Honeycombs with structured core for enhanced damping

Abstract

Honeycomb sandwich panels, formed by bonding a core of honeycomb between two thin face sheets, are in wide use in aerospace, automotive and marine applications due to their well-known excellent density-specific properties. There are many technological methods of damping vibrations, including the use of inherently lossy materials such as viscoelastic materials, viscous and friction damping and smart materials such as piezoelectrics. Some have been applied to damping of vibrations, in particular to sandwich panel and honeycomb structures, including viscoelastic inserts in the cell voids. Complete filling of the cell with foam, viscoelastic or particulate fillers have all been demonstrated to improve damping loss in honeycombs. However, the use of an additional damping material inside the core of a sandwich panel increases its mass, which is often deleterious and may also lead to a significant change in dynamic properties. The work presented in this thesis explores the competing demands of vibration damping and minimum additional mass in the case of secondary inserts in honeycomb-like structures. The problem was tackled by initially characterising the main local deformation mechanism of a unit cell within a sandwich panel subjected to vibration. Out-of-plane bending deformation of the honeycomb unit cell was shown to be the predominant mode of deformation for most of the honeycomb cells within a sandwich panel. The out-of-plane bending deformation of the honeycomb cells results in relatively high in-plane deformation of the cells close to the skins of the sandwich panels. It was also highlighted that the magnitude and loading of the honeycomb unit cell are dependent on its location within the honeycomb or sandwich panel and the mode shape of the panel. An optimisation study was carried out on diverse honeycomb unit cell geometries to find locations at which the relative displacement between the honeycomb cell walls of the void is maximal under in-plane loadings. These locations were shown to be dependant of the nature of the loading, i.e. in-plane tension/compression or in-plane shear loading of the honeycomb unit cell and the unit cell geometry. Analytical expressions and finite element analyses were used to investigate the partial filling of the honeycomb unit cell with a damping material, in this case a viscoelastic elastomer, in the target locations identified previously where the relative displacement between the honeycomb cell walls is maximal. Damping inserts in the form of ligaments partially filling the honeycomb cell void have shown to increase the density-specific loss modulus by 26% compared to cells completely filled with damping material for in-plane tension/compression loading. The form of the damping insert itself was then analysed for enhancement of the dissipation provided by the damping material. The shear lap joint (SLJ) damping insert placed in the location where the relative displacement between the honeycomb cell walls of the void is maximal under in-plane loadings was characterised with very significant damping improvements compared to honeycomb cells completely filled with viscoelastic material. A case study of a cantilever honeycomb sandwich panel with embedded SLJ damping inserts demonstrated their efficiency in enhancing the loss factor of the structure for minimum added mass and marginal variation of the first modal frequency of the structure. Partial filling of the cells of the honeycomb core was shown to be the most efficient at increasing damping on a density basis.