Tomographic observation and modelling of three-dimensional fibre waviness in prepreg draping

Abstract

Draping of carbon fibre prepregs is widely used to form composite components with complex geometries, but imperfections generated during this process become locked into the cured composite and can strongly affect its structural performance. Here, we use X-ray tomography to investigate the formation of fibre waviness in prepregs subjected to controlled bending deformations representative of typical draping operations. The measurements reveal a three- dimensional (3D), higher-order buckling mode that develops perpendicular to the plane of bending and is therefore inaccessible to conventional surface imaging and digital image correlation (DIC). We systematically investigate how the wavelength of this instability depends on consolidation pressure, ply thickness, layup and fibre type. A laminate model is developed to provide a mechanistic description of the instability and the resulting fibre waviness. The model shows excellent quantitative agreement with the tomographic measurements and reveals how the buckling wavelength is governed by the material and geometric properties of the prepreg. Importantly, a comparison between model predictions and experiments enables quantitative inference of mechanical properties of soft, uncured prepregs that are otherwise difficult to measure directly. Together, these results provide a mechanistic framework for understanding and predicting the formation of fibre waviness during prepreg draping.