Compressive failure of unidirectional NCF composites

Abstract: With more people flying every year, new technologies are needed to reduce our impact on the environment. One option is to reduce the energy used in flight by introducing lighter materials such as carbon fibre reinforced polymers (CFRP). This type of material can be used in cold to moderately high temperature regions of the aero-engine and the current trend is to maximize its use. The current use of CFRP in aerospace is dominated by tape-based composites (pre-pregs), which are processed in an autoclave. This offers optimum performance but at high cost. 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. Composites based on dry textile  reinforcements such non-crimp fabric (NCF) offers potential cost savings over tape-based pre-preg materials, with good mechanical properties in general. One problem with the textile composites is their relatively low strength in longitudinal compression. This is due to the higher degree of fibre waviness generated by the textile architecture. The aim with this research projects is to develop failure criteria and computational methods needed for reliable and efficient design of aero-engine components from textile composites. When longitudinally arranged fibres in a composite are wavy, local misalignments are generated with respect to the load axis. These induce shear stresses, critical to compressive failure. The sensitivity to fibre misalignments is generally well known. Yet, no systematic measurements have previously been conducted of its spatial distribution. Existing models for strength prediction consider fibre misalignment representations as either, a scalar value, periodic or random. Our approach is instead based on measurements of fibre misalignment with high accuracy and high spatial resolution in a large number of samples. Misalignment data has been used for statistically based direct assessments of compressive strength. The misalignment data has also been used to calibrate models for strength prediction and for numerical studies to increase understanding. The diversity in studied fibre misalignments are not generated by artificial means, but instead reflect upon the relevant material architecture and processing principles. Many studies on compressive failure seek to model or understand details on kink-band formation. We have instead maintained a clear focus on failure initiation, relevant to aero-engine components. We have addressed the extreme sensitivity of kink-band initiation to fibre misalignment angle that subsequently lead to compressive failure within a ply. We conclude that kink-band initiation in practical fibre composites is a coordinated kinematic event. It requires studies of regions with real (measured) spatial distributions of fibre misalignment angles. These studies are preferably conducted on 2D micrographs parallel to the kink-plane.