Characterization of room-temperature
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INTRODUCTION
M A T E R I A L S selection for structural applications is usually based upon mechanical strength rather than intrinsic acoustic properties, e . g . , intrinsic damping capacity. Here, the term "damping" has been loosely used to define a material's intrinsic ability to dissipate vibrational mechanical energy as heat. In general, the term "damping" also includes mechanisms that are associated with frictional elements in a structure, such as bolted assemblies, or with the frictional resistance imposed by the environment surrounding the vibrating structure, e . g . , air damping. Internal friction is the generally accepted name for intrinsic material damping, but the phrase "internal friction" usually conveys a low level of energy dissipation. However, there exists a class of materials, called Hidamets, that possess very high levels of internal friction. Hidamet is derived from the words high damping metals. These metals exhibit specific damping capacities (SDC) as high as 68 pct, Ill where SDC has been defined as the ratio of energy dissipated as heat to the maximum vibrational energy stored per cycle per unit volume. From the designers point of view, the selection of materials based upon intrinsic damping has been complicated by the many definitions used to quantify the internal friction, e . g . , SDC, tan 6, logarithmic decrement, inverse quality factor, and loss factor. Specific damping capacity is perhaps the most intelligible for the designer, but the choice of terminology is often dictated by the testing method. The apparatus used for the present work measures damping in terms of tan 6, where the angle 6 has the physical interpretation as the phase angle by which the deflection (strain) lags behind the applied load (stress). For phase angles much less than 0.1 radians, there exists the following relationship between tan 6 and pct SDC: pct SDC = 200 7r tan 6
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ASHWANI K. MALHOTRA, Graduate Research Assistant, and DAVID C. VAN A K E N , A s s i s t a n t P r o f e s s o r , are with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109 2136. Manuscript submitted September 11, 1990. METALLURGICAL TRANSACTIONS A
Materials which exhibit an SDC greater than 6 pct, or tan 6 greater than 0.01, are considered Hidamets, and Figure 1 shows a graph of tan 6 vs yie!d strength at room temperature for some common engineering alloys of which several are considered to be Hidamets. Although this graphic representation is a useful comparison for the materials engineer, it does not convey the temperature, frequency, and strain amplitude dependence of the damping for these materials. It should also be noted, with the exception of sintered aluminum powder ( S A P ) , 12'31 that most aluminum alloys exhibit very low damping. A number of attempts have been made in recent years to improve the damping of aluminum via composite engineering using graphite fibers 14-71 or graphite particles L81 in an effort to make an aluminum analog of gray cast iron. I9'l~ For gray c
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