Background to the elastic creep-fatigue rules of the ASME B&PV Code Case 1592

Recent modifications associated primarily with elastic analysis have been made to the creep-fatigue rules of the ASME B&PV Code for use in design of elevated temperature nuclear components. The changes involve: (1) a modified equation for predicting the total strain range per cycle based on a more accurate approximation to the Neuber equation; (2) a variable Poisson's ratio in calculating stresses and strain using elastic equations to account for local thermal strain concentrations; (3) a modified approach for determining the equivalent strain range and; (4) an adjusted design fatigue curve. The modified Neuber approach accounts for the beneficial effects of a residual stress related to the relaxation strength of the material and includes a strain concentration factor greater than the elastic stress concentration factor for geometrical notches for mechanical loads. Notch concentration factors are taken equal to the elastic stress concentration factors for peak thermal and creep strains. The background and intent for these rules are discussed. Correlations of the strain range predictions to those obtained using more rigorous detailed inelastic analysis and test data are presented.