A varying torque of 30 kN m to 80 kN m is applied to the end of a shaft. The shaft material has a yield strength of 30 MPa and the endurance limit (S’e) of 250 MPa with derating factors ka =0.85, kb = 0.82, kc = 0.6. Notch sensitivity q=0.9 and stress concentration factors kt= 1.39. Find out the shaft diameter. Take factor of safety (FOS)=1.6.