Microstructural Modeling And Mechanical Analysis Of 3-D Braided Composites

In recent years, structural textile composites have established themselves as the advanced material system of choice in many load-bearing applications including aerospace components, automobile components, and biomedical implants. The yarn architecture of three-dimensional (3-D) braided composites produced by the four-step 1×1 braiding technique and their effective elastic properties were studied in this dissertation with experimental investigation and a numerical method, respectively.Three-dimensionally braided structural composites have distinct structure that is fully integrated, continuously spatial fiber-network impregnated with ductile material. The new innovative materials have not plies as conventional composites have, and put an end once and all to low interlaminar strength showing in laminate materials. Because of their enhanced stiffness and strength in the thickness direction, near-net-shape design and manufacturing, superior damage tolerance and specified aerospace function, the braided composites are gaining more and more attention of industry and academia. The research in this dissertation began with braiding process of three-dimensional textile perform. A new fabric architecture model (FAM) was set up. The geometric structure of braided composites was recognized and the mechanical properties of the composites were analyzed. The primary research can be listed as follows.In this thesis, a control volume method was employed to model the microstructure of 3-D braided performs produced by the four-step 1×1 square braiding process and their reinforced composite materials. According to the movement traces of yarn carriers on the braiding machine bed, a perform was divided into three regions, i.e. interior, surface and corner, and distinct control volumes were defined for each region. Analyzing the control volume of each region, the yarn architecture of perform was described and three kinds of local unit cell were identified. Then the relations between the braiding parameters of the perform were derived. Based on the analysis of the perform, two local unit cells for the 3-D braided composite material were obtained and the relations between the braiding parameters were modified. Finally, the method of designing a 3-D braided composite was given. The braiding parameters include the dimensions of sample, the braiding yarn of main part, the volume proportion of each region to whole structure, the size of the braiding yam, the yarn packing factor, the fiber volume fraction, the braiding angle and the braiding pitch length.Furthermore, a methodology based upon the concept of fabric unit cell structure was developed for studying the elastic properties of three-dimensional braided composites. In order to facilitate the analysis of the minimum repeated unit cell, the following geometrical characteristics were assumed: (a) All the yarn segments parallel to a diagonal direction or braiding axis in all unit cells lying in the samelayer of the perform are treated as forming inclined lamina or horizontal lamina after matrix impregnation, (b) Fibers were considered to be straight and unidirectional. Fiber interlocking and bending due to the interaction at the centers of the unit cell were not taken into account, (c) A unit cell can be further considered as an assemblage of four inclined unidirectional lamina and one horizontal unidirectional lamina. Each inclined unidirectional lamina was characterized with a unique fiber orientation and had the same thickness. Furthermore, the fiber volume fraction of each inclined and horizontal unidirectional "lamina was assumed to be the same as that of the composites. Three-dimensional stress-strain analysis was applied to each unidirectional lamina that was assumed to be transversely isotropical under on-axis coordinate system. Carrying out the integration and averaging of stiffness yielded the effective elastic moduli of 3-D braided structural composites. Based upon failure mechanisms of 3-D braided composites, a kind of micro-structural strength criterion was s...