General mixed mode I/II failure criterion for composite materials based on matrix fracture properties

The available mixed mode I/II fracture failure criteria for orthotropic materials have been proposed mostly for cracks along fibers. The important fracture parameter in these criteria is the mode I fracture toughness that can be derived by utilizing standard tests. Whenever the crack is not along the fibers, the prediction of mode I fracture toughness is impossible due to mode mixity. Therefore, a comprehensive criterion that can predict the crack initiation instance has not been proposed so far. In this study, in line with the definition of equivalent fracture toughness for off-axis orthotropic materials, a general mixed mode I/II fracture failure criterion is proposed, which is independent of the orthotropic fracture properties. As per this criterion, it is assumed that the fracture occurs in the isotropic matrix and it is the effect of the fibers modeled as tensile or shear reinforcements. On the other hand, only elastic and fracture properties of the isotropic matrix are included in the proposed criterion, which can be derived easily by simple standard experimental tests. In providing this general criterion, the strain energy release rate approach was considered. The concept of crack initiation and propagation energy is employed for investigating the wasted energy in the fracture process zone due to toughening micro-mechanisms. Moreover, comprehensive experimental tests and the required finite element simulations are performed for validation of the proposed criterion.