Experimental characterisation of three-phase NiTi wires under tension

• Three-phase NiTi wires are extensively studied thermo-mechanically.
• A pseudo-diagram of a three-phase NiTi alloy is built.
• Cyclic behaviour changes under quasi-static loadings are discussed.
• Temperature and strain rate influences on dissipated energies are quantified.
• The damping power due to superelasticity effect dissipation is studied.

NiTi shape memory alloys (SMAs) wires exhibit R-phase frequently. This intermediate phase has interesting properties under cyclic loadings and fatigue. These wires have a good potential in civil engineering applications for their damping capacity. In accordance with these applications, in this study, a complete characterisation of NiTi wires is proposed. First the pseudo phase diagram concerning R-phase and its existence comparing to austenite or to martensite is obtained by realising tensile tests at different temperatures and strain amplitudes. Then, the cyclic behaviour under tension at various amplitudes is studied. Based on these loadings, the significant observations are: a small residual strain, the decrease of transformation yield stresses and the evolution of the hysteresis size with the number of cycles. To consider the real use conditions, the effect of strain rate has been studied at two temperatures. The obtained results show that, due to thermo-mechanical coupling, the mechanical behaviour is affected by the evolution of strain rate. Eventually, the damping effect of the studied wires has been evaluated. All the obtained results confirm the potential use of NiTi wires exhibiting R-phase in civil engineering applications. They also constitute an interesting reference for models development describing R-phase occurrence and ease their identification and validation procedures.