(d) True or false? A: The stress at a crack tip decreases with the crack length. B : The stress at the crack tip increases with larger radius of curvature. C: The applied tensile stress does not affect the stress at the crack tip. D: The stress at the crack tip is high because the radius of curvature is small. (e) The Young's modulus of aluminum oxide is 215GPa and that of silica (quartz) 66GPa. The fracture surface energies of both ceramics are approximately 3J/m2. Which material has a higher critical stress for crack propagation and is more resistant to fracture? (f) Using the information in (e), compute the critical stress at the crack tip for crack propagation in aluminum oxide, assuming a crack length of 0.5mm. (g) Explain why higher temperature can lead to more ductile failure for many materials, and not to more brittle failure. (h) Do materials with a fatigue limit form cracks below the fatigue limit? (i) True or false: Creep failure involves variable stress on the material. Explain your