Influences of stress states and loading directions on the anisotropic fracture of magnesium alloy AZ31B sheet under tension-dominated forming conditions

The crystallographic textures of metal sheet induced by the rolling process cause its anisotropic fracture behavior via plasticity anisotropy. This research aimed to characterize the anisotropic fracture behavior of magnesium alloy AZ31B sheet during the conventional tension-dominated forming conditions. Four specimens with different shapes were designed to cover diverse stress states, and were respectively tension-tested to fracture along the rolling direction (RD), diagonal direction (DD), and transverse direction (TD) of rolled sheet. Almost all specimens failed in the shear fracture mode with slight necking localization. The distinct differences among load response, strain distribution as well fracture strain for three directions revealed the severe anisotropic fracture characteristic. To characterize the fracture anisotropy, the isotropic modified Mohr-Coulomb (MMC) fracture criterion was revised into an anisotropic one by considering the effect of loading direction. The updated unified fracture model together with the Yld2000-3d anisotropic yield function had better performances in describing load responses of tension tests for scaled modified compact-tension (SMCT) specimens under three loading directions, especially blindly predicting their crack locations. Three SMCT specimens all failed due to the tension-dominated stress state with the stress triaxiality higher than 1/3. As a comparison, the isotropic MMC model separately calibrated by tests along the RD, DD, and TD can only predict the fracture behavior of SMCT specimen in the corresponding loading direction, but it failed to judge the fracture features of other two directions.