Recovery and Residual Stress of Shape Memory Alloy Wires and Its Application

Two kinds of shape memory alloy wires, NiTiNb and NiTi, are manufactured and pre-elongated during the manufacturing process. After fixing the pre-elongated wires, heating on the wires induces recovery stress on them. Several tests to measure the recovery stress are conducted with varying initial strains. Stress-reduction of the recovery stress is observed after the temperature of the wires is cold down; the remaining stress is called residual stress that is also measured. Also, this study measures the stress-strain curves of the SMA wires under the state that the residual stress is working. The tension behavior of the wires under residual stress is very unique. The NiTi is martensitic state at room temperature, thus a residual deformation remains after applying a large strain. However, under residual stress, the tensile behavior of the wire does not remain residual deformation and return to the original position by unloading; this looks like viscoelastic behavior. The residual stress of the SMA wires can be used as active confinement for civil structures and the behavior under residual stress can be used as like an elastic spring with hysteretic damping. This study confines concrete cylinders and reinforced concrete columns using the pre-elongated SMA wires to introduce an active confinement on them. The SMA wire jackets on concrete cylinders increase the peak strength and the ductility so much comparing to the plain concrete cylinders. Also, the wires on reinforced concrete columns increase the ductility so much without the reduction of flexural strength.