Finite element modelling and high speed testing of countersunk composite aircraft joints

Abstract

Bolted joints feature extensively in composite aircraft structures, facilitating disassembly for maintenance while offering a cost effective joining method. However, inadequate tools for predicting failure of composite bolted joints have led to extensive physical testing programs and conservative design solutions. As usage of composites in airframe design has increased, the optimisation of composite bolted joints has become a key priority for aircraft manufacturers. The phasing out of metal in large commercial aircraft has now reached the fuselage structure, with next-generation aircraft such as the Airbus A350 featuring all-composite designs. Bolted fuselage skin joints tend to be single-lap and incorporate countersunk fasteners for aerodynamic reasons. In order to investigate the mechanical behaviour of single-lap countersunk composite joints, detailed 3D FE models have been developed. Initially, this difficult contact problem was studied elastically using the implicit FE solver, Abaqus/Standard. The mechanical response and hole boundary stress distributions were studied for clearance levels both inside and outside typical aerospace tolerances. Increased clearance delayed load take-up, reduced stiffness and altered hole boundary stresses. Significant convergence issues in these elastic implicit analyses prompted the choice of an explicit solver for challenging simulations of bearing failure in single and multi-bolt joints. A composite damage model was developed for the simulaton of bearing failure in 3D Abaqus/Explicit joint models. Physically-based failure criteria and a crack band model ensured accurate and objective solutions. Predictions of bearing failure in quasistatically loaded, carbon-epoxy fuselage joints correlated well with experiment and gave a novel insight into the development of failure in countersunk composite joints. As part of a dynamic experimental test series, these fuselage joints were shear loaded at 5 m/s and 10 m/s. The final failure mode significantly affected energy absorption, which governs crashworthiness. The 3D explicit approach was finally applied to study issues of concern to industry, which included dynamic simulations. The quasi-static mechanical behaviour of large, panel-level joints was also investigated. An automated Python scripting approach, developed to create 3D composite bolted joints in Abaqus, was presented as a GUI which dramatically reduces model preparation times