|dc.description.abstract||Sandwich composite materials have been widely used in recent years for the construction of spacecraft, aircraft, and ships, mainly because of their high stiffness-to-weight ratios and the introduction of a viscoelastic core layer, which has high inherent damping. One of the main objects of this research is the measurement and estimation of the bending stiffness and damping of sandwich structures. Knowledge of the elastic properties of the core and face sheet of the sandwich structures is indispensable for the analysis and modeling of sandwich strictures. However, traditional methods to determine the elastic properties are not suitable for the core, which is usually brittle, and the face sheets, which are usually very thin. A set of special techniques has to be used to estimate the elastic properties of these materials. The dynamic bending stiffness of such materials is difficult to measure because it depends on frequency unlike ordinary non-composite materials. A simple measurement technique for determining the material parameters of composite beams was used. The damping is an important property used for the analysis of the acoustical behavior of the sandwich structures, especially for the characterization of the sound transmission loss. An interesting fact is that damping can be measured as a byproduct in the procedure of the measurements of dynamic stiffness.
Another main object of the research is to analyze the sound transmission loss of sandwich structures and to simulate their acoustical behavior using the statistical energy analysis method (SEA). While solving vibroacoustic problems, FEM and SEA are commonly used. However, common vibroacoustic problems involve a very large number of modes over a broad frequency range. At high frequencies these modes become both expensive to compute and highly sensitive to uncertain physical details of the system. Many processes involved in noise and vibration are statistical or random in nature. So SEA is suitable for the high frequency problems such as vibroacoustic problems.
The materials used in the research include sandwich structures with polyurethane foam-filled honeycomb cores and sandwich structures with closed-cell polyurethane foam cores. Foam-filled honeycomb cores possess mechanical property advantages over pure honeycomb and pure foam cores. The honeycomb structure enhances the stiffness of the entire structure; while the foam improves the damping. Closed-cell polyurethane foam is CFC-free, rigid, and flame-retardant foam. Both foam-filled honeycomb and closed-cell foam cores meet the requirements of many aircraft and aerospace manufacturers. Also, foam-filled honeycomb and closed-cell structures have high strength-to-weight ratios and great resistance to water absorption, and will not swell, crack, or split on exposure to water.||en_US