Synchrotron holotomography for in-situ damage monitoring of single fibre composites

A common bottleneck in state-of-the-art composite micromechanics models is their reliance on constituent properties, such as interface properties. The last round-robin activity to measure interfacial strength occurred two decades ago and reported large variability for the same carbon-epoxy interface. Despite this outcome, test methods to extract interfacial properties remain largely unchanged. They typically rely on microscopy to record parameters needed by conventional data reduction schemes, such as the fibre break density and the interfacial debond lengths. However, microscopy images the material's surface. Therefore, it neglects influential 3D damage characteristics. This paper presents the results of an in-situ tensile test conducted at the ID16B beamline, ESRF (European Synchrotron Radiation Facility). The tests were performed on carbon and glass single-fibre composites using nanotomography (holotomography, pixel size = 150 nm), which enabled 3D observations of fibre breaks, longitudinal interfacial debonds, and transverse matrix cracks. This novel experimental methodology allowed us to extract damage parameters used in conventional reduction schemes, such as the debond length, and critically assess them by considering damage features in 3D, such as the debond symmetry and uniformity and the influence of transverse matrix cracking.