| In part 1, intergranular cavity growth in regimes, where both surface diffusion and deformation enhanced grain boundary diffusion are important, is studied. In order to continuously simulate the cavity shape evolution and cavity growth rate, a fully-coupled numerical method is proposed. Based on the fully-coupled numerical method, a gradual cavity shape change is predicted and this leads to the adverse effect on the cavity growth rate. As the portion of the cavity volume growth due to jacking and viscoplastic deformation in the total cavity volume growth increases, spherical cavity evolves to V-shaped cavity. The obtained numerical results are physically more realistic compared to results in the previous works. The present numerical results suggest that the cavity shape evolution and cavity growth rate based on the assumed cavity shape, spherical or crack-like, simply cannot be used in this regime.; In part 2, intergranular creep failure of high temperature service material under a stress-controlled unbalanced cyclic loading condition is studied. The experimentally verified Murakami-Ohno strain hardening creep law and Norton's creep law are incorporated into the Tvegaard's axis-symmetric model for the constrained grain boundary rupture analysis. Based on the physically realistic Murakami-Ohno creep law, it is shown that the cavity growth becomes unconstrained upon the stress reversal from compression to tension. This leads to the prediction that the material life under a cyclic loading condition is shorter than that under a constant loading. Based on the classical Norton's law, the predicted material life under a cyclic loading condition remains the same as that under a constant loading. The obtained numerical results qualitatively match with recent experimental results by Arai, where the life under a cyclic loading can be much shorter than that under a constant loading. There are many cases where engineers use a simple Norton's creep law because of its simplicity. The present work suggests that more physically realistic creep laws should be used when cyclic loading must be considered. |
