Hygroscopic expansion: A key point to describe natural fibre/polymer matrix interface bond strength

Abstract 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 (β f,R  = 1.14 e/Δm) than thermal expansion (α f, R  = 78 10 −6 e/°C). Thus, its contribution to the development of residual stresses σ rad 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/σ rad ) ≈ 0.46 are calculated. Therefore, the interfacial bond strength of natural fiber/polymer systems should be described by taking into account their hygroscopic behavior.