Tests on superelastic Ni-Ti SMA bars under cyclic tension and direct-shear: towards practical recentring connections

Shape memory alloys (SMAs) are smart metals featuring either superelastic effect or shape memory effect. While current emphasis on the civil engineering application of SMA is mainly given to dampers and isolators, recent research has been directed to superelastic SMA bolts or tendons for recentring connections. However, available information on the mechanical shear response of SMA bars is still inadequate. Consequently, this knowledge barrier can cause significant uncertainties when such components are intended to resist shear action. In this respect, this paper presents an experimental study on SMA bars subject to cyclic direct-shear actions, and uniaxial tensile tests were also undertaken on the bars of similar sizes for comparison. Key results including stress-strain/force-displacement hysteretic response, recentring ability, and energy dissipation are discussed in detail. The results show sound recentring and moderate energy dissipation/damping capabilities of the bars under cyclic tension below a strain level of 3 %, but beyond this the properties start to degrade. The SMA bars with a smaller diameter seem to perform better than those with a larger diameter. On the other hand, the cyclic direct-shear performance of SMA bars is generally unsatisfactory with poor recentring, energy dissipation, and damping characteristics. Therefore, the direct use of SMA bars subject to shear loading is undesirable. For both tension and shear tests, fracture of the bars within the threaded area is the most typical failure mode, which should be minimised in future applications to ensure a ductile response of seismic resistance devices. Recognising both advantages and potential shortcomings of the SMA bars, an innovative recentring connection type is proposed, and the preliminarily findings indicate that such connections are feasible.