Moisture Diffusion, Compression and Degradation Mechanisms of Alfa Natural Fibers (Stipa tenacissima L.) Reinforced Epoxy: Experimental, Morphological and Numerical Analysis

The hygroscopic behavior of natural fibers plays a crucial role in determining the lifetime of biocomposites exposed to humid environments. This study investigates the hygroscopic stresses in Alfa fiber/epoxy composites under various relative humidity (RH) conditions (60%, 83%, 95%) and during immersion. A coupled hygro-mechanical model of multi-physical behavior, integrating both moisture diffusion and bidirectional mechanical responses, provides a detailed prediction of the stress evolution during water sorption, incorporating the experimentally determined hygroscopic swelling of Alfa fibers. Results show that the Alfa/epoxy composite exhibits hydrophilic behavior at RH levels above 60%, with significant anisotropic swelling during water immersion. In particular, out-of-plane expansion is much higher than longitudinal or transverse expansion. Moisture content increases considerably for fiber volume fractions from 0 to 13%, with less pronounced changes above 13%. The differential hygroscopic expansion between the composite and matrix generates compressive stresses, leading to interfacial debonding. The coupled hygro-mechanical loading results in both tensile and compressive stresses in the longitudinal and transverse directions, contributing to potential material damage. These observations are confirmed by scanning electron microscopy (SEM) analysis.