This paper deals with durability of glass fiber-reinforced polymer composites under sustained loads and simultaneously exposed to either saltwater or tap water. The tensile properties of the specimens before and after conditioning were evaluated along with their moisture uptaking behavior to reveal the durability of the specimens under combined effects. The mass change curves under saltwater and tap water immersion exhibited dual-stage diffusion process. The first stage followed Fickian profile, while mass loss occurred in the second stage with increasing immersion time, attributed to hydrolysis of resin. The weight gain in saltwater was greater than that in tap water. Moreover, the mass change curves under varying percentage of sustained loadings exhibited similar trends, while the maximum moisture uptake was obviously affected by the loading conditions. Based on Fick’s second law, the diffusion coefficients in the first stage under different conditions were determined through the least-square regression technique. It was observed that the tensile strength and modulus were increasing initially upon immersion, which was attributed to curing beyond the initial cure during manufacture. Then, the tensile properties decreased with continued immersion in saltwater or tap water. After 180 days of immersion, the tensile properties degraded at a smaller rate, both for specimens with and without sustained loading. For the saltwater immersion of 360 days, the tensile properties decreased significantly as the sustained loading increased. However, the tap water immersion had less detrimental effect on tensile properties of glass fiber-reinforced polymer. Furthermore, based on Arrhenius concept coupled with fracture mechanics principle, a new model was proposed to describe the combined effects of immersion and sustained load.
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