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Residual stress, fatigue and stress relief
"Metal Fatigue in Engineering", Ralph I. Stephens, Ali Fatemi, Robert R.
Stephens, Henry O. Fuchs, Ali Faterni http://www.amazon.com/gp/reader/0471...08#reader-link Page 247-8 describe creation of beneficial residual stress at a notch by overloading in tension, whereby the stress concentrating effect of the notch brings the material above yield in the immediate notch vicinity, followed by a residual compressive stress when the overload is relaxed. The undesirable residual stresses created by bending to form parts (skin tension caused by forming compression) are also mentioned. Page 257 describes stress relieving via yielding. Page 259 describes modifying residual stress by overloading: "In springs, as in other parts that are primarily loaded in one direction, an overload applied early in life is beneficial because it introduces desirable residual compressive stresses at the proper surface. Springs, hoists and pressure vessels are strengthened by proof loading with a higher load than the expected service load" |
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#2
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Residual stress, fatigue and stress relief
Peter Cole wrote:
"Metal Fatigue in Engineering", Ralph I. Stephens, Ali Fatemi, Robert R. Stephens, Henry O. Fuchs, Ali Faterni http://www.amazon.com/gp/reader/0471...08#reader-link Page 247-8 describe creation of beneficial residual stress at a notch by overloading in tension, whereby the stress concentrating effect of the notch brings the material above yield in the immediate notch vicinity, followed by a residual compressive stress when the overload is relaxed. The undesirable residual stresses created by bending to form parts (skin tension caused by forming compression) are also mentioned. Page 257 describes stress relieving via yielding. Page 259 describes modifying residual stress by overloading: "In springs, as in other parts that are primarily loaded in one direction, an overload applied early in life is beneficial because it introduces desirable residual compressive stresses at the proper surface. Springs, hoists and pressure vessels are strengthened by proof loading with a higher load than the expected service load" see previous post. existence of residual stress does not mean it causes spoke fatigue. simple observation shows the truth. spokes are not observed to have their cracking initiate in regions of high residual stress, but in regions of high applied stress. as one might expect given that spoke elbows, by definition, are subject to bending as a function of being offset from the spoke axis. simple observation of the facts. i suggest you try educating people on basic scientific method rather than leaping to conclusions. or trolling. |
#3
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Residual stress, fatigue and stress relief
"jim beam" wrote: existence of residual stress does not mean it causes spoke fatigue. simple observation shows the truth. spokes are not observed to have their cracking initiate in regions of high residual stress, but in regions of high applied stress. as one might expect given that spoke elbows, by definition, are subject to bending as a function of being offset from the spoke axis. simple observation of the facts. i suggest you try educating people on basic scientific method rather than leaping to conclusions. or trolling. ^^^^^^^^^^^^^^^^^^^^^^ Residual COMPRESSIVE stress would tend to reduce fatigue failure, since fatigue cracks grow ONLY under tensile stress. That observation does not depart from the scientific method. leap to conclusions, and certainly is not trolling. Under tension, spoke bends try to straighten out, which creates tensile stress on the inside of the bend. Any residual compressive stress in that area as a result of the formation of the bend would have the effect that Peter Cole referred to. |
#4
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Residual stress, fatigue and stress relief
Leo Lichtman wrote:
"jim beam" wrote: existence of residual stress does not mean it causes spoke fatigue. simple observation shows the truth. spokes are not observed to have their cracking initiate in regions of high residual stress, but in regions of high applied stress. as one might expect given that spoke elbows, by definition, are subject to bending as a function of being offset from the spoke axis. simple observation of the facts. i suggest you try educating people on basic scientific method rather than leaping to conclusions. or trolling. ^^^^^^^^^^^^^^^^^^^^^^ Residual COMPRESSIVE stress would tend to reduce fatigue failure, since fatigue cracks grow ONLY under tensile stress. That observation does not depart from the scientific method. leap to conclusions, and certainly is not trolling. Under tension, spoke bends try to straighten out, which creates tensile stress on the inside of the bend. Any residual compressive stress in that area as a result of the formation of the bend would have the effect that Peter Cole referred to. but the region of highest residual stress is near the neutral plane, not the outer parts of the bend where fatigue is always observed to initiate. and in fact, fatigue is still observed to initiate where, if there is any, compressive residual is compressive, on the outer part of the elbow. again, observe the facts, bother to understand the whole story, and don't leap to conclusions. |
