Correlations Between Spall Damage Mode Preference and Microstructure in Shocked Polycrystalline Copper: A 3-D X-Ray Tomography Study

Nondestructive characterization techniques are essential for maximizing the amount of high fidelity data obtained from post mortem mechanically tested materials, particularly for the complex experimental techniques used for shock loading. Polycrystalline copper samples of varying thermomechanical histories were shock loaded to induce incipient spall damage conditions via plate impacts at low pressures. Electron backscattering diffraction (EBSD) and 3-D X-ray tomography (XRT) were utilized to obtain microstructural and void volume data, respectively. Voids were fit to ellipsoids using their inertia tensors obtained from the 3-D XRT data, and the results compared to EBSD data. This, in turn, enabled establishing quantitative correlations between void shapes and preferential damage mode, either inter- or trans-granular, for each thermomechanical history. Greater than 60 % of voids present in heat-treated samples were spherical, indicative of transgranular damage preference. Greater than 90 % of voids present in as-received rolled samples were flat sheet-like voids or unsymmetrical oblate ellipsoids, indicative of intergranular and coalesced damage modes. Analyzing grain boundary damage statistics in conjunction with global damage mode distributions indicate that boundaries become less dominant as preferential void nucleation sites within the spall plane as the bulk strength of the material decreases from heat treatments.