From the thermal and kinematical full-field measurements to the analysis of deformation mechanisms of NiTi SMAs

Abstract
Intensive experimental investigations have been carried out to understand deformation mechanisms associated with the superelasticity of polycrystalline nickel-titanium shape memory alloys. This paper analyses results of superelastic tensile tests on thin plates and thin walled tubes of NiTi. During the deformation, thermal and kinematical full-field measurements were simultaneously carried out using a visible and an infrared camera. Heat source fields were estimated from the thermal measurement during both homogeneous and localized stages of deformation of these tests. Local heat energy values were then obtained by integrating heat sources over time for any given location on the sample. This provides in-situ local calorimetric measurements similar to a virtual local differential scanning calorimeter all along the specimens. Deformation mechanisms can be analyzed from the simultaneous knowledge of heat source-strain rate fields or of heat energy-strain fields. It is shown that forward and reverse transformation deformation mechanisms are involved in all homogeneous stages usually considered as elastic deformation of austenitic and of martensitic phases. The kinematic and morphology of deformation localization during deformation stages over the stress plateaus are also investigated. It is shown that forward (loading) and reverse (unloading) transformations are not complete inside the localized bands.