A gear box’s shaft is made of a hollow circular steel tube with allowable yield stress equal to σa????????o???? . The shaft is loaded by a combination of bending moment ???? 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-DistortionEnergy) failure theory. Present a step-by-step derivation.