Describe the mechanism of crack propagation for both ductile and brittle fracture modes. Explain in your own words why the Griffith criterion for fracture, which worked exceedingly well for glass, had to be modified by Orowan to accommodate the behavior of metals. 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 x 104 mm and a crack length of 3.8 x 10-2mm when a tensile stress of 140 MPa is applied?