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Characterizing the Acoustic Behavior of Hexagonal Periodic Cellular Cores

Ahmed, Ammar
A Master of Science thesis in Mechanical Engineering by Ammar Ahmed entitled, “Characterizing the Acoustic Behavior of Hexagonal Periodic Cellular Cores”, submitted in December 2020. Thesis advisor is Dr. Maen Alkhader and thesis co-advisor is Dr. Bassam Abu-Nabah. Soft copy is available (Thesis, Completion Certificate, Approval Signatures, and AUS Archives Consent Form).
Cellular solid cores are used in composite sandwich structures due to their high stiffness to weight ratio. However, owing to their porosity, they are inherently weak and are susceptible to damage due to improper loadings. As damaged cores can potentially lead to the failure of sandwich structures, core damage should be detected, preferably using nondestructive evaluation techniques. However, common nondestructive techniques, such as ultrasound, have limited effectiveness in inspecting cellular cores due to their dispersive properties. Since cellular cores are less dispersive at sub-ultrasound frequencies, inspecting them using sub-ultrasound frequencies has been introduced as a promising alternative to ultrasound inspection. However, this approach requires a priori knowledge of the acoustic characteristics in the inspected material, which is not available for most available cores. This work utilizes finite element computations to characterize the low frequency acoustic characteristics, namely phase velocity and dispersive properties, in commercial aluminum honeycombs made by bonding thin corrugated sheets. Results illustrate that the dispersive behavior and acoustic anisotropy of the studied honeycombs are more significant at higher porosities and higher frequencies. Moreover, results identify the frequencies below which honeycombs are least dispersive. To allow for realizing cores with tunable acoustic properties, the effect of admissible deformation modes, density, and geometric features on the phase velocities and dispersive properties is investigated. Accordingly, the acoustic behavior of honeycomb-based lattices that promote the two main admissible deformation modes in cellular solids, namely bending and stretching modes, is investigated. Results show that asymmetric waves in the bending dominated lattice are more direction dependent and less dispersive than in the stretching dominated lattice, whereas symmetric waves are generally independent of direction and dispersive in bending and stretching lattices. Results show that phase velocities of symmetric and asymmetric waves scale linearly with relative density in the bending dominated lattice and nonlinearly with relative density in the stretching dominated lattice; however, maximum phase velocities are higher in the stretching dominated lattice.
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