Characterization of the pseudoelastic damping behavior of shape memory alloy wires using complex modulus

The pseudoelastic stress/strain hysteresis behavior observed in nickel-titanium (Ni-Ti) shape memory alloys (SMAs) above the austenite finish temperature can be exploited to provide passive structural damping in a variety of applications. The present study characterizes the damping behavior of Ni-Ti SMAs using the complex modulus approach, commonly used in structural dynamics for the characterization of damping materials. Results indicate that as excitation frequency increases, the loss modulus (a measure of the damping) undergoes a rapid initial decrease. The value of loss modulus (and available damping) at 6-10 Hz is approximately 50% of that at low frequencies, but does not show significant reduction thereafter. As the cyclic strain amplitude increases, the storage modulus (a measure of the stiffness) initially undergoes a rapid decrease, implying that the material softens with increasing motion amplitude. As the static strain offset increases, the loss modulus decreases, and the storage modulus increases. The loss modulus decreases at temperatures above 90 degrees F while the storage modulus shows a significant increase above 130 degrees F associated with the SMA operating outside its ideal pseudoelastic temperature window. The experimental stress/strain hysteresis loops and the idealized loops based on complex modulus characterization compare very well for small cyclic strain amplitudes, but may differ for higher amplitudes. However, the energy dissipation estimates from the idealized and experimental hysteresis loops compare very well over the entire range of cyclic strain amplitudes, indicating that complex modulus characterization is well suited for estimating the damping capability of SMAs.