Effect of pre-training on low-cycle fatigue of NiTi pseudoelastic shape memory alloy wires

This paper investigates the impact of mechanical pre-training on the structural fatigue of NiTi pseudoelastic shape memory alloy (SMA) wires. Pseudoelastic NiTi wires were subjected to various training processes, incorporating different stress amplitudes, frequencies, and numbers of cycles. Subsequently, stress- and strain-controlled fatigue tests were conducted. A theoretical model was then proposed to elucidate the mechanism of the training effect, based on the energy stored in transformation-induced plasticity (TRIP). The local plasticity density was evaluated using an electrical resistivity-based approach. The results reveal several key findings: (i) the stored energy of TRIP from over-stressed training improves fatigue resistance by mitigating concentrated local stress; (ii) macroscopic plastic strain exhibits independence from local plasticity, suggesting its limited relevance to the training effect; (iii) the local plasticity density, measured by electrical resistivity, serves as an indicator of low-cycle fatigue life in trained NiTi SMAs. Based on these insights, a fatigue prediction model for trained NiTi SMAs was established and experimentally validated. Through an appropriate training process, the structural fatigue lifetime of NiTi SMAs can be extended up to tenfold.