"Anisotropic behaviour of thin walled medical tubes in Nickel-Titanium superelastic shape memory alloys"

 

Place : Salle Mazaré (RDC) – Bâtiment Boucherle – La Tronche

 

Thesis supervisor:

 

Anisotropic behaviour of thin walled medical tubes in Nickel-Titanium superelastic shape memory alloys

 

 

Thin walled tubes of Nickel-Titanium shape memory alloys (NiTi SMA) are widely used in the fabrication of self-expandable stents. The operation of stents relies on the superelastic effect (SE), as many other applications of NiTi SMA in the biomedical field. The SE is a reversible crystallographic phenomenon that gives SMA the ability to recover large strains through simple unload. Due to the crystallographic nature of the SE, thermomechanical properties related to this effect are expected to be affected by the inherent anisotropy of the tube, which emerges from its fabrication process. However, NiTi is still often treated as isotropic in the design and optimization of such devices. One of the difficulties preventing the use of anisotropic models is a lack of mechanical characterizations about the NiTi tube anisotropy. The present work aims to perform such characterization for a thin walled NiTi superelastic tube. In an experimental campaign, the tensile behaviour of the tube is analysed at different orientations and temperatures. Digital Image Correlation (DIC) technique is used to monitor the strain distribution during tensile tests. Results show that all the analysed properties related with SE are anisotropic. All the orientation dependencies are nearly symmetrical with respect to 45° from the tube axis. Some properties are also dependent on temperature, a dependence that is also anisotropic. A thermodynamic approach based on the Gibbs free energy is used to analyse these orientation and temperature dependencies. With this analysis it was possible to relate the SE stress hysteresis and thermodynamic irreversible energy contributions. Finally, the influence of anisotropy on the strain distribution of tensile samples is verified. Focus is given to the analysis of the strain localization phenomenon throughout loading and unloading. The inclination of the localization front band is characterized and evaluated with a plasticity approach. The front angle observed with DIC is predicted using strain rate data calculated from stress-strain curves.