Measurement and Analysis of the Biaxial Loading and Unloading Behavior of AZ31 Mg Alloy Sheet

A commercial AZ31 magnesium alloy sheet has been tested under proportional biaxial tensile loading using cruciform specimens. In order to quantitatively determine the elastic‐plastic deformation behavior of the test material, the contours of plastic work and the directions of plastic strain rates were measured over a range of equivalent plastic strain, $0.001 ⩽ε0p⩽0.008.$ The measured work contours and the directions of plastic strain rates have been compared with those predicted using the von Mises, Hill’48 and the Yld2000‐2d yield functions. The validity of these yield functions is discussed from viewpoints of the yield surface shape and normality flow rule in the first quadrant of the stress space. The r‐value of AZ31 was found to increase with plastic strain. The effect of the changes in r‐value on the shape of the calculated yield locus has been discussed. Furthermore, biaxial unloading tests following biaxial loading were carried out for several stress ratios. The nonlinearity of the stress‐strain curves during uniaxial and biaxial unloading was measured and quantitatively evaluated.