Abstract

As efficient lightweight components, sandwich elements are becoming increasingly important in machine tool design and automotive industry, based on the combination of a very high ratio of rigidity and flexural rigidity to weight. Bending and shear loaded fasteners such as bolts, rivets and screws are used most frequently to connect the often semi-finished parts. A major challenge in the design of screw connections is the absorption of load and moments, usually supported by various reinforcement structures. Therefore, both the geometry and the reinforcement principle must be considered. A local, precisely placed load introduction element for composite sandwich structures is analysed and validated as a purely mechanical and form-fitting solution using experimental and numerical investigations in several iteration steps. The sandwich structures, made of carbon fibre reinforced plastic face sheets and a phenolic resin-impregnated Nomex random fibre paper honeycomb core, with screw connection are prone to failure due to this geometry discontinuity and the material transition caused by the direction of load and bending moments. The analysis considers an appropriate modelling approach of the failure mechanisms occurring during a pull-out test according to the ESA insert design handbook of a developed insert connection using explicit simulation codes and special algorithms. For this, extensive preliminary investigations were carried out to determine the necessary, relevant parameters for the simulation. The presented work focuses on the material-specific design and the direction-dependent elasto-plastic material behaviour using a suitable material model to depict buckling and strength failure which depend significantly on the semi-finished fibre product used and the matrix system and manufacturing imperfections like cracks, pores and resin conglomerations in corners. A specific example illustrates the non-linear method considering the material behaviour, contact conditions and occurring failure mechanisms. The global load-displacement curve from experiment and calculation shows a good coincidence. The mapping of the successive failure and the resulting failure modes enables a targeted improvement of the insert connection design.