A shear-lag model was extended to account for the effect of fibers bridging across the longitudinal split in finite-width double edge notched unidirectional composites. The analysis was used to measure the longitudinal splitting fracture toughness of composites of Hercules AS4/3501-6 and Avimid K-polymer/Magnamite IM-6 graphite. The effect of resin toughness was further investigated using laminates from a series of experimental resins based on the K-polymer chemistry. The inclusion of fiber ...