Mean Stress Correction
This section describes the ways the influence of a mean stress dependence can be included in cases where the S–N curve does not have an explicit dependence on the mean stress, or indirectly through the load ratio.
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In Stress life fatigue evaluation, the amplitude stress that is being used in the S–N Curve model, the Basquin model, or the Approximate S–N Curve model can be modified to account for the mean stress.
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In Harmonic vibration fatigue evaluation, the amplitude stress that is being used for the S–N curve for amplitude stress option can be modified to account for mean stress.
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In Cumulative damage fatigue evaluation, the amplitude stress that is being used for the S–N curve for amplitude stress option can be modified to account for mean stress.
The mean stress σm is defined as the average of the minimum and maximum stresses during a load cycle. The scalar stress measure used is the same as in for defining the amplitude stress. If you select Gerber as Method, the amplitude stress is modified using the mean stress as
where σu is an ultimate tensile strength that you need to specify.
If you select Goodman as Method, the amplitude stress is modified using the mean stress as
The Goodman and Gerber methods differ only by the shape of the interaction curve between amplitude stress and mean stress. In the Goodman case, the allowed amplitude stress decreases linearly with the mean stress, whereas in the Gerber case the interaction curve is parabolic.
If you select Soderberg as Method, the amplitude stress is modified using the mean stress as
where σys is a yield stress that you need to specify. The Soderberg method is similar to the Goodman method in the sense that the interaction curve is a straight line. The Soderberg method is however more conservative, since it predicts that failure occurs when the mean stress is at the yield limit, rather than at the ultimate tensile stress.