$($a$)$ A solid shaft of diameter $D=50mm$

rotates and carries the minor shock

loading as shown in figure. Calculate

the fatigue safety factor n and

yielding factor of safety $n_y$ for the

shaft. Given: $D=75mm,S_y=550$MPa,

$S_u=660$MPa. The torque fluctuates

$5\%$ each way from the mean value,

and the shaft is to be machined from

unnotched steel. $[5$ Marks$]$