Understanding the effect of hygroscopic cycling on the internal stress and stiffness of natural fibre biocomposites

Abstract

This article investigates the development and the evolution of hygroscopic stresses within flax/ Polypropylene + Maleic Anhydride grafted Polypropylene (PP + MAPP) asymmetric biocomposites when exposed to various hygroscopic cycles that simulate a wide range of outdoor applications with large Relative Humidity (RH) variation (from 11 to 50% RH, 50–99% RH, 11–99% RH) and with various cycling periods (1-week and 24-hour cycles between 50 and around 90% RH). The transversal and longitudinal tensile properties of biocomposite laminates are characterised, and the weight and curvature of the asymmetric samples are tracked throughout the cycles. The study shows that the hygroscopic cycles entail a reduction of internal stress due to damage development and a relaxation mechanism that are illustrated by permanent hygroscopic deformations. Two parameters have been identified to play a role on the damage development: the magnitude of the RH level and the duration of the cycles.