INCIPIENT SPALLATION FRACTURE IN LIGHT METALS FROM 3D X-RAY TOMOGRAPHY, 2D MICROSCOPY, AND MOLECULAR DYNAMICS SIMULATIONS

摘要

The dynamic fracture of ductile metals is known to occur through the nucleation and growth of microscopic voids. As the voids grow, the surrounding metal is plastically deformed to accommodate the change in void volume. In order to gain better insight into void growth, gas gun recovery experiments were used to study incipient spallation fracture in light metals (Al, Cu, V). In addition to in-situ free surface velocity wave-profiles, the recovered samples were first analyzed using 3D X-ray tomography and then sectioned for 2D microscopy. The void size and spatial distribution were determined directly from the X-ray tomography. The single crystal samples show a bimodal distribution of small voids with large (50 -100 micron) well separated voids. The plastically damaged region surrounding the large voids is quantified using optical and electron backscattering microscopy. Microhardness measurements indicate this region to be harder than the surrounding metal. Concurrently, a molecular dynamics model of void nucleation and growth at high strain-rate was developed. The model is consistent with experimental observations, e.g. voids nucleate at the weakest points in the metal such as inclusions and grain boundary junctions. The nature of void growth in a single crystal is sensitive to the crystal structure with initially spherical voids in FCC metals growing into octahedral shapes as observed in experiment. Details of the dislocation mechanisms of void growth will be presented.