Comparison of Fatigue Damage Estimates Using the ASME Code and Other Methods

Abstract

Fatigue damage calculations have been performed in a specific design application using the method outlined in the ASME Code Section III as well as the local strain approach. For both methods, the finite element stress analysis results for a structural component subject to a specified set of transient loadings have been considered. The local strain approach is based on computing strain ranges from the elastic stresses using the material stress strain curve and Neuber’s rule. The allowable number of cycles is determined from the strain ranges and the continuous cycling fatigue curve for the material. A comparison of the fatigue damages predicted by the two methods demonstrates some of the conservatisms of the ASME Code procedure over the local strain approach. The sources of conservatism lie in the low cycle fatigue strain concentration factors and inherent safety factors in the design fatigue curves of the ASME Code. Some of the non-conservatisms in the ASME Code fatigue evaluation could primarily arise from the low cycle fatigue strain concentration factors for stress ranges in the vicinity of 3Sm for the material, a result based on experimental and finite element studies. We have also included an assessment approach based on a material distance parameter for the same problem.