Evolution of the martensitic transformation in shape memory alloys under high strain rates

Abstract

The speci c properties of the shape memory alloys are mainly due to the martensitic transformation occuring in the material when mechanical or thermal loadings are applied. Here, the e ect of strain rate on the transformation on an NiTi SMA is studied in tension. Di erent tests were performed at di erent strain rates in the range of 0,0001 /s to 15 /s. Two distinct methods were used to measure the extension rate of the martensitic phase region in the specimen: digital image correlation technique and infrared thermography (IR during quasi-static tensile tests only). For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45'000 fps. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and results show that the temperature peak, as expected, follows the transformation front. As a consequence of the former validation of the DIC procedure, the velocity of the transformation front at high strain rate is deduced from space-time gures. As a conclusion, in the range of strain rates investigated in this paper, no strain rate sensitivity is observed for dynamics of extension of the transformation region.