Methods for measuring material parameters of hardened steel core for failure models

The complex material behavior of hardened steel projectile cores in impact scenarios places high demands on constitutive modeling. To measure material parameters for failure models, miniaturized round tensile, flat tensile, compression and compression-torsion specimens were produced and quasi-static characterization tests were carried out on a material testing system machine. The specimens were made directly from the projectile cores of 14.5 mm caliber AP ammunition, which severely limited the maximum feasible specimen sizes. By using specimens with different geometries and notch radii, failure of the specimens was initiated at different multiaxial stress states. The compression-torsion specimen was used to generate different combinations of stress triaxialities and Lode angles at failure without changing the geometry by setting different ratios of torsional to compressive loads, including combined compression-torsion, pure compression and pure torsion loads. It was shown that the simulated failure locations of the compression-torsion specimen were highly dependent on the ratio of compression and torsion. A set of plastic failure strains as a function of stress triaxiality and the Lode angle was derived specifically for 14.5 mm high-hardness steel cores, which are characterized by a particularly high hardness and relatively strong brittleness, marking a novel contribution to the field. The global and local strains that occurred until failure were measured by means of digital image correlation. The specific stress states were then determined by accompanying numerical simulations using the commercial hydrocode Autodyn. The experimentally observed variances of the failure strains were used to evaluate the uncertainties of the stress states. Further, a concept for the simulation-based determination of stress states was presented in cases where the location of failure cannot be determined in the experiments. Ultimately, it was demonstrated that the measurement results can be used to improve the simulated fragmentation of a B-32 projectile core in a ballistic scenario.