Analysis of the Size Effects on the Pseudoelastic Behavior of Shape Memory Alloy Micro-pillars

Size dependent properties of Shape Memory Alloys (SMAs) in micro and nano scales have gained an increasing attention due to the existing and potential applications of SMAs in microelectromechanical systems (MEMS) and small scale biomedical devices. Such applications exploit the pseudoelastic and shape memory properties the SMAs. In order to enhance the applicability of SMA micro and nano structures, the size dependency of the thermo-elastic behavior of SMAs should be understood. In this study, the dependency of the pseudoelastic behavior of Nickel-Titanium (NiTi) micro-pillars on their diameter was analyzed. Isothermal compression experiments from literature of bulk and micro-pillars were analyzed to determine the critical transformation stresses for different pillar diameters. The analysis of experimental data shows that the critical transformation stresses increase as the micro-pillar average diameter decreases. The relations between the critical transformation stresses and the average pillar diameter were represented using power functions. It was assumed that the elastic modulus and Poisson’s ratios of the austenite and martensite phases, the transformation strain parameters, and the stress influence coefficients were unaffected by the micro-pillar size. Parametric studies were performed using the finite element analysis to find the effects of the taper angle and the aspect ratio on the micro-pillars behavior. Comparisons of the results found from finite element simulations and experiments show that the model accurately predicts the pseudoelastic response of bulk and micro-pillars. The results of the parametric studies show that the hysteresis of the compression response decreases as the taper angle increases. The effect of the micro-pillar diameter on the compression response is less significant for micro-pillars of higher aspect ratios and higher taper angles