A thin plate with a width of $250$ mm and a thickness of $10$ mm is loaded as shown in the figure. Observations indicate a crack with a length of $2$ mm located $300$ mm from the free end. The crack at the surface is propagating towards the interior of the plate. Determine the minimum required critical stress intensity factor, $K_{kc}($in MPa.m ${?}^{\frac{1}{2}})$ for the plate material. Consider a factor of safety of $2\mathrm{.}8.$ Do not consider the transverse shear stress.

Dimensions are given in millimeters