Engineered metallic foam for controlling sound and vibration
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Many structural acoustic and vibration designs rely extensively on materials that are light-weight, stiff, and highly damped. Advanced materials such as metallic foams can be engineered to achieve these properties in order to control sound and vibration for a variety of aerospace, maritime, and ground transportation applications. In this work, the structural and acoustic properties of commercially available and digitally designed metallic foams are analyzed through numerical and experimental methods. Furthermore as a post-manufacturing process, metallic foams can be engineered in order to preferentially alter the microstructure and achieve material property enhancements. In this work, the following engineering methods are proposed and investigated: plastic deformation and material saturation. When a metallic foam is plastically deformed, the foam's porosity and pore shape are dramatically altered. This transformation in microstructure can lead directly to changes in bulk properties. In this work, a method for triaxial hydrostatic compression of metallic foams is proposed and demonstrated experimentally. The structural properties of transformed foams are tested using a load cell with digital image correlation. Transformed foams exhibit higher compliance, higher toughness, and a reduced Poisson ratio. Measurement and analysis of acoustic properties indicate that the transformed foams can absorb significantly more sound than the conventional samples of equal thickness in the test range of 0.25 - 4.50 kHz. Due to their open-cell microstructure, metallic foams can be filled with saturating materials. In this work, metallic foams saturated with viscous liquids are investigated for reducing vibration transmissibility in a structure. For the best performing saturated foam subject to a transient excitation, an order of magnitude increase in damping ratio is measured. Additionally, a composite foam (consisting of metallic foam saturated with polyurethane foam) is fabricated to enhance acoustic properties. For the best performing composite foam at normal incidence, the sound absorption coefficient is improved by a factor of 6 near 0.60 kHz and by a factor of 2 up to 4.5 kHz. Lastly, two methods for estimating acoustic absorption in metallic foams are presented which utilize finite element analysis and boundary layer theory. The proposed methods are discussed for commercially available foams as well as for representative digital designs. Limitations and assumptions of the methods pertaining to size scales and boundary layer features are addressed.
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