The objective of this work is to evaluate the interlaminar fracture toughness, in terms of the total critical strain energy release rate (G_Tc), for unidirectional glass/epoxy composites under mixed Mode I + II loading. Several bending tests proposed in the literature, such as the DCB, MMF, ENF, and CLS tests, were performed. In addition, the mixed mode CBON and IDCB tests were developed. The combination of these tests gave fracture toughness results that covered a complete mixed mode range: 0 ≤ G_II/G_Tc ≤ 1. The G_Tc and the individual components, G_I and G_II, were determined analytically by the beam theory and by finite element calculation. Measurements of these parameters using experimental compliance calibration were compared with the analytical results to ensure their validity. Both the testing and analytical methodologies are discussed. The influence of specimen configuration on the G_Tc values was also investigated. Fractography provided a way to understand the fracture mechanisms and to associate the fracture surface morphology with the variation of the G_Tc.

Based on experimental data obtained, a new mixed mode delamination criterion has been proposed.