Shape memory alloys (SMAs) are frequently used in a broad spectrum of commercially valuable and innovative applications due to its high specific energy density as an actuator and simple thermal actuation. This paper presents a study seeking to understand a recently discovered property of Nickel Titanium (NiTi) SMAs where they can repeatedly produce stable residual stresses after undergoing constrained recovery and returning to a low temperature, martensitic, state. The underlying mechanisms of this post constrained recovery residual stress (PCRRS) are still under investigation.
Experiments on multiple formulations of NiTi based SMAs (including a ternary material) confirmed the PCRRS generation in every SMA tested and following the exposures to further application of small cyclic strain (both tensile and compressive) from PCRRS states revealed a cyclic softening like phenomena where the magnitude of residual stresses over every small strain exposure were incrementally reduced, but the repetition of the constrained recovery process reproduced the initial residual stress. This paper presents the results of new experiments that help explain the nature of the PCRRS phenomena, discussion of the cyclic softening phenomena with small strain applications and a conceptual study to understand the underlying mechanism of PCRRS based modulus comparison in different loading states during PCRRS generation.
The capability to produce PCRRS and its repeating regeneration provides a new way for the material to be used as an actuator that may be beneficial to applications like self-healing material and could support new applications like using a dispersion of NiTi particles to inhibit fatigue cracking in material structures. Novel actuation mechanism can be developed and designed through PCRRS based on the potential energy stored in the structures to use in different applications and smart material design.