CFRP/Metal Hybrid Material for Improving Composite Bolted Joints

Kurzfassung

One effective method of increasing the mechanical joint efficiency of highly loaded composite joints entails the reinforcement of the joining area with thin high-strength metal foils. These foils are locally embedded into the composite laminate by means of ply substitution techniques, thus avoiding any local laminate thickening and providing for a local hybrid laminate with high bearing and shear capabilities and high robustness. The feasibility and suitability of this advanced reinforcement technique has been extensively studied in view of its application on highly loaded bolted joints of cylindrical and conical spacecraft structures and monolithic and sandwich laminates aiming at structural weight savings, assembly work reductions, robustness increase and overall structural design optimizations. The future Ariane 5 composite Booster, a payload adaptor and an ATV distancing module are the main applications being considered within the study. The presented activities include numerical and experimental studies which are focused on the mechanical performance improvement in comparison to the reference conventional design. An extensive sample and component test program has been performed which analyzes and evaluates the impact of different design parameters and load conditions on the load bearing capabilities and hence the reinforcement efficiency. The single bolt bearing strength of the hybrid laminates on the one hand and the coupling efficiency of the transition zone, which is characterized by the gradual replacement of certain composite plies, on the other represent the two basic issues being focused on. Considerable bearing strength improvements of up to 2.8 times the bearing strength capabilities of the reference pure CFRP have been reported. The bearing behavior of the bolt loaded holes of hybrid laminates as well as the delamination and strength behavior of the ply substitutions within the transition region have been studied and predicted by means of numerical simulations which consider the progressive damage response of the composite material and the elastoplastic behavior of the reinforcing metal. The correlation with the test results shows an excellent accuracy of the modeling strategy. The verification of the hybrid technique’s processability, inspectability and compatibility with standard industrial processes has been successfully demonstrated by the manufacturing of a payload adaptor by the fibre placement technique featuring continuous and discrete hybridized regions for the upper and lower root joint as well as for local tank and shear panel attachments.