S/N Curve and Fatigue Testing

What is the S/N Curve?

With reference to the fatigue graph (S/N curve) below answer the following questions:

i) What is the name used for the horizontal section of graph exhibited by the 1045 steel sample and explain why it is a desirable feature for materials to show.

ii) Your boss has asked you to calculate the minimum allowable diameter for an aluminium component subjected to a load cycle of +/- 3000 N, where the component must last at least one million cycles (i.e. 10⁶ cycles).