Vibration and Acoustical Properties of Sandwich Composite Materials

Abstract

In applications where the use of lightweight structures is important, the introduction of a viscoelastic core layer, which has high inherent damping, between two face sheets, can produce a sandwich structure with high damping. Composite sandwich structures have several advantages, such as their high strength-to-weight ratio, excellent thermal insulation, and good performance as water and vapor barriers. So in recent years, such structures have become used increasingly in transportation vehicles. However their fatigue, vibration and acoustic properties are known less. This is a problem since such composite materials tend to be more brittle than metals because of the possibility of delamination and fiber breakage. Structures excited into resonant vibration exhibit very high amplitude displacements which are inversely proportional to their passive damping. The transmission loss of such composite panels is also poor at coincidence. Their passive damping properties and attempts to improve their damping at the design stage are important, because the damping properties affect their sound transmission loss, especially in the critical frequency range, and also their vibration response to excitation.

The research objects in this dissertation are polyurethane foam-filled honeycomb sandwich structures. The foam-filled honeycomb cores demonstrate advantages of mechanical properties over pure honeycomb and pure foam cores. Previous work including theoretical models, finite element models, and experimental techniques for passive damping in composite sandwich structures was reviewed. The general dynamic behavior of sandwich structures was discussed. The effects of thickness and delamination on damping in sandwich structures were analyzed. Measurements on foam-filled honeycomb sandwich beams with different configurations and thicknesses have been performed and the results were compared with the theoretical predictions. A new modal testing method using the Gabor analysis was proposed. A wavelet analysis-based noise reduction technique is presented for frequency response function analysis. Sound transmission through sandwich panels was studied using the statistical energy analysis (SEA). Modal density, critical frequency, and the radiation efficiency of sandwich panels were analyzed. The sound transmission properties of sandwich panels were simulated using AutoSEA software. Finite element models were developed using ANSYS for the analysis of the honeycomb cell size effects. The effects of cell size on both the Young’s modulus and the shear modulus of the foam-filled honeycomb core were studied in this research. Polyurethane foam may produce a negative Poisson’s ratio by the use of a special microstructure design. The influence of Poisson’s ratio on the material properties was also studied using a finite element model.