Buckling and Postbuckling Characteristics of the Superelastic SMA Columns – Numerical Simulation

Numerical simulation has been carried out to analyze several unique buckling and postbuckling behaviors of superelastic shape memory alloy (SMA) columns observed from experimental evidences. Comparison of the experimental critical load with analytical predictions verifies the fact that physical nonlinearity should be taken into account to comprehensively analyze the buckling behaviors of SMA columns. Thus, based on the nonlinear stress–strain relations as well as the large deformation theory, the load–deformation curves of the columns during a complete loading–unloading cycle have been predicted by using the FEM code ANSYS. Simulation has also been performed for the stainless steel (SUS304) columns. The total strain distributions in the column materials have been calculated at different states of loading. Precise and quantitative analyses of the results verify that the unique behaviors of the SMA columns could be attributed to the special nature of the stress–strain curves. It is found that the slender SMA column buckles in the austenite phase. Due to SMA's high strength for large strains, the slender columns can sustain the load with a very small change in magnitude for the postbuckling compression. A special method has been devised to simulate the unique unloading path of the slender superelastic SMA columns and its usefulness has been demonstrated. It has been found that defining a few intermediate unloading curves between the actual loading and unloading curves can help to minimize the stress discontinuity at the beginning of unloading. Simulation results also verify that because of the elastic shape recovery of the buckled slender SMA columns, the load increases when they are unloaded.