고체역학 (Gere) — Torsion: Problem 3.12-2

3.12-2 A stepped shaft with diameters \(D_1 = 40 \text{ mm}\) and \(D_2 = 60 \text{ mm}\) is loaded by torques \(T = 1100 \text{ N}\cdot\text{m}\) (see figure). If the allowable shear stress at the stress concentration is \(120 \text{ MPa}\), what is the smallest radius \(R_{\min}\) that may be used for the fillet?