چكيده به لاتين
In this research, a physics-based theory based on the critical strain energy density approach has been developed to predict the strain energy release rate at which the crack growth onset microscopically occurs under mixed mode I/II loading together with the initial crack growth angle. This approach, unlike the current procedures based on performing delamination tests, can fully characterize the fracture behavior of the material under any mixed mode I/II loading by having only mode I critical strain energy release rate and the material properties, without any needs for manufacturing mixed mode bending specimens and performing time-consuming and costly experiments. To evaluate the theory, the results have been compared with the available experimental data for both isotropic and orthotropic materials. Also, a series of experimental investigations, including quasi-static delamination experiments and the acoustic emission technique have been performed to evaluate the theory in predicting the delamination behavior of laminated composites. Since the fracture process zone has a significant effect on the behavior of the mixed mode I/II delamination in laminated composites, a novel micromechanical model called “mixed mode I/II micromechanical bridging model” has been proposed based on the calculation of the delamination crack bridging zone energy absorption. To implement the proposed model, only some physical and mechanical parameters such as fiber, matrix and interface properties that can easily be extracted from the literature are needed. To demonstrate the effectiveness of this model, the results as the energy absorption of fiber bridging zone have been compared with the available experimental data. By considering the absorbed energy in the fracture process zone, a new mixed mode I/II failure criterion has been proposed. By defining a suitable damage factor and using the mixed mode I/II micromechanical bridging model, the absorbed energy in the fracture process zone has been considered. It causes the new criterion to be more compatible with the nature of the failure phenomena in orthotropic materials, unlike the available conservative ones. The implementation of the proposed criterion for prediction of the mixed mode I/II crack growth is straightforwardly possible by considering the mode I fracture toughness, elastic properties of the material and the energy absorbed by the fracture process zone. The validity of the present criterion has been assessed by comparing the fracture limit curves obtained for both laminated composites and wood species with the available experimental data.
