Freshly made light bulb strength demonstration

Tom's thesis advisor Bob (Uhlmann/Rose — see editorial register) would obtain freshly-made light bulbs from the Sylvania North Shore plant and demonstrate in 3.091 that a bulb thrown against the wall would bounce when fresh and shatter two days later, illustrating humidity-driven Griffith-criterion surface flaw formation.

When the glass is first formed, it has very perfect surfaces and extremely high strength. When we make fiberglass — if we make the fiberglass and then within minutes coat it with plastic and keep the moisture off — it turns out the humidity in the air will slightly attack the glass on the atomic scale, and create flaws that will embrittle the glass per the Griffith criterion. I've never done it, but my thesis advisor lives up on the North Shore, he's retired now. Sylvania had a glass factory up on the North Shore where they made light bulbs, and Bob would stop in the morning — he'd worked out a deal with the guy there — and he'd get a freshly made light bulb, and he'd bring it in when he was lecturing 3.091. He could take that light bulb and throw it across and hit the wall, and it would bounce, because it was freshly made. If he did that two days later, after it sat in the humid Cambridge atmosphere, it would shatter. But freshly made, it had no flaws on the surface, it had not been corroded by the moisture in the air, it had tremendous strength.

Combined with the Bob Rose anecdote; explains why fiberglass is coated with epoxy immediately after drawing — to preserve the 200 ksi surface strength before moisture corrosion creates micron-scale flaws that destroy fracture toughness.

The other story on glass is freshly made glass, as it's blown, has a very perfect surface. Glass actually corrodes in the moisture in the air — it's a silicate, and it forms silicate hydroxides. The strength of glass will change dramatically within a few days after its manufacture. One of the secrets of fiberglass is they draw the fibers and coat them with epoxy or whatever plastic resin, and they keep the moisture away from attacking the surface of the glass, so those fibers maintain fantastic strength — 200 ksi if you could actually measure them.