$($a$)$ The white circles in the plot represent experiments. Interpret what these results tell us about fatigue behavior when the mean stress is in compression $($be clear and specific$)[2$pts$[$

$N=\frac{1}{C}{\displaystyle \underset{a_i}{\overset{a_f}{}}}\frac{da}{(\sqrt[\phantom{2}]{a}{)}^m}$

$($b$)$ Above is the Paris equation used in fracture mechanics. What is the physical significance of $a_f$ in terms of the figure shown in problem #$1.$ Be specific. $[2$ pt$]$