Structure and mechanical properties of three-dimensional braided composites

Textile preforming is one of the manufacturing processes that offers the potential to make composite structures cost-effective compared to aluminum structures. The thermoelastic properties of the composites made from these textile preforms depend on the fiber architecture (three-dimensional yarn structure of the preform), which in turn depends on the preform processing parameters.; In this thesis, the structure of the preform in the 4-step (1 x 1) 3-D braiding process is investigated and the fiber architecture of the braided composite is established based on the movement of the fiber carriers on the machine bed. It is demonstrated that the fiber architecture in the interior, the boundaries, and the corners of the braided composite are different. It is shown that the yarns inside the braided composite do not change direction to create interlocking points. The yarns only change direction in the boundaries and the corners.; Five parameters are needed to define the geometry of the preform and the composite. The relationship between the geometric parameters, which may be measured from the surface of the specimen, and the interior fiber architecture of the specimen is demonstrated. A graphical notation, called "compact form", is introduced for depicting the fiber architecture of the preform. This notation facilitates tracing the trajectory of any yarn in the composite.; A finite element method is proposed for modeling the deformation behavior of the braided composite. In this method the matrix and the fiber architecture identified for the preform are modeled separately. The fiber and the matrix are distinct elements with no smearing as is customary in lamination theory. The model of the braided composite is obtained by superposition of these two models. The variation of the thermoelastic constants of the braided composite as a function of braiding angle and fiber volume fraction is demonstrated.; The approach proposed in this thesis for establishing the fiber architecture of the preform is general enough to be applicable to other textile composite processes. The modeling technique of this thesis may be applied to calculate the thermoelastic properties of braided composite parts including axial yarns. It can also be used to study the response of braided parts of complex shapes under complex boundary conditions and loading.