Corrected Endurance Limit (Shigley Marin Factors)

The corrected endurance limit Se from Shigley's Marin equation Se = ka kb kc kd ke Se'.

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Example

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You get

Details, formula, and sources

Which turns the ultimate strength into the real fatigue strength. Se' = 0.5 Sut (steel, Sut <= 200 ksi); ka corrects for surface finish (ground/machined/hot-rolled/as-forged, a Sut^b), kb for size (0.879 d^-0.107 rotating, 1 axial), kc for load (1 bending, 0.85 axial, 0.59 torsion), kd for temperature, ke for reliability (0.897 at 90%, 0.814 at 99%). Sut 105 ksi steel, machined, 1 in, rotating bending, 99% reliability gives ka 0.79, kb 0.88, ke 0.81 and Se 29,500 psi - a 44% cut from the raw 52,500 psi, the reason a part sized on Se' alone can be fatigue-unsafe. Feed Se into a fatigue safety-factor check for the full stress-life workflow. Steel; stress concentration (Kf) and finite-life S-N are separate. A design aid; Shigley and the engineer of record govern.

Se = ka kb kc kd ke Se'; Se' = 0.5 Sut (steel, Sut<=200 ksi, else 100 ksi); ka = a Sut_ksi^b (ground 1.34/-0.085, machined 2.70/-0.265, hot-rolled 14.4/-0.718, forged 39.9/-0.995); kb = 0.879 d^-0.107 (0.11-2 in) / 0.91 d^-0.157 (2-10 in) bending, 1 axial; kc = 1/0.85/0.59; ke = 0.897/0.868/0.814/0.753 at 90/95/99/99.9%.

The Marin modification factors and the rotating-beam endurance limit Se' = 0.5 Sut (Shigley, Mechanical Engineering Design, Ch. 6 -- Marin surface, size, load, temperature, and reliability factors), by name; feeds the fatigue-safety-factor tile.

The Marin equation and its factor tables are standard published machine-design results; the ultimate strength, diameter, finish, load type, and reliability are the designer's inputs.

Estimate. AHJ and licensed professional govern.

Field names used by the API: ultimate_strength_psi, surface_finish, diameter_in, load_type, reliability_pct, temperature_factor_kd, endurance_limit_psi, ka

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