A gear box's shaft is made of a hollow circular steel tube with allowable yield stress equal to dallow- The shaft is loaded by a combination of bending moment M and torque T. Assume that the tube's inner diameter is half of the outer diameter. Determine the minimal outer diameter D of the steel tube according to von Mises (or called Maximum-Distortion- Energy) failure theory. Present a step-by-step derivation. M T.