Thermal and mechanical considerations in using shape memory alloys to control vibrations in flexible structures
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INTRODUCTION
T H E performance of structures, such as aerospace, submarine, and machinery in general, can be affected severely by vibration and noise. New design concepts leading to lightweight, slender structural components can increase the vulnerability of the components to failure due to excessive vibration. Components of rotating machinery, such as jet engines, are designed to keep the ratio of the vibratory stress to steady stress below a predetermined minimum. An increase in vibratory stress therefore reduces the allowable steady-state stresses, requiting an increased area of cross section. The key requirement of modem submarine structures to operate undetected requires considerable control of vibration and noise. Discomfort to pilots and passengers in rotorcraft arises primarily due to annoying levels of noise--both airborne and structure borne. Requirements of thirdgeneration space structural systems include a vibrationfree environment, so that not only, machinery and personnel can function effectively, but the entire craft can be positioned accurately. In all of such situations, the traditional solution is to consider the adequacy of structural, material, and aerodynamic damping through an estimate of the same and redesign the structure if needed. The latter almost always means an increase in stiffness of the component, resulting in increased cost. Apart from the extremely low damping potential of modem metals, estimates of structural damping arising out of rubbing at joints have essentially defied all attempts at accurate measurement and analysis. Similarly, calculation of aerodynamic damping, which has been a subject of serious study in the past, yields results which are not always in agreement with observation. It is in this context that a study of the potentials of
A.V. SRINIVASAN, Manager, Applied Mechanics Research, United Technologies Research Center, East Hartford, CT, is currently on leave with the Air Force Offme of Scientific Research, Washington, DC. D.G. CUTTS, Research Engineer, is with United Technologies Research Center, East Hartford, CT. L.M. SCHETKY, Chief Scientist, is with Memory Metals Corporation, Norwalk, CT 06854. This paper is based on a presentation made in the symposium "Acoustic/Vibration Damping Materials" presented during the TMS Fall Meeting, Indianapolis, IN, October 1-5, 1989, under the auspices of the TMS Physical Metallurgy Committee. METALLURGICAL TRANSACTIONS A
using shape memory materials to control vibration and noise in structural components becomes important. The basic properties of shape memory alloys (SMAs) are discusssed in detail below. Suffice it to say that shape memory metals exhibit the capacity to hysteretically recover significant deformation so that large amounts of energy are absorbed in the process. Laboratory testing conducted at the United Technologies Research Center has shown that this characteristic remains intact under alternating forcing. Thus, the energy absorption capability can be used to damp vibration. In addition, these shape memory
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