Cyclic Properties of Superelastic Shape Memory Alloy Wires and Bars

Abstract

This study evaluates the properties of superelastic Ni.Ti shape memory alloys under cyclic loading to assess their potential for applications in seismic resistant design and retrofit. Shape memory alloy wire and bars are tested to evaluate the effect of bar size and loading history on the strength, equivalent damping, and recentering properties of the shape memory alloys in superelastic form. The bars are tested under both quasistatic and dynamic loading. The results show that nearly ideal superelastic properties can be obtained in both wire and bar form of the superelastic Ni.Ti shape memory alloys. However, the wire form of the shape memory alloys show higher strength and damping properties compared with the bars. The recentering capabilities (based on residual strains) are not affected by section size. Overall, the damping potential of shape memory alloys in superelastic form is low for both wire and bars, typically less than 7% equivalent viscous damping. Cyclical strains greater than 6% lead to degradation in the damping and recentering properties of the shape memory alloys. Strain rate effects are evaluated by subjecting the shape memory alloys to loading rates representative of typical seismic loading. The results show that increased loading rates lead to decreases in the equivalent damping, but have negligible effects on the recentering properties of the shape memory alloys.