If a shape memory element is mechanically deformed in the martensitic state, a high elongation (up to approximately 8%) occurs in the form of a plateau when a critical stress is exceeded. If the temperature is subsequently increased to such an extent that a phase transformation of martensite into austenite occurs, the reshaping of the shape memory element takes place. This process is hysteretic and reversible . In this way, the shape memory wire shown in Figure 1 (below) can be switched back and forth between states 1 and 2 under load by electrically heating it up. During the material conversion, a significant change in the electrical resistance can be detected.
An FG wire actuator usually consists of a NiTi alloy and can generate maximum tensile stresses of 400 MPa in continuous operation, 800 MPa for one-off operations . For example, a 1g dead weight FG wire can move loads of 5000g. An electromagnet of the same power class would weigh more than 200g.
Because of the above properties, for example, the realization of very lightweight and compact haptic elements is possible.