A solid circular bar of copper (G = 45 GPa) with length L = 0.75 m and

Question:

A solid circular bar of copper (G = 45 GPa) with length L = 0.75 m and diameter d = 40 mm is subjected to pure torsion by torques T acting at the ends (see figure).
(a) Calculate the amount of strain energy U stored in the bar when the maximum shear stress is 32 MPa.
(b) From the strain energy, calculate the angle of twist ( (in degrees)