Effect of Microstructural Parameters on the Relative Densities of Metal Foams
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INTRODUCTION
AIRCRAFT noise is a major environmental concern especially during takeoff and landing.[1] Compared with the 1960s, commercial airplanes are significantly quieter today because of several improvements in design and materials technology.[2,3] Nevertheless, additional reductions in these noise levels are desirable. Thus, the National Aeronautics and Space Administration (NASA) has identified an ambitious goal of reducing aircraft noise by –52 dB relative to the International Civil Aviation Organization Chapter 4 certification standards by the year 2020 under its Subsonic Fixed Wing project.[4] It is expected that these noise reduction goals will be achieved through a combination of design changes and through the development of suitable materials.[3,4] Open cell foams and other cellular materials have been used successfully for sound absorption in many nonaerospace applications.[5,6] More recently, metal foams have been proposed for use in jet engines as an acoustic treatment over rotors,[7] fan blades,[8] and other applications.[9] The flow resistance[10,11] and sound absorption property[12–15] studies on metal foams have established their effectiveness as sound absorbers especially at higher frequencies typically greater than 500 Hz.[12] Unlike materials that reflect acoustic waves to isolate noise, foams and other cellular materials absorb sound energy. Although the physics of sound absorption in porous materials is fairly complex,[5] it is S.V. RAJ, Research Engineer, is with the NASA Glenn Research Center, Cleveland, OH 44135. Contact e-mail: [email protected] JACOB A. KERR, Engineer, formerly with the Pennsylvania State University, State College, PA 16801, is now Process Engineer with Piezo Kinetics, Inc., 660 E. Rolling Ridge Dr., Bellefonte, PA 16823. Manuscript submitted June 22, 2010. Article published online January 4, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
generally agreed that sound energy is dissipated primarily within the porous microstructure through viscous and thermal losses.[12,13] The properties of foams, including sound absorption, are dependent on their relative density q*/qs, where q* and qs are the densities of the foam and the solid material, respectively, and microstructure.[5,11,13,14,16] Although the wavelengths of the sound waves are much larger than the cell dimensions,[6] it is expected that the dimensions of the cells and their three-dimensional connectivity are likely to influence their sound absorption properties as energy is transferred to pumping the air columns within the cells (Figure 1). Simple formulae exist for correlating the foam relative densities and their microstructures with static flow resistance and sound attenuation.[5,10–14] Other factors, such as the relative area fractions of closed and open cells, are likely to influence the acoustic properties.[12] A qualitative and quantitative understanding of the role of the foam microstructure on the acoustic and mechanical properties of metal foams is important if they are to be used as acoustic liners in aircraft
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