Use Fatigue Limit to Set Minimum Part Thickness

Fatigue limit is the stress level below which a material can endure an infinite number of cycles. For steels, this threshold is real. If you design for infinite life, you must keep cyclic stress below this limit. Thickness drives stress: a thinner part sees higher stress under the same load. Therefore, thickness often determines whether a part survives.

Don't make everything thicker blindly. Identify critical locations using FEA, and focus on stress concentrations. At a notch or hole, local stress can be several times the nominal stress. If that local stress exceeds the fatigue limit, increase thickness there. This is more efficient than thickening the entire part.

Remember that fatigue limit is a material property, not a part property. Surface finish, corrosion, and residual stresses all affect the real limit. A rough surface or sharp edge can cut the endurance limit by half. If you must keep a part thin, consider shot peening or surface rolling to raise the fatigue strength.

In practice, set your minimum thickness based on the stress at the worst-case location. Keep that stress below the fatigue limit with an appropriate safety factor. Then verify with testing if the part is critical. Thickness decisions guided by fatigue limits will save weight, cost, and warranty failures.

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