Load attachment for honeycomb panels in racing cars

Abstract Lightweight, honeycomb core, sandwich panels form the major part of the body structure of Formula One racing cars. Key properties include high specific stiffness and specific strength, ease of fabrication, and the ability to tailor impact resistance. The engine and gearbox, which in current designs are placed at the rear of the car, are load-bearing components and must be attached to the sandwich body shell. Although techniques for introducing loads into the skins of sandwich panels, via inserts, are well established, for Grand Prix cars, where every gram of weight must be saved, efficiency is at a premium. Thus the shape of the insert is critical and the choice of the bonding adhesive and insert's material important. While it is possible to undertake preliminary design using simple methods, if a complex insert geometry is used, recourse to finite element (FE) analysis is inevitable. However, particular difficulties arise when using FE methods in these circumstances, notably the relatively small thickness of the panel skins and the discontinuity at the insert boundary. In the current paper these issues are addressed for a particular application. To assess the efficiency of load transfer, three different insert geometries are considered. The influence of element type, mesh size and boundary conditions are addressed and their influence on stress levels and discontinuities assessed. Ways of representing the honeycomb core are also considered, as is the need to involve non-linear effects, such as the shear stress-strain properties of the adhesive. The FE results are compared with experimental data obtained from testing a representative panel to which a photoelastic coating had been applied. Encouraging agreement between prediction and experiment was obtained, together with guidance as to the optimum insert shape.