Investigation On The Mechanical Behavior Of Three-Dimensional Four-Directional Braided Composites

Since three-dimensional braided composites have fine properties, it is widely used in the fields of high technology such as aeronautics and astronautics, etc. It is very important to investigate the mechanical properties of three-dimensional braided composites, which decide whether the three-dimensional composites can be applied in engineering fields. At present, it is preliminary to understand the mechanical properties of three-dimensional braided composites. There are many problems which need to be solved. The fracture mechanism, stiffness and strength of three-dimensional four-directional carbon/epoxy braided composites are investigated by using experimental, theoretical and numerical methods in this dissertation.The developing process of braided composites is introduced in Chapter one and the advances on the mechanical properties of three-dimensional braided composites are reviewed from several aspects, including experiment, stiffness prediction and strength analysis. In addition, the background and significance of the present project are demonstrated in this chapter.In Chapter two, the mechanical properties of three-dimensional four-directional carbon/epoxy braided composites were investigated by quasi-static experiments. On the one hand, the mechanical properties of the 3D braided composites, including stiffness, strength and Poisson's ratio, were investigated by the tensile experiments. Furthermore, the fracture surfaces and damage models of specimens were observed under the optical microscopy. On the other hand, a set of specific compressive clamps was designed for the compressive experiments. The compressive mechanical properties of the 3D braided composites were investigated and the fracture surfaces and damage models with different braiding technical parameters were analyzed.The failure processes of three-dimensional (3D) carbon/epoxy braided composites were investigated by the acoustic emission (AE) technique in Chapter three. The AE event rate, amplitude, and the peak frequency of a dominant frequency band after treated with the fast Fourier transform (FFT) were analyzed. At the same time, combining with the P -δcurve varying feature, the fracture processes were divided into different stages to deeply understand the damaged mechanisms of the braided composites. Results reveal that the behavior of AE parameters described well the fracture process in the 3D braided composites with different braiding angles from the macroscopic view of point, and the damage mechanisms of the composites can be successfully identified by AE characteristics from the microscopic view of point.Taking into account the material microstructure, an improved three-cell model was used to calculate the effective elastic constants of three-dimensional four-directional braided composites in Chapter four. The topological structure of three-dimensional braided composites is divided into three distinct kinds of unit cells namely interior, surface and corner unit cells. The bending characteristics of yarns in the three-cell model were considered and the fill factor was proposed to distinguish the different cross-sectional variation of yarns in the three-cell model. Finally, the model to predict the stiffness of 3D braided composites was established in terms of the stiffness volume average method, and the effective elastic constants of 3D braided composites were obtained.In Chapter five, combining the finite element method with the micromechanics and according to the material microstructure, a finite element model of a unit cell is presented. The curvature and the circular cross-section of fibers and no friction between fibers and matrix were supposed. The effective elastic constants of 3D four-directional carbon/epoxy braided composites were predicted and compared with experimental results. A good consistency between numerical results and experimental data was found. The stress field of three-dimensional four-directional braided composites was obtained by FEM. In addition, a finite element method which can be used to predict the strength of three-dimensional four-directional braided composites is established according to the different kinds of failures and the strength of three-dimensional four-directional carbon/epoxy braided composites was estimated, and the variation for the strength of the materials with the braiding angle was discussed.