Process, Structures And Properties Of Hexagonal Braiding And Integrated Design Of Braided Structural Composites

A braid is a complex structure or pattern formed by intertwining three or more yarns or fibers along fabric forming direction. The braiding technology mainly includes two dimensional (2D) and three dimensional (3D) braiding techniques.2D braiding technology is an skill in which a braid is formed by interwineing two sets of yarns.3D braiding technology is an extension of2D braiding technology. Several types of2D braiding machines exist such as flat braiding machine, circular braiding machine,radical braiding machine and so on.3D braiding can be classified as Catesian (track and column) braiding and rotary3-D (horngear) braiding. To meet the requirement of new fileds, development of a new braiding machine which can use micofilament to braid is promising. The novel hexagonal braiding machine can meet the need and is capability of fabricating polygonal cross-sectional preforms. Therefore, it is of benefit to investgate the process, structures, properties of hexagonal braiding. Meanwhile, it is of great value to develop a integrated design package to guide the design of braided preforms based on the principles of traditional2D braiding,3D braiding and novel3D hexagonal braiding.The main research in the thesis includes:(1) The principle and building of hexagonal braiding machine. According to principle of hexagonal braiding, achievement of hexagonal braiding was investigated from hardware and software system aspects.(2) Simulation of hexagonal braiding. A software package for simulating the hexagonal braided structures was developed based on hexagonal braiding process by Matlab programme.(3) Hexagonal braided structures with polygonal cross-sections were analysized. The method to fabricate polygonal structures using hexagonal braiding was introduced and a type of braids with triangular cross-section was studied in detail.(4) Software package for integrated design for manufacturing of braided preforms and parametric study. According to principles of2D braiding and3D braiding technology and hexagonal braiding, a software package for integrated design of braided preform was programmed. Based on design results of different preforms, some important parameters of braiding process and braided preforms were studied. It provides theoretical basis for designing braided preforms.Conclusion:(1) The hexagonal braiding machine built is very flexibility, ease to enlarge as it is controlled by high precision digital step motor and control software system is programmed through open Labview platform. The braider is able to precisely fabricate fabric using microfilaments. Maximum number of yarns can be loaded in limited area.(2) It is proved the software package developed by Matlab can effectively process simulation of hexagonal braided preforms. The program is open and could be further expanded. It could be an important tool to investigate interior structures of hexagonal braided performs.(3) It is found that hexagonal braiding has unique capability of fabricating preform with polygonal cross-section. Through microscopic analysis of braided structure with triangular cross-section, it can be seen the preform is consist of interior cells, facial cells and corner cells. The properties of hexagonal braided composites were studied. The results show some differences between hexagonal braided composite and4step braided composites.(4) The software package developed using can be used as a useful tool to process design of2D braiding,3D braiding and hexagonal braiding. Its reliability and practicality is shown by some samples.(5) Based on integrated design software package, some important parameters were studied. The results show braiding speed and braiding angle have a great matter and the linear density and their ratio have great effect on fiber volume fraction of preform for2D braiding. For3D braiding, the dimension of preform is determined by number of braiding yarn and braiding angle. Also, given parameters of a preform, the machine set-up can be derived; Braiding angle and linear density of braiding yarn have effects on fiber volume fraction of braided preform. Fiber volume fraction of hexagonal braided preform without axial yarn is lower than that of4-step braided preform without axial yarn. But it is converse with axial yarns.