Quasi-static extension of shape memory wires under constant load

The transformation of nickel-titanium (NiTi) shape memory wires under constant load is studied by hanging a mass from the end of the wire and measuring the subsequent elongation. The experimental relationship between the weight of the mass and the elongation rate is compared with the theoretical model of Lee, Shield and Bruno. In displacement controlled experiments this model was shown to predict accurately the rate effects due to heat transfer at austenite-martensite interfaces in the wire. In the load controlled case these rate effects are manifest as a relationship between the elongation rate and the applied load. Very good agreement between the model and the experiments is found in the load controlled case. It is also observed that the load controlled experiments damage the wire when sufficiently large loads are applied. Copyright (C) 1996 Acta Metallurgica Inc.