Experimental Exploration of Post Constrained Recovery Mechanics of NiTi

Abstract

Shape memory alloys (SMAs) have tremendous potential use as actuators in mechanical systems due to their high specific energy density. Large recovery stresses can be generated when Nickel Titanium (NiTi), the most widely used SMA, undergoes constrained recovery where it is held in a deformed geometry and heated from a detwinned martensite phase to austenite phase. Recent experimental results have found that residual stresses can also be generated in NiTi after returning to a low temperature geometrically constrained state. This paper presents experimental results performed on NiTi wire samples where wire was: 1) deformed from a low temperature twinned martensite state to produce a strain that would be recoverable in an unloaded state 2) held at that strain state and heated above the austenite finish transition temperature and then cooled back below the martensite transition finish temperature while recording the forces generated. It was found that a residual load was produced in the low temperature state. Results from further testing beyond this point showed repeatability with application of small and large strains. Post constrained recovery stresses have the potential to be used to generate residual stresses in structures in a low energy, un-actuated state with a remaining potential for thermal actuation.

Explorations of Post Constrained Recovery Residual Stress of Shape Memory Alloys in Self-healing Applications

“…Afterward, the same experiments were reiterated with small strain of 0.5% loading without thermal cycling as PCRSS can be gained continually for every cycle of small strain application. These two experiments and subsequent loading procedures were presented thoroughly in Haider et al Haider, Correa, Moghadam, Yan, & Salowitz, 2018;Haider, Yazdi, Rezaee, Tsai, & Salowitz, 2019). Furthermore, a circumstantial analysis was reported on magnitudes and trends of the recurring PCRRS due to both large and small strain cyclic applications.…”

“…Recent experimental investigation has revealed a phenomenon wherein the material can be actuated to a high temperature state, constrained from returning to its original geometry, and upon ceasing actuation and returning to a low temperature, the material will continue to exert a force on the constraint. This property has been named post constrained recovery residual stresses (PCRRS) Haider, Yazdi, Rezaee, Tsai, & Salowitz, 2019;Haider, Correa, Moghadam, Yan, & Salowitz, 2018). This capability provides a new way for the material to be used as an actuator that may be beneficial to previous applications like self-healing material and could support new applications like using a dispersion of NiTi particles to inhibit fatigue cracking in material structures.…”

“…The SME refers to the ability of a SMA to undergo deformation at one temperature, and then recover its original, undeformed shape upon heating above its reverse transformation temperature. Recovery stress appears when this shape recovery is restrained during heating Haider, Yazdi, Rezaee, Tsai, & Salowitz, 2019;Haider, Correa, Moghadam, Yan, & Salowitz, 2018). This recovery stress is used in many applications, such as actuators, SMA pipe couplings, robotic applications, and SMA composites (Kapgan & Melton, 1990;Yamauchi, Ohkata, Tsuchiya, & Miyazaki, 2011).…”

Investigation into post constrained recovery properties of nickel titanium shape memory alloys

“…Nickel titanium alloys possess unique shape memory and superelastic properties that have made them attractive as actuators in many applications leading to studies of their properties [1][2][3]. The shape memory effect has been employed in high tech applications from heart stints [4] to aircraft components [5] and recent research has employed the shape memory effect in self-healing materials [6][7][8]. NiTi SMAs can withstand 10% strain in their low temperature martensite state, undergoing a transition from a twinned to detwinned martensite structure.…”

“…Recent experimental investigation has revealed a phenomenon wherein NiTi materials can be subjected to constrained recovery and produce a residual stress upon return to their low temperature, martensite, un-actuated state [6][7][8][12][13][14]. The ability to generate residual stress that persists without continued application of heat energy could lead to new capabilities in previous applications like selfhealing materials and could support the use of NiTi SMAs in new applications like using a dispersion of NiTi particles to inhibit fatigue cracking in material structures [8]. However, the nature, origin, and extent of the phenomena remain poorly understood.…”