Enhancing flexural performance of pultruded GFRP core sandwich beams using thin and thick CFRP/GFRP face sheets

Abstract In this study, the flexural behavior of sandwich beams with GFRP core produced by pultrusion method and thin (~ 330 g/m²) and thick (800–1200 g/m²) CFRP/GFRP fabrics applied on the outer surfaces were investigated. Sixteen specimens with eight different layer configurations were subjected to three-point bending tests and the maximum load carrying capacity, peak displacement at maximum load and stiffness values were compared. Experimental results showed that thick surface fabrics increased the maximum load carrying capacity by 10–30% and stiffness by 34.8–295.0%; however, they limited the deformation capacity by reducing the deformation capacity at maximum load by 15.9–70.6%. Symmetrical and cross-layered specimens (CGPGC, GCPGC) with CFRP on the top surface showed the best performance in terms of stiffness-displacement balance (139–246% stiffness increase, 48–66% deformation decrease). Multi-stage fracture behavior (secondary/tertiary peak) was observed in all specimens, which allowed the thick fabric specimens to partially maintain their post-peak bearing capacity. The thin-faced beams showed brittle failure with sudden fiber breakage and delamination, while the thick-faced beams exhibited gradual damage progression with more diffuse delamination and local debonding mechanisms. In conclusion, in sandwich beam design, not only fiber type but also fiber placement strategy and fabric thickness are critical for optimizing the balance of bearing capacity, stiffness and strain capacity. In the future, it is recommended to further investigate these trade-offs with ± 45° or biaxial fabrics, adhesive thickness, environmental conditions and high cycle fatigue tests.