A comprehensive study on the fracture behavior modeling of adhesively bonded composite joints under mixed-mode loading using a compliance-based beam method

In this study, the fracture response of adhesively bonded composite joints under different modes of I, II and mixed-mode I/II conditions was characterized by employing the double cantilever beam (DCB), end notched flexure (ENF), and mixed-mode bending (MMB) tests, respectively. In addition, various mechanical tests were executed for determining the mechanical properties of the glass fiber/epoxy composite substrates. The flexural modulus and equivalent crack length relations for the MMB specimen were derived using the compliance-based beam method (CBBM). This method enabled the determination of strain energy release rate (SERR) and fracture energy only using the load-displacement curve. Furthermore, the proposed method simplified the process of obtaining resistance curves (R-curves), as the crack length did not need to be measured during the test and considered the effects of energy loss in the fracture process zone that are neglected in classical methods. The R-curves obtained from the proposed method were compared with the direct beam theory for pure mode I and corrected beam theory for pure mode II and mixed mode loadings. According to this comparison between the classical methods and CBBM, SERR achieved by the proposed method was higher than that obtained by classical methods. The cohesive zone model and corresponding traction-separation laws developed by a direct method were used in conjunction with the finite element method to validate the acquired experimental data. Moreover, the failure patterns occurred during testing was determined after investigating the fracture surfaces under the loading modes of I, II, and mixed I/II.