Shape memory alloys are a unique metallic alloy which can undergo large deformations while reverting back to their undeformed shape through either the application of heat (shape memory effect) or the removal of the load (superelastic effect). The ability to recover their shape provides recentering, which can limit residual drifts, while the flag-shape hysteresis provides supplemental damping to control the response of the structure (figure 1). In order to explore the use of shape memory alloys for seismic applications, superelastic NiTi shape memory alloys are focused on as they are the most commonly used shape memory alloy in the United States.

Most past research on NiTi shape memory alloys has been in conjunction with biomedical, aerospace, and commercial applications which have focused on small wires and thin plate specimens. Typical structural engineering applications require larger diameter specimens for which the behavior has not been characterized. The experimental side of this research focuses on the characterization of the material properties of large diameter NiTi shape memory alloys under loadings and strain levels equivalent to those undergone by structural members during an earthquake. Cyclic tension and compression tests are conducted on bar specimens of varying diameters in order to determine how strain rate and strain level affect the residual strain, equivalent viscous damping, loading plateau stress, unloading plateau stress, and initial elastic modulus. Results have shown that large diameter specimens maintain good superelastic properties with residual strain values slightly above 1%. The equivalent viscous damping values tend to be lower, suggesting that superelastic NiTi shape memory alloys cannot be used in purely damping applications. Close collaboration with the material science department has taken place in order to link the optimal mechanical properties with the microstructural properties. Advanced material testing specific to the use of shape memory alloys in seismic applications has also been conducted. The experimental results are then being used to develop simplified phenomenological models which can be incorporated into structural engineering platforms such as OpenSEES. These models are used to perform nonlinear time history analyses for structures which implement innovative bracing systems consisting of superelastic shape memory alloy segments. Preliminary results have shown a significant reduction in the residual drifts and maximum inter-story drifts.


Selected Publications and Presentations

DesRoches, R., McCormick, J., and Delemont, M.A. (2004) "Cyclic Properties of Superelastic Shape Memory Alloys," ASCE Journal of Structural Engineering , January, 2004.