Estimate the theoretical fracture strength of a brittle material

Estimate the theoretical fracture strength of a brittle material if it is known that fracture occurs by the propagation of an elliptically shaped surface crack of length 0.25 mm (0.01 in.) and having a tip radius of curvature of 1.2 ( 10-3 mm (4.7 ( 10-5 in.) when a stress of 1200MPa (174,000psi) is applied.