What is the magnitude of the maximum stress that exists at the tip of an internal crack having a radius of curvature of 1.9 × 10-4 mm (7.5 × 10-6 in.) and a crack length of 3.8 × 10-2 mm (1.5 × 10-3 in.) when a tensile stress of 140 MPa (20,000 psi) is applied?