고체역학 (Gere) — Torsion: Problem 3.9-3

3.9-3 A stepped shaft of solid circular cross sections (see figure) has length \(L = 2.6\text{ m}\), diameter \(d_2 = 50\text{ mm}\), and diameter \(d_1 = 40\text{ mm}\). The material is brass with \(G = 81\text{ GPa}\). Determine the strain energy \(U\) of the shaft if the angle of twist is \(3.0^\circ\).