Dealing with a small crack in a titanium head tube
09-13-19, 10:05 AM
#26
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The evidence I have thatthere are internal stresses in the frame is that the headtube cracked at the weld. A piece of metal removed from the forge or mill will generally have it's forces balanced, but if you weld it, those forces can become unbalanced. Melting the metl will almost always cause warping or distortion, but since the metal of the head tube is bonded fast to another piece of metal (which would likely also be warped by welding, but not necessarily in the same direction or amount), the warping isn't being allowed and the result is unseen forces inside the material. Generally speaking, post weld 'heat treating' is partly to remove those forces and to align grain structure.
I hadn't read closely enough to realize that your inside replacement tube is very thick, so hopefully that keeps everything together.
I hadn't read closely enough to realize that your inside replacement tube is very thick, so hopefully that keeps everything together.
09-13-19, 02:43 PM
#27
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Thanks. Could well be -- we'll see how it holds up.
One idea I take from your post -- that there are internal stresses lurking in the frame like it's a Popsicle stick bomb -- seems a little suspect to me. I think a piece of metal is generally just resting in equilibrium. The forces that would break it are external, and some combination over the years cracked a very thin tube wall near the weld where it may have been more brittle due to how it cooled or escaped some of the Argon during manufacturing or something. As evidence for my way of thinking, I don't know of any frames spontaneously cracking, and when metal is cut, it doesn't uncoil or spring into some new shape. I know that there is some surface tension created in forging that lends additional strength, but this isn't a forged piece.
The carbon I put inside is about the same thickness as the headtube itself. My hope is that it's well bonded to the titanium and that this hybrid tube can absorb whatever forces I pound into it by riding on a smooth, flat bike path at 16 miles an hour or so.
There are a couple failures that seem possible to me. First, the bond between the carbon and titanium might not be what I hope it is. If the titanium tube flexes a lot more easily than the carbon insert, and flexes even more where it's cracked and I've drilled out material, it could de-laminate there. Or the head tube itself may just be at the end of its fatigue life in a lot of areas. Maybe it had a crack because someone drove the fork into their garage. Maybe it's had 80,000 hard miles pounded into it, and it's about to crack near the top tube or something. It's easy to summon fears about the way failure can arrive.
But who needs to borrow trouble? I enjoyed this project, already learned from it, and now I'll get to see how my work fares in the world. The carbon patch on the outside is pretty useless to the structure, but maybe it will act as a canary in the coal mine to show if indeed a Popsicle stick bomb is about to blow.
One other thing I meant to include somewhere is the observation that this frame has two much larger holes drilled into it by the manufacturer to allow the brake cable to pass in and out of the top tube. These are just drill holes -- no guide or metal on the internal side -- done at an oblique angle to get an oval.
One idea I take from your post -- that there are internal stresses lurking in the frame like it's a Popsicle stick bomb -- seems a little suspect to me. I think a piece of metal is generally just resting in equilibrium. The forces that would break it are external, and some combination over the years cracked a very thin tube wall near the weld where it may have been more brittle due to how it cooled or escaped some of the Argon during manufacturing or something. As evidence for my way of thinking, I don't know of any frames spontaneously cracking, and when metal is cut, it doesn't uncoil or spring into some new shape. I know that there is some surface tension created in forging that lends additional strength, but this isn't a forged piece.
The carbon I put inside is about the same thickness as the headtube itself. My hope is that it's well bonded to the titanium and that this hybrid tube can absorb whatever forces I pound into it by riding on a smooth, flat bike path at 16 miles an hour or so.
There are a couple failures that seem possible to me. First, the bond between the carbon and titanium might not be what I hope it is. If the titanium tube flexes a lot more easily than the carbon insert, and flexes even more where it's cracked and I've drilled out material, it could de-laminate there. Or the head tube itself may just be at the end of its fatigue life in a lot of areas. Maybe it had a crack because someone drove the fork into their garage. Maybe it's had 80,000 hard miles pounded into it, and it's about to crack near the top tube or something. It's easy to summon fears about the way failure can arrive.
But who needs to borrow trouble? I enjoyed this project, already learned from it, and now I'll get to see how my work fares in the world. The carbon patch on the outside is pretty useless to the structure, but maybe it will act as a canary in the coal mine to show if indeed a Popsicle stick bomb is about to blow.
One other thing I meant to include somewhere is the observation that this frame has two much larger holes drilled into it by the manufacturer to allow the brake cable to pass in and out of the top tube. These are just drill holes -- no guide or metal on the internal side -- done at an oblique angle to get an oval.
Cheers
09-13-19, 03:02 PM
#28
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Well, the patch only covers about 2 cm x 0.5 cm and doesn't go to any edges, so if the headtube is able to use that as cover as it launches a surprise explosion, more power to it.
What seems more likely to me is it will demonstrate the strength of the carbon to titanium epoxy bond in an easily visible way.
What seems more likely to me is it will demonstrate the strength of the carbon to titanium epoxy bond in an easily visible way.
