Supersonic wind tunnels store air under high pressure in cylindrical pressure vessels - the pressure, when released, produces hypersonic rates of flow. The pressure vessels are routinely prooftested to ensure that they are safe. If such a cylinder, of diameter 400mm and wall thickness 20 mm, made of a steel with a fracture toughness of 42 MPa-m1/2, survives a proof test to 40 MPa (400 atmospheres), what is the length of the largest continued penny-shaped interior crack it might contain?