Buckling and fracture analysis of three-dimensional four-directional braided composite tubes with cutouts under axial compression

The mechanical properties of cutouts are a key issue in the engineering application of three-dimensional four-directional(3D4D) braided composite tubes. In this study, numerical and experimental methods were used to investigate the effect of different hole diameters on the buckling properties of 3D4D braided composite tubes under axial loading. In the numerical part, microscale RVE model and mesoscale RVE model were established to predict the elastic properties by applying the multiscale finite element method, and the accuracy of them was verified by theoretical calculations. A macroscale-mesoscale coupling model was established to perform linear buckling and nonlinear analysis of braided tubes with different hole sizes to predict the buckling behavior of 3D4D braided composite tubes. The simulation results show that the buckling load decreases with the increase of the hole sizes. In the experimental part, specimens were prepared by braiding process, axial compression experiments were carried out until failure and full-field displacements and strain distribution were recorded by digital image correlation(DIC) technique. Smaller holes enable fiber-dominated strain transfer networks, medium-sized holes trigger matrix-fiber cooperative failure, whereas larger holes induce strain localization and premature failure through interface debonding.