Recovery stress behavior of Fe-SMA under fatigue and thermal loading

Abstract

The present paper investigates mechanical properties and recovery stress behavior of an iron-based shape memory alloy (Fe-SMA) with a special focus on the effect of fatigue and thermal loading. Change in recovery stress of activated Fe-SMAs subjected to fatigue at RT, −20 °C, 60 °C, and −20 to 60 °C was monitored. A second activation after fatigue was performed to investigate the retrievability of the lost recovery stress owing to fatigue. Coefficients of thermal expansion (CTEs) of the samples with different activation histories were also calculated. Results showed that the maximum reduction in recovery stress after two million cyclic loads of Δɛ=0.070% was approximately 20% under RT and −20 °C. A high temperature (60 °C) weakened such detrimental effect owing to higher critical stresses for martensitic transformation at higher temperatures, leading to only 10% reduction in recovery stress. Loss in the recovery stress due to fatigue was pronouncedly retrieved by a reactivation regardless of the temperature scenarios. This study extends understanding of recovery stress behavior of Fe-SMAs subjected to fatigue and thermal loading, and explores the retrievability of relaxed recovery stress by a second activation.