$8\mathrm{.}1$ Look at Fig. $8\mathrm{.}32,$ and perform the following tasks:

$($a$)$ Obtain approximate values of fracture toughness $K_{Ic}$ for AISI $4340$ steel heat treated to yield strengths of $800$ and $1600$ MPa $.$

$($b$)$ For each of these yield strengths, calculate the transition crack length $a_t,$ and comment on the significance of the values obtained.