#5
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Residual stress, fatigue and stress relief
On 2008-04-23, Leo Lichtman wrote:
"jim beam" wrote: existence of residual stress does not mean it causes spoke fatigue. simple observation shows the truth. spokes are not observed to have their cracking initiate in regions of high residual stress, but in regions of high applied stress. as one might expect given that spoke elbows, by definition, are subject to bending as a function of being offset from the spoke axis. simple observation of the facts. i suggest you try educating people on basic scientific method rather than leaping to conclusions. or trolling. ^^^^^^^^^^^^^^^^^^^^^^ Residual COMPRESSIVE stress would tend to reduce fatigue failure, since fatigue cracks grow ONLY under tensile stress. That observation does not depart from the scientific method. leap to conclusions, and certainly is not trolling. Under tension, spoke bends try to straighten out, which creates tensile stress on the inside of the bend. Any residual compressive stress in that area as a result of the formation of the bend would have the effect that Peter Cole referred to. 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. 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. |
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Residual stress, fatigue and stress relief
Ben C wrote:
The evidence we would expect to see for residual stress being a factor just isn't there. The point I made by posting the source was that overloading was a recognized technique to manipulate residual stress -- either to reduce or increase it depending on the desired outcome. If a spoke is laced with an elbow angle that is too large, there will be a bending stress in operation (load stress) that will put the outside skin in tension. If the angle is too small, the load stress will be tension on the inside skin. If the load path for a spoke is straight from the hub to the rim, there will be no moment (bending stress), only uniform tension across the cross section and shear stress. By overloading the spoke, any existing notch conditions (small cracks, threads) yield in tension and after unloading have residual compressive stress which retards crack growth (see reference). The important factor is that the static load plus overload plus residual totals to greater than yield, if only in very local spots where stresses become naturally concentrated. As for the claim that spokes always crack from the outside of the elbow (which doesn't agree with my limited experience), it's a certainty that cold forming a ~90 degree bend will leave micro cracks on the outside skin. Stress relief will yield these and generate beneficial (compressive) residual stress in the immediate vicinity (see reference). It does not matter if the residual skin stress from forming was compressive, the stress relief will mitigate the fatigue effect of surface flaws and provide additional benefit. As Jobst has frequently pointed out, these effects are at the microscopic level, the source I cited explains the mechanism. Stress relief by brief overload before a part is put into service is a well established method for improving fatigue life. The only requirement is that the overload be applied in the same direction as the service load. The literature abounds with examples, I just cited one source. This can only be controversial via willful ignorance. |
#7
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Residual stress, fatigue and stress relief
Peter Cole wrote:
Ben C wrote: The evidence we would expect to see for residual stress being a factor just isn't there. The point I made by posting the source was that overloading was a recognized technique to manipulate residual stress -- either to reduce or increase it depending on the desired outcome. If a spoke is laced with an elbow angle that is too large, there will be a bending stress in operation (load stress) that will put the outside skin in tension. If the angle is too small, the load stress will be tension on the inside skin. If the load path for a spoke is straight from the hub to the rim, there will be no moment (bending stress), only uniform tension across the cross section and shear stress. By overloading the spoke, any existing notch conditions (small cracks, threads) yield in tension and after unloading have residual compressive stress which retards crack growth (see reference). The important factor is that the static load plus overload plus residual totals to greater than yield, if only in very local spots where stresses become naturally concentrated. As for the claim that spokes always crack from the outside of the elbow (which doesn't agree with my limited experience), it's a certainty that cold forming a ~90 degree bend will leave micro cracks on the outside skin. Stress relief will yield these and generate beneficial (compressive) residual stress in the immediate vicinity (see reference). It does not matter if the residual skin stress from forming was compressive, the stress relief will mitigate the fatigue effect of surface flaws and provide additional benefit. As Jobst has frequently pointed out, these effects are at the microscopic level, the source I cited explains the mechanism. Stress