Time and dose-dependent deformation of SiC/SiC composites With off-axis fiber alignment

Full text of publication follows: The use of SiC-reinforced composites for fusion reactors or other nuclear applications will not be restricted to 0/90 aligned fiber architecture in all cases. It is important to understand the role of fiber orientation in the strength, toughness, and time-dependent properties for such materials. The use of high-strength ceramic fibers for composites is predicated on optimizing the strength, fracture resistance, and retained strength in aggressive environments, which argues for the best use of fiber strengths, namely on-axis loading for full load transfer to the high-strength fibers. Relatively few researchers have systematically studied the effects of fiber orientation on composite properties, and none have, to the best of our knowledge, performed any time-dependent testing of composites with off-axis fiber orientations. We have performed mechanical property tests on Hi-Nicalon Type-S fiber SiC/SiC composites as a function of fiber orientation. The mechanical testing consisted of 4- point bend strength, 4-point single-edge notched bend fracture toughness, and 4-point bend slow crack growth testing on two composite architectures from ambient to 1600 deg. C (1873 K). The two composite materials that were tested included a ±55 deg.-braided-weave composite with Type-S fibers inclined at 55 deg. to the principal composite axes to simulate a tubular-weave architecture and a Type-S 0/90 satin-weave composite as a reference material. A time-dependent fiber-bridging model that accounts for fiber orientation has been developed and its predictions are compared to the strength and crack growth data. The level of agreement suggests that existing models of off-axis bridging fibers are adequate for fusion reactor designs using SiC/SiC composites in off-axis orientations. However, the strength data suggests that off-axis orientations are much weaker than aligned fiber orientations and, thus, care must be taken to ensure that some fraction of fibers should be aligned with the principal stress axes, but it is difficult to specify what that fraction should be without a detailed mechanics model of the structure and thermal-mechanical loadings.(authors)