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I've found a Titanium bike frame on eBay. It's very cheap and the owner says that it has a very small crack on the seat tube where it connects to a bottom bracket. See in the image above:
I've never gone for titanium frames. How bad is this damage, and what is likely to happen in the future - will it get worse?
If you look at titanium frame bikes on the web you can see their warranties. Most have very long or life time warranties that cover everything but crashes and deliberate damage to the frame. So I would assume this frame was involved in a crash.
There are three types or cracks in titanium frames (from best to worst):
You can weld titanium (but you need an inert gas shield to prevent O2 & N2 absorption while the material cools). Seam and weld cracks are relatively easy to fix. (But still pricey.) A stress crack may not be repairable at all, and I'm not sure I would trust it in any case on the BB.
So without anymore information, I would say you have a crashed frame with frame failure. It is in the worst possible place. A repair will be expensive and may not even be possible. The repair was also not covered under the frames original warranty.
After looking at the crack again, there is another possibility. When you weld titanium you have to shield the metal with an inert gas as it cools. Otherwise it can absorb O2 or N2 from the air. If the titanium absorbs either gas it becomes brittle and will crack over time. This crack is right near the BB at the seat of two major welds. The tube would have been very hot there and if it was not properly shielded by argon during welding and cooling, it could have absorbed air during that period. That effectively wrecks the titanium. This type of problem can not be fixed. (It should have been covered under warranty though.)
Sounds like a good frame to avoid no matter how cheap the price.
Am I looking at the right thing? The little line running across the seat tube about 1cm above the bottom bracket?
It's hard to say, but personally I'm not sure that's a crack at all. It looks more like a scratch. I say that because:
a) it looks shallow, and
b) that is a really, really weird place for a crack to occur. There are four types of stress (if I remember my solid mechanics correctly) that could cause a frame to crack: shear (picture gripping a pipe with your hands next to each other and pushing with one and pulling with the other), bending, torsion, or tension. Compression wouldn't lead to a crack.
There's basically no way that you could apply enough shear force, bending force, or torsional force at that location on the frame to crack it (even a fatigue crack over time), so we'll discount those (plus, you'd expect to see other deformation from bending). That leaves tension.
It is totally possible to apply a huge tensile load to a seat tube, for instance by coming down on the pedals really hard after a drop. However, I would imagine that 999 times out of 1,000 it would fail at the welds, not in the middle of the tube like that.
I'm not saying you should buy the frame, but it might be worth asking the guy for more info.
I'm way late to this party, but in case anyone else wanders in, I'll say - based on countless postings of similar Ti-frame problems - that this and similar joined areas are common locations to have a crack on a titanium frame. Gary E (above) already talked about the need to have an inert gas atmosphere when welding titanium or you will find that the titanium becomes brittle in the area that was heated by the welding process. There is one other thing. Welding sets up thermal stresses also, and unrelieved residual stresses further compromise the embrittled titanium. This can be relieved to some extent by shot-peening (not media-blasting) the work, or by heat-treating it in an inert gas atmosphere. Finally, I would have no problem trusting a stress crack that had been repaired by welding, as long as I were certain that the weld was done by an expert. Such a repair will restore ductility to the surrounding metal and result in a strong and dependable component. Success rests entirely on the skill and professionalism of the welder, obviously.
It will get worse. Buying a used frame is risky, generally, even if there are no visible problems. It could be bent, which most people can't diagnose. Titanium and aluminium aren't good materials for a frame, in spite of widespread use nowadays. But they are cheap and sold for nearly the same price as a top steel frame would cost, which is much more difficult to make, in every aspect. I hope you did not buy that nonsense.