relief by brief overload before a part is put into service is a well established method for improving fatigue life. The only requirement is that the overload be applied in the same direction as the service load. The literature abounds with examples, I just cited one source. This can only be controversial via willful ignorance. yet again, fatigue is NOT observed to be initiating in regions affected by high residual stress, "relieved" or not. and it's independent of whether any residual is either compressive or tensile. but it IS observed to be originating in regions of high /applied/ stress. and that high applied stress is entirely a function of the design of the component. if you want to fix spoke breakage, change the design - don't waste your time clutching at straws demonstrating ignorance and inability to observe. move to straight pull spokes. that's what the smart manufacturers with research budgets and engineers that have done their homework have done. |
#8
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Residual stress, fatigue and stress relief
On 2008-04-23, Peter Cole wrote:
Ben C wrote: The evidence we would expect to see for residual stress being a factor just isn't there. The point I made by posting the source was that overloading was a recognized technique to manipulate residual stress -- either to reduce or increase it depending on the desired outcome. Interesting point and thank you for posting it. The idea of creating residual compressive stress at a notch as you describe is not something I've heard before. If a spoke is laced with an elbow angle that is too large, there will be a bending stress in operation (load stress) that will put the outside skin in tension. If the angle is too small, the load stress will be tension on the inside skin. If the load path for a spoke is straight from the hub to the rim, there will be no moment (bending stress), only uniform tension across the cross section and shear stress. By overloading the spoke, any existing notch conditions (small cracks, threads) yield in tension and after unloading have residual compressive stress which retards crack growth (see reference). The important factor is that the static load plus overload plus residual totals to greater than yield, if only in very local spots where stresses become naturally concentrated. As for the claim that spokes always crack from the outside of the elbow (which doesn't agree with my limited experience) I don't know who's claiming that. My understanding was that if residual stress were a factor, we would expect to see the majority of outbound spoke failures starting from the inside and the majority of inbound spokes failures starting from the outside. Let me just check I got that the right way round... Yes I think so since residual stress is tensile on the outside of the bend for a spoke whose angle you made less acute (inbound), and the other way round for the other ones. The highest residual stresses I think jim beam has been saying are in the interior of the spoke and not on the skin at all. But, we don't see any particular pattern of whether failure starts on the outside or inside, or on the exterior or in the interior, for outbound or inbound spokes one way or the other. But as I said we don't have much evidence that there isn't such a pattern either, since most people don't look at their broken spokes in such detail. It's a pity Jobst didn't since he reports experiencing a big change in number of broken spokes after he started stress-relieving. Examination of the broken spokes might have helped confirm the theory that residual stress was a significant factor in why they broke. But I think you're saying with this new link that fatigue would be mitigated at notches on either side of either kind of spoke anyway. , it's a certainty that cold forming a ~90 degree bend will leave micro cracks on the outside skin. Stress relief will yield these and generate beneficial (compressive) residual stress in the immediate vicinity (see reference). It does not matter if the residual skin stress from forming was compressive, the stress relief will mitigate the fatigue effect of surface flaws and provide additional benefit. As Jobst has frequently pointed out, these effects are at the microscopic level, the source I cited explains the mechanism. I don't remember Jobst mentioning anything about this mechanism of notches resulting in compressive residual stress but never mind. Stress relief by brief overload before a part is put into service is a well established method for improving fatigue life. The only requirement is that the overload be applied in the same direction as the service load. The literature abounds with examples, I just cited one source. This can only be controversial via willful ignorance. The controversy here is not that brief overload relieves stress or that stress relief improves fatigue life. 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. |
#9
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Residual stress, fatigue and stress relief
On Apr 23, 7:57 am, Peter Cole wrote:
Ben C wrote: The evidence we would expect to see for residual stress being a factor just isn't there. The point I made by posting the source was that overloading was a recognized technique to manipulate residual stress -- either to reduce or increase it depending on the desired outcome. If a spoke