Subcritical Crack Growth in Hi-Nicalon Types Fiber CV1-S1C/S1C Composites

Abstract

SiC continuous-fiber composites are considered for nuclear applications but concern has centered on two major issues for this material at elevated temperatures in neutron environments. One is the differential materials response of the fiber, fiber/matrix interphase (fiber coating), and matrix under thermal-mechanical loads and irradiation-induced swelling. The other is subcritical crack extension when time-dependent and dose-dependent fiber creep can occur. In this study, both experiments and simulations were employed to understand and predict this behavior. Constant stress tests at elevated temperatures in inert environments without radiation damage are being used to explore subcritical crack growth in Type-S SiC-fiber composites. Additionally, a continuous-fiber composite is simulated by four concentric cylinders to explore the magnitude of radial stresses when irradiation swelling of the various components is incorporated. The outputs of this model were input into a time-dependent crack-bridging model to predict crack growth rates in an environment where thermal and irradiation creep of SiC-based fibers is considered. Under assumed Coulomb friction the fiber-matrix sliding stress decreases with increasing dose and then increases once the pyrocarbon swelling reaches turn around. This causes an initial increase in crack growth rate and higher stresses in crack bridging fibers at higher doses. An assumed irradiation creep law for fine-grained SiC fibers is shown to dominate the radiation response, however.