고체역학 (Beer) — Torsion: Problem 3.108

3.108 A shaft of mild steel is machined to the shape shown and then twisted by torques of magnitude 40 kip·in. Assuming the steel to be elastoplastic with \(\tau_Y = 21 \text{ ksi}\), determine (a) the thickness of the plastic zone in portion CD of the shaft, (b) the length of portion BE that remains fully elastic.