Creep-fatigue Life Prediction of Normalized and Tempered 21/4Cr-1Mo Steel by Life Fraction Rule and Strainrange Partitioning Method

An investigation was made on the creep fatigue life which is an important factor in the design of structural components for elevated temperature applications under severe load condition. The material used for investigation was a normalized and tempered 21/4Cr-1Mo Steel. Two kinds of prediction methods, the Life Fraction Rule (LFR) and the Strain-range Partitioning (SRP) method, were used. After obtaining fundamental data necessary for the prediction of the creep fatigue life by the LFR and the SRP method, uniaxial creep-fatigue tests were performed under strain-controlled conditions in order to investigate the accuracy of the life prediction. Observed creep-fatigue lives were compared with those predicted by the LFR and the SRP method using stress-strain behavior obtained from both experiments and inelastic analysis. The results were as follows: (1) When the stress-strain values obtained from the experimental results were used, the creep fatigue life prediction agreed with the experimental values in both the LFR and the SRP method. (2) When the analytical stress-strain behavior was used, the life prediction was very conservative in the LFR, but agreed with the experimental values in the SRP method. This is because the estimated stresses were higher than the experimental values and the calculation of creep demage in the LFR is stress-based and the damage is sensitive to the values of stress used.