On design, yarn architecture and mechanical behavior of three-dimensional braided composites

In recent years three-dimensional (3-D) braiding has emerged as a viable method for the fabrication of monolithic, near-net shape structures with 3-D fiber reinforcement. The premise of this thesis is to investigate the interrelationships between design, fabrication, testing and analytical modeling of these braided composite shapes. The investigative approach taken begins with the design of the 3-D braiding apparatus, the actual braiding of the preforms, their consolidation into composite specimens followed by mechanical testing and measurement of the resulting properties. Concurrently, analytical modeling of each of these developmental stages was carried out in close coordination with numerical simulation of the actual situations. Several structural shapes with varying degrees of complexity were designed and braided using different fiber systems; the braiding methodology used in the making of the complex-shaped preforms was developed based on the existing four-step 1x1 braiding process; the braided preforms were consolidated with a resin matrix using the Resin Transfer Molding process to yield the final composite specimens. A descriptive methodology to analyze the yarn architecture of these composites was developed based on the design and braiding processes; the unit-cells and cell-composition of the composite were determined from the yarn architecture and were used in micromechanics models for the prediction of local (unit-cell) and global (cell-composition) properties. The mechanical properties predicted by these models were correlated with experimental data obtained under conditions of axial loading and pure bending. A Scanning Electron Microscopic analysis of the failure modes associated with the tested specimens under conditions of pure bending led to the establishment of a preliminary failure criterion.