Thermoelastic studies on Nitinol stents

In this paper a methodology for applying Thermoelastic Stress Analysis to Nitinol self-expanding stents is described. A test rig has been designed that allows the stents to be loaded under a pseudo physiological loading of internal pressure. High resolution thermoelastic data obtained from a stent is presented and features in the data are identified where high signal and potentially high stresses exist. The data is 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 further understand 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 is characterised throughout its nonlinear elastic range. The effect of the mean stress on the signal is identified experimentally and compared with theoretical calculations, and it is proposed that if testing is conducted at elevated temperature (>37oC) 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