Swelling and time-dependent crack growth in SiC/SiC composites Charles H. Henager Jr. * Pacific Northwest National Laboratory, 902 Battelle Boulevard, MS: P8-15, Richland, WA 99352-0999, USA Abstract SiC continuous-fiber composites are considered for nuclear applications but concern has centered on the differential materials response of the fiber, fiber/matrix interphase (fiber coating), and matrix. In our study, a continuous-fiber com- posite 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 pyro- carbon 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 radi- ation response. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction SiC-based continuous-fiber composites are con- sidered for nuclear applications as structural components, but concern has centered on the differ- ential materials response of the fiber, pyrocarbon fiber/matrix interphase (fiber coating), and matrix under irradiation [1]. A modeling approach using concentric cylindrical regions to simulate continu- ous-fiber composites to give the stress distribution in each region was developed to explore thermo- mechanical loading effects [2–5]. This model pro- vided an elastic solution to the fiber, fiber–matrix interphase, matrix, and far-field composite regions for thermo-mechanical strains and irradiation- induced strains as a functions of temperature and neutron dose [5]. The influence of pyrocarbon den- sity on swelling was shown to determine the radial stress present at the fiber–matrix interface as a func- tion of dose for composites containing stoichiome- tric SiC fibers, which can ultimately influence composite mechanical properties through the fric- tional sliding stress, s. Time-dependent, and thus dose-dependent, prop- erties of interest for SiC-composites include retained strength, dimensional stability, and creep-crack growth [1], which have been addressed partly by a dynamic crack growth model developed to predict SiC-composite lifetimes [6–8]. However, crack growth models have omitted dose-dependent swelling until the development of the modified four-cylinder model. 0022-3115/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2007.03.088 * Tel.: +1 509 376 1442; fax: +1 509 376 0418. E-mail address: chuck.henager@pnl.gov Journal of Nuclear Materials 367–370 (2007) 742–747 www.elsevier.com/locate/jnucmat