In situ transverse single-fibre and bundle tests in synchrotron nano-tomography to unravel interfacial debonding in carbon and glass fibre-epoxy composites

Interfacial normal (mode I) properties are crucial for understanding and predicting the transverse failure behaviour of fibre-reinforced composites. The experimental methods to extract them, such as the Broutman and the transverse single-fibre tests, are typically coupled with 2D monitoring techniques to characterise the involved damage mechanisms. Inevitably, these techniques struggle to visualise the debond growth in situ and in 3D, which results in questionable values and the need for significant assumptions in models. This paper reports a state-of-the-art methodology integrating transverse single-fibre testing with synchrotron holo-tomography. This nano-imaging technique enables the in situ detection and monitoring of the 3D debond profile and its interaction with other damage mechanisms, such as matrix cracking. In situ tests were conducted for carbon and glass single-fibre epoxy specimens and a specimen with a transversely embedded carbon fibre bundle. By combining these different tests, novel insights were obtained into the influence of fibre type and packing on mode I interfacial damage initiation and propagation. Despite observing the same three damage phases — debond initiation, slow propagation, and tunnelling — in all specimens, our findings suggest that debond initiation occurs at lower stress levels in closely packed fibres while tunnelling is independent of fibre packing. The damage phases are quantitatively expressed via a series of parameters, offering direct input for numerical modelling of the transverse tensile failure of composites.