고체역학 (Gere) — Torsion: Problem 3.9-4

3.9-4 A stepped shaft of solid circular cross sections (see figure) has length \( L = 0.80 \text{ m} \), diameter \( d_2 = 40 \text{ mm} \), and diameter \( d_1 = 30 \text{ mm} \). The material is steel with \( G = 80 \text{ GPa} \). Determine the strain energy \( U \) of the shaft if the angle of twist is \( 1.0^{\circ} \).