Continuous analysis and evaluation of production-related fibre misalignments in thick-walled fibre-reinforced plastic composite structures

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In order to increase the range of applications for fibre-reinforced plastic composites (FRP) in highly stressed structural components, it is necessary to produce thick-walled multi-layer laminates. For the efficient production of thick-walled FRP components, the braiding technique and the high-pressure resin transfer moulding process (HP-RTM) are suitable. The stacking of a large number of textile reinforcement layers, the high infiltration pressures of the HP-RTM process and curing effects of the polymeric matrix material often lead to fibre misalignments due to production. These are mainly formed in the laminate plane, but with increasing laminate thickness, they also become more pronounced in the laminate thickness direction. Altogether, the fibre misalignments are probably responsible for the documented disproportionate degradations of the quasi-static in-plane compressive strength with increasing laminate thickness.The phenomena that lead to the occurrence of fibre misalignments in thick-walled FRP structures are to be detected, analysed and evaluated within this research project using the example of a generic test structure made of glass fibres and polyurethane resin. The sequential manufacturing steps from the insertion of the dry preform into the infiltration tool to the high-pressure infiltration and curing are examined with the aid of computer tomography and other NDT-methods and modelled using a digital twin. In thick-walled composite structures, the impregnation behaviour is a key factor in the development of composite imperfections. Within the scope of the planned project, the impregnation behaviour of thick-walled braided structures in the thickness direction will be characterized with a novel test method. The results of the experimental investigations serve as a basis for the further development of modelling approaches, which describe the direction-dependent impregnation behaviour with the aid of representative volume elements.The analysis of the pressure conditions during impregnation provides information on the phenomenol-ogy of the development of fibre misalignments. The effects of the recorded phenomena can be evaluated by comprehensive thermo-mechanical investigations under quasi-static and cyclic, as well as uni- and multiaxial loads. Through the synergetic combination of online process monitoring, structure elucidation together with material and structure tests, the foundations can be laid for subsequent work on process simulation and process control strategies with regard to lower composite imperfections.