고체역학 (Beer) — Pure Bending: Problem 4.22

4.22 A 900-mm strip of steel is bent into a full circle by two couples applied as shown. Determine (a) the maximum thickness \( t \) of the strip if the allowable stress of the steel is 420 MPa, (b) the corresponding moment \( M \) of the couples. Use \( E = 200 \text{ GPa} \).