Influence of Stress on the Transition Behaviour of NiTi Shape Memory Alloys for Actuator Applications

Shape memory alloys (SMA) stand out due to the ability that they can be deformed and then return to their initial shape by heating. This process results in a high actuation energy density. All SMA manufacturing methods, regardless of which matrix material is used, result in heat input into the SMA, which could induce a phase transformation and therefore disable the actuator function. In this regard, options to increase the transition temperatures of NiTi alloys above the processing temperatures are fundamental. In this study, the influence of the pre-tension on the transformation temperatures of SMA wires was investigated as a way to prevent phase transformation due to heat impact during the production of SMA-polymer-actuators. Applying a pre-tension of 400 MPa to the NiTi wire, the austenite start temperature could be increased by a factor of 1.9 whereas it could be increased by a factor of 2.2 with a pre-tension of 550 MPa. Therefore, after preloading the wires, a phase transformation should not be induced when the wire contacts the polymer droplets. However, the shift of the phase transformation has to be further investigated.