Deformation and failure of a rolled homogeneous armour steel under dynamic mechanical loading in compression

A Rolled Homogeneous Armour (RHA) steel was tested at strain rates in excess of 10³ s^-1 in compression using direct impact Hopkinson Bar. The dynamic stress-strain curves and microstructural evolution in this material at high strain rates were obtained. They were found to be dependent on the extent of thermal softening occurring during deformation and the accompanying visco-plastic instabilities, which lead to localization of shear strain along adiabatic shear bands (ASBs). The formation and the type of shear bands are found to depend on the impact momentum and strain rates. Depending on the loading mode, there is a strain-rate threshold below which plastic deformation is homogeneous. At slightly higher strain rate, deformation becomes localized leading to formation of deformed ASBs. As impact momentum and strain rate increase, another threshold value is reached above which strain localization become so intense that it triggers the formation of transformed ASBs. Ductile shear fracture occurs along these transformed bands.