Methods are described for assessing acceptable defect sizes for plant in high temperature service. At one extreme of behavior, that of creep ductile response, failure is governed by continuum damage mechanics which may be analyzed using finite element methods or more approximately by reference stress techniques. In less ductile cases failure can occur through the initiation and growth of a dominant creep crack. For this case the paper outlines how the assessment requirements may be based on an estimate of the initiation time and a method for calculating this initiation time is given. When the initiation time is small, the assessment also requires an estimate of the time taken in the creep crack growth stage and both empirical correlation and statistical approaches for analyzing this stage are described. Finally the paper outlines how fast fracture and ductile instability can be included in determining repair criteria for use with the various assessment methods.

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