Suppose that the fatigue data for the cast iron in Problem 8.20 were taken for bending-rotating tests, and that a rod of this alloy is to be used for an automobile axle that rotates at an average rotational velocity of 750 revolutions per minute. Give maximum lifetimes of continuous driving that are allowable for the following stress levels:
(a) 250 MPa (36,250 psi),
(b) 215 MPa (31,000 psi),
(c) 200 MPa (29,000 psi), and
(d) 150 MPa (21,750 psi).