Bilinear softening model and double K fracture criterion for quasi-brittle fracture of pultruded FRP composites

This study presents an experimental study on the mode I fracture property of pultruded fiber reinforced polymer (FRP) composites along the fiber direction. A quasi-brittle fracture exists for this material. The bilinear softening model is used to describe the constitutive relation in the cohesive fracture zone. Furthermore, a double K fracture criterion is applied to analyze the crack propagation and fracture property. The orthotropic effect on the fracture property of mode I along the fiber direction is rather small and ignored in this study. According to the bilinear softening model, the cohesive fracture energy is divided into two parts, the microcracking fracture energy and the fiber bridging fracture energy. The ratio between the two fracture energies and the model parameters are decided with the test results. For the double K fracture criterion, a cohesive fracture toughness in introduced to calculated the initial cracking fracture toughness. Based on the linear elastic fracture mechanics and the bilinear softening relation, the two fracture toughness of this criterion are obtained with the test results.