고체역학 (Beer) — Pure Bending: Problem 4.107

4.107 A milling operation was used to remove a portion of a solid bar of square cross section. Knowing that \( a = 30 \text{ mm} \), \( d = 20 \text{ mm} \), and \( \sigma_{\text{all}} = 60 \text{ MPa} \), determine the magnitude \( P \) of the largest forces that can be safely applied at the centers of the ends of the bar.