Abstract

One feature of creep fatigue data is the large amount of scatter present, even for test specimens taken from the same heat of material and tested under nominally identical conditions. This has led to considerable discrepancies between methods proposed during the last 40 years for fatigue life prediction of high temperature components. Presently, there is no general agreement on optimum approaches.

The inadequacy of the Manson-Coffin law for high temperature use is discussed. The frequency modified fatigue life model and the strain range partitioning method, two popular extensions of the Manson-Coffin law for high temperature use, are studied. Major differences in terms of mechanistic interpretations and mathematical formulations between these two models are investigated. Also, difficulties in the realistic application of these methods are studied and some simplified solutions are proposed.

The time-dependent damage rule of the ASME Boilerand Pressure Vessel Code is studied for its application to notched components and issues such as creep rupture under compressive stress, creep rupture time of notched components, and cyclic creep rupture curves vs. static creep rupture curves are discussed. Difficulties in the calculation of the time fraction term of this method are investigated for notched components and a simplified solution is proposed.

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