$1\mathrm{.}1$ Two solid cylindrical rods $AB$ and $BC$ are welded together at $B$

and loaded as shown. Knowing that $d_1=30mm$ and $d_2=50mm,$

find the average normal stress at the midsection of $($a$)$ rodAB,

$($b$)$ rodBC.

Fig. P$1\mathrm{.}1$ and P$1\mathrm{.}2$