Martin N. Raftenberg
Description
A model for introducing the effects of adiabatic shear banding into a penetration calculation was installed into the EPIC wavecode. These effects are
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deemed to be reduction in the ratio of flow stress to the value predicted by the Johnson-Cook strength model and increase in spall pressure. A strain-rate- and temperature-dependent instability strain is determined from small-amplitude perturbation of constant-strain-rate simple shear. Imposed alterations in flow stress ratio and spall pressure commence at the "localization strain", separated from the instability strain by a fixed strain increment. The alterations proceed linearly with increasing effective plastic strain and terminate after an additional fixed strain increment, at the "failure strain." The values imposed on the flow stress ratio and the spall pressure at the failure strain are functions of local pressure at the time step when localization strain was reached. The nonzero value imposed on the flow stress ratio in the case of positive localization pressure reflects the phenomenon of fracture suppression within a fully formed shear band. The two fixed strain increments are evaluated from a torsional Kolsky bar test. The pre-shear-banded spall pressure is evaluated from plate-on-plate impact data. The flow stress ratio and spall pressure at and beyond the failure strain introduce two currently "free" parameters. The model was applied to a set of problems involving steel plate perforation by a tungsten rod, and reasonable agreement with experiment was obtainable in terms of the final target hole size and the length and speed of the tungsten residual.
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