Multiscale characterization of moisture-induced fiber-resin interface degradation in glass fiber composites

Abstract

Glass fiber reinforced polymers (GFRP) are widely used in industrial components. They consist of glass fibers, typically in a woven or stitched fabric, a polymer matrix, and sizing. Sizing, applied during manufacture of the fiberglass fabric, facilitates fiber-matrix bonding and forms the 'interphase' region where bonds enable effective load transfer. This research investigates how moisture affects the interphase across different polymer matrices. Four studies were conducted at increasing structural scales. Atomic force microscopy measured the stiffness of neat epoxy surfaces, revealing a measurable decrease in elastic modulus after twenty days of moisture exposure, confirming polymer degradation over the moisture conditioning period. At the composite level, a high-precision 3D scanner quantified moisture-induced swelling in thick laminates and neat resin, comparing control and porous specimens to evaluate the influence of defects. A third study examined composites with two epoxy and two vinyl ester matrices, using standardized mechanical tests on control and moisture-conditioned laminates to isolate interfacial degradation effects. Finally, fatigue tests were performed under various environmental conditions with a custom apparatus that allowed fully submerged loading. Combined moisture and cyclic loading significantly affected epoxy specimens, with less pronounced effects in vinyl ester specimens. Understanding water absorption and its impact on the fiber-resin interphase provides a basis for improving the durability of marine hydrokinetic structures, which experience simultaneous moisture diffusion and cyclic loading. Moisture-induced polymer breakdown and interfacial deterioration accelerate damage, while interfacial cracks facilitate further moisture ingress. By integrating mechanical testing, optical microscopy, and atomic force analysis, this study illustrates the interactions between moisture, mechanical damage, and interfacial degradation, guiding the design of more robust submerged composites.