Heat-activated prestressing of NiTiNb shape memory alloy wires

Abstract

Pre-strained shape memory alloys (SMAs) can develop significant level of recovery stress (prestress) when heated above their transformation temperature in the constrained conditions. In this study, thermo-mechanical tests were conducted to investigate heat-activated prestressing (HAP) of NiTiNb SMA wires. The tests were conducted to investigate: (1) the effect of pre-strain on reverse transformation temperatures (As and Af) of the NiTiNb SMA; (2) the effect of strain rate and the pre-strain level on the recovery stress; (3) post-HAP behaviour under monotonic and cyclic loading; and (4) reusability of the NiTiNb SMA wires for HAP applications. The digital image correlation technique was used to gain deeper insight into micro-mechanical behaviour of the SMA. The study highlights the complex material behaviour of the NiTiNb SMAs. The results from this study suggest that NiTiNb SMAs can be effectively used for HAP. Recommendations on optimal strain rate for pre-straining NiTiNb SMAs and the optimal pre-strain level for achieving maximum recovery stress in NiTiNb SMAs are given in this paper.