Effect of fiber surface treatment on delamination fatigue under mode II loading was investigated for unidirectional CF/Epoxy laminates. Two types of laminates were made from surface-treated-carbon fiber or surface-non-treated fiber, and a common epoxy matrix. Tests were carried out using end notched flexure (ENF) specimens. Stabilized mode II static tests showed that the fracture toughness of the surface-treated CFRP was 30% higher than that of non-treated CFRP. Fatigue crack growth resistance of the surface-treated CFRP was higher than that of non-treated CFRP at higher crack growth rate. However, the effect of fiber surface treatment was negligible near the threshold region. At higher growth rate, the interfacial fracture occurred prior to the matrix fracture near the crack tip for the non-treated CFRP. Then, the fracture mechanism was controlled by the interfacial fracture. On the other hand, the resin fracture with plastic deformation occurred prior to the interfacial fracture near the crack tip for the surface-treated CFRP. Then, the fracture mechanism was controlled by the resin fracture. Near the threshold region, the ratio of the resin fracture was rather large without respect to the fiber-surface-treatment. The main fracture mechanisms near the threshold region was only controlled by the matrix resin. This fact was well correlated to the fact that the threshold value was insensitive to the fiber surface treatment.
