Influence of Surface Etching on the Endurance Limit of TiNiCu Shape Memory Alloy Spring Actuators

Energy harvesting from natural heat sources has emerged as an alternative to fossil fuels. Heat engines driven by shape memory alloys (SMAs) are one of the approaches used for low-temperature geothermal energy conversion. However, the endurance of SMA actuators under cyclic actuation is still a significant concern. This study investigated the influence of surface etching on the endurance limit of the cyclic deformation and phase transformation of TiNiCu SMA spring actuators under elevated-temperature conditions. TiNiCu SMA springs with and without surface etching were tested under cyclic loading at different extension ratios in an 80°C water bath. The results showed that fatigue life decreased with increasing extension ratio for both conditions. The SMA spring actuators exhibited stable cyclic behavior for more than 10⁶ cycles without fracture at lower extension ratios. It was found that non-etched springs exhibited higher endurance limit than etched springs. DSC results showed changes in transformation peaks in etched samples after fatigue testing, suggesting increased variation in phase transformation after cyclic loading. The results indicated that etching process has influenced the fatigue properties of TiNiCu SMA actuators due to the change in the endurance limit. This work contributes to the understanding of cyclic deformation behavior and mechanical reliability of TiNiCu SMA actuators for heat-engine applications.