Effect of cyclic hydrothermal accelerated ageing on the mechanical behavior of components in adhesively bonded aluminum/composite joints reinforced with z-pins

Abstract

This study presents an in-depth investigation into the moisture absorption and mechanical behavior of Glass Fiber Reinforced Polymer (GFRP) composites and adhesive bonded joints under hydrothermal aging conditions. The aim of this research is to assess the effects of moisture exposure and aging cycles on the performance of GFRP composites and their bonded joints. The experimental program involved preparing GFRP composite specimens and adhesive samples with specific thicknesses and subjecting them to hydrothermal aging cycles. Moisture absorption and desorption behavior of the adhesive and composite specimens were evaluated using Fick’s law, and equilibrium moisture content and diffusion coefficients were determined. Tensile tests were conducted to assess the mechanical properties of the materials, including elastic modulus, ultimate strength, and ultimate strain. The results demonstrated that both the adhesive and GFRP composite exhibited continuous moisture absorption and desorption behavior, following Fick’s law. The moisture uptake reached a saturation point, indicating equilibrium moisture content. The diffusion coefficients of the composite were found to be significantly influenced by the saturated moisture concentrations, highlighting the role of the fiber/matrix interface. Furthermore, the mechanical properties of both the adhesive and GFRP composite were evaluated under hydrothermal aging conditions. The elastic modulus of the adhesive increased after the first aging cycle, indicating the completion of the curing process. However, subsequent cycles resulted in reduced mechanical properties due to plasticization, molecular network changes, and swelling caused by moisture. The GFRP composite exhibited a reduction in ultimate strength and elastic modulus with increasing aging cycles, along with a decrease in strain. The study revealed that the moisture absorption affected the failure modes of the bonded joints, leading to a transition from fiber failure to adhesive failure. Hybrid bonded joints, incorporating pins to distribute stress, exhibited improved stability and higher ultimate loads compared to simple joints. This work contributes to the existing body of knowledge by providing a comprehensive understanding of the moisture absorption/desorption behavior and mechanical properties of GFRP composites and adhesive bonded joints under hydrothermal aging. The findings highlight the need for further investigation into the underlying mechanisms, long-term durability, and development of moisture barrier coatings to enhance the performance of GFRP composites in challenging environmental conditions.