The Effects of Training on the Actuator and Sensing Characteristics of NiTi Actuator Wires

Commercially available SMA actuator wires are usually preconditioned by the manufacturer for a stable behavior at up to 200 MPa. For many applications it is of interest to increase the actuation stress to gain a higher force output, less installation space or higher transformation temperatures. This work covers the experimental investigation of two fundamental methods for an additional training of SMA wires, with the goal to reach a stable actuator behavior at high loads and also improved sensor properties. The combination of actuation and sensing functionality in a single system opens up new possible fields of application and enables possible self-sensing control strategies without additional sensors.

Therefore, a custom designed multifunctional test rig is used to perform the training as well as the electro-thermo-mechanical characterization of the SMA wire specimen. Focusing on the parameters relevant for applications, the actuator stroke output and the electrical resistance of Joule heated wires are examined. The experiments are performed with varying loads up to 400 MPa on a NiTi wire with 72 µm of diameter. The results outline the influence of mechanical and thermal training on the actuator stroke and its stability under different loads as well as at increased ambient temperatures of up to 100 °C. Simultaneously, the effects of training on general shape and hysteresis of the resistance vs. stroke diagram are investigated and discussed.