Mode II fracture toughness tests of unidirectional GFRP were conducted using a four-point shear loading test method. This test method was a modified Iosipescu test method which was identified to be the most reliable one for obtaining a shear stress-strain relation in composite materials. In this study, a leverage ratio of the loading and a pre-crack length ratio of the specimen were examined to perform the fracture toughness tests. Fracture behavior of the unidirectional GFRP under shear loading was investigated through a fractographic observation. The results obtained are summarized as follows:
(1) The four-point shear loading test method can be assessed at a simplified in-plane mode II fracture toughness test. To prevent premature failures without reinforcement attached to the specimen, it is necessary for the pre-crack length ratio, a/W to be larger than 0.3 under the definite leverage ratio.
(2) The mode II fracture toughness evaluated in terms of stress intensity factors at the maximum load points was found to be dependent on the pre-crack length ratio.
(3) The Hackle patterns were confirmed at a stable crack propagation region which are typical fracture traces on the matrix under the shear loading. The formation mechanism of the Hackle at different parts was investigated in detail, and then the physical meaning of the fracture toughness obtained in this experiment was discussed.