Peridynamic modelling of functionally graded interphases in spray-coated natural fibre composites

Natural fibre-reinforced polymer composites are increasingly considered for sustainable structural applications; however, their performance is often constrained by weak fibre–matrix interfacial adhesion. Nano-reinforced biocoatings have emerged as an effective means of enhancing this adhesion by creating a transitional interphase with spatially varying stiffness and toughness. Despite their physical presence and mechanical significance, such coatings are commonly represented in computational models as idealised zero-thickness interfaces, neglecting their finite geometry and graded properties. This study introduces a functionally graded interphase (FGI) model for spray-coated flax/epoxy composites within an ordinary state-based peridynamic (PD) framework. The nanobiochar (NBC) coating is explicitly resolved as concentric layers of finite thickness surrounding each fibre. A modified three-phase Halpin-Tsai formulation is employed to determine the radial variation of elastic stiffness and fracture toughness based on local nanoparticle volume fractions. The model is validated against transverse tensile experiments conducted on unidirectional flax/epoxy composites coated with 5 wt% NBC. The simulations conducted by using PeriLab successfully reproduce the nonlinear pre-peak stress–strain response and capture experimentally observed microscale damage mechanisms, including tortuous crack paths that propagate around intact fibres. Furthermore, analyses of randomly generated microstructures reveal that local fibre clustering plays a dominant role in controlling post-peak variability and fracture morphology. These results demonstrate that the explicit FGI formulation provides a physically representative and predictive framework for modelling coated natural fibre composites and offers a robust tool for the design and optimisation of advanced bio-based composite materials.