Mechanical and vibro-acoustic aspects of composite sandwich cylinders

Abstract

Designing a fuselage involves many considerations such as strength and stability, fatigue, damage tolerance, fire and lightning resistance, thermal and acoustic insulation, production, inspection, maintenance and repair. In the background of the application of composite sandwich structures on the aircraft fuselage, the focus of the thesis is to investigate the vibration and acoustic behaviours of sandwich structures. As a preliminary design of aircraft fuselages, a sizing work of sandwich cylinders was conducted with respect to the strength and stability. FE models for the buckling prediction of the sandwich cylinder were validated with the analytical expressions. Under a typical flight loading, the sizing results of a sandwich cylinder and a laminated cylinder were compared and it was found that the mechanical efficiency of the sandwich cylinder is comparable to that of the traditional stiffened cylinder. Subjected to the diffuse acoustic field, the sound transmission loss (TL) of composite sandwich cylinders was investigated using an analytical method and the Statistical Energy Analysis (SEA) method at 100-16000 Hz. The SEA method showed a good agreement with the analytical method. The parameters, including the fibre orientation, facing materials, cylinder geometry, core thickness, sandwich layup and core shear stiffness, were studied for their influences on the TL of cylindrical structures. A uniform laminated, a stiffened and a sandwich cylinder with the equivalent mass were compared for the sound insulation performance. The laminated cylinder had the largest TL below the coincidence frequency and the sandwich cylinder had the largest TL above the coincidence frequency. The structural velocities and noise reductions of laminated and sandwich cylinders were experimentally tested at 1-4000 Hz under a point acoustic excitation, and a mechanical excitation respectively. The wave propagation in the sandwich structure was compared with that in the laminated structure, as an explanation of the noise reduction difference of the two structures. As the coincidence frequency plays an important role on the sound transmission, influence parameters of the coincidence frequency of sandwich structures were also studied. To investigate the vibro-acoustic performance of sandwich structures under different kinds of external excitations, the FEM/BEM numerical method was used to analyze the noise reduction of sandwich cylinders at low frequencies. Under a force excitation, some parameters including the core shear stiffness, sandwich layup, core thickness and facing orientation were studied for their influences on the sound transmission. Results showed that there exist optimal values for these parameters to achieve the best sound insulation performance. Therefore, an efficient optimization technique using the acoustic transfer vector (ATV) and the genetic algorithm (GA) was applied to optimize a typical sandwich cylinder for the best noise insulation. In addition, taking a fuselage section as an example, a multi-objective optimization (weight & noise insulation) was conducted considering the mechanical constraints under flight load cases. The noise control treatment such as the addition of absorption layers is one of the common methods for the noise control of the transport vehicles. Thus the sound transmission of sandwich panels with open-cell foam was studied. The transfer matrix method (TMM) was used for the TL prediction of sandwich panels with porous foams. This method was validated by experimental results. A sensitivity study of the flow resistivity, tortuosity and porosity on the TL of sandwich panels was conducted. Then four kinds of absorption materials were studied for their influences on the TL of sandwich cylinders. Finally the TLs of a stiffened cylinder and a sandwich cylinder were compared in case of addition of absorption layers. As the damping plays an important role on the vibro-acoustic behaviors of sandwich structures, the damping properties of composite sandwich structures were studied using the modal strain energy (MSE) method and experimental measurements. The hysteresis method and the half power method were used for the damping measurement. Compared to the facing, the cores usually have much higher damping and they make the main contribution on the sandwich damping. Therefore the material damping properties of two kinds of foams (PMI & PVC) were measured at low frequencies using the hysteresis method. The measured results have been validated by numerical models. The damping of the PVC foam were also measured using the half power method and results showed a good agreement with those measured using the hysteresis method. For the damping prediction of sandwich structures, the MSE method was verified by the measurements using the half-power method. Finally, the effects of the core thickness and core properties on the damping of sandwich structures were studied.