AN EXAMINATION OF THE ROLE OF THE ASSUMED YOUNG'S MODULUS VALUE AT THE HIGH CYCLE END OF ASME CODE FATIGUE CURVE FOR STAINLESS STEELS

摘要

When the ASME Code fatigue curves (S-N curves) are used in the assessment of high frequency cyclic stresses (such as those produced by flow-induced vibrations), the question arises as to the need for an E correction (i.e., multiplying the calculated cyclic stress by the ratio of the E value at the room temperature and the E value at the temperature used in the stress analysis). This question becomes significant for materials such as stainless steels when the two sets of S-N curves up to the 2007 Edition of the Code are specified: ⅰ) the first curve covered the cyclic range of 10 to 10 cycles and specified an E value. This curve covered mostly the strain controlled fatigue data for which the correction for E is required. ⅱ) The second curve covered the cyclic range of 10~6 to 10~(11) cycles and didn't provide a specific E value. This curve covered mostly the load controlled fatigue test data for which the correction for E is not required since the stress was independent of E (stress was either P/A for axial loading or Mc/I for bending). However the 2010 and subsequently the 2013 Editions of the Code combined the two curves into a single curve with a cycle range of 10 to 10~(11) cycles with E value specified at room temperature. This means that the E correction applies across the board for the entire cyclic range of 10 to 10~(11) cycles including the high cycle end where the test results are independent of E. The inclusion of the E correction for the high cycle fatigue range presents a problem for the evaluation of components with vibratory loading. The present paper describes the results of a thorough review of the past technical basis papers for the ASME Code S-N curves and examines the necessity for E correction at the high cycle end of the Code S-N curves for stainless steels.