Contrasting Effects of Natural Aging on the Quasi-Static Properties and Fatigue Life of Pultruded GFRP Composites

This study investigated the quasi-static properties, fatigue behavior, and surface and fracture morphologies of pultruded glass/epoxy composites subjected to 24 months of natural outdoor exposure in Guangzhou, Hangzhou, and Suihua, China. Quasi-static tests were performed at scheduled exposure intervals, whereas fatigue tests on exposed specimens were conducted after 24 months. The quasi-static moduli and strengths fluctuated without consistent monotonic degradation. At both selected tensile stress levels, all exposed groups exhibited lower mean tension–tension fatigue lives than the unexposed reference, with the Guangzhou specimens showing the lowest means. Compression–compression fatigue exhibited greater scatter and no uniform reduction across the exposure groups. SEM observations revealed surface resin loss, fiber exposure, and local fiber–matrix separation, with more pronounced surface changes in the examined Guangzhou specimens. These observations suggest that local matrix and interfacial damage may contribute to the reduced fatigue resistance. Under the investigated conditions, tension–tension fatigue life showed a more pronounced response to 24-month natural exposure than the measured quasi-static properties, supporting its use as a complementary indicator in integrity assessment of pultruded GFRP composites.