7. a) Explain why in case of short cracks or of very long cracks the linear elastic fracture criteria are no longer valid, even for relatively brittle materials b) In fracture mechanics inclusions are generally subdivided in three categories classified according to their size. The largest category is that of inclusions ranging from 0.5 to 50m. What role do these inclusions play in the following phenomena? i) Ductile fracture ii) Fracture toughness, KIc