Pre-training effects on buckling-restrained NiTi SMA behavior under tension-compression cycles

Shape memory alloys (SMAs) have been applied in seismic engineering due to their superior self-centering and remarkable energy dissipation capacity. However, research on the mechanical performance of SMA bars under cyclic tension-compression loading in confined concrete conditions remains limited, particularly with regard to the influence of pre-training. To evaluate the effects of different pre-training strain amplitudes, loading protocols, and the presence or absence of pre-training treatment on the key mechanical behaviors of SMA bars, five NiTi alloy bars with a dog-bone shape were designed in this study. The mechanical behaviors including martensitic transformation starting stress, secant stiffness, residual strain, and energy dissipation capacity were analyzed and compared. A comparative analysis of the microstructural morphology of SMA bars before and after pre-training treatment was conducted using scanning electron microscopy (SEM). The experimental results demonstrated that SMA bars exhibited characteristic flag-shaped hysteretic behavior with distinct tension-compression asymmetry and excellent self-centering capacity under cyclic tension-compression loading. Various pre-training treatments moderately enhanced the martensitic transformation stress by 28.5–39.3% and secant stiffness by 14.5–40.7% at 6% compressive strain. Pre-training had a limited effect on reducing residual strain, and when the strain exceeded 2%, the non-pre-trained specimen exhibited better energy dissipation. Consequently, for SMA bars equipped with buckling-restraint device (BRD) under cyclic tension-compression loading, pre-training may not be essential.