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 γ 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 γ 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  γ= 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  γ= 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.