Mechanical, morphological and micromechanical characterization of multiscale walnut shell powder, E-glass fibre and graphene reinforced epoxy hybrid composites

Abstract

The immense advancement in materials science has led to a progression across engineering disciplines, changing structural requirements from monolithic conventional materials to highly structured, multiphase composite materials. Hybrid composites utilize the properties of individual components, substituting for the deficiencies of the matrix and other primary reinforcements. This research focuses on advanced multiscale hybrid composites fabricated using an epoxy matrix reinforced with natural Walnut Shell Powder (WSp), synthetic E-glass fibres, and nanoscale graphene, prepared with the hand lay-up method, and cured at ambient temperature. The hybrid composite samples were tested for their performance in tensile, flexural, and compressive loads. The experimental results demonstrate that while the addition of 5wt% WSp significantly enhances compressive strength by 75.30% whereas its improvement in tensile characteristics is confined by particle agglomeration and interfacial void formation. In contrast, the multiscale hybrid composite sample reinforced with 15 wt% continuous E-glass fibres and a 0.1 wt% graphene showcased the optimal mechanical synergy. The E-EgG hybrid demonstrated a significant rise of 71.66% in ultimate tensile strength, 123.81% in flexural strength, and 355.71% in compressive strength in comparison to the neat epoxy sample. Morphological evaluation via Scanning Electron Microscopy (SEM) confirmed that the nano-scale graphene fundamentally alters the properties, bridging micro-cracks, improving the strength, and inducing significant crack deflection. The experimental trends were further confirmed through analytical micromechanical modeling. Rule-of-Mixtures and Halpin–Tsai predictions aligned with E-Eglass within 2%, while Halpin–Tsai substantially underpredicted the E-Gr modulus (−44.5), while a two-step homogenization approach, wherein graphene is initially assumed to be a matrix-toughening phase before superposing the macro-scale reinforcement, predicted the E-EgG modulus within 3.8% of the experimental value (7.81 vs. 9.22 GPa). Notably, the E-WSpG hybrid outperformed its corresponding two-step prediction by ∼46%, providing quantitative, model-based evidence for a genuine interdependent interaction between the micro-scale WSP and nanoscale graphene phases.