Mechanical Property Of Three-Dimesional Four-Directional Braided Composites With Cracks

The three-dimensional braided composites, as their a serial of advantages which can not be substituted , and are 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 .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 stiffness of three-dimensional braided composites with defects is investigated by using theoretical and numerical method in this dissertation.In chapter one, the background and significance of the present project are demonstrated. In addition, the advances in the mechanical problems of three-dimensional braided composites is reviewed from stiffness. In chapter two, the basic theories that are used in this dissertation are introduced.It includes three contents.The first is braided premodling technique including 2-Step braiding and 4-Step braiding. The second is variational theory with certain area. The last is The minimal complementary energy theory. In chapter three and chapter four,a laminate model is developed to predict the effective moduli of three-dimensional four-directional braided composites with initial periodic boundary cracks and periodic internal cracks. Firstly, a mechanical model of three-dimensional four-directional braided composites with periodic edge cracks and periodic internal cracks are established by using the stiffness averaging method and considering the microstructure of the material.Secondly, the minimal complementary energy theory is used to obtain the expressions of the stress field in the representative unit. Finally, the effective moduli of three-dimensional four-directional braided composites with periodic boundary cracks and periodic internal cracks are predicted by means of Hashin's average theory.The effective moduli that are predicted in this dissertation of three-dimensional four-directional braided composites with initial cracks has an important significance for the development and application of braided composites.