Local effects induced by dynamic load self-heating in NiTi wires of shape memory alloys

The use of shape memory alloys wires in dampers devices for Civil Engineering applications is well documented in the literature. This paper develops a critical discussion on the wire macroscopic behavior and on the associated temperature effects with emphasis on the wire diameter. Vibration damping requires the absorption of mechanical energy and its conversion to heat via the action of hysteresis cycles. The experimental study is carried out on NiTi wires of different diameters. The flat cycles shown by thin wires (i.e., 0.5 mm of diameter or less) and the nonclassical S-shaped cycles (e.g., for diameter 2.46 mm) establish clear differences in the response of the samples. For this reason, a supplementary investigation is here reported to show that the flat cycles are coherent with the classical treatment of shape memory alloys as a first-order phase transition, but the S-shaped cycles of thick wires can be associated to an anomaly in the heat capacity.