Free Response and Musical Pitch of Shape Memory Alloy Wires Under Voltage Loading

An experimental setup, on which the shape-memory effect is based, is developed to measure the free-response frequency and musical pitches of shape memory alloy (SMA) wires. In this article, the voltage, force, and sound signals of SMA wires are determined simultaneously under two kinds of experiments: 1) constant-temperature uniaxial tensile tests and 2) fixed-length thermal cycle tests. Based on constitutive equations and thermodynamic equations, an empirical formula for the driving voltage and the steady-state stress of SMA wires is proposed, and the undetermined coefficients at radii of 0.050 and 0.070 mm are determined by experiments. The musical pitches of SMA wires under a constant voltage are obtained by combining the empirical formula with the string vibration equation. The theoretical results are compared with the actual musical pitches measured by the sound signals, and the theoretical results are highly consistent with the actual musical pitch. Just for an SMA wire with a radius of 0.070 mm, the free-response frequency range reaches 640–1120 Hz, which covers 6 whole musical tones in total. Thus, the SMA wire can be used as a string to play simple music.