Franklin Veaux (tacit) wrote,
Franklin Veaux

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Some thoughts on fatigue

A long time ago, when I was going to school in Pennsylvania, I had a friend named Barry Kramer. Barry's father is a machinist, and he gave Barry a metal rod, about a foot long and an inch or so in diameter, that had been mis-machined.

Barry used this rod (which weighed a good five or ten pounds) to beat things up--road signs, walls, pavement, that sort of thing. There's something inherently, irrationally satisfying about holding a heavy piece of steel in your hands and really whacking the holy hell out of something...but I digress.

Anyway, one afternoon, a mutual friend of ours went over to Barry's house, picked up the heavy steel bar, and delicately tapped on the ringer for the doorbell with it.

The steel bar failed instantly--it cracked in half and fell in two pieces to the floor.

I got to thinking about Barry this evening as I was leaving the office. I have a large laptop carrying case I've used for years; I've had it for so long it's starting to come apart, and the strap is frayed. I walked outside and was just ambling along toward the car, minding my own business and thinking about cognitive limits and modeling of human intelligence, when I heard a distinct Tink! and the strap of the laptop bag went slithering over my shoulder, sending the bag plummeting to the ground.

I caught the bag before anything bad happened, and hauled it up expecting to see that the strap had failed. But no. The metal latch at the end of the strap had failed and split, very cleanly, in two.

Metal fatigue is really interesting. Most metals do not have an infinite fatigue life. In fact, with most metals, if you take a rod of metal capable of holding, say, 100 pounds of weight, bolt it to your ceiling, and hang a 90-pound weight from it, eventually the metal will fail and the weight will fall. Titanium doesn't behave this way, but many other metals do.

The split where the metal latch failed is surprisingly clean. The metal broke precisely in an almost perfectly straight line; the geometry of fatigue failure is not fractal, which isn't what I would expect.

But goddammit, now I have to buy a new laptop case.
Tags: science
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