Tensile properties of advanced SiC/SiC composites for nuclear control rod applications

Recent progress toward development of irradiation resistant SiC/SiC composites has shown great promise for their application in nuclear systems. In this paper, we report the results of a study on the evaluation of the mechanical properties of a nuclear-grade SiC/SiC composite, which is being considered as a candidate control rod material for a very-high temperature gascooled reactor. Specifically, this study evaluates the anisotropy in tensile properties of the composites to provide a basis of the practical component design. The materials were satin-woven or biaxially braided Hi-Nlcalon™ Type-S fiber reinforced chemical-vapor-infiltrated (CVI) SiC matrix composites with multilayered interphase. Results indicate excellent axial and off-axial tensile fracture behaviors for the satin-woven composites. In contrast, the braided composites failed at unexpectedly lower stresses. The primary cause for this difference was the varied inplane shear properties, on which off-axial tensile properties significantly depend. Superior inplane shear properties for the satin-woven composites were achieved by increasing the volume fraction of transverse fibers normal to the fracture plane. Considering the failure modes depend on the off-axis angle, the anisotropy of proportional limit tensile stress and fracture strength were preliminary' evaluated by a simple stress criterion model. It is worth noting that specimen size effect on axial and off-axial tensile properties seems very minor for nuclear-grade SiC/SiC composites with rigid CVI-SiC matrix.