This paper studies intergranular creep failure of high-temperature service material under a stress-controlled unbalanced cyclic loading condition. 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 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.

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