A Study On Geometric Mircrostructure And Mechanical Performances Of 4-directional Rectangular Braided Composites

Braided composites are advanced materials come out twenty years ago. Owing to their many superior behaviors, they have been used in aeronautics, astronautics and general industries. Theoretic approach and prediction for their performances have become increasingly important.This dissertation investigates interior, surface and comer architecture of 4-directional rectangular braided composites in detail. Vector method is used to deduce mathematic expressions, the path of fiber tows in the material, position of contact point and shape of contact line between any two fiber tows, and common normal of any two fiber tows through their contact place, are all determined analytically, some important conclusions are drawn. These efforts result in provisions of geometric basis and data needed by following finite element analysis. Removing several shortcomings of existing methods dividing the material into unit cells, a correct division method and corresponding unit cell's graphics which are more approximate to the real, are proposed. Formulas to determine the numbers of unit-cells become unity.A finite element method is developed to study mechanical response of composites under longitudinal tension. The fiber tows are divided into spatial beam units, and matrix of the materials is omitted. But matrix's effect on connecting fiber tows is expressed in adopted supposition that no relative displacements occur among all fiber tows at their contact points, which leads to establishment of constrained relation between node displacements of beam units, and leads to the formation of material's augmented structural-stiflf-matrix. Programming and calculating obtain the macro mechanical measurements such as strain and Young's module of the composites. Besides, forces and moments, stress's maximal value as well as occurring places in each fiber tow, force and moment exerted on any fiber tow by its adjacent fiber tows become all known. The numerical solutions agree with experiments well, proving the effectiveness and feasibility of the method. The method may be also available for other types of load bearing on the materials.Work done above fills several gaps in study on geometric microstructure of the material, initiates a new way to analyze and predict the material's module and strength. For the both explorations, this dissertation has made a substantive progress.