A model stress analysis of swelling in SiC/SiC composites as a function of fiber type and carbon interphase structure Charles H. Henager Jr. a, * , Edward A. Le b , Russ H. Jones a a Pacific Northwest National Laboratory, 902 Battelle Blvd., MS: P8-15, Richland, WA 99352, USA b Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA Abstract A continuous fiber composite was simulated by four concentric cylinders (consisting of fiber, fiber/matrix interphase coating, matrix, and surrounding composite) to explore composite stresses when irradiation swelling of the various components is included to study radial debonding at the fiber-coating interface as a function of neutron dose. SiC Type- S and Hi-Nicalon fibers, and three types of transversely isotropic carbons for the fiber coating were considered. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction SiC-based continuous-fiber composites are consid- ered for nuclear applications as structural components due to their high specific stiffness and strength, but the differential response of the fiber, fiber/matrix interphase (fiber coating), and matrix under irradiation raises concerns [1]. A model using concentric cylindrical re- gions to simulate continuous fiber composites and give the composite stress distribution has been developed to explore thermo-mechanical loading effects, including irradiation swelling [2–4]. When considering a coated fiber in a surrounding matrix, four elastically distinct cylindrical regions are required [2], which can be ex- panded to any number of cylindrical regions [3]. Such models are used to study residual and thermals stresses in composites with a variety of fibers, matrices, and fiber coatings [2–5]. El-Azab and Ghoniem [6] modified a two-cylinder model of a continuous fiber composite to study irradi- ation swelling and fiber shrinkage, plus irradiation and thermal creep of both fiber and matrix, on the time- dependent stresses in a composite with SCS-6 or Nicalon fibers. They showed that Nicalon fiber shrinkage under irradiation leads to fiber/matrix debonding [7,8]. How- ever, none of the previous studies of SiC-based contin- uous fiber composites under irradiation has considered irradiation-induced dimensional changes to fibers, fiber coatings, and SiC-matrices simultaneously. 2. Simulation details 2.1. Four cylinder model A continuous fiber composite is simulated by four concentric cylinders [2]. The surrounding composite is the outermost cylinder, while the matrix, fiber coating, and fiber are the remaining cylinders, with the fiber being the innermost cylinder, denoted as domains n ¼ 1, 2, 3, 4 with radii r 1 , r 2 , r 3 , and r 4 , respectively (Fig. 1). The cylinders are subjected to three independent boundary conditions; axisymmetric temperature change, DT ðrÞ, uniaxial applied stress, r 0z , and biaxial applied stress, r 0r , where r and z are the radial and axial com- ponents in cylindrical coordinates ðr; h; zÞ. Stress relax- ation was not allowed during irradiation or during cooling from the fabrication temperature and all com- ponents remain elastic and perfectly bonded. * Corresponding author. Tel.: +1-509 376 1442; fax: +1-509 376 0418. E-mail address: chuck.henager@pnl.gov (C.H. Henager Jr.). 0022-3115/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2004.04.110 www.elsevier.com/locate/jnucmat Journal of Nuclear Materials 329–333 (2004) 502–506