16) What is the critical fiber length of a fiber in a composite and why is it important? How does the fiber length affect mechanical properties of composites?

17) Explain possible mechanisms of polymer 'work hardening'.

18) Explain how and why the applied stress changes over time for a polymer for a fixed strain.

19) What is the difference between PAN and Pitch fibers and why is one stronger than the other?

20) Describe the toughening mechanism for fiber reinforced ceramics and how the fiber/matrix interface plays a role in mechanical properties.