Experimental study on the influences of static strain aging, apparent size effect and heating input level on stress recovery behavior of Fe-SMA bars

In this study, six groups of Fe-SMA bars with nominal diameters of 10, 14, and 30 mm were first designed, and then heating activation tests (with activation temperatures of 100–350 °C) were conducted after strain aging times ranging from 0 to 48 h, simulating typical construction timelines. Test results demonstrate that the strain aging more significantly altered the balance between austenite transformation and thermal expansion during the heating stage compared to the cooling stage. Short-term strain aging (within 48 h) slightly improved the recovery stress with a maximum increase of about 5.0%, occurring at 30 mm-diameter Fe-SMA bars. The recovery stress of Fe-SMA bars was negatively correlated with their diameters. This diameter-dependent stress recovery behavior, i.e. an apparent size effect, was macroscopically associated with differences in yield strength, which correlated microscopically with variations in grain size governed by the Hall–Petch relationship. Furthermore, increasing the heating input level offered limited enhancement to the recovery stress, and mitigating grain coarsening during material processing presents a potential strategy to optimize the recovery stress of large-diameter Fe-SMA bars.