Divergent tensile and flexural responses of pultruded glass fibre/epoxy composites under accelerated UV aging: a coupled DSC, FTIR, and mechanical investigation

Pultruded glass fibre reinforced polymer (GFRP) composites are increasingly used in structural applications owing to their high strength-to-weight ratio and corrosion resistance. In outdoor service, however, UV radiation can alter the resin matrix. In this study, pultruded unidirectional glass fibre/epoxy laminates (anhydride-cured bisphenol-A epoxy, fibre-free resin-rich surface layer of 257 µm) were exposed to accelerated UV aging for 200, 400 and 800 h (UVA-340 lamps, ISO 4892–3 Cycle 2). Thermal, chemical and mechanical responses were characterised by DSC, ATR-FTIR, and tensile and bending tests. FTIR spectra normalised to the aromatic band at 1508 cm−1 showed increasing carbonyl, hydroxyl and C–O bands, with the carbonyl index doubling within 400 h and stabilising thereafter, indicating photo-oxidation of the exposed surface. In contrast, the DSC midpoint glass transition temperature remained between 94.9 and 101.5 °C at all exposure times, showing the bulk matrix was unmodified. Tensile strength (576 MPa) and modulus (53 GPa) were unaffected by aging, whereas flexural strength and modulus increased by 19.8% (785 to 940 MPa) and 19.1% (47 to 56 GPa) after 800 h, and the fracture path became nearly straight. A layered-section stiffness model showed that stiffening of the resin-rich skin alone cannot explain the flexural modulus increase, whereas the change in failure mode is consistent with an embrittled surface layer on the tension face. These results show that surface-confined UV-induced chemical changes affect the flexural and tensile responses of pultruded GFRP differently, providing a baseline for UV durability assessment.