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Through-thickness modulus, Ez, GPa

Figure 28. Relationship between longitudinal modulus and through-thickness modulus of 3-D composites.

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Through-thickness tensile modulus, Ez GPa

Figure 29. Relationship between inplane shear modulus and through-thickness modulus of 3-D composites.

relationship between Gxy and Ez for the linear and curvilinear reinforcement systems. For the curvilinear system, Gxy increases as Ez increases. This increase reflects the respective sensitivity of the linear and curvilinear-reinforced systems to fiber volume fraction distribution and fiber orientation.

Concluding Remarks

Textile preforms have much to offer in the toughening and manufacture of nextgeneration high-performance structural composites. With a large family of highperformance fibers, linear fiber assemblies, and 2-D and 3-D fiber architectures, a wide range of composite structural performances may be tailored to meet specific requirements.

An examination of the literature indicated that only a limited number of systematic studies have been carried out on fabric-reinforced CCC's (carbon-carbon composites). A well-established data base is needed to stimulate the usage of fabric-reinforced CCC's for structural applications.

The literature suggests a trend toward using 3-D fiber architecture for CCC structural toughening which poses important technical challenges. The first challenge is the question of converting high-modulus yarns to textile structures. The processing difficulty with brittle fibrous structures calls for an innovative combination of materials systems such as the concept of material and geometric hybridization.

The infiltration or placement of matrix material in a dense, 3-D fiber network also creates new challenges and demands an understanding of the dynamics of the process-structure interaction. Questions that must be answered relate to the optimum pore geometry for matrix infiltration, the pore distribution, and the bundle size.

As the level of fiber integration increases, the opportunity of fiber-to-fiber contact intensifies at the crossover points. Guidance is required to select the fiber architecture and matrix placement method best suited to reduce the incidence of localized fiber-rich areas.

To take advantage of the attractive features that textile structural composites offer, a sound data base and design methodologies need to be developed. The fabric geometry models developed so far establish a necessary, but not entirely complete, first step in the modeling of CCC's. Future work in the modeling of fabric-reinforced CCC's requires a better understanding of the dynamic interaction among fiber, matrix processing condition, and fiber architecture.

Acknowledgments

Much of the work on fiber architecture reported herein, and specifically 2-D and 3-D fiber architecture, has been supported by the Office of Naval Research and the Air Force. The assistance provided by Mitchell Marmel of the Fibrous Materials Research Center in the preparation of this manuscript is greatly appreciated.

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