An experimental investigation was conducted to determine the pseudoelastic hysteresis damping characteristics of Ni-Ti Shape Memory Alloy (SMA) wires. The comprehensive study examines the effects of cycling, oscillation frequency, strain amplitude, temperature, and static strain offset on the pseudoelastic stress-strain hysteresis of SMA wires under axial loading. Experimental data are obtained for complete austenite-martensite transformation hysteresis as well as partial transformation hysteresis. The results indicate that as the frequency of excitation increases, the reverse phase transformation from detwinned martensite to austenite commences at higher stress levels, and the area of the hysteresis loop decreases. This results in a rapid initial decrease in energy dissipation, but approaches a stable level by about 10 Hz. The energy dissipation at 10 Hz was found to be about 50% of that observed at very low frequencies. The energy dissipated at higher temperatures was found to be up to 40% lower than that observed at 90'F. The energy dissipated per cycle was found to be larger when lower values of static strain offset were used.
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 , while the storage modulus shows a significant increase above 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.
The Department of Civil and Mechanical Engineering at the United States Military Academy (USMA) offers a course in thermodynamics that is well known among the Corps of Cadets, because of its uniqueness and applicability. Students from every department in the USMA enroll in the course and are taught by a faculty that is composed of both military and civilian professors. The classroom and laboratory experiences that have been designed over the past decade provide students with a broad introductory exposure to thermodynamics, while focussing on very relevant applications. This paper presents an overview of the thermodynamic experience created at the USMA and offers several examples of methods to enhance similar courses at other institutions.
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