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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 ε/Δm) than thermal expansion (α f, R = 78 10 −6 ε/°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. [ABSTRACT FROM AUTHOR]

The purpose of this article is to understand the influence of typical composite parameters (interfacial bond strength, surface-area and fibre mechanical properties) on the tensile properties of nonwoven composite materials. The materials investigated were flax, hemp and Poly-(propylene) (PP) and Maleic Anhydride-grafted PP (MAPP), which provide different configurations in terms of fibre mechanical properties, bundle individualization and fibre/matrix interface. Whereas hemp fibres exhibit poorer tensile properties and lower bundle individualization than flax fibres, their higher lignin content evaluated by FT-IR analysis improves the interfacial shear strength (IFSS) with PP and MAPP. However, the tight interface developed with hemp fibres has only a weak effect at the scale of composites. With low-IFSS systems such as plant fibres associated with polyolefin matrices, mechanical properties at the macroscale are governed by the fibre mechanical properties and bonding area rather than by the interfacial bond strength. [ABSTRACT FROM AUTHOR]