Development of Fatigue Design Curves for Pressure Vessel Alloys Using a Modified Langer Equation

The Jaske and O’Donnell [1] curve-fitting procedure for analyzing fatigue data generated between room temperature and 427° C (800° F) for several pressure vessel alloys is reexamined in the present paper. Substantial improvements over their best-fit curves to the data are found to result from two proposed modifications to their procedure, namely 1) the use of a variable exponent in the Langer equation, and 2) minimization of the sum of the squares of the errors in the logarithms of the cyclic-stress amplitudes rather than in the stress amplitudes directly. Likewise, important differences are observed for the resultant allowable stress-amplitude values for design purposes. In particular, the present analysis permits higher allowable stress amplitudes in the critical low-cycle fatigue-life region for the austenitic stainless steels, alloy 800, and alloy 600. Two best-fit curves and the associated sets of allowable stress amplitudes, corresponding to the inclusion or deletion of load-controlled data, are obtained for alloy 718.