Deformation of high strength alloys at high strain rates

In this study, the plastic deformation behavior of RHA (Rolled Homogenous Alloy) steel, Aluminum 5083 and Tungsten A 90S were investigated at high strain rates. These three metallic materials are used in armor plate and military applications. The aim of this work is to evaluate dynamic response of the selected armor materials used by Department of National Defense (DND) to mechanical loading at high strain rates and to evaluate the modes of failure in these materials.;The influence of strain rate and impact momentum on shear strain localization was in the materials of interest were investigated. The test specimen samples were tested at different firing pressure ranging from 180-400 kPa for steel samples, 80-200 kPa for aluminum alloy and 200-340 kPa for tungsten samples. This range of firing pressure of the gun produces impact momentum that varies between 40 kg.m/s and 60 kg.m/s for the hardened steel projectile weighing about 1.905 kg.;Microscopic evaluation of the test samples that were cut from RHA steel after high velocity impact shows formation of both white etching bands and deformed bands depending on the impact momentum. At impact momentum lower than 44.53 kg.m/s, no shear band was observed. Deformed bands were observed in samples impacted at 46.6 kg.m/s while formation of white etching bands occurred in samples impacted at samples impacted at momentums above 47 kg.m/s. The hardness in the shear bands region is much higher that in the bulk material. It was also found that the width and the hardness of the white etching bands that are formed in the steel specimens are influenced by the applied impact momentum. The width of the shear bands becomes wider and the hardness of the shear bands increases as the impact momentum increases.;Deformed bands were observed in both tungsten and aluminum alloys. As the impact momentum increases for RHA Steel and tungsten alloy, the ultimate stress increases until it reaches the impact momentum that causes the formation of adiabatic shear bands. The flow stress then decreases with further increase in impact momentum. In aluminum alloy, as the impact momentum increases, there is no significant changes in maximum flow stress, and the plastic deformation is dominated by adiabatic heating and occurrence of shear band.;The investigated materials were subjected to plastic deformation in compression at high strain rates using direct impact Hopkinson Pressure Bar (HPB). This testing procedure involves striking cylindrical specimens of the materials with a projectile at high impact momentum. The firing pressure of the projectile was varied to produce different impact momentums. The strain rate produced in the test materials on impact is a function of the impact momentum. Dynamic stress strain curves showing deformation stages were generated. The impacted samples were subjected to microscopic evaluation to determine microstructural evolution in the materials during deformation. The objective is to determine failure and deformation mechanisms under extreme loading conditions as in the case of ballistic impact. These investigations show that thermo-mechanical instabilities leading to strain localization and occurrence of Adiabatic Shear Bands (ASBs) dominate the deformation and failure mechanism of these materials at high strain rates and high strain. ASBs are regions of extreme strain localization and are usually harder than the bulk material. They provide preferred crack initiation sites at high strain rates.