Measurement of phase transformation properties under moderate impact tensile loading in a NiTi alloy

Abstract

Pseudoelasticity is one of the main characteristics of shape memory alloys (SMAs), allowing them to recover their initial state after undergoing large deformation. This is due to the martensitic transformation (MT) occurring in the material, turning the austenitic phase into a stress-induced martensitic phase when a mechanical load is experienced. By conducting tests in quasi-static and dynamic ranges, studies have reported the strain rate dependence of the macroscopic behavior of SMAs. This paper investigates the influence of the strain rate applied to a NiTi SMA at the level of the MT, providing experimental data of the heterogeneous strain field occurring during quasi-static and dynamic tensile loading. Experiments were conducted at three different levels of prescribed velocity: 0.01mm/s, 1mm/s and 1000mm/s, using a classical loading machine for the quasi-static cases and a Split Hopkinson Tensile Bar (SHTB) for the dynamic cases. The observations of the heterogeneous strain field during the tests were made using a digital image correlation (DIC) technique which was validated by infrared thermography (IRT) measurements, considering that MT is exothermic. The analysis of the tests shows that the velocity of growth of the transformed zone is related to the applied velocity by a constant factor. Moreover, quantitative results about the strain level in the heterogeneous strain field are analysed. This study gives complementary results to the studies already made on MT in the quasi-static case, extending the observation by 103 to 105 times regarding the applied displacement rate.