FBD & Equilibrium equations

Using the FBDs$,$ of cut sections between A&B and between C&D$,$ shown

${\displaystyle \underset{?}{\overset{?}{}}}{M}_x=0=>T_{AB}=T$

${\displaystyle \underset{?}{\overset{?}{}}}{M}_x=0=>T_{CD}=T$

Design w$.$r$.$t max. shear stress in $AB$

Design $w.r.t$ max. shear stress $inCD$

Design w$.$r$.$t max. shear stress in CD

The allowable torque $Tis(kip.in)$ and $isa$

The allowable torque $T$ is $($

kip.in$)$ and is a