Design-oriented fracture criteria for orthotropic composites based on minimum strain energy density theory

Accurately and efficiently predicting the fracture behavior of composite materials under mixed-mode loading I/II remains a significant challenge. This study presents novel design-oriented fracture criteria based on the Minimum Strain Energy Density Theory (MSEDT) to effectively address these challenges. To improve the accuracy of predictions for the critical stress intensity factor (CSIF) related to fiber-aligned cracks, the classical Strain Energy Density (SED) criterion is enhanced by including micromechanical crack initiation angles. Subsequently, a statistically averaged initiation angle, derived from forty-two highly orthotropic materials, is used to significantly reduce computational costs while maintaining acceptable accuracy. This approach achieves a balance between precision and simplicity. Furthermore, to account for the variability observed in experimental results due to T-stress effects at arbitrary crack-fiber orientations, two complementary criteria based on micromechanical and macromechanical assumptions are proposed. The micromechanical version offers a high-risk estimate, whereas the macromechanical variant provides a conservative prediction. Additionally, a semi-empirical criterion that explicitly considers T-stress effects is introduced for the frequently encountered case of cracks that are perpendicular to fibers. All proposed criteria show strong agreement with experimental data, proving reliable for preliminary design and structural assessments. Therefore, these criteria can support effective design decisions and structural integrity evaluations in aerospace, automotive, and other composite-utilizing industries.