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Well I learned a lot reading the stuff after my post. I wanted to make the point that metal fatigue is real, not some imagined thing, but there's no way I can relate it to bicycle frame behavior, nor did I (although I did post it in a "frame fatigue" related thread).
Personally I have no experience with riding multiple steel frames. I only ever had one steel framed serious bike and had it for two years, and it was about as bad a frame as you could get, Columbus Zeta tubing. I used to joke that Zeta was Italian for "Worst". Currently I think it's been reincarnated as a fixed gear frame under an aspiring frame-builder's torch.
At any rate I've seen one Pinarello frame fail when the downtube broke in half. No crashes on it, it was a few months old. I sold the (new) frame to the rider, a friend, he was riding it, and suddenly his pedals were scraping the ground. That was a steel frame. I think the tube broke by the downtube shifter mounts so it was probably related to overheating the tube. However the tubing was intact at some point. Now I don't know what made it fail, if it wasn't fatiguing a weak part of the frame.
I've had a few aluminum frames crack, chainstays (Specialized M2, Giant TCR, Cannondale original). I used them for years and with the TCR I was pretty overweight. I had two more aluminum TCRs after that with no problems (still have them in the basement). Now I understand that all cycles on aluminum frames "count".
My main lesson is that a structure can fail even if the materials are still okay. The real world one for me is the Tappan Zee Bridge, which basically had a "use by date" that expired several years ago (2005 or so). Commuting on it was a bit sketchy as I'd keep an ear out on traffic reports for "hole in the deck in the second lane" etc. Chunks of the bridge fell out, and there was one day where I saw maybe 10-15 cars pulled over on the bridge with broken rims/etc, another 20-ish limping across the rest of the span on flat tires, and yet another 10-20 cars at the tolls changing flats, all from a 3 or 4 foot wide hole "in the deck". I was fortunate enough to miss that one, and another one that I was close enough to to be able to look down into it (I just saw bridge stuff, not water like I expected). I realize now that it may not be the steel/metal that is the issue but things like construction techniques/materials, corrosion, etc. A lot of the bridge has been fixed and apparently it's much better than it was in 2007, the last year I was commuting over it.
I just read that they're starting construction of a new bridge now. Also I read there is no redundancy in the bridge, so if a single support failed the bridge could potentially fail. I also read that there are 8 floating things supporting the bridge (caissons, pressurized hollow concrete columns). This makes me want to take the Newburgh bridge rather than the TZ.
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