AbstractHexcel F185 neat resin and T6T145/F185 graphite fiber‐reinforced composite were subjected to Mode I loading in the compact tension (CT) geometry (fibers parallel to the crack) and the energy per unit area of crack extension,JIC, determined to be 8100 and 1600 J/m2respectively.In‐situfracture studies using scanning electron microscopy on a CT‐type specimen of F185 showed extensive microcracking in a damage zone ahead of the crack tip, which was similar to the microcracking observed in the whitened area ahead of the crack tip in the macroscopic CT specimens. A simple calculation using a rule of mixtures approach suggests that the diminished size of the damage zone and the presence of rigid fibers in the damage zone in the composite are not a sufficient explanation for the significantly lower delamination toughness of the composite compared to the neat resin. From this it may be inferred that the strain to failure locally in the damage zone ahead of the crack in the composite may also be lower than that which can be tolerated in the neat resin. Evidence for this idea comes from the observation that microcrack coalescence seems to occur preferentially at the fiber/resin inte