Study of the Load-Response Effect of NiTi-Based Thermal Actuators. Application to a Robotic Gripper

The paper summarizes the main characteristic features of martensitic transformation, firstly in plain carbon steels and then in NiTi shape memory alloys (SMAs), from the point of view of crystallographic mechanism of martensitic transformation and functional properties. The structure of an experimental setup is described, which is meant to test NiTi SMA wires in the force-tensile deformation-temperature space, during heating–cooling under constant applied force. The specimens have been soft in martensitic state at room temperature and were elongated under the effect of the different applied loads of 2, 5, 10, 15, 20, 22, 27 and 29 N. During heating, they developed work generating shape memory effect (SME), by shrinking while lifting the applied loads. In none of the eight experiments the heating temperature did not exceed 60 °C. Based on the tensile displacement–temperature records, graphs were plotted to emphasize the specimens ‘shrinkage during heating (when they lifted the loads by work-generating SME) and their reversible expansion during cooling, under applied load. By means of the tangent method, the critical temperatures for the start of martensite reversion to austenite (As) were determined. Using the slopes of tensile-displacement-variations during heating the SME-generated shrinkage rates (Rh) were calculated. The effect of the load response on both As and Rh was discussed. Finally, a novel self-centering device that could be used as a robotic gripper was designed, actuated by a NiTi wire developing stretching-SME by resistive heating and meant to grip light loads.