A multi-scale approach for the localization of the moisture content in composites with two hydrophilic constituents

Abstract

Predicting the service life of structural parts with hydrophilic constituents requires accounting for moisture-induced aging during the design phase. This involves determining the moisture content distribution through the material’s thickness over time. Traditionally, composites with at most one hydrophilic constituent are considered. For materials with multiple heterogeneous hydrophilic constituents, traditional methods for estimating macroscopic moisture content are no longer adequate. To address this, this work introduces an original, general methodology to localize moisture at the constituent scale (matrix and hygroscopic reinforcements) from the knowledge of the macroscopic laminate moisture field and the sorption isotherms of each constituent. The proposed framework employs an algorithm to correlate macroscopic water content fields with local moisture distributions in composite constituents. Model validation was conducted by comparing predicted moisture profiles and diffusion characteristics with experimental data on a flax/epoxy composite from literature. Using the Hailwood-Horrobin equation, the model accurately represents the sorption isotherms of flax/epoxy composites, with a high correlation coefficient (R² = 0.9905) to experimental data. For the epoxy matrix, the predicted maximum moisture content capacities are 1.4 wt% and 2.5 wt% at 45% and 75% relative humidity, in close agreement with the experimental values of 1.2 wt% and 2.4 wt%, respectively. For flax fibers confined within the matrix (composite), the calculated maximum moisture content capacities are 2.7 wt% at 45% relative humidity and 6.8 wt% at 75% relative humidity, compared with 4.2 wt% and 7.0 wt% reported for free fibers, thereby highlighting the confinement effect of the surrounding matrix.