Mechanical behaviour of NiTiNb shape memory alloy wires– strain localisation and effect of strain rate

Abstract

Over the past few years, shape memory alloys (SMAs) have received increased attention in civil engineering research due to their unique features such as superelasticity, shape memory effect (SME), high ductility and good corrosion resistance. SMAs, however, have complex material behaviour which depends on many parameters such as chemical composition, thermo-mechanical treatment and ambient temperature. SMAs exhibit high strain-rate sensitivity and strong strain localisation during the stress-induced martensitic (SIM) phase transformation. Both have serious implication in civil engineering applications. Strain rate effects and the strain localisation in SMAs have been commonly investigated separately in the literature which has been mostly focused on NiTi SMA. This paper investigates the strain localisation phenomenon, the effect of strain rate and their interaction in NiTiNb SMA. The digital image correlation (DIC) technique was used to investigate the strain localisation phenomenon. The effect of strain rates is investigated within the quasi-static range and varies from 3.3 × 10−5/s to 3.3 × 10−2/s in strain-controlled tests and 2 × 10−5/s to 2 × 10−1/s in displacement-controlled tests. Significant non-uniform strain distribution is observed during the SIM phase transformation, which progresses through the nucleation and broadening of the transformation bands (TBs). The nucleation of the TBs and the unloading strain recovery within the TBs show high strain-rate sensitivity. At high strain rates, the nucleation of a TB is accompanied by a significant stress-drop.