Problem $6(4$ points$).$ Estimate $S_e^{\text{'}}$ for the following materials in kpsi.

$($a$)$ AISI $1020$ CD steel

$($b$)$ AISI $1080$ HR steel

$($c$)$ Aluminum with $S_{ut}=30k$

$($d$)$ AISI $4340$ steel quenched and tempered at $315C$