A multi-scale reinforced natural composite sandwich panel concept for vibroacoustic applications

Abstract

The integration of vibroacoustics constraints in the design of sandwich panels is nowadays of utmost importance. Countless industrial sectors and applications now require multifunctionality in terms of mechanical, vibrational and acoustical performances. The main challenge dwells in the fact that good mechanical properties of a sandwich panel are normally related to poor acoustic performances. Numerous solutions have been proposed to enhance their vibroacoustic properties, ranging from porous and viscoelastic insertions or layers to Helmholtz resonators. However, the main drawback of such additions is the increase of weight which undermines the lightweight advantage of the panel. Thus, the complexity of designing structures which are mechanically and vibroacoustically efficient at the same time is further aggravated by the necessity of maintaining the additional mass at low levels.
This thesis illustrates the development of a novel sandwich panel for vibroacoustics applications with emphasis on vibration damping, paramount indicator of vibroacoustics performances of sandwich structures. The design strategy adopted revolves around the utilisation of novel materials and viscoelastic inserts as passive damping mechanisms. Specifically, a flax-fibre reinforced composite laminate is chosen for the square-cell core and the skins. Natural-fibre composites generally provide higher vibration energy dissipation than commercial materials traditionally used for sandwich and composite structures. Carbon-nanotube reinforced viscoelastic inserts are embedded in critical locations within the lattice core — junction corners between interlocking walls — to further increase its vibration damping capabilities and, by consequence, to potentially increase its strength as well. Restrictions to the value of additional mass brought by the inserts are imposed to ensure the preservation of the lightweight advantage of the sandwich panel.
Mechanical static analyses are carried out to guarantee good-quality structural performances. The quasi-static flatwise compression characterisation of bare and insert-reinforced core specimens shows that both configurations outperform other comparable benchmark cases present in open literature, in terms of normalized stiffness and strength. The reinforced configuration, in particular, has 24.5% higher normalized strength compared to the bare panel. Two different types of tests are performed to investigate vibroacoustic performances of the sandwich panels. Results of out-of-plane vibration transmissibility tests show that the bare configuration outperforms that with inserts due to Coulomb dry friction effects which are triggered during the test at the interlocking areas between walls. On the other hand, the reinforced panel offers the highest viscoelastic damping values under free vibrations. In this case, indeed, bending deformation shapes are excited and the dry friction mechanisms in the bare lattice are not initiated.

Date of Award	23 Mar 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Fabrizio Scarpa (Supervisor) & Dmitry Ivanov (Supervisor)