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[FAQ]

The Goodman and Gerber mean stress correction theories are used in the StressLife (SN) fatigue methodology to account for the detrimental effect that tensile mean stresses have on fatigue life. In SN fatigue, the calculated stress amplitude is the maximum stress range for a cycle of loading. It is used in conjunction with a stress vs cycles (S-N) curve to determine the number of cycles to failure for that loading. The Goodman/Gerber theories modify the calculated stress amplitude to determine an effective stress amplitude that includes the effect of the mean stress. The effective stress amplitude is the calculated stress amplitude multiplied by the mean stress correction factor (1.0 minus the ratio of the mean stress to the ultimate stress raised to an exponent). Sa = [1-(Sm/Su)^n] * Seff For the Goodman theory, the exponent (n) is 1.0 (linear correction). For the Gerber theory, the exponent is 2.0 (parabolic correction). Note: The Goodman correction is more conservative, but comparisons to test results indicate that the Gerber correction is usually more accurate.