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) 250MPa (36,250psi), 

(b) 215MPa (31,000psi), 

(c) 200MPa (29,000psi), and 

(d) 150MPa (21,750psi).