On the Multi-scale Vibroacoustic Behavior of Multi-layer Core Topology Systems

In this last decades, honeycomb sandwich panels have been the subject of intensive researches. Indeed, their high mechanical performances combined to a low stiffness to weight ratio result in a reduced acoustic efficiency.  Therefore, many designs are usually proposed to overcome this issue. Besides, different methods are developed to model more complex structures using the periodic structure theory to study the wave propagation allowing to investigate the vibroacoustic parameters.

The main purpose of this thesis is to investigate the vibroacoustic multi-scale behavior of multi-layer core topology systems which consist on stacking layers of honeycomb cores leading to an impedance mismatch between layers. In addition, such structures allow to increase the design space up to now limited to standard sandwich panels made of a single honeycomb core. Therefore, it is possible to obtain many configurations keeping the mass constant with simple shifting process between layers. A parametric model is proposed allowing to extract the unit cell through the thickness of the panel and to apply the periodic structure theory.

Modelling multi-layer core topology systems has been performed using the wave finite element method, and an extended method has been proposed to solve the acoustic transmission problem. The study is focused on transition frequencies, the sound transmission loss as well as veering effects and internal resonances, to finally optimize the geometrical parameters and to analyze their influence on the acoustical and mechanical performances of the structure.

Although the out-of-plane compression properties of multi-layer core topology systems are reduced, it is possible to strongly improve the in-plane compression properties. These later are studied by comparing a multi-layer hexagonal core and a standard single hexagonal core.

Finally, using multi-layer core topology systems and a perforated upper skin, it is possible to increase the energy dissipation occurring inside the core and thus, improve the sound absorption coefficient. Therefore, the thermo-viscous effect is considered. The acoustic behavior is similar to porous media and the Johnson-Champoux-Allard parameters are retrieved to characterize the acoustic fluid flow.

An improvement of the sound transmission loss and the sound absorption coefficient is obtained in a broadband frequency and the obtained resonance frequencies can be modified. However, this leads to lower mechanical properties especially the compression modulus and the dynamic rigidity.

Keywords: multi-layer core systems, vibroacoustic, transmission loss and sound absorption coefficient, transition frequency, veering, optimization.