Development of a shape memory alloy: Activated clamping device for split hopkinson tension bars

The testing of structural metals at high strain rates is a subject of great interest in many modern technological areas. The split Hopkinson tension bar (SHTB) allows to investigate the ductile failure at nominal strain rates between 102 and some thousands of s-1. The tensile wave in SHTBs can be generated in various ways, but the simpler one from the viewpoint of its realisation is that of pre-tensioning a partial length of the input bar with an hydraulic cylinder and an opportune clamping device, and then suddenly releasing the clamp. So, a tensile stress wave is originated which travels towards the unstressed free end of the bar and towards the output bar through the specimen, breaking it. In this work a special clamping system is developed, integrating a Shape Memory Alloy actuator which allows the breaking of a fragile pin and initiating the rapid release of the input bar, without the need of a secondary hydraulic cylinder which is usually adopted in these devices. The clamp developed is much lighter than the standard clamping systems, and does not require the large centring and orientation efforts typical of heavier frame-fixed clamps.