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#21
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Residual stress, fatigue and stress relief
On Apr 23, 4:11 pm, "Leo Lichtman"
wrote: 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. I'm applying the equations for static equilibrium. If some part of your spoke is under residual compression, the material around that zone must be in tension to keep it there. |
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#22
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Residual stress, fatigue and stress relief
On Apr 23, 4:20 pm, Peter Cole wrote:
wrote: This is because residual compression on one side of the bend is residual tension on the other, and trying to produce just the right amount of residual stress in a spoke by hand is like aligning microscope lenses with a framing hammer. It works great as long as you never look into the eye piece. It doesn't matter for the overload method of stress relief. That's what makes it such a useful technique. So the magnitude doesn't matter? Proof loading of things like gun barrels is done to a fairly precise stress level. To think that without doing any calculations on spoke yielding and cracking or using any instrumentation in application other than your bare hands will achieve this level of precision is absurd. I'm willing to believe that residual stresses may improve fatigue life under the right proof loading conditions, but I will not accept that the average wheel builder (myself included) has taken the time to figure out exactly what those conditions are and ensured that they're being followed. |
#23
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Residual stress, fatigue and stress relief
In article ,
daveornee wrote: 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. Thereby putting the wheel out of true. Spoke wind up is never present when attention is paid while turning the nipples. Turn until the nipple turns with respect to the spoke, then back the spoke wrench until the spoke is not wound up. The process also temporarily overloads spokes in the direction of the working load.... No it does not. All strain is elastic when the spoke wrench turns the nipple. 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... No it won't. Nipples unwind when the spoke tension is insufficient to the job of carrying the cyclic load at the contact patch. sometimes to the point where nipples will back-off and the wheel will have spokes that are totally slack. Oil the threads and spoke bed. Use a sufficient number of spokes to carry the load. -- Michael Press |
#24
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Residual stress, fatigue and stress relief
Michael Press Wrote: In article , daveornee wrote: 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. Thereby putting the wheel out of true. Spoke wind up is never present when attention is paid while turning the nipples. Turn until the nipple turns with respect to the spoke, then back the spoke wrench until the spoke is not wound up. The process also temporarily overloads spokes in the direction of the working load.... No it does not. All strain is elastic when the spoke wrench turns the nipple. 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... No it won't. Nipples unwind when the spoke tension is insufficient to the job of carrying the cyclic load at the contact patch. sometimes to the point where nipples will back-off and the wheel will have spokes that are totally slack. Oil the threads and spoke bed. Use a sufficient number of spokes to carry the load. -- Michael Press I build my way and it always works. I properly lubricate the threads and I make sure there isn't windup as I tighten nipples. If the wheel goes out of true while stabilizing it isn't likely to stay true when riding. I use the proper numbers of spokes for the intended loads in my builds. I don't uderstand what you mean: "No it does not. All strain is elastic when the spoke wrench turns the nipple." Pick apart as you want! -- daveornee |
#25
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Residual stress, fatigue and stress relief
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#26
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Residual stress, fatigue and stress relief
daveornee wrote:
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. indeed. and it's easy to test this observation at home - it's not like this is inaccessible rocket surgery. |
#27
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Residual stress, fatigue and stress relief
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 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. so how freakin' hard is it to go to a bike shop and get them to save a bunch of broken spokes for you??????????? neither you nor jobst can manage this? how about find a magnifier and bother to look at the fatigue patterns????? Follow his faith-based analysis if you want. try observing fact - it makes life a lot easier than trying to swallow misinterpreted garbage from a pair of lazy presumptive bull****ting amateurs!!! |
#28
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Residual stress, fatigue and stress relief
Peter Cole wrote:
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. er, actually, the residual stress at the root of cold rolled threads is compressive, not tensile. you *don't* want to "relieve" that. and your cite is for the phenomenon of residual stress and mechanical stress relief. nobody contests that it exists. what's being contested, and the point you /still/ don't seem to be able to grasp, is that OBSERVATION OF THE FACTS contradicts the supposition that it plays any material role in spoke fatigue. supposition, not matter how hotly defended, is simply not fact, and no amount of repetition by either you or jobst can make it so. |
#29
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Residual stress, fatigue and stress relief
Ben C wrote:
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. no, he's completely unclear on either concept. 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. it's not confusing, it's just plain wrong. see above. 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. why bother to muddy the water with something not observed to be the case? if we want to discuss irrelevancies, let's discuss stress corrosion cracking and hydrogen embrittlement as well! 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. and he failed to correlate his use of the new breed of spoke, made of fatigue resistant vacuum degassed stainless steel, with prolonged life, instead attributing it to the process he "invented" [copied] instead. that kind of, er, "oversight" might be good for selling books to those with insufficient scientific or engineering background, but it's just plain insulting to those who do. "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. |
#30
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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. jobstian "stress relief" theory is presumptive nonsense that has no correlation with observed fact. it's as well grounded as his assumption that fatigue can be eliminated from spokes made of stainless steel, a material with no fatigue endurance limit. absent sufficient knowledge and absent proper application of scientific method, you're just perpetuating jobstian mistakes - and looking foolish to boot. |
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