The development and use of a torsional Hopkinson-bar apparatus

A technique is described by means of which torsional waves of large, essentially constant amplitude can be generated in an elastic bar. Waves with rise times of order 25 μs and maximum angular velocities of order 10³ rad.s^?1 have been achieved and used to test tubular specimens at shear-strain rates up to 15×10³ s^?1. Results are presented for mild steel tested at 2×10³ s^?1, and it is shown that the flow stress correlates well with the trend found at lower rates using conventional methods. The measured drop of stress at yield, however, was considerably smaller in the present tests than in earlier work; this is attributed to the generation of flexural waves which reach the specimen at the same time as the torsional wave.