Flexural performance and microstructural correlation of natural silk fabric-reinforced epoxy composites

This study investigates the flexural behaviour and microstructure-property relationships of natural silk fabric-reinforced epoxy composites fabricated using a cast moulding route. Three rectangular specimens (100 × 20 × 5 mm) were tested under three-point bending in accordance with ASTM D790 with a 60 mm support span. The composites were produced at moderate fibre volume fractions (approximately 20-40 vol%), and their density and void content were evaluated using mass-volume measurements and optical image analysis. The measured flexural strength ranged between 100 and 130 MPa, with results reported as mean values along with standard deviation. The composites exhibited a pseudo-ductile response characterized by an initial linear elastic region followed by progressive damage and gradual post-peak softening. Although peak stresses were comparable, clear differences in strain-to-failure and post-peak stability were observed, indicating that deformation behaviour is governed primarily by interfacial integrity and microstructural heterogeneity. Multiscale characterization using optical microscopy and scanning electron microscopy (SEM) was carried out to establish structure-property relationships. Optical observations revealed that voids and non-uniform resin impregnation influence crack initiation and propagation. SEM-based fractography identified dominant failure mechanisms including matrix cracking, interfacial debonding, fibre pull-out, and localized fibre fracture. Specimens with improved impregnation showed better interfacial continuity and crack-bridging, whereas defect-rich regions exhibited premature damage initiation and unstable crack growth. The results demonstrate that load transfer and flexural stability are governed by interfacial stress transfer and processing-induced heterogeneity. The study provides a mechanistic interpretation of flexural behaviour and offers a basis for positioning silk-epoxy composites within natural fiber reinforced systems.