Effect of UV‐water weathering on the mechanical properties of flax‐fiber ‐reinforced polymer composites

Abstract This study examines the influence of weathering aging on the mechanical properties and durability of flax fiber‐reinforced polymer composites (FFRP) intended for outdoor applications. Employing an accelerated weathering environment replicating natural outdoor conditions, encompassing UV radiation, condensation, and water spray, the FFRP samples with 2 and 4 mm thicknesses underwent exposure durations of 0, 500, 1000, and 1500 h. Mechanical tests characterized tensile, flexural, and inner‐plane shear properties, while a custom‐made apparatus measured the long‐term tensile creep behavior. Additionally, scanning electron microscopy allowed for the observation of microscopic changes in fiber and matrix. The findings illustrate a consistent decline in tensile and flexural properties with increased weathering exposure, resulting in reductions of 17% and 38% in the tensile strength and modulus, and 24% and 52% in the flexural strength and modulus after 1500 h. Intriguingly, inner‐plane shear strength exhibited a slight increase after 500 h before a subsequent decrease. Furthermore, samples with the same thickness demonstrated an increased creep development with longer weathering durations. Despite this, the mechanical degradation rate induced by weathering was comparatively slower for thicker samples. Overall, the diminished mechanical properties of FFRP post‐weathering can be attributed not only to the polymer but also to the substantial role played by flax fibers in this process. Highlights UV weathering degrades the mechanical properties of flax/polyester composites. Both the instantaneous elastic properties and the long‐term creep behavior are affected by weathering. Thickness influences the mechanical degradation of the composites caused by UV weathering. SEM confirmed UV‐water weathering resulted in the degradation of both natural fiber and polymer matrices.