Stress Concentration Kt

Two kinds of case. The closed-form ones — a small hole in a wide plate (Kt = 3, exact), an elliptical hole (Inglis), a hole in a finite-width plate (Heywood) — are formulas. The rest is the whole of Shigley’s Table A–15, seventeen charts, digitised from the curves themselves rather than read by eye; it reproduces every reading the book takes off them to within about 2%. Each chart defines its own nominal stress, shown with the case — Kt is meaningless against the wrong one.

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What the material actually feels — Kf

Kt is geometry. A soft steel shrugs part of it off and a hard one feels nearly all of it, so the fatigue notch factor is K_f = 1 + q(K_t − 1), with the notch sensitivity q from the Neuber constant (Shigley eq. 6–33 to 6–35, steels). Use the radius of the feature itself: d/2 for a hole, the fillet radius for a shoulder.