On The Martensitic Transformation In SMA Under High Strain Rates

Shape memory alloys (SMA) have specific properties that are mainly due to the martensitic transformation occurring in the material when mechanical or thermal loadings are applied. To study the effect of strain rate on the transformation occurring on an NiTi SMA different tests were performed at different strain rates in the range of 0,001 /s to 15 /s. For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45'000 fps used to measure the extension rate of the martensitic phase region in the specimen. For quasi-static tests, an additional infrared thermography measurement was used. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and the 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 figures. 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.