Anisotropic behaviors of moisture absorption and hygroscopic swelling of unidirectional flax fiber reinforced composites

Abstract

This work aims to study the anisotropic moisture absorption characteristics of unidirectional flax fiber reinforced composites (FFRCs) and evaluate the variations of the internal stress caused by the hygroscopic deformation, which might lead to the deterioration of the mechanical properties of the composites. FFRCs were prepared using vacuum-assisted resin infusion molding (VARIM) technology. Moisture absorption tests were designed by sealing the designated surfaces with a polyurethane waterproof coating to keep the moisture absorption of FFRCs only in the desired directions. The deformation caused by moisture absorption in the width direction was also monitored. It confirms that the moisture absorption of FFRCs conformed to the 3D Fick's law, and the influence of moisture absorption in the width direction cannot be ignored, especially for the unidirectional transverse tensile test specimen (UTFRC). The hygroscopic deformation of the unidirectional FFRCs showed that the material expands perpendicular to the fiber direction but shrinks along the fiber direction. The modified 3D Fick’s law based on heat conduction equation was performed to reveal the mechanism of the above moisture absorption and hygroscopic deformation. According to the finite element inversion, it was found that the axial diffusion coefficient along flax fiber bundles was much larger than the transverse diffusion coefficient perpendicular to the fiber bundle direction due to the existence of the unique lumen structure inside flax fibers. In addition, the high internal stress caused by the mismatched swelling properties between the fiber and the matrix was the main reason for the damage of the FFRCs, especially at the interface, which may seriously affect the service life of the composites. The outcome of this study can be used to accurately analyze the hydro-elastic behaviors of plant fiber reinforced composites, which is a prerequisite for predicting the mechanical properties affected by moisture absorption.