Methods for fibre-orientation characterisation in monolithic carbon-fibre composites

The route to lighter composite aerostructures requires advanced 3D non-destructive characterisation methods to provide confidence that the as-built structures conform to the design expectations. Whilst X-ray Micro-CT imaging is an excellent NDT method for 3D characterisation, it can rarely be applied in production. In its place, Ultrasound is an ideal vehicle for exploring the detailed local response of a composite structure to (acoustic) stress and map this across the whole 3D structure. Its wavelength, bandwidth and beamwidth can be optimised for just the right sized volume element to characterise plies and fibre tows, whilst providing data that can be inverted to give 3D fibre direction, ply spacing, fibre volume fraction and, in the future, 3D porosity distribution. The first material property that must be determined to enable full characterisation and materials modelling of as-manufactured composite components is the vector field representing the fibre direction at every point. Ideally, an inversion method for converting NDT data sets into 3D profiles of material properties would work with both X-ray CT and ultrasound data. Following an extensive study of the literature for characterising other kinds of fibrous or textured images (fingerprints, hair, textiles, etc) the most promising methods have been evaluated and compared. This paper presents the findings of this evaluation and demonstrates the ability of the optimum method to map in 3D the actual fibre orientations throughout a composite component.