Appropriate fitted point on fracture limit curve of orthotropic materials to include the fracture process zone effects on mixed-mode I/II fracture criterion

The fracture phenomenon in orthotropic materials is associated with the formation of the fracture process zone at the crack tip. Several toughening mechanisms in this zone delay the fracture by absorbing energy. The activation of these mechanisms and the extent of their effects depend on the loading mode. In this paper, the behavior of the fracture process zone under different loading modes is investigated. In the following, considering the fracture process zone effects, a new mixed-mode I/II fracture criterion is presented to investigate the crack growth behavior of orthotropic materials. In this criterion, by extending the minimum strain energy density theory to orthotropic materials and fitting the fracture limit curve to a midpoint (a mixed-mode I/II point) instead of the pure mode I point, a greater portion of the energy absorbed in the fracture process zone is considered. To validate the presented criterion, the related fracture limit curves are compared with the available experimental data for laminated composites and wood species as orthotropic materials. The results show that the newly proposed criterion, unlike the available conservative ones, especially around predominantly mode II, is more compatible with the nature of the failure phenomena in orthotropic materials.