Meaning of Ke in Design-by-Analysis Fatigue Evaluation

Abstract

The ASME Section III Design-by-Analysis rules for pressure-retaining components include a detailed fatigue evaluation based on elastically-predicted primary, secondary, and peak stresses. A pre-requisite for the fatigue analysis is that the primary-plus-secondary stress range does not exceed 3Sm. If this limit is exceeded, the code provides “Simplified Elastic-Plastic Analysis” rules for the fatigue evaluation. A Ke penalty factor is applied to the elastically-predicting alternating stress. The maximum value of Ke (3.3 or 5) is a severe design limitation. Test data indicate that the code specified value of Ke is very conservative. The simplified elastic-plastic rules were originally developed for piping and published in B31.7. When the piping rules were incorporated into Section III in 1971, the B31.7 procedure was replaced by a less complex procedure. The development of the simplified elastic-plastic analysis approach is reviewed to establish the technical basis for the present code rules. The concepts of fatigue, shakedown to elastic action, thermal bending, elastic follow-up, notch factor, and strain redistribution are discussed. Recommendations for changes to the plastic strain correction factor are provided.