A multiphysics dynamic model for shape memory alloy actuators

Shape Memory Alloys (SMAs) are increasingly being used in actuator technologies owing to their high reliability, advantageous specific power and fast actuation capabilities. This results from the extremely simple architecture and actuation mechanisms, which are linked to the thermally-induced shape recovery properties of SMAs. Owing to the elevated power-to-weight ratio, small diameter wires can be directly used as linear actuators, whose activation can be achieved by electric currents, exploiting the Joule effect. Fast actuation represents a design requirement in several engineering fields, ranging from aerospace to automotive applications, and it can be easily obtained by a high electric current pulse owing to the extremely small thermal inertia of SMA wires. However, dynamic effects caused by fast actuations, in terms of overloads and stroke oscillations, could represent a threat to material integrity at both structural and functional levels. For these reasons the thermal-mechanical response of the SMA wires under both static and dynamic conditions must be taken into account in designing SMA-based actuators, especially when dealing with fast actuation applications. Unfortunately, SMAs exhibit a very complex constitutive response, with intricated electro-thermal-mechanical coupling mechanisms, whose modeling represents one of the main challenges within the technical and scientific communities. Within this context, an effective multiphysics simulation model was developed combing basic underlying equations for electric, thermal, and mechanical problems. The model was implemented in MatLab/SimuLink environment and solved numerically. Experiments were also carried out, by using an ad-hoc developed testing rig, to capture the dynamic response of SMA wires during fast actuation experiments. The accuracy of the model was validated by systematic comparisons with experimental results, that is under different applied mechanical stresses and electric actuation conditions.