Estimate material fatigue life in cycles from stress amplitude and ultimate tensile strength.
The endurance limit (or fatigue limit) is the stress level below which a material can theoretically endure infinite cycles. For steel, it's approximately 0.5× ultimate strength. Aluminum and most non-ferrous metals have no true endurance limit.
Surface finish, stress concentrations (notches, holes), temperature, corrosion, mean stress level, and loading type (bending vs. torsion) all significantly affect fatigue life. Real-world fatigue life is often much shorter than calculated.