1. The Origins of the Hopkinson Bar Technique

Although Bertram Hopkinson had a research interest in the dynamic strengths of materials, the bar technique he published in 1914 was developed to roughly determine the shapes of stress pulses produced by bullet impact and explosions. His pressure bar proved useful to the British explosives and armour industry during the Great War allowing them to improve their munitions. It also allowed the phenomenon of spall of metal plates produced by ballistic impact or by explosions to be understood. Another greater war had to occur before Hopkinson’s pressure bar was applied by Taylor, Davies and Volterra in the UK to the measurement of the dynamic compressive properties of explosives and polymers. Unlike after the First World War, why this research was thought important to the British armed forces during the Second World War was not explained in the declassified reports published once the war was over. Soon after the end of the Second World War, Kolsky made a number of improvements to the double Hopkinson bar system including using detonators to substantially increase the stresses that could be applied to specimens. This made possible for the first time the accurate determination of the dynamic compressive stress-strain curves of metals. Detonators are, however, impractical for all but defence labs to obtain and use. So an alternative method of loading was needed, and this was developed by Krafft and co-workers at the Naval Research Laboratory in Washington DC in the early 1950s. Their design used a gun to launch a projectile rod against one end of the double bar system and this American method eventually became standard in all laboratories that use the split Hopkinson pressure bar technique. However, it was not until the end of the 1970s that split Hopkinson bars began to be built in laboratories in an ever increasing number of countries around the world.