DEVELOPMENT OF A SHAPE MEMORY ALLOY - ACTIVATED CLAMPING DEVICE FOR SPLIT HOPKINSON TENSION BARS

The testing of high performance materials at high strain rates is a subject of great interest in many technological areas. The tensile Hopkinson bar (SHTB) has a promising architecture because allows very high strains and failure at strain rates between 102 and 105 s-1. The tensile wave in SHTBs can be induced in various ways but the more versatile is that of pre-tensioning a partial length of the input bar through an hydraulic piston and a special clamp, then suddenly releasing the clamp. So a tensile stress wave travels toward the unstressed free end of the bar and the specimen, breaking it. In this work a special clamp is developed, integrating a Shape Memory Alloy actuator which allows the breaking of a fragile pin and the rapid release of the input bar without the need of a secondary hydraulic piston which is usually adopted in these devices. The clamp developed is much lighter of standard clamping systems, and does not require the large centering and orientation efforts typical of larger frame-fixed clamps. Also, the proper adoption of conical springs into the loading chain seem to allow the fine tuning of the wave rise time, which is a critical parameter of releasing devices in SHTBs.