Stress dependent electrical resistivity of orthodontic wire from the shape memory alloy NiTi

The aim of this study was to compare and evaluate the stress induced martensitic phase under different loading conditions in orthodontic wire from the Shape Memory Alloy Nickel–Titanium (NiTi). For this purpose we investigated the phase transformation from austenite to martensitic due to the different loading conditions by measuring the electrical resistance which could show the type of deformation which occurs at the beginning of phase transformations. In this framework we developed two special devices for measurements of electrical resistance in different types of load and their combination on the orthodontic wire. These results were compared with the analytically calculated stresses in the orthodontic wire. It was shown that they caused complex or multi-axial stress state phase transformation rather than other more simple load such as uniaxial loading. Finally, the article presents the deformation which occurs at the change of phase that is nearly connected to the useful superelasticity effect of the Shape Memory Alloy NiTi.