The competing effects of failure by net section stress rupture and failure due to crack growth in creeping structural alloys have been investigated to determine the dominant failure process under various constant load service conditions. Two sets of experimental information were utilized: times to failure by quasi-homogeneous damage accumulation represented by Larson-Miller master curves, and creep crack growth rate information correlated either with C* for creep-ductile materials or with K for creep-brittle materials. A criterion, phrased in terms of the applied stress, temperature and initial crack length, has been established to justify the rate-determining fracture process. A number of materials, ranging from ductile materials such as Type 304 stainless steel, IN800H and low alloy steels obeying C*-controlled crack growth, to brittle materials such as Ni-based superalloys obeying K-controlled crack growth, have been studied. Both the configurations of infinite bodies and finitesize specimens were considered. The results show that for those materials where crack growth is C*-controlled, eventual failure is governed by quasi-homogeneous creep damage accumulation; whereas, for the materials with K-controlled crack growth, creep crack growth is likely to govern the final failure of engineering structures.

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