A deployment-scale array of locally resonant membrane-type acoustic metamaterials (MAMs) is fabricated. The acoustic performance of the array is measured in a transmission loss chamber, and a complex interaction between the individual cell and the array length scales is shown to exist. Transmission behavior of both the membrane and the array are independently studied using analytical models, and a method for estimating transmission loss through the structure that combines vibroacoustic predictions from both length scales is presented and shown to agree with measurements. Degradation of transmission loss performance often associated with scaling individual MAM cells into arrays is explained using analytical tools and verified using laser vibrometry. A novel design for hierarchical locally resonant acoustic metamaterials is introduced, and experimental and analytical data confirm this approach offers an effective strategy for minimizing or eliminating the efficiency losses associated with scaling MAM structures.
Out-of-autoclave/vacuum-bag-only (OoA/VBO) composite processing has emerged as an alternative to autoclave cure, addressing economic, environmental, and production flexibility limitations associated with autoclave production. VBO processing can produce defect-free components under ideal processing conditions; however, adverse process conditions (e.g. poor vacuum) commonly encountered in manufacturing environments result in unacceptably high scrap rates, preventing more widespread adoption of such techniques. This work explores how modifications to prepreg format can increase process robustness. A unidirectional (UD) prepreg was produced with a customized, discontinuous resin distribution, henceforth referred to as semipreg. The semipreg exhibited through-thickness permeability orders of magnitude greater than conventional hot-melt VBO prepregs. The semipreg also was less sensitive to variations in process conditions than conventional VBO prepreg. In situ process monitoring allowed observation and identification of two defect formation mechanisms arising during cure of the custom prepreg. Resin feature topography played a critical role in these mechanisms, indicating its importance to the design of next generation VBO semipregs.
As the importance of sound attenuation through weight-critical structures has grown and mass law based strategies have proven impractical, engineers have pursued alternative approaches for sound attenuation. Membrane-type acoustic metamaterials have demonstrated sound attenuation significantly higher than mass law predictions for narrow, tunable bandwidths. Similar phenomena can be achieved with plate-like structures. This paper presents an analytical model for the prediction of transmission loss through rectangular plates arbitrarily loaded with rigid masses, accommodating any combination of clamped and simply supported boundary conditions. Equations of motion are solved using a modal expansion approach, incorporating admissible eigenfunctions given by the natural mode shapes of single-span beams. The effective surface mass density is calculated and used to predict the transmission loss of low-frequency sound through the plate–mass structure. To validate the model, finite element results are compared against analytical predictions of modal behavior and shown to achieve agreement. The model is then used to explore the influence of various combinations of boundary conditions on the transmission loss properties of the structure, revealing that the symmetry of plate mounting conditions strongly affects transmission loss behavior and is a critical design parameter.
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