On matrix-driven failure in unidirectional NCF composites - A theoretical and experimental study

Licentiate thesis, 2016

With ever increasing traffic levels, the civil aircraft industry is in constant need of new technologies to make air travel environmentally sustainable. One such technology is light materials to reduce the energy consumption in flight. Currently, up to 53 weight percent of the Airbus A350XWB aircraft is made from carbon fibre reinforced polymer (CFRP) material, mainly in the fuselage and wings. There are other areas in the aircraft where components traditionally made from metals can be replaced with composite material, for instance the cold and moderately high temperature parts of the engines. CFRP fanblades were successively introduced in a civil aircraft engine (GE90) in 1994 and there is now an interest to increase the use of CFRP deeper into the engine. The current research project is carried out in response to an industrial need of cost-effective CFRP components in load carrying parts of the engine. The preferred route is an automated, out-of-autoclave manufacturing method using resin transfer moulding (RTM) with a non crimp fabric (NCF) as reinforcement. Carbon fibre/Epoxy composites reinforced with NCF offer potential cost savings over tape based prepreg materials, with good mechanical properties - close to that of prepreg type composites. For this reason, NCF-reinforced composites provide an interesting alternative to prepregs for the aerospace industry. One limiting factor for their use in primary structures is their relatively low strength in compression. The overall goal of this research project is to further understand the compressive behaviour of NCF composites and to develop a strength assessment method for the aerospace industry. In the first part of the project, we increase the fundamental understanding of the composite material on a meso-scale level, where the NCF have a specific architecture, consisting of fibre tows in various configurations. A new failure criterion has been developed to take into account the orthotropic properties in the transverse (2-3) plane. Local interaction between two bundles out-of-plane was simulated with a finite element model and it was proved to be one possible explanation for the reduced strength out-of-plane. In the second part of the thesis, we investigate the influence of intrinsic material variations on the performance of an NCF composite loaded in compression. These intrinsic variations to the material have been identified as potentially likely to occur in future aero-engine composite structures. In conjunction with the compression test campaign, a method to measure fibre misalignment angles out-of-plane has been developed based on microscopy and a Matlab script. The fibre waviness was found to have a strong adverse effect on the compressive stiffness and strength of the material. The measured compressive strength was reduced to half when the mean fibre misalignment angle was doubled.