A micromechanical framework for the ductile-to-brittle transition in particulate biocomposites: Validation on starch/olive stone systems

This study presents a unified micromechanical framework to predict the ductile-to-brittle transition in particulate-reinforced biocomposites. While bio-based matrices are inherently ductile, incorporating rigid natural fillers triggers a mechanical shift toward damage-dominated failure. This behaviour is formulated by integrating elasto-plastic hardening with continuum damage mechanics through a multi-regime constitutive law. To capture non-linear stiffening, a hybrid Mori–Tanaka Cellular Solid model is proposed, utilising Gibson-Ashby scaling laws to describe the percolating filler network formation. Failure limits are governed by modified Pukánszky and Nielsen-Saturation models, introducing a physical threshold for structural network emergence based on the effective maximum packing fraction. The framework was validated using a starch/olive-stone-powder (OSP) system across 0%–30% (w/w) filler loadings, accounting for plasticiser and residual moisture. Notably, results reveal an optimal composition at 15%–20% (w/w) loading, where the biocomposite doubles the matrix’s tensile strength while retaining internal plasticity. Benchmarking via Ashby property maps demonstrates the material bridges the gap between commodity polyolefins and rigid engineering plastics like PLA and ABS, ultimately migrating toward a wood-like cellulosic domain at higher fractions. This unified framework serves as a robust design tool for transforming variable agricultural waste streams into tailored, high-performance biocomposites with calibrated strength and toughness.