Effect of Training Procedures on the Functional Fatigue of Shape Memory Alloy Wires

Thermal shape memory alloys (SMAs) made of nickel–titanium enable compact and lightweight actuator systems with high power density and noiseless activation. Especially in wire form, they offer an efficient alternative to conventional actuators that are driven by electric current. Commercially available SMA wires are typically pre-trained by the manufacturer for stable operation up to 200 MPa. When these stress limits are exceeded, functional stability and lifetime decrease significantly. The main causes are increasing residual strains due to irreversible plastic deformation and material fatigue resulting from progressive microcrack formation during repeated phase transformations.

This work investigates the influence of two different training approaches—mechanical and thermomechanical—on the lifetime and functional behavior of SMA wires under cyclic electro-thermo-mechanical loading. The objective is to achieve a deeper understanding of how training affects fatigue behavior and to provide practical guidance for selecting suitable training methods in actuator design. The experiments are performed using test benches that enable controlled electrical activation, mechanical loading, and long-term cyclic testing under defined boundary conditions.

Preliminary findings indicate that both training procedures can stabilize material behavior above 200 MPa, although their effects depend strongly on the applied control strategy and mechanical constraints. The study shows that no single training method is universally optimal. Instead, the appropriate approach should be chosen according to the specific application requirements, balancing lifetime, achievable stroke, and operational stability to ensure reliable actuator performance.