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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.

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Comments

( 14 comments — Leave a comment )
pstscrpt
May. 1st, 2007 02:12 am (UTC)
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
Have you ever taken apart an old metal shed with a crowbar? Damn, that's a lot of fun, especially first thing in the morning.
tacit
May. 1st, 2007 02:40 am (UTC)
I haven't! It sounds like great fun, though.
joreth
May. 1st, 2007 03:04 am (UTC)
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

That's why (what we call) Chainsaw Load Outs are so much fun!

Too bad about your laptop bag though
sylvar
May. 1st, 2007 03:27 am (UTC)
...wait, this isn't going to be a metaphor about relationships and communication?

So much for predictability. :)
tacit
May. 1st, 2007 03:36 am (UTC)
Heh. Not this time; I do think about other things occasionally! :P
sylvar
May. 1st, 2007 11:09 am (UTC)
Well, sure, but they usually involve breasts.

Now hold on a minute... you WERE talking about cleavage!
(Deleted comment)
tacit
May. 4th, 2007 07:46 pm (UTC)
Nope, I've never taken any materials classes. I can see where the lectures might be tedious, but the manner in which things fail is oftentimes very interesting.

I understand that the aircraft industry had problems with composite materials failing through delamination for a while.
gipsieee
May. 1st, 2007 02:00 pm (UTC)
Not directly related to this post, but I've enjoyed peering through your posts. Thank you for sharing them. Mind if I add you to my f-list?
tacit
May. 4th, 2007 07:47 pm (UTC)
Not at all...welcome aboard!
meandering
May. 2nd, 2007 03:21 am (UTC)
From what little chemistry I currently remember, aren't the atoms in metal arranged in a crystalline pattern? If they are, I think a "shearing" sort of break would result in said breaks as the first bit of fatigue would rapidly run through the "crystal".
mantic_angel
May. 2nd, 2007 04:12 pm (UTC)
Y'know, I've read most of xeromag and followed links to your LJ at least three times. *settles in and adds you to her f-list* So, hello there :)
tacit
May. 4th, 2007 07:23 pm (UTC)
Howdy, and welcome board!
quadrapop
May. 3rd, 2007 02:08 pm (UTC)
please consider a Crumpler bag as a replacement - designed and (used to be) made in Australia (I think some models still are but most are now made next door in SE Asia) - very funky and they have coloured interiors so you can see all those black things that fall to the bottom of the bag. The bags all have funky names - we have a "Sheep Scarer" and a "Team Player" whilst I have some photography friends with "Six Million Dollar Homes". They have a cool website too. Dub Dub Dub dot crumpler dot com do au

My DH adds: they are incredibly reliable even under the trying circumstances of an alpha geek with oftem more than one laptop (plus o'reilly journals).

he's had his "team player" for about 4 years and it's showing no signs of wear.
tacit
May. 4th, 2007 07:27 pm (UTC)
Those look like neat bags! I've already replaced mine (Selly had one she wasn't using), but when the time comes to buy a new one, I'll have to look at those again.
( 14 comments — Leave a comment )