Galileo's beam-bending sketch

I might as well tell you about toughness right now. What is toughness? We start out historically, we measured the strength of a material in terms of its tensile strength. And the first person to do something like this — [Tom shows Galileo's beam-bending sketch.] this is a picture from Galileo's notebook showing the bending of a beam under a load. He described the strength of a beam under a load, and he actually understood that they tend to break back where they attach to the tree trunk. Why is that? It's the maximum bending moment, if you know anything about mechanics. So people knew about strength of materials in the 1880s and they would run a tensile test. [Tom holds up a tensile test bar.] Here's the tensile test bar, I'll pass it around. Has what we call a cup-and-cone fracture on the top, which you can't see very well, but if I pass it around you probably can.