Effects of macroscopic defects on the damping behavior of aluminum and Zn-27 Pct Al alloy
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I. INTRODUCTION
IT has been known for many years that materials may be tailored to exhibit high damping capacity by incorporating high intrinsic damping phases into them or by utilizing component phases to dramatically modify the matrix microstructures.[1,2] For example, addition of graphite, SiC, or Al2O3 particulates of several microns to Al alloys may produce a certain increase in damping capacity. Such reinforcements, however, have been shown to provide limited or slight improvements in the resultant damping capacity of metal matrix composites (MMCs), since only relatively small quantities of reinforcements can be added to the alloys as a result of the difficulties in production. More recently, the work of Han et al.[3] has revealed that the damping capacity of Al can be substantially improved through incorporating macroscopic porosity with higher proportion into it and, generally, at a rate of several times higher than that of the parent material. It suggests that introducing a quantity of high damping phases of macroscopic dimension into materials can modify their damping capacity more dramatically than those with microscopic reinforcements and can result in a new category of high damping material that is more likely to find applications in some passive attenuation of noise and vibration structures where stiffness, strength, and weight are critical. However, the selected matrixes in almost all the previous studies are low damping materials; there is little research concerning combining a high damping reinforcement into a high damping matrix. We have, as yet, little understanding of what damping behavior materials have with different matrixes and different reinforcements with microscopic or macroscopic level, what damping mechanisms operate, and how the structures 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 mechanisms of materials and to give some information on applications. II. EXPERIMENTAL A. Specimen Preparation In order to examine the effects of the damping properties of matrixes on the damping behavior of the composites, two materials with very different damping capacity are selected: commercially pure Al and Zn-27 pct Al (ZA27) alloy. It is well known that the damping capacity of commercially pure Al is rather low, but that of ZA27 alloy is quite high. Considering the effect of interface and the intrinsic damping capacity of the reinforcements, two macroscopic defects with different moduli are introduced into the materials: pores and graphite particulates, which will induce different stress and strain state at the interface. The specimens studied were produced by an infiltration process. NaCl and graphite particulates, with the size of 0.001 ⫾ 0.0005 m and the volume fraction ranging from 19 to 94 pct, were used as preforms, into which commercially pure Al liquid or ZA27 alloy liquid was forced by application of high
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