Use of Neuber's rule to estimate the fatigue life of notched specimens of ASME SA 106-B steel piping in 288°C air

Abstract

Fatigue strain-life tests were conducted on notched specimens of ASME SA 106-B piping steel at PWR operating temperatures (288°C (550°F)), under completely reversed loading. Fatigue limits at 10 7 cycles were estimated for smooth specimens to be 185 MPa (26·8 ksi) at 24°C and 232 MPa (33·7 ksi) at 288°C. The higher fatigue strength observed at the PWR temperature is postulated to be caused by dynamic strain aging processes. However, a reduction in fatigue strength in the low cycle fatigue regime was observed in 288°C air environment tests, which may indicate that the current ASME Section III design curve for carbon steels is nonconservative in its positioning. Notch strain histories were estimated for the notched specimen tests using various interpretations of Neuber's rule. It was concluded that the use of the fatigue notch concentration factor (K f) in the Neuber relation in conjunction with the uniaxial cyclic stress-strain curve provided the best correlation of notched specimen fatigue data with results obtained from smooth sp̀ecimen tests. The notched specimen strain-life results derived from the application of Neuber's rule alone proved to be conservative when compared with smooth specimen test results to such an extent that Neuber-generated notch stresses and strain amplitudes cannot accurately be compared with the mean data curves derived from the ASME Section III fatigue curves for carbon steels which are based on net section stress measurements.