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Fatigue — Mean-Stress Criteria

Endurance limit Se = kₐkᵦk꜀·(0.5·Sᵤₜ), then the safety factor by Goodman, Soderberg (uses yield) and Gerber (parabolic). The fatigue notch factor Kf amplifies the alternating stress — notches drive fatigue, not the steady part. Stresses in MPa.

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Sᵤₜ ultimate (MPa)

S_y yield (MPa)

Kf notch factor

σₐ alternating (MPa)

σₘ mean (MPa)

kₐ surface

kᵦ size

k꜀ load

Se corrected

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σₐ effective (×Kf)

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Se′ (0.5·Sᵤₜ)

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n — Goodman

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n — Soderberg

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n — Gerber

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Soderberg (vs yield) is most conservative; Gerber (parabolic) least. Goodman is the common default.

Steel-oriented (Se′ capped at 700 MPa). A helper, not a substitute for engineering judgment.