Mean stress effects in strain-life fatigue

A mean stress equation can be incorporated into the strain-life curve in a manner that is consistent with the stress-based use of the same equation. Doing so for the Walker mean stress relationship gives excellent results for a number of strain-life data sets with non-zero mean stresses, including data on steels, one titanium alloy and aluminium alloys. This approach has a number of advantages: All data at all mean stresses can be combined into a single fitting procedure to determine the constants for the stress-life curve, which values also apply to the elastic strain term of the strain-life curve. The Walker parameter gamma that also arises from this fitting is related to the sensitivity of the material to mean stress, giving this approach a versatility that is not possessed by other common mean stress methods. Where non-zero mean stress data are not available to obtain gamma from fitting, an equation based on existing fitted values can be used to make estimates for steels. For precipitation-hardened aluminium alloys in the 2000 and 7000 series, an estimate of gamma = 0.5 may be applied, so that the method becomes similar to that of Smith, Watson and Topper. For other metals, a default estimate of gamma = 0.5 is suggested. For life estimates using the strain-based approach, it is recommended that the Walker mean stress method, incorporated into the strain-life curve, should be employed as an alternative to other methods, or perhaps to even replace them entirely.