Compression Characteristics of Thin NiTi Wires

Medical NiTi implants are frequently made by shape setting thin superelastic NiTi wires into complex structures. Since NiTi wires in implanted structures deform in a general multiaxial way, modeling is necessary to evaluate distribution of stresses and strains within the shape set structure. Thin NiTi wires compared to bulk NiTi exhibit excellent thermo-mechanical cycling stability, commonly ascribed to their unique nanosized microstructure which results from high degree of cold work homogeneously distributed throughout the wire volume followed by suitable heat treatment. At the same time, these wires are strongly textured due to the multistep forming employed to decrease the wire diameter to few tens of microns. If we want to evaluate material parameters necessary to introduce the impact of wire texture into mechanics models of superelatic NiTi wires, we need to perform not only tensile but also compression and torsion tests on them. This work complements the already known characteristics of thin NiTi wires obtained by tensile and combined tension-torsion test by gathering data from unique compression loading experiments on thin wire samples. Cylindrical shaped micro samples (100 µm  thin and 150 µm  long) were fabricated from heat treated superelastic NiTi wire. Compression tests are carried out using a purposely developed apparatus at temperature and special attention is paid to the alignment of the loading axis and precise strain measurement. The obtained data are used to parametrize mechanics model used to simulate medical devices made from thin NiTi wires.