Journal of the American Ceramic Society--Fujita et al Vol. 87. No. 2 OOM/OA 100M 0M4 60M40A Fig 4. Fracture surfaces of compacts that cover the range of alumina content, from O% (a, b) to 40%(c, d). following a 1000 h aging at 1200 C. Examples of fractured particle junctions are highlighted by the arrows between them broken. The simulation proceeds by incrementally where Ti is the intrinsic junction toughness. The results of the increasing the remote displacement while allowing the junctions at simulations ( Fig. 5)are well described by the crack tip to lail he junction stress reaches a critical bining Eqs. (4) and (9) yields the time dependence of the (10) rr=12(aR)2 predictions of modulus and toughness are compared with rimental measurements in Fig. 4. The best correlations are by taking n= 7 in Eqs. (7)and (10). The inference is that growth occurs predominantly by surface diffusion. For this mechanism, the reference time is given by 0.1 =1126Dy where k is Boltzman's constant, T is absolute temperature. 8, is the 0.2 030.40.5 effective surface thickness, D. is the surface diffusion coefficient. Relative junction size, a/R y is the surface energy (I J/m"), and n is the molecular volume (2.2x 1028 m for mullite Fig. 5. Results of fracture simulations based on the discrete element A further assessment of the models was made in the following way: The numerical values of the various parameters for mullite