31 sheet.