Pressure-shear impact and the dynamic viscoplastic response of metals

Pressure-shear plate impact experiments are used to investigate the viscoplastic response of metals at shear strain rates ranging from 10⁵ s^?1 to 10⁷ s^?1. Flat specimens with thicknesses between 300 μm and 3 μm are sandwiched between two hard, parallel plates that are inclined relative to their direction of approach. Nominal stresses and strains in the specimens are determined from elastic wave profiles monitored at the rear surface of one of the hard plates. Results are reviewed for two fcc metals: commercially pure aluminum and an aluminum alloy. New results are presented for bcc high purity iron, a high strength steel alloy and vapor deposited aluminum. For commercially pure aluminum the flow stress increases strongly with strain rate as strain rate increases from 10⁴ s^?1 to 10⁵ s^?1. At strain rates above 10⁵ s^?1 the flow stress, based on results for thin vapor-deposited aluminum specimens, increases strongly, but less than linearly, with increasing strain rate until it saturates at strain rates between 10⁶ s^?1 and 10⁷ s^?1. Preliminary results for high purity alpha-iron indicate that the flow stress increases smoothly over eleven decades of strain rate, and faster than logarithmically for strain rates from 10² s^?1 to greater than 10⁶ s^?1. In contrast, for a high strength steel alloy the flow stress depends only weakly on the strain rate, even at strain rates at high as 10⁵ s^?1. Such contrasting behavior is attributed to differences in the relative importance of viscous glide and thermal activation as rate controlling mechanisms for dislocation motion in the various metals. Numerical studies indicate that experiments performed at the highest strain rates on the thinnest specimens are not adiabatic, thus requiring a full thermal-mechanical analysis in order to interpret the data.