High-temperature pressure-shear plate impact experiments on ofhc copper

Abstract

Experimental results are presented on the dynamic plastic response of OFHC copper at strain rates of 10 5 10 6 s -1 and temperatures up to 700°C. Measurements in this previously unexplored regime of high temperatures and very high strain rates are made possible by recent extensions of the pressure-shear plate impact methodology to allow testing at high temperatures. Athin foil of the specimen material is sandwiched between two pure tungsten carbide plates which have su.cient high temperature strength to remain elastic under the temperature and loading conditions of the experiment. This target assembly is heated by an induction heating coil. To overcome possible misalignment of the impact face of the target due to thermal expansion of the target supports, a laser beam reflected from the rear surface of the target is used as an optical lever to detect changes in the orientation of the target. Remote controls are used to make the necessary adjustments in the orientation of the target assembly to maintain its original alignment with the impact face of the flyer plate. To withstand the high temperatures, the photoresist gratings which normally provide the diffracted beams used in recording the transverse velocity of the target assembly are replaced by titanium phase gratings produced by SEM lithography. Over temperatures from 300 to 700°C and strain rates from 10 5 to 10 6 s -1 the flow stress of OFHC copper increases with increasing strain rate, and decreases with increasing temperature. Numerical simulations of the experiments, based on popular constitutive models, exhibit lower flow stresses than those measured in the experiments. The models also fail to predict the softening that is observed at large strains in the tests at strain rates of 10 6 s -1 . 1998 Elsevier Science Ltd. All rights reserved.