Damping behavior of Al-Li-SiC p composites processed by stir casting technique
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I. INTRODUCTION
DAMPING capacity is the measure of a material’s ability to dissipate elastic strain energy during mechanical vibration or wave propagation. Structural materials that exhibit high damping capacity are useful in the passive attenuation of noise and vibration in structures. This has led the way for development of materials possessing high damping capacity in combination with high stiffness and low density including metal matrix composite (MMC). In MMCs, damping capacity can be improved through the additions of reinforcing phases that possess high intrinsic damping or that dramatically modify the matrix microstructure in such a way as to increase damping capacity. Discontinuously reinforced MMCs are particularly suitable for this purpose since these can be mass produced and also possess promising mechanical properties,[1,2,3] and high damping capacity.[4–9] SiC, Al2O3, and graphite are the most frequently used reinforcements in these MMCs. Both positive and negative changes in damping capacity with the addition of SiC particles have been reported.[5,9] There are reports[4,6,10–15] of substantial improvement of damping capacity of Al alloys with addition of graphite particles. Composites having a combination of SiC and graphite reinforcements (hybrid reinforcements) have also been processed and reported to possess better damping capacity.[16,17,18] Al-Li alloys possess low density and high specific strength and stiffness, which can make them potential candidates to serve as the basis for high damping materials. However, most of the damping studies on Al MMCs have been done on systems based on 6061 Al or 2519 Al as matrices.[12–19] To the best of the authors’ knowledge, the damping behavior of MMCs based on the 8090 Al alloy has not yet been reported. This investigation aims at characterizing the damping behavior and mechanisms of damping in 8090 Al-SiCp composites processed by stir casting technique. The damping data obtained using a dynamic mechanical analyzer (DMA) is discussed on the basis of matrix microstructure. RANJIT BAURI, Research Scholar, and M.K. SURAPPA, Professor, are with the Department of Metallurgy, Indian Institute of Science, Bangalore–560 012, India. Contact e-mail: [email protected] Manuscript submitted June 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. CHARACTERIZATION OF DAMPING CAPACITY The damping capacity can be characterized by different quantities. Specific damping capacity, , is given by c
¢W W
[1]
where W is the energy lost per cycle and W is the total stored energy per cycle. The damping capacity is also characterized by a loss factor or loss tangent. For the periodic stress imposed on a material, the expression for stress, , and strain, , can be given by[20] s so exp (ivt)
[2]
o exp [i(vt f)]
[3]
where o and o are the stress and strain amplitudes, respectively; 2 f is the circular frequency, and f is the vibrational frequency; is the loss angle by which strain lags behind stress. In an ideally elastic material, 0 and /
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