Micromechanical modelling for wood–fibre reinforced plastics in which the fibres are neither stiff nor rod-like

Plant fibres distort to curved or kinked shapes during injection moulding, while glass fibres are relatively stiff and remain rod-like. The consequences of these differences were investigated for an example of wood fibre prepared by thermomechanical pulping to reinforce polypropylene in tensile test specimens. Krenchel’s orientation factor increased with distance from the gate, reaching values similar to those published for glass-reinforced plastic. Halpin–Tsai and Mori–Tanaka micromechanical models predicted the tensile modulus within ±7% at low strain, despite implicit and incorrect assumptions of rod-like shape. Both models assumed elastic reinforcement with perfect fibre–matrix bonding, and therefore overestimated the tensile stress at higher strains.