Full - Scale Mechanical Response Characteristics And Meso - Damage Analysis Of Three - Dimensional Five - Direction Braided Composites

Three-dimensional (3D) braided composites have been increasingly incorporated into engineering industry because of their low density, high strength and/or stiffness to weight ratio, good damage tolerance, impact resistance and fatigue resistance. They can fundamentally overcome the fatal shortcomings of low delamination resistance of laminated composites. With the braiding techniques and the machining automation developing, it is expected to realize the specific material performance and the near net shape capacity by aid of the variable structure and the strong design based on the premise of weight loss and low cost. Thus,3D braided composites with high load-bearing capacity and special function can be believed to become the ideal structural materials in the field of aerospace. However, the interlaced structure of 3D braided composites is extremely complicated. Moreover, the orientation, squeezing cross sections, and distribution proportion of the fiber bundles could be changed with the variation of the component shapes and performance requirement. Hence, the full-field microstructural parameters are highly inhomogeneous, which results in the considerable difficulties in the analysis of the mechanical properties of 3D braided composites. In this dissertation, the full-field mechanical responses and meso-scale damage behaviors of 3D five-directional braided composites are analyzed by using experimental, theoretical and numerical methods. The main contents are as follows:(1) The Shimadzu universal material machine and the digital image correlation (DIC) equipment are used to evaluate the mechanical behaviors of 3D braided composites subjected to different loads (longitudinal tensile, longitudinal compression load, transverse tensile and transverse compression). The boundary effect on the full-filed mechanical behaviors and the failure mechanisms of 3D braided composites are particularly discussed. The results show that the yarns at the edges are discontinuous because of cutting. Meanwhile, the defects at the edge of specimens can be produced and extended. These phenomena result in the change of the full-filed strain distribution and the failure mechanisms.(2) According to the motion law of yarn carriers of four-step 3D five-directional braiding techniques, the yarn movement traces and the meso-scale topology structures in the interior, surface and corner regions are studied. Also, combined with the real cross sections and squeezing shapes of yarns, the macro bar model and the macro homogeneous model are established, respectively. Moreover, after generating the fiber embedded matrix finite element model and the multi-scale finite element model, a commercial finite element software ABAQUS/Standard is employed to investigate the full-field mechanical responses of 3D braided composites. Also, the influences of the braiding angle and the boundary condition on the mechanical properties of 3D braided composites are discussed. It can be found that the full-field displacement, strain and stress distributions of 3D braided composites subjected to longitudinal loads can be successfully captured by the fiber embedded matrix model. Furthermore, the predicted results of the fiber embedded matrix model agree well with the experimental ones.(3) Based on the interior solid models, the meso-scale progressive damage model, which is related to the damage initiation criteria, the damage evolvement model and the damage stiffness matrix, is developed. A user-defined material subroutine is prepared to predict the strength value of 3D braided composites subjected to different axial loads. Also, the damage initiation, evolution and failure processes of 3D braided composites are simulated under different loads. Moreover, the meso-scale failure mechanisms of 3D five-directional braided composites are discussed. Compared with experimental results, the reliability of the numerical results is verified.(4) By calculating the biaxial strength characterizations of 3D five-directional braided composites, the macroscopic failure criterion of the braided composites is constructed. Moreover, the failure envelopes are drawn and compared with those of the current strength criteria, Tsai-Hill and Tsai-Wu. Results indicate that the proposed strength criteria of this paper can better reflect the biaxial mechanical behaviors of 3D braided composites to a certain extent.(5) Based on the multi-scale analytical model, the customized development platform of ABAQUS is used to develop new user interface. Also, the python language is used to write the kernel script with respect to the virtual design and the analysis of mechanical properties. Moreover, the plug-in is selected to realize a series of functions, such as the elastic constant prediction of the three unit cells, the import of the macro components and the analysis of the macro components etc. Furthermore, starting from the material and structure scales at the same time, the integrated design method related to "materials-braiding technology-structure" is proposed based on the fiber embedded matrix model.