Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states, three kinds of representative triaxial stress states have been adopted, namely smooth tensile, notch tensile, and pure shear. The results of the study indicate the following. During the notch tensile test, a relatively higher stress triaxiality appears in the root of the notch. With the applied loading increasing, the volume fraction of microvoids in the root of the notch increases continuously. When it reaches the critical volume fraction of microvoids, the specimen fractures. During the pure shear test, the stress triaxiality almost equals to zero, and there is almost no microvoids but a shear band at the center of the butterfly specimen. The shear band results from nonuniform deformation constantly under the shear stress. With stress concentration, cracks are produced within the shear band and are later coalesced. When the equivalent plastic strain reaches the critical value (equivalent plastic fracture strain), the butterfly specimen fractures. During the smooth tensile test, the stress triaxiality in the gauge of the specimen remains constant at 0.33. Thus, the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids. The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test, respectively. The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well. In addition, the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.