This paper investigates the translation of fracture toughness of polyester resins to the mode I interlaminar fracture toughness of polyester/glass fiber composites. An unsaturated polyester resin (UPR) and a vinyl ester resin (VER) were toughened by two very distinctive methods: (1) incorporating a very low loading (0.04 wt%) of alkyl or vinyl modified graphene oxide (mGO); or (2) adding 5 wt% of core-shell rubber (CSR). Glass fiber reinforced composites (GFRPs) were fabricated using the toughened resins as the matrix and various types of fiber mat as reinforcements. Results from compact tension tests revealed that 0.04 wt% loading of mGO increased the fracture toughness (GIC_Resin) of UPR and VER by as much as 49% and 31%, respectively, without lowering the modulus and glass transition temperature (Tg). On the other hand, CSR is a more effective toughener, though at much higher loading. CSR enhanced the GIC_Resin of UPR and VER by 375% and 600%, respectively, with a slight decrease in flexural properties and Tg. Flexural properties and mode I interlaminar fracture toughness (GIC_Comp) of the GFRPs were evaluated. The incorporation of mGO and CSR did not alter the flexural properties of the GFRPs, regardless of resin type and fiber architecture. CSR was not able to increase the GIC_Comp of UPR laminates; however, CSR increased the GIC_Comp of VER laminates by 33%. The observed toughening effects do not translate directly from resin to GFRPs (i.e. a 600% increase in GIC_Resin of VER by 5 wt% of CSR led to only a 33% enhancement in GIC_Comp). Scanning electron microscopy of the fractured specimens revealed a weak fiber-matrix bonding for all GFRP specimens. During the interlaminar fracture toughness test, the cracks preferentially propagated through the relatively weak fiber-matrix interface; therefore, the matrix toughening effect provided by mGO or CSR was significantly diminished in the polyester/glass fiber composites.
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