What is the magnitude of the maximum stress that exists at the tip of an internal crack having a radius of curvature of 4.5 × 10-4 mm (1.772 × 10-5 in.) and a crack length of 3.5 × 10-2 mm (1.378 × 10-3 in.) when a tensile stress of 180 mpa (26110 psi) is applied?