고체역학 (Beer) — Pure Bending: Problem 4.201

4.201 Three 120 × 10-mm steel plates have been welded together to form the beam shown. Assuming that the steel is elastoplastic with \(E = 200 \text{ GPa}\) and \(\sigma_Y = 300 \text{ MPa}\), determine (a) the bending moment for which the plastic zones at the top and bottom of the beam are 40 mm thick, (b) the corresponding radius of curvature of the beam.