On the structural mechanics of 3-D braided preforms for composites

Three dimensional (3-D) braiding technology using high performance fibers for advanced composite structures is receiving great attention as a result of the outstanding mechanical and thermoelastic properties of the materials produced. There are now mainly two groups of 3-D braiding techniques, namely the 4-step and the 2-step processes. The objective of this research is to analyze the structural geometries of the braided preforms and study the feasibility of automating these two processes.; Many efforts have been devoted to develop understandings of the microstructures of the 3-D braided preforms for composites. Based on close observations and extensive investigations, theoretical models for the braided structures are established using few assumptions. Structural geometries of the preforms are analyzed and discussed according to the theoretical models. Mathematical relations among the structural parameters such as the fiber orientation, yarn volume fraction, preform contour sizes as well as their dependencies on machine operating conditions are derived. It is found that for certain constituent yarns and braiding plans, all the structural parameters can be expressed as functions of the preform pitch length, which is controlled by the process operating conditions. The extreme values of the parameters due to geometric constraints of the structures are also analyzed and determined. These extreme values dictate the limiting conditions in process operation. To verify the validity of the analytical models of the structures, experimental investigations have also been carried out. The results strongly support the theoretical predictions.; Feasibility of automating the 3-D braiding processes has been investigated, and the results are encouraging. Two laboratory scale automatic machines, one for each process, have been designed, constructed and computer controlled. Efforts are mainly made to embody the braiding concepts, and some new devices are developed to accomplish the automation of the processes.; Capabilities of the 3-D braiding processes to fabricate near net-shape preforms for composites have been studied and proven. Using the specially developed techniques, the processes are capable of braiding preforms with complex cross-sectional shapes as well as with variations along the preform lengths.