Novel two-stage superelastic SMA bars with enhanced ductility and graded pseudo-yielding for seismic applications

This study proposed a novel design concept of two-stage superelastic shape memory alloy (SMA) elements to achieve graded pseudo-yielding and enhanced ductility that are suitable for multi-level seismic design. This design concept utilizes the natural stiffness hardening of superelastic SMA after martensite finish stress to activate two transformation stress plateaus (i.e., graded pseudo-yielding points) of two-stage SMA elements. Consequently, the small- and large-diameter parts can be activated under small and rare earthquakes, respectively. This study discussed the design concept, experimental tests, and numerical simulations of two-stage SMA bars with two different diameters. The cyclic behaviour and failure modes of the two-stage SMA bar observed in the tests successfully validated the expected merits. The ductility of the tested two-stage SMA bar increased by 50% compared with that of a normal one-stage SMA bar. Meanwhile, the self-centring (SC) capacity was not compromised when parts of the bar were in the martensite phase during the graded pseudo-yielding. The two-stage SMA bar showed desirable cyclic performance during the whole loading process. Finally, a finite element model was established based on the testing results to facilitate parametric and comparative studies. The two-stage SMA bar showed obvious graded pseudo-yielding behaviour, larger ductility, and greater post-pseudo-yielding stiffness, which can potentially benefit seismic applications.