Effect of high strain rate on plastic deformation of a low alloy steel subjected to ballistic impact

Some results of an experimental study on high strain rate deformation of a low alloy steel are discussed in this paper. Cylindrical specimens of the steel in quench-hardened and tempered condition were subjected to high strain rate deformation by direct impact using a modified Split Hopkinson Pressure Bar (SHPB). The deformation process is dominated by stress collapse due to thermal softening effect of adiabatic heating in some narrow bands within the material under deformation. The time for the onset of the stress collapse leading to strain localization was found to increase with strain rate. The critical strain at which adiabatic shearing occurs depends on the strain rate and microstructure. Steel samples that were tempered at 425°C exhibits greater tendency to adiabatic shearing than those tempered at 315°C. The time and critical strain for the onset of stress collapse and adiabatic shearing are lower. Whereas single white adiabatic shear bands were observed to initiate and propagate during the high strain rate deformation in samples tempered at 315°C, multiple shear bands were found to initiate and dynamically propagate in the samples tempered at 425°C. Of these multiple shear bands, the one closest to the centre of the cylindrical test pieces were white etching bands while the rest appeared as deformed bands. The higher tendency of the samples tempered at 425°C to formation of adiabatic shear bands has been attributed to increased perturbation in the steel samples as a result of the heat treatment procedure. The type of shear bands formed in the steel at high strain-rates can be explained as due to the accompanying fragmentation of martensite laths and dissolution of carbides during adiabatic heating and shearing, which eventually determines the size of the micro-constituents of the adiabatic shear band and the resolvability by optical microscopy.