Methodology for determining spall damage mode preference in shocked FCC polycrystalline metals from 3D X-Ray tomography data

Three-dimensional X-ray tomography (XRT) provides a non-destructive technique to determine the location, size, and shape of spall damage within shock loaded metals. Polycrystalline copper samples of varying thermomechanical histories were shocked via plate impacts at low pressures to ensure incipient spall conditions. Additionally, samples of similar heat-treated microstructures were impacted at various loading rates. All 3D XRT volumetric void data underwent smoothing, thresholding, and volumetric sieves. The full inertia tensor was found for each void, which was used to create best fit ellipsoids correlating shape to damage modes. Density distributions were plotted for the best-fit ellipsoid semi-axes aspect ratios alc and blc, where, a≤b≤c. It was found that >60% of voids in heat-treated samples resembled transgranular damage, whereas >70% of voids in the rolled sample resembled intergranular damage. Preliminary analysis also clearly indicates an increase of void coalescence with decreasing tensile loading stress rates for impacted samples of similar microstructures.