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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 stressstrain
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.