A tension bar is made from $2024-$T$3$ aluminum alloy with the dimensions $b=40mm,t=5mm,$ and $h=100mm.$ If it contains a pre$-$existing cextral crack of length a $=6\mathrm{.}55$amm as illustrated, what is the critical stress for failure by fracture, in MPa?

For $=\frac{a}{b}$ and $\frac{h}{b}1\mathrm{.}5,$

$Y=\frac{1-0\mathrm{.}5+0\mathrm{.}326{}^2}{\sqrt[\phantom{2}]{1-}}$