Rate-Dependent Hole-Expansion Experiments on Plastically Anisotropic Sheets

The hole-expansion test with a flat-headed punch is a unique way to explore a material's plastic anisotropy because all the stress states found in the first quadrant of the yield surface can be observed from a single test. Traditionally, hole-expansion testing is conducted at quasi-static rates, and the effect of strain rate on the anisotropic behavior of the sheets is not considered. This study conducts hole-expansion tests on thin anisotropic sheets of aluminum 6061-T6 and 304 stainless steel at various punch velocities, including dynamic loading from a drop tower. Anisotropic material behavior is observed in all tests for both materials. In the AA6061-T6 anisotropic thinning and fracture occur at 0 degrees, 75 degrees, or 105 degrees from the rolling direction; in the SS304, these occur at 45 degrees and 135 degrees. Clear rate dependence is observed in the punch force displacement; however, the orientational dependence (anisotropy) for these materials is not rate dependent, although their magnitude might be. Slight misalignments in the setup are observed, which lead to further investigation leveraging full-field DIC techniques and finite element modeling to decouple material anisotropy and experimental boundary conditions. This work provides novel methods for exploring the constitutive behavior of sheets incorporating anisotropy, rate dependence and multi-axial loading in a single test.