On the Nature of Post‐Constrained Recovery Residual Stress of NiTi Shape Memory Alloys

Abstract

This paper investigates the nature of the post‐constrained recovery residual stress (PCRRS) observed in nickel titanium (NiTi) shape memory alloys (SMAs) subjected to constrained recovery and returned to a low temperature state while remaining constrained. After performing the thermomechanical cycling to produce the PCRRS state, four experiments were performed with different deflection profiles applied from the PCRRS state, and the changes in the residual stress was observed. All tests produced a gradual decrease in stress after every load cycle, similar to cyclic softening. However, repetition of the process used to initially generate PCRRS restored the initial magnitude of the PCRRS, indicating that the decrease in residual stress was a stable, repeatable, and reversible phenomenon, and not true material degradation. To better understand the nature of PCRRS, an analysis was performed comparing the modulus relating stress and strain at different points along the thermomechanical loading path. This analysis showed NiTi has a combination of twinned and detwinned martensite in the PCRRS state. Understanding the nature of PCRRS, properties and its stability have the potential for advanced engineering applications such as self‐healing and fatigue‐resistant materials by generating stresses without continuous actuation over a long period of time.This article is protected by copyright. All rights reserved.