Effects of anisotropy on material hardening and burst in the bulge test

The hydraulic bulge test provides a means for testing sheet metal under a nearly equibiaxial stress state. Failure is delayed, allowing measurement of the material response at significantly larger strains than in the traditional uniaxial test. This study uses experiment and analysis to develop a methodology for incorporating anisotropy in the extraction of the material stress–strain response from a bulge test. A custom six-inch bulge testing facility is used to test aluminum alloy discs to failure. The curvature and strains at the apex of the bulge are monitored via stereo digital image correlation (DIC). Anisotropy is modeled via the 18-parameter non-quadratic yield function of Barlat et al. (2005), which is calibrated through independent tests on specimens from the same sheet as the bulge test specimens. The extraction of the material response uses the measured deformation at the apex and a flow rule based on the calibrated yield function. An equibiaxial state of stress or strain at the apex is not assumed. The extracted material response and the anisotropic yield function are subsequently used to simulate numerically the bulge test using solid elements. The results illustrate the effect of anisotropy on the extracted material stress–strain response and on the onset of localization that precedes failure.