Damping behavior of foamed aluminum
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I.
INTRODUCTION
DAMPING capacity is a measure of a material’s ability to dissipate strain energy during mechanical vibration under cyclic loading or wave propagation.[1] The damping behavior of metallic materials is closely related to defects in the structures, which include point defects, grain boundaries and interfaces, dislocations, micro- or macropores, etc.[2] According to the classical Zener’s theory,[3] high linear damping can be at its largest when the mechanical interaction between the defects is optimized. Alternatively, high nonlinear damping can be utilized if a mechanical amplification mechanism is built into a material. In light of this theory, high-damping-capacity materials have emerged in large numbers in recent years. Materials, for instance, that are essentially low damping may be tailored to exhibit high damping by incorporating high-intrinsic-damping phases into them or by utilizing component phases to dramatically modify the matrix microstructures.[1,2] It can be rationalized that porous materials will also show an increased damping capacity. A number of experiments have demonstrated that pores can play a very important role in the damping responses of both metallic and nonmetallic materials. Many articles reported that the damping capacity of porous materials increased as the porosity increased, and, generally, at a rate several times higher than that of the parent materials.[4–7] Foamed aluminum (FA) is a porous metallic material with a porosity of as high as 60 to 85 pct and has attracted considerable attention as a result of its high damping capacity and its potential applications in some passive attenuation of noise and vibration structures.[8–11] Since its pore structure is very different from that of traditional porous materials, the damping behavior will also have unique features. However, there is little research concerning this topic. We have, as yet, little understanding of what damping features FA has and how the structure and testing conditions influence this behavior. Damping measurements are, therefore, carried out over a wide range of temperatures, frequencies, and strain amplitudes to improve our understanding of the damping behavior and operative FUSHENG HAN and ZHENGANG ZHU, Professors, CHANGSONG LIU, Doctoral Student, and JUNCHANG GAO, Engineer, are with the Laboratory of Internal Friction and Defects in Solids, Chinese Academy of Sciences, Hefei 230031, China. Manuscript submitted May 8, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanisms of FA and to give some information on applications. II.
EXPERIMENTAL
A. Specimen Preparation Two metals, commercially pure aluminum and an Al-10 pct Mg alloy, are chosen as the matrices of FA. The specimens are manufactured by a melt foaming process.[12] The specimen’s density is determined from its weight and physical dimensions. From this density, the porosity (P) is given by P512
r rs
[1]
where r and rs are the densities of FA and the pore wall material, respectively (for commercially pure aluminum, rs is taken to be 2.
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