RISE-TIME MEASUREMENTS OF SHOCK TRANSITIONS IN ALUMINUM, COPPER, AND STEEL

Time-resolved measurements of shock-wave rise times have been accomplished for aluminum, copper, and steel to stress levels of 41, 96, and 139 GPa, respectively, using velocity-interferometer techniques. To within the time resolution of the technique, the shock transition is found to occur within 3 ns in all materials. Based on this upper limit for the transition time, limiting viscosity coefficients of 1000, 3000, and 4000 P are obtained for 6061-T6 aluminum, OFHC copper, and 4340 steel, respectively, at strain rates above 108 s-1. It is found that the effective viscosity can be expressed as parameters in a Maxwellian relation for an elastic-plastic solid, in which the viscosity is related to an effective relaxation time. It is also shown that viscosity is inversely proportional to mobile-dislocation density, which implies that the density of mobile dislocations obtained during shock compression in these materials is well over 109/cm2.