Effect of intrinsic damping on vibration transmissibility of nickel-titanium shape memory alloy springs
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INTRODUCTION
THERE are numerous applications in which metallic isolation devices can offer definite advantages over rubber or other elastomeric mounts and isolators. The advantages are most promising in areas where gasses, oils, corrosive agents, and high temperatures or greatly fluctuating heat levels are present. It is also noted that there are certain disadvantages associated with metal springs as isolation devices, as pointed out by Sykes.tq From a mechanical vibration standpoint, springs deviate from ideal behavior due to standing wave resonances developed at frequencies above the primary natural frequency of the spring-mass system. High frequency standing waves are unavoidable in any mount, but sharp peaks in transmissibility can be suppressed with high damping. One of the aims of this study was to see if sharp standing wave peaks in transmissibility could be alleviated through use of a metal with intrinsic high damping. Shape memory alloys (SMAs) such as nickel-titanium (Ni-Ti) offer many useful characteristics for vibration isolation and noise suppression, including a high intrinsic damping capacity (peak loss factor between 0.02 and 0.04) and a capability for force actuation. The mechanical characteristics and microstructural properties of these alloys are described in many literature sources.tZ-sJ A brief review is offered here for the purpose of describing damping in Ni-Ti SMAs. In SMAs, the martensite transformation is the key to the shape memory effect (SME); it is a first-order displacive process in which a body-centered cubic parent phase (ausEDWARD J. GRAESSER, Research Engineer, is with the Machinery Structures Acoustics Branch, Carderock Division Naval Surface Warfare Center, Annapolis, MD 21402-5067. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
tenite phase) transforms by a shearing mechanism to martensite which is ordered and twinned. The martensite transformation can be driven by changes in temperature or by applied stress. The essential process of the crystalline phase change is the same in either case. When the SMA is above its martensitic starting temperature, Ms, the lattice arrangement of atoms is disordered. Cooling to M, leads to displacive reorganization and an ordered structure; this is due to free energy minimization. Below Ms, another energy minimization produces shear transformation to a close-packed symmetric martensite. Further cooling to temperatures below the fully martensitic temperature, M~ results in highly twinned self-accommodating martensite groups (platelike variants). Between M~ and Mj, martensite takes up a fraction of the material volume (0 pct at M~ and 100 pct at Mi). There are 24 possible variants of martensite in SMAs. When holding the temperature below M r and subjecting the material to an applied stress, these variants (due to their multiplicity) und
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