Size effect of natural fibre reinforcement on mechanical performance for the development of thin bio-composite laminates: do fibre bundles represent a limitation?

As a consequence of the scale effect, thinner laminates generally exhibit fewer defects than thicker ones. However, the use of natural fibres in thinner composites (approximately 500 µm), such as composite membranes, remains limited due to the inherent geometric and mechanical variability of natural fibres, which significantly affect properties at reduced thicknesses.

This study investigates the effect of laminate thickness on the longitudinal tensile properties of unidirectional flax/PPMA biocomposites, with thicknesses ranging from 220 µm to 1778 µm.

Cross-sectional analyses examine changes in reinforcement distribution and the degree of flax fibre individualisation, specifically, whether fibres are dispersed as elementary fibres or retained in bundles. Strain fields are analysed using Digital Image Correlation (DIC) during tensile tests to localise high-strain regions.

Results show that when flax fibre bundles dominate the microstructure, mechanical performance decline. In contrast to conventional expectations of the scale effect, there appears to be a critical thickness threshold (approximately 550 µm), below which tensile properties decline. At the lowest tested thickness, tensile performance decreased by up to 20 % relative to the maximum. Back-calculated laminate properties closely match those of the bundles, indicating that conventional scale effect theory does not adequately reflect the heterogeneous structure of flax fibre bundles, and must be reconsidered for natural fibre-reinforced biocomposites. This study highlights the relevance of mechanical characterisation at the final object scale rather than at the norm scale as it exhibits a different behaviour. Undermined by the increased heterogeneity in reinforcement structure mainly dependant on bundles for their mechanical behaviour.