A thin large sheet of a maraging steel has a through-thickness internal crack, which is 4.0 mm long and orientated perpendicular to the stressed direction. For this steel, E can be taken as 200 GPa, the energy of fracture surface as 2.0 J/m, and the work of plastic deformation of each crack tip is 2.0 104 J/m. Determine the critical stress for brittle fracture.