Interlaminar toughening of flax/Elium® composites using asphaltene-derived carbon fibre interleaves

Natural fibre composites exhibit limited resistance to interlaminar crack growth, particularly under mode II loading, which restricts their applicability in structural components. This study examines a composite interleaving strategy using short asphaltene-derived carbon fibres produced by centrifugal spinning. A wet-laying process adapted from papermaking was used to fabricate 72 gsm interleaf without binders, consolidating fibres into randomly-oriented mats without extensive processing. Similar mats were also produced using commercial PAN-derived carbon fibres to establish a performance benchmark. Flax/Elium® laminates with an interleaf layer in the mid-plane were manufactured by vacuum-assisted resin infusion and tested according to ASTM D5528 (mode I) and ASTM D7905 (mode II). Neither interleaf type significantly changes mode I fracture toughness, despite some mechanistic changes. In contrast, the asphaltene-derived interleaf increases mode II fracture toughness by 40%, and the commercial carbon fibre interleaf increases it by 86%. Scanning electron microscopy confirms that both interleaf types induce crack deflection in the interleaf layer and increase propagation path tortuosity, resulting in higher shear-driven energy dissipation. This study presents a feasible manufacturing route for interleaved composites using wet-laid nonwoven fibre mats and demonstrates that asphaltene-derived carbon fibre interleaves can effectively improve mode II delamination resistance while largely maintaining the mode I fracture toughness in natural fibre thermoplastic composites.