Physically-based strain criteria for mixed mode fracture and fatigue thresholds in orthotropic composite laminates

This study proposes a strain-based framework for predicting mixed mode fracture and fatigue threshold in orthotropic composite laminates. By analyzing the stress field at the crack tip and combining constitutive relations and compatibility conditions, maximum principal strain (MP ε ) and maximum shear strain (MS ε ) criteria for static crack initiation prediction are established. By introducing the anisotropy of the critical fracture distance, r -related criterion forms ( r -MP ε and r -MS ε ) that reflect both material anisotropy and characteristic damage length are obtained. Notably, the proposed criteria can not only perform forward predictions but also identify unknown parameters inversely and support model construction with limited experimental data. Furthermore, inspired by the evolution of the cohesive model, the proposed static fracture criterion is extended to the fatigue threshold prediction by assuming a similarity between the static fracture envelope and the fatigue threshold envelope. This enables the prediction of the fatigue delamination threshold under different stress ratios and mixed mode ratios. The proposed model has been validated using experimental results from different composite systems. The results show good agreement between the model and experimental results and demonstrate its ability to link fracture behavior to underlying mechanisms. Furthermore, a dimensionless parameter is introduced to characterize the combined effects of elastic anisotropy and fracture toughness. This parameter acts as a unified indicator for the occurrence of major failure mechanisms and overshoot phenomena. The proposed framework provides a physically meaningful and unified method for assessing mixed mode fracture and predicting fatigue life in composite structures.