Experimental Investigation on Buckling and Post-buckling Behavior of Superelastic Shape Memory Alloy Bars

This paper examines the buckling and post-buckling behavior of superelastic shape memory alloy (SMA) bars. A NiTi SMA bar with a diameter of 12 mm was used in all the experimental tests. First, the tensile and compression responses of NiTi bar were characterized under monotonic loading up to failure. A total of 15 specimens with slenderness ratios that range from 25 to 115 were tested to study the critical buckling load and post-buckling behavior of SMA bars. Digital image correlation (DIC) system was implemented to monitor full-field surface displacements. The interaction between material nonlinearity due to phase transformation and geometric nonlinearity was explored. Data obtained from the DIC measurement system were further analyzed to identify the onset of buckling and to extract experimental critical buckling loads. The effect of loading rate on buckling response of SMAs was investigated by conducting additional testing at higher loading rates on the specimens with three selected slenderness ratios. The temperature field on the surface of the specimens was recorded by an infrared camera. The analytical critical buckling loads were computed and compared with experimental results. All specimens exhibited a unique buckling behavior characterized with almost a complete shape recovery upon unloading.