Microstructure Analysis And Effective Elastic Properties' Prediction Of Two-Step Rectangular Three-Dimensional Braided Composites

In this thesis, the yarn architecture of the two-step rectangular three-dimensional braided composites was studied comprehensively and the effective elastic properties have been analyzed by means of two kinds of micro mechanical method.The properties of components and the microstructure play an important role in the mechanical properties of two-step three-dimensional braided composites. Analyzing the microstructure of composites truly, three kinds of unit cell structures have been identified, i.e. large unit cells, sub unit cells and micro unit cells. The deformations of axial yarns packed by braider yarns have been calculated through the changes of fiber packing fractions of axial yarns. The relations between the braiding parameters of the composites were derived and the fiber volume fraction of the composites was computed exactly. The process of designing a two-step 3-D braided composite was given. The window of braiding process was given with the yarn jamming condition.The model of the analogous laminar unit cell has been proposed based on the micro unit cells. The classical laminated plate theory was utilized to analyze the mechanical properties of two-step 3-D composite materials. In this methodology, the effective elastic properties were predicted through the properties of fiber and resin, the geometry of the laminated plate model and the fiber volume fraction.Furthermore, the finite element model has been established on the basis of the large unit cells of the composites. The composite material was considered as an assemblage of a finite number of unit cells. In this model, axial yarns, braider yarns and resin were treated as spatial beam elements. Each unit cell is then treaded as space truss. Three-dimensional finite element method can be employed for the mechanic analysis. Utilizing the finite element software MARC, the monotonic tension tests of the beam samples were simulated. Parametric study showed the range of variability in the elastic modulus of the composites.The parameters include the linear density ratio of axial yarn to braider yarn and the braiding pitch length.The two model predictions were found to be in good agreement with the results of monotonic tensional tests performed in this study. The resolution method and the numerical method can be used to analyze the mechanical properties of two-step three-dimensional braided composites effectively.