Recovery stress of direct tensioned or cold drawn NiTi SMA wires according to applied strain and temperature

Abstract

Shape memory alloys (SMAs) with shape memory effect are used to induce prestress or to close cracks developed in cementitious materials. For the purpose, the SMA wire should be prestrained; however, a direct tensioning method is not practical for many applications. Thus, a cold drawing method is appeared to induce prestrain in the SMA wire and it is verified that the cold-drawn SMA wires showed the shape memory effect and recovery stress. However, the relationship between direct tensioning and cold drawing is not clearly explained. Therefore, this study is planned to investigate recovery and residual stresses of shape memory alloy (SMA) wires, which are direct tensioned or prestraind by cold drawing method, with considering diameter of the SMA wire and heating temperature to induce the shape memory effect. This study also investigates the tensile behavior of the SMA wires under the recovery stress. This study uses NiTi SMA wires with diameters of 1.0, 0.7, and 0.5 mm. Each type of the wire is cold drawn considering area reduction. The prestrained SMA wires are heated with a few temperatures of 100°C, 200°C, and 300°C, which are higher than the temperature Af that is completing temperature to transform to Austenite state. Through this study, it will be clear that the recovery and residual stress in the cold drawn SMA wires and their tensile behavior under the recovery stress.