Experimental investigation and modeling of low cycle fatigue behaviour of Ni-Ti shape memory alloy (SMA) rebar

Evaluating the low-cycle fatigue (LCF) response of Ni-Ti shape memory alloy (SMA) bars is essential for their reliable use in seismic-resistant structural components subjected to large strain reversals. Experimental investigations are conducted in this study to understand the LCF performance of Ni–Ti SMA bars with aspect ratios (L/D) of 5, 7, and 10 under cyclic tension–compression loading at strain amplitudes of 4% and 5%. The strain recovery capacity, degradation mechanisms, and fatigue life of the bars are examined to clarify their response under repeated inelastic deformation. A numerical model of a bridge pier equipped with Ni–Ti SMA reinforcement in the plastic hinge region is also developed in OpenSees to assess structural behavior following rebar fatigue failure. SMA fatigue degradation and the resulting post-failure column response are simulated using LCF model developed in a companion study. Results show that Ni–Ti SMA bars demonstrate favorable LCF characteristics, though reductions in strength and stiffness occur under reversed cyclic loading. LCF performance is strongly influenced by bar slenderness, with higher aspect ratios showing increased vulnerability to buckling-induced degradation. While the LCF performance of Ni-Ti SMA bars is promising, results highlight the need for careful optimization of rebar geometry and loading conditions to maximize their potential in enhancing structural resilience.