Thermoelastic studies on Nitinol stents

Abstract

In this paper a methodology for applying thermoelastic stress analysis (TSA) to superelastic, nickel-titanium (Nitinol), shape memory alloy self-expanding stents is described. A test rig has been designed that allows the stents to be loaded under a pseudophysiological loading of internal pressure. High-resolution thermoelastic data obtained from a stent are presented, and features in the data are identified where high signal and potentially high stresses exist. The data are examined in detail and it is demonstrated that the thermoelastic signal is linearly related to internal pressure but is also dependent on mean stress. To gain further understanding of the thermoelastic response of Nitinol, a series of calibration-type experiments were conducted on thin-walled Nitinol cylinders so that the thermoelastic response of Nitinol was characterized throughout its non-linear elastic range. The effect of the mean stress on the signal was identified experimentally and compared with theoretical calculations. It is suggested that, if testing is conducted at elevated temperature (>37°C), these effects can be eliminated and quantitative analysis can be achieved. Other issues such as non-adiabatic behaviour and strain rate dependence are also discussed.