Interleaving light veils to minimise the trade-off between mode-I interlaminar fracture toughness and in-plane properties

Interleaving heavy micro-fibre veils, commonly used to improve interlaminar fracture toughness, severely compromises tensile and compressive properties. To reduce the trade-off, this study interleaved two different types of light (4 g/m(2)) co-polyamide veils into a thin-ply quasi-isotropic baseline laminate. Sub-micron resolution X-ray micro-computed tomography provided insight into the after-manufacturing state of the veil fibres as well as into the fracture process zone of mode-I interlaminar fracture toughness specimens. The veil fibre diameter was the key parameter in determining the tensile properties (the veil with thinner fibres avoided resin accumulation at the interfaces and left the baseline properties unaffected). Both veils decreased the compressive strength by up to 9%. Mode-I crack propagation was controlled by the adhesion between the veil fibres and resin. Veils fibres with the higher adhesion deflected crack propagation to the surrounding 0 degrees plies, which improved mode-I initiation and propagation interlaminar fracture toughness (by 101% and 43%).