| With the development and application of 3D braided composites, more and more studies on it have been done. So far, all the studies on 3D braided composites have concentrated on those with simple rectangle cross-section. Few on complex rectangular cross-section has been published. The complex rectangular shape refers here to the kind of the geometrical shape formed by joining together two or more rectangles, such as T-shape, L-shape, etc. Therefore it is very necessary to do some research on the microstructure, technical design and mechanical performance of 3D braided composites with complex rectangular cross-section in order to instruct the production and application of this composite with complex rectangular cross-section.The goal of this thesis is as follows:(1) To establish unit cell models in joint region, which can reflect and describe the special microstructure, arrangement, braiding angle and interlacement of each braiding yarn.(2) To present process and method for calculating and designing each parameter of 3D braided composites with complex rectangular cross-section.(3) To evaluate the effect on mechanical performance of 3D braided composites with complex rectangular cross-section engendered by special microstructure in join region.First of all, this thesis analyzes the microstructure of 3D braiding with complex rectangular cross-section, and establishes unit cell geometrical models that can describe the arrangement and interlacement of braiding yarns. Following the formation rule of the braiding process, the paths of the carriers are traced in 3D braiding preform with L-shape cross-section. It is found that the moving rule of carriers is special when they go through the surface, interior and corner in joint region. Then it can be confirmed that compared with simple rectangular cross-section, there are special microstructure in the interior, surface and corner of joint region. Accordingly,: there are six different unit cells in a 3D braided preform with complex rectangular cross-section: in addition to the interior, surface and corner cells of a simple rectangle introduced by earlier researchers, three new types of unit cell are formed in the joint region, namely the unit cell of interior in the joint region, the unit cell of surface in the joint region and the unit cell of corner in the joint region.Microstructure of unit cells in joint region is studied by means of the control volume method. Three unit cell models, unit cell of interior in the joint region, the unit cell of surface in the joint region and the unit cell of corner in the joint region, are established. The shape, dimension and braiding angles of yarns in each unit cell are given.It is demonstrated that the interior unit cell in the joint region is a cube, of which the dimension is the same as the interior unit cell of a simple rectangle cross-section. There are five braiding yarns in the interior unit cell of joint region totally. Apart from two yarns following the same braiding rule of a common four-step braiding for a simple rectangle, there is a straight yarn with a greater braiding angle and two spatially orientated yarns with smaller braiding angle than that in a common four-step braiding with simple rectangular cross-section.Compared with the corner cell in a simple rectangle, the corner unit cell in the joint region is not a pentagon prism, but a heptagon prism. Inside the heptagon, there are four braiding yarns totally. There are not only three curving yarns, but also a straight yarn. The braiding angles of these yarns are respectively same with that in the corner cell and the interior cell of a common four-step braiding preform.The surface unit cell in the joint region is a pentagon prism, of which the dimension is the same as the surface unit cell of a simple rectangle cross-section. There are four spatial orientated braiding yarns in the surface unit cell of the joint region. The braiding angle of one of the braiding yarns is greater and the other three smaller than that in the surface cell of a common four-step braiding preform.Three testing method, slice-up in long direction, slice-up in cross direction and X-ray microtomograph are undertaken to verify the unit cell models established. It indicates that the unit cell models established are correct.The calculation and design of parameters for 3D braided preform with simple rectangular cross-section are not suitable for those with complex rectangular cross-section because there are special microstructures in complex shaped preform. According to the unit cell models established, the methods for calculating the parameters of 3D braided preform with complex rectangular cross-section are given. First of all, "disassemble-assemble method" is presented to confirm the number and arrangement of six unit cells: transform the complex cross-section to simple cross-section, and then assemble the simple cross-section to complex cross-section. After giving the generation rule of new unit cells, the number and arrangement of six unit cells can be got. According to the unit cell models and "disassemble-assemble method", the method for computing the amount and volume of each unit cell is presented, the equation for calculating the fiber volume fraction is derived, and the relationships between preform parameters are developed. Based on the equations and relationships established, the method for calculating the dimension of cross-section is given. The procedure for designing the numbers of yarns arrangement is also presented from preform dimensions, yarns linear density, fiber density and fiber volume fraction. A preform with complex rectangular cross-section was designed and produced. The procedure for designing is supported by experimental results.Next, in order to confirm and judge the effect on the mechanical performance of 3D braided composites due to the special microstructure in the joint region, bending tests are taken to compare the simple rectangular cross-section and complex rectangular cross-section.It is found that the bending damage of 3D braided composites with simple rectangular cross-section include four phase as follows: initial phase of damage, transitional phase of damage, serious phase of damage and wholly failure phase. The destroy of 3D braided composites is multiple, which includes destroy of matrix, craze of interface and breakage of fiber. The damage of matrix and interface is the main mode. There are few fibers breaking.Although the bending damage of 3D braided composites with complex rectangular cross-section is also composed of same four phase, the special microstructure will result in the craze of interface and destroy of matrix happening in the joint region under lower load and earlier time, which will emit much more acoustic emission events with low amplitude. This conclusion is verified by observing section and analyzing parameters of acoustic emission. This results in that compared with the simple rectangle, the mechanical performance of 3D braided composites with complex rectangular cross-section transforms from linearity to non-linearity earlier. In addition, the total number of acoustic emission events of complex rectangle is more than that of simple rectangle in whole procedure of bending damage because of the special microstructure in the joint region, namely the damage is comparatively serious. That is the reason why the ultimate bending load of complex rectangle is a little bit lower than that of simple rectangle. Moreover, the braiding angle of most yarns in joint region of 3D braided composites with complex rectangular cross-section is smaller than that of yarns in corresponding region of the simple rectangle. This results in that modulus of 3D braided composites is a little higher than that of simple rectangle under bending load.
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