Mean stress effects in stress-life fatigue and the Walker equation

Mean stress effects in finite-life fatigue are studied for a number of sets of experimental data for steels, aluminium alloys and one titanium alloy. Specifically, the agreement with these data is examined for the Goodman, Morrow, Smith-Watson-Topper and Walker equations. The Goodman relationship is found to be highly inaccurate. Reasonable accuracy is provided by the Morrow and by the Smith-Watson-Topper equations. But the Morrow method should not be used for aluminium alloys unless the true fracture strength is employed, instead of the more usual use of the stress-life intercept constant. The Walker equation with its adjustable fitting parameter gamma gives superior results. For steels, gamma is found to correlate with the ultimate tensile strength, and a linear relationship permits gamma to be estimated for cases where non-zero mean stress data are not available. Relatively high-strength aluminium alloys have gamma approximate to 0.5, which corresponds with the SWT method, but higher values of gamma apply for relatively low-strength aluminium alloys. For both steels and aluminium alloys, there is a trend of decreasing gamma with increasing strength, indicating an increasing sensitivity to mean stress.