Acircular bar is constructed by bonding an aluminum shell around a solid steel cylinder. the radius of the steel cylinder is rs = 10 mm, and the outer radius of the aluminum shell is ra = 20 mm. the overall length of the bar is given by l = 1 m and a torque t = 1 kn-m is applied at the ends. the shear moduli for the aluminum and steel are ga = 28 gpa and gs = 76 gpa, respectively.

(1) find the maximum shear stress in the steel and in the aluminum

(2) determine the total twist angle of the bar

(3) determine the torsional stiffness

(4) find the allowable torque for a safety factor of 2 when the yield stresses for both materials is 300 mpa.