On the use of the Hopkinson bar for the determination of the transient dynamic behaviour of pure iron

The split Hopkinson pressure bar device is used to study the dynamic response of pure cold-drawn and annealing irons to a sudden decrease of strain rate. Different strain rate history effects are pointed up for the both cold-drawn and annealing states. Applying different unloads to several dynamic microstructural states, allows to access the internal stresses and to characterize the main deformation mechanisms. These mechanisms are firstly thermally- activated, then they change into viscous-drag mechanisms when the internal stress saturates and the strain rate become high enough. (orig.)