Extended maximum principal stress (EMPS) criterion for fracture assessment of orthotropic materials with cracks along and across to the fibers

In the present study, maximum principal stress (MPS) criterion is incorporated into the reinforced isotropic solid (RIS) model to investigate the fracture behavior of orthotropic materials. Cracks are assumed along and across to the fibers in the linear elastic fracture mechanics context. Our experimental observations have shown that in macro point of view cracks in orthotropic materials always occur and grow between the fibers in the isotropic matrix media of orthotropic materials. When the composites are subjected to the pure mode I of loading which is across the fibers, the fibers do not react to the applied load. It means that they do not have effects on load bearing. On the other hand, when the mixed mode I/II of loading is applied to the same material, the fibers play a significant role in load bearing. In the present research, these effects are proposed in the form of reinforcement isotropic solid (RIS) coefficients. Taking an analytical approach, RIS coefficients are embedded into the MPS formulation to obtain the new extended maximum principal stress criterion (EMPS) with high accuracy. For the case of cracks across to the fibers, the crack kinking phenomenon has also been used and proved that when the cracks collide with the fibers, they kink and propagate along the fibers. To validate the proposed criterion, center notch disk tension (CNDT) specimens as appropriate ones for mixed mode I/II fracture test of orthotropic materials are fabricated which can cover the different range of mixed mode I/II loadings. Critical forces range from 452 to 1554 N for cracks along the fibers and 730–2399 N for cracks across the fibers. The fracture limit curves in comparison with the obtained experimental data indicate the compatibility of this criterion with the nature of fracture of the orthotropic materials.