A three dimensional model for high temperature deformation and failure of an advanced silicon nitride, NT-154, is presented. The model uses a nonsymmetric flow surface to account for different creep behavior in tension and compression. An isotropic strain hardening variable is used to describe the continuous drop in creep rates with deformation. The failure criteria uses a second order symmetric tensor as an internal damage variable to describe the anisotropy of cavity formation under creep conditions. A one dimensional version of the model is compared with experimental creep tests at various stresses and temperatures. The model captures the deformation and failure well to within the scatter in experimental data.
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