A machine part in a turbine-generator unit operates near room temperature and is made of ASTM A470-8 steel (yield strength = 620 MPa, fracture toughness = 60 MPa*m1/2). A surface crack has been found that is roughly a semi-ellipse, with surface length 2c = 50 mm and depth a = 15 mm. The stress normal to the plane of the crack is 250 MPa, and the member width and thickness arc large compared with the crack size. What is the safety factor against brittle fracture?