Porosity and Spall Fracture of Shock Loaded Metals,
BALLISTIC RESEARCH LABS ABERDEEN PROVING GROUND MD
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Common to encouters between warhead and armor at extremes of strength and ductility is an interplay between plastic flow and fracture failure of the interacting materials. The optimum effectiveness of either a penetrating device or armor is dictated by controlling this interplay. A mechanism for shock fracture has been delineated covering the material reactions from the sub-microscopic to the macroscopic scale, showing how the various observed phenomena are related. A simple shock stress criterion is not sufficient to describe fracture. Combinations of stress and time are required for creation, migration and condensation of vacancies to produce pores which produce fracture nucleation cites. Vacancies are created during the compressive cycle but more are required for producing fracture. These come from the interaction of dislocations in the micro-cell walls during the tensile shock phase of the cycle. It is now possible to calculate energies required to produce failure, migration speeds of vacancies and to arrive at a quantitative description of the process based on cell-sizes, vacancy energetics and effective stress results. This is being done to provide shock fracture criteria. The average size of spall fragments should be predictable based on studies of dislocation cells in metals subjected to compressive shocks and basic data on vacancy migration in the metal.