The present article aims to investigate the contribution of hygroscopic expansion of flax fibres to interfacial radial stresses and Interfacial Shear Strength (IFSS) of Maleic Anhydride grafted PolyPropylene (MAPP)/Flax biocomposites.

During manufacturing of thermoplastic biocomposites and storage at 50% RH, a weight variation is observed, attributed to water content evolution within plant cell-walls. The hygroscopic radial expansion coefficient βr flax of single flax fibres estimated by Environmental Scanning Electron Microscopy (ESEM) observation is many orders of magnitude higher (βf,R = 1.14 ε/Δm) than thermal expansion (αf, R = 78 10−6 ε/°C). Thus, its contribution to the development of residual stresses σrad during processing should be prevalent. A multiscale analysis of interfacial stress state and hygroscopic contribution is performed with the use of a cylindrical concentric model at microscopic scale and asymmetric composite laminates [0, 90°] curvature generation at macroscopic scale. Similar radial stresses are obtained, while relevant values of μ (IFSS/σrad) ≈ 0.46 are calculated. Therefore, the interfacial bond strength of natural fiber/polymer systems should be described by taking into account their hygroscopic behavior.