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In college, for engineering breadth requirements, I took an intro to materials science course. Nearly 20 years later, three demos still stand out in my mind:
1) The first was a hands-on demo of work hardening, using copper bars. Each student was handed one and told to bend it, which was easy enough to do. Bending it back was immensely more difficult due to defects interlocking. A good way to bring together crystal lattices, defects, stress-strain curves and make it very tangible. The professor explained they baked the bars afterwards to restore them for later demos.
It's described here under the "cold working" section, and I guess this demo works well w/ paper clips too:
http://matse1.matse.illinois.edu/metals/prin.html
2) The professor had a demo involving a (sledge)hammer and
a) a (big) thick sheet of ice,
b) sheets of newspaper, both of which readily yield to a hammer. Then
c) was a thick sheet of ice w/ sheets of newspaper layered in, and it was remarkable how hard it was to break through it with a hammer. This was a vivid demonstration of composites.
youtube has a less impressive demonstration of this: https://www.youtube.com/watch?v=r6RlKFKB2BE
3) A demonstration of how a metal bar is massively weakened when a small notch is cut in it, due to the concentration of stress. This isn't what people regularly see or experience, and it really drove home that there's some interesting stuff going with the properties of materials. Most people appreciate that metal is strong, but I don't know that very many realize how much a simple bar of metal can be weakened by a little cut.
Hope this helps!
Dennis
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