A circular shaft is subjected to a torque Ta and a bending moment Ma. Assuming that the diameter of the shaft is d, determine the expressions for the principal stresses and maximum shear stress in terms of Ma, Ta and d. Next, determine the size of the shaft so that the maximum shear stress in the shaft does not exceed 80 MPa. Assume that the applied torque is 50 kN-m and the bending moment is 12 kN-m.