Thermomechanical Characterization of NiTiNb Shape Memory Alloy for Concrete Active Confinement Applications

Despite growing interest in the use of shape memory alloys (SMAs) in civil structures, much is still to be understood about their behavior in real loading environments. This experimental work focuses on characterizing the thermomechanical behavior of a thermally prestressed NiTiNb SMA under thermal and loading conditions pertinent to civil structural applications. The work aims at exploring the feasibility of using this type of alloy in developing seismic retrofitting spirals that could be used to actively confine vulnerable reinforced concrete columns to improve their flexural ductility and shear strength under earthquake loading. Differential scanning calorimetry tests are conducted to determine the transformation temperatures of the used alloy. A testing program is carried out on 2-mm-diameter NiTiNb wires to examine the recovery stress of prestrained wires and the monotonic and cyclic behaviors of thermally prestressed wires under various testing durations and ambient temperatures. A thermal chamber is used to induce a realistic range of ambient temperature (−10°C to 55°C) during testing. The results show that the recovery stress is quite stable within the studied range of temperature. The prestressed NiTiNb alloy exhibits nonlinear behavior with a modulus that is highly dependent on ambient temperature.