3-D simulation of macroscopic erosion of CFC under ITER off-normal heat loads

Because of their high heat conductivity carbon fiber composites (CFC) are considered as armour material for the vertical target in the divertor strike point region where high heat loads are expected during off-normal heat loads. The newly developed CFCs have a complex 3-D framework from ex-PAN and ex-pitch carbon fibers. The framework is filled with a carbon matrix. Both fibers are anisotropic materials. The linear thermal expansion coefficient, the heat conductivity and the Young’s modulus along and across the fiber direction are 10-50 times different. This anisotropy and the difference of the linear thermal expansion coefficient of the matrix and the fibers cause larger internal thermostress in CFC as compared to graphite. The thermostress concentrates at the interfaces between fibers and matrix, especially close to the sites of the perpendicular intersection of the fibers. There is experimental evidence of formation of large macroscopic pits at such intersections under high heat loads [1] and if those pits combine there occurs large macroscopic erosion by brittle destruction [2]. Indeed first experimental results for CFC samples heated by quasistationary surface heat loads to temperatures of about 3050 K have shown rather large macroscopic erosion by brittle destruction of fibers, parallel to the CFC surface [3]. ... mehrFor the simulation of the thermomechanical properties of the CFC materials an existing 3-D lattice model previously used for the simulation of fine grain graphite has been modified to reproduce the CFC structure with fibers and matrix. Simulation of CFC erosion under high surface heat loads, typical for transients and off-normal regimes has been performed. The 3-D numerical simulations show that brittle destruction of CFC occurs under such heat loads and might result in large macroscopic erosion of the CFC armour. As a consequence the lifetime of the CFC armour is drastically reduced and considerable amounts of dust are produced.