Implications of manufacturing-induced variation in tow architecture on the damage behaviour of braided composite tubes under torsion

The aim of this study was to examine the extent to which different resin infiltration processes can lead to different tow architectures and hence different mechanical properties in braided composites based on the same textile preforms (i.e. having the same fibre content). To this end single-layer carbon/epoxy (±45°) braided tubes have been manufactured by vacuum assisted resin infusion using: (a) resin transfer moulding (RTM) with rigid inner and outer moulds, and (b) vacuum infusion (VI) with a rigid inner mould, but with a flexible outer mould. Their 3D architecture and the resulting torsional behaviour has then been investigated by in-situ synchrotron X-ray CT and image-based finite element modelling. The VI process gave a significantly reduced tube wall thickness (∼35%) relative to RTM, higher effective fibre volume fraction (∼55% VI vs ∼33% RTM) and flatter (half-lenticular) tow geometries with greater in- and out-of-plane crimp and higher crossover density. Taken together these differences in mesoscale architecture led to the VI braided tube exhibiting ∼16% higher shear modulus yet ∼30% lower torsional rigidity compared to the RTM specimens. Image-based modelling showed that the flattened tows and increased crimp angles in the VI braided tubes amplified the local stresses, leading to earlier damage initiation (∼0.5% shear strain) and intra-tow cracking. More widely this study exemplifies how different resin infiltration processes can lead to different post-cure fibre architectures and mechanical performance, even when based on the same textile preform.