Comparative repair of drilled flame-retardant GFRP composites using UV-curable and unsaturated polyester resins

Glass fiber-reinforced polyester (GFRP) composites are widely used in structural applications because of their high specific strength, corrosion resistance, and easy processing. However, drilling during assembly can induce local damage, reduce load-bearing capacity, and affect fire behavior. In this study, drilled flame-retardant GFRP laminates were repaired using ultraviolet (UV)-curable resin or conventional unsaturated polyester (UP) resin, and the effects of repair route and flame-retardant type on mechanical and burning performance were compared. Aluminum trihydrate (ATH), ammonium polyphosphate (APP), and zinc borate (ZB) were incorporated into the polyester matrix at 10, 15, and 20 wt.%. Undamaged, drilled, and repaired specimens were characterized by tensile and three-point bending tests, glass fiber content analysis, horizontal burning tests, delamination measurements, and scanning electron microscopy. Both repair methods improved the mechanical response compared with unrepaired drilled specimens, although full recovery was not achieved. Under the fixed curing conditions investigated, UV-based repair system achieved tensile and flexural recovery closed to UP-based repair system for the unfilled control and selected flame-retardant formulations, while reducing the nominal curing duration. Under the UL 94 HB test conditions applied in this study, the samples containing APP exhibited self-extinguishing behavior after the ignition source was removed.