Eagar plexiglass brittle-fracture demonstration

Now I have a demo of brittle versus ductile, both plastics. [Tom holds up two plastic samples.] Polyethylene or polypropylene, I'm not sure which one it is, but I can bend it all I want. This one's been bent into a nice U before, comes back mostly. I can bend it back the other way. If I keep doing this forever, longer than you and I want to be here, it will break in fatigue. This is a piece of polymethyl methacrylate, better known as plexiglass. They didn't take the paper backing off that keeps it from looking scratched. If I break that — [snap] I never got three pieces before. But if you look carefully, you will see a little what we call a shear lip on the compression side — a positive shear lip on the compression side, and a brittle fracture in between. I'll pass the other one around, except it's just a bar of white plastic.