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#11
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Residual stress, fatigue and stress relief
wrote: This is because residual compression on one side of the bend is residual tension on the other, (clip) ^^^^^^^^^^^^^^^^ No. You are evidently applying the equations for bending stress, with symmetry about the neutral axis. If there are tensile stresses present, the bending stresses add on one side and subtract on the other. Then, if the higher value (either tensile or compressive) passes the yield point, the symmetry is gone, and the residual stress could have an effect on fatigue afterward. |
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#12
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Residual stress, fatigue and stress relief
Ben C wrote:
The controversy here is not that brief overload relieves stress or that stress relief improves fatigue life. Not true. It's the claim that this is known to be _the significant beneficial effect_ of spoke-squeezing, the Mavic method, and other "stabilization" practices that people do when wheel-building. Not true. The specific claim (originally by Jobst) is that spoke squeezing causes stress relief by the exact mechanism described in the sources I cited. "Stabilization" is your word -- and a meaningless one, too. Stress relief is a specific term. That there are residual stresses in spokes is not a matter of faith. Overloading in the direction of the working load will either diminish undesirable residual stresses or create desirable residual stresses or both. That is the whole point. It needs no other qualifications. |
#13
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Residual stress, fatigue and stress relief
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#14
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Residual stress, fatigue and stress relief
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#15
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Residual stress, fatigue and stress relief
Ben C wrote:
I don't remember Jobst mentioning anything about this mechanism of notches resulting in compressive residual stress but never mind. Never mind, yourself. If threads aren't notches, I don't know what are. Jobst claimed that his technique of stress relief would improve failure rates at the threads, too. The published material I cited supports this claim. |
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Residual stress, fatigue and stress relief
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#17
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Residual stress, fatigue and stress relief
Ben C wrote:
I think jim's point is that no-one has shown that spoke fatigue starts on the inside of the bend significantly (or at all) more often than it starts on the outside. "jim beam" doesn't correct his spoke lines. He will have a moment under normal load. That will likely be larger than any residual stress contribution. The evidence we would expect to see for residual stress being a factor just isn't there. Having said that many people (who aren't jim beam) don't scrutinize the broken spoke carefully through a magnifying glass, but just chuck it in the trash, so we wouldn't know. People like Jobst & I are at a disadvantage from a "forensics" angle -- we don't break spokes, so have no samples to analyze. "jim beam" seems to have plenty. Follow his faith-based analysis if you want. |
#18
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Residual stress, fatigue and stress relief
On 2008-04-23, Peter Cole wrote:
Ben C wrote: The controversy here is not that brief overload relieves stress or that stress relief improves fatigue life. Not true. It's the claim that this is known to be _the significant beneficial effect_ of spoke-squeezing, the Mavic method, and other "stabilization" practices that people do when wheel-building. Not true. The specific claim (originally by Jobst) is that spoke squeezing causes stress relief by the exact mechanism described in the sources I cited. "Stabilization" is your word -- and a meaningless one, too. The idea is to choose a term that does not rule out that spoke squeezing (etc.) may have a beneficial effect but that isn't because it relieves residual stress. Stress relief is a specific term. That there are residual stresses in spokes is not a matter of faith. No, but that they make any practical difference to how quickly the spoke breaks or not is. |
#19
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Residual stress, fatigue and stress relief
On 2008-04-23, Peter Cole wrote:
Ben C wrote: I think jim's point is that no-one has shown that spoke fatigue starts on the inside of the bend significantly (or at all) more often than it starts on the outside. "jim beam" doesn't correct his spoke lines. He will have a moment under normal load. That will likely be larger than any residual stress contribution. The presence of moment under normal load is my preferred likely candidate for what causes most spoke failures. And in fact part of what Jobst calls "residual stress" is I believe just a convoluted description of moment under load. He says that when you put the bend in the elbow of an outbound spoke the bend can't spring back because the flange is in the way but is held there by spoke tension. He calls that "residual stress", which is confusing, but that's what he calls it. For that to be possible there must be quite a bit of moment, since spoke tension is not high enough to hold a bend in an elbow that is close to flush with the flange. Overloading reduces that tension by bending the elbow and/or deforming the hub and reducing the moment. Jobst says instead overloading deforms the fibres or something and relieves the "residual stress". But it is actually basically the same thing (although I think Jobst denies the hub can possibly deform at this point). That is my conclusion from all the discussions I have read about this on RBT. Residual stress deserves mention as a possible mechanism that may also be involved, but no more than that. Beam is right: straight-pull spokes are a better design. On the other hand with aluminium hubs and quality steel spokes breakages are rare these days so it's not really as big a problem as a casual reader of RBT might take it for. The evidence we would expect to see for residual stress being a factor just isn't there. Having said that many people (who aren't jim beam) don't scrutinize the broken spoke carefully through a magnifying glass, but just chuck it in the trash, so we wouldn't know. People like Jobst & I are at a disadvantage from a "forensics" angle -- we don't break spokes, so have no samples to analyze. I thought Jobst did used to break them before he started stress-relieving. "jim beam" seems to have plenty. Follow his faith-based analysis if you want. I know you don't really want to be a troll. Fight the urge. |
#20
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Residual stress, fatigue and stress relief
Peter Cole Wrote: Ben C wrote: The controversy here is not that brief overload relieves stress or that stress relief improves fatigue life. Not true. It's the claim that this is known to be _the significant beneficial effect_ of spoke-squeezing, the Mavic method, and other "stabilization" practices that people do when wheel-building. Not true. The specific claim (originally by Jobst) is that spoke squeezing causes stress relief by the exact mechanism described in the sources I cited. "Stabilization" is your word -- and a meaningless one, too. Stress relief is a specific term. That there are residual stresses in spokes is not a matter of faith. Overloading in the direction of the working load will either diminish undesirable residual stresses or create desirable residual stresses or both. That is the whole point. It needs no other qualifications. "Stabilizing" is a term used Barnett Bicycle Institute in thier wheel building classes. It is not meaningless. Stabilizing makes sure the spokes are embeded and residual windup removed. The process also temporarily overloads spokes in the direction of the working load.... likely better than the spoke squeezing method. I can see and immediately measure the results of the process. I know that if a wheel isn't stabilized it will detension as it is ridden... sometimes to the point where nipples will back-off and the wheel will have spokes that are totally slack. -- daveornee |
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