A Comparative Study of Ke Factor in Design by Analysis for Fatigue Evaluation

Abstract

ASME Section III Design by Analysis rules provide a “Simplified Elastic-Plastic Analysis” approach for the fatigue usage evaluation when the primary plus secondary stress range exceeds the limit of 3Sm. A penalty factor Ke is applied to the alternating peak stress calculated by assuming elastic material behavior. There is general consensus that the Ke factor determined using current code equations can be overly conservative for some conditions, and can lead to severe design limitations. The ASME code committees have been actively working on revising the Ke equations. The work in this paper intends to assess the conservatism of the Ke procedure in current ASME NB-3228.5, and to evaluate the two proposed alternative ASME Code Ke procedures through a comparative study. The evaluation focuses on the Ke procedure related to the effect of through-wall temperature gradient induced stresses on fatigue life for austenitic stainless steel. The conservatism of current ASME NB-3228.5 rules for the Ke factor is demonstrated through the comparison of predicted fatigue life with the test result of a stainless steel stepped pipe. In view of the technical background that the Ke factor represents the ratio of the actual strain range to the elastically calculated strain range, the theoretically accurate Ke factor can be calculated from elastic and elastic-plastic analyses. This directly calculated Ke factor is found to be in good agreement with the fatigue life from the test. A comparative study of procedures for calculating the Ke factor using current ASME NB-3228.5 rules and its counterparts in various codes is then conducted. B31.7 code, French RCC-M code and Japanese JSME code are considered. The two proposed alternative ASME Code Ke methodologies are evaluated. The differences in calculated Ke values are illustrated through the sample stepped pipe problem with various thermal transients. The directly calculated Ke factor using elastic and elastic-plastic analyses is used as reference value for comparison and conservatism assessment.