Localized deformation of NiTi wires loaded in tension

Localized deformation of NiTi wires loaded in tension

Lukáš Kadeřávek¹, Petr Šittner¹, Orsolya Molnárová¹ and Luděk Heller¹

Deformation/transformation mechanisms in NiTi shape memory alloy wire loaded in tension can be investigated either in isothermal tensile test or via isostress thermal loading. If we exclude R-phase, there are basically four deformation/transformation processes (martensitic transformation, martensite reorientation, plastic deformation of martensite and plastic deformation of austenite) which can be investigated in both isothermal and isostress loads [1].

A complication in case of NiTi is that the deformation/transformation processes activated during thermomechanical loading tests tend to proceed in macroscopically localized manner. Macroscopic strain localization takes the form of either necking or propagation of Luders band fronts. Although the latter is very well known for stress induced martensitic transformation in isothermal tensile tests, macroscopic strain localization frequently occurs also in martensite reorientation [1] or plastic deformation of martensite [2]. Moreover, strain localized in macroscopic Luders band fronts was observed also in wires subjected to isostress heating or cooling.

Recently, we employed in-situ digital image correlation method to track the macroscopic strain localization phenomena in various isothermal, isostress and general thermomechanical experiments on thin nanocrystalline NiTi wires with controlled austenitic microstructures. The results of these experiments will be presented from the point of view of the strain localization phenomenon. The role of austenitic microstructure, temperature, stress in strain localization phenomena will be discussed.

Keywords: Shape Memory Alloys, NiTi, DIC, tensile test, strain localization