Properties of large-diameter shape memory alloys under cyclical loading

This study evaluates the properties of superelastic shape memory alloys under cyclical loading to asses their potential for applications in seismic resistant design and retrofit of civil engineering structures. Shape memory alloy bars are tested to evaluate the effect of bar size (diameter) and loading history on the strength, equivalent viscous damping, and recentering properties of the shape memory alloys in superelastic form. The bars are tested under both quasi-static and dynamic loading. The results show nearly ideal superelastic properties can be obtained in large diameter shape memory alloy bars. However, comparing these results to previous studies, the more common wire form of the shape memory alloys show higher strength and damping properties compared with the large bars. The recentering capabilities (based on residual strains) are not affected by the section size of the bar. Overall, the damping potential of superelastic shape memory alloys is low for large diameter bars, typically less than 7% equivalent viscous damping. Degradation of the superelastic properties of the shape memory alloys occurs for cyclical strain greater than 6%, leading to increased residual strains and reduction in energy dissipated. Finally, strain rate effects are evaluated by subjecting the shape memory alloys to loading rates representative of typical seismic loadings. The results show that increased loading rates lead to slight decreases in the equivalent damping, but have negligible effect on the recentering of the shape memory alloys.