In situ tomography reveals mode II delamination migration in multidirectional composites

Interlaminar fracture under mode II-dominated loading is a critical damage mechanism in fibre-reinforced composite laminates, but conventional edge-based measurements cannot resolve non-planar crack growth in multidirectional architectures. Here, in situ X-ray computed tomography is combined with a bespoke four-point bending rig to visualise delamination in carbon-fibre-reinforced laminates. A unidirectional 0° laminate showed planar crack growth confined to the original interface, enabling conventional fracture-toughness interpretation. By contrast, a ±45° laminate exhibited successive crack-migration events, producing a non-planar zig-zag fracture path involving interlaminar fracture and intralaminar splitting. These observations show that crack propagation in multidirectional laminates is non-self-similar and that conventional data-reduction methods may not yield a physically meaningful interlaminar fracture toughness. Instead, the measured response represents an apparent global fracture resistance influenced by laminate architecture, crack-front distortion, and crack-path evolution.