Ductile fracture of 2024 aluminum under unequal biaxial in-plane tension and out-of-plane compression

The ductile fracture of 2024 aluminum loaded under unequal in-plane biaxial tension and out-of-plane compression is investigated. This stress state is induced in materials during ballistic impacts. A novel experimental design is presented which utilizes an elliptical shaped punch to induce unequal amounts of tension along the in-plane axes of a thin specimen plate. An annealed copper backing plate placed behind the specimen induces out-of-plane compression. Fracture is observed directly with Digital Image Correlation during continuous loading of unbacked specimens. Interrupted testing and acoustic emissions are used to determine the instant of fracture in tests where a backing plate is used. Numerical simulations are performed using an assumed plasticity model, and the model is validated by matching the simulated force-displacement response and strain with the experiments. The equivalent plastic fracture strain is determined for various stress states that correspond to averaged triaxiality values ranging from 0.14 to 0.61 and averaged Lode parameter values ranging from -0.72 to -0.29.