Hysteretic behavior of NiTi and NiTiNb SMA wires under recovery or pre-stressing stress

Shape memory effect of shape memory alloy (SMA) wires generates recovery stress and provides external confining pressure on concrete. Previous studies showed the effectiveness of SMA wire jackets to increase peak strength and failure strain of concrete through experimental tests. However, the behavior of SMA wires under recovery stress was not investigated seriously. Thus, this study plans a series of tensile tests for NiTi and NiTiNb martensitic SMA wires under recovery stress with varying pre-strain. This study also assesses the behavior of SMA wires under pre-stressing. Remaining residual stress is reduced with increasing additional loading and unloading strain subsequently. In general, initial and effective stiffness of the hysteretic curves are stable with increasing strain, however, unloading stiffness decreases with increasing strain. Damping ratio also decrease generally with increasing strain. When the SMA wires are heated up to the transformation temperature under pre-stress, the stress of the wires increases due to transformation of the state. Also, the stress decreases with decreasing temperature of the wires down to room temperature. The stress of the NiTiNb wires with heating is higher than the pre-stress though that of the NiTi SMA wires is less than the pre-stress. Thus, the developed stress under pre-stress seems to depend on the composition and temperature windows of SMAs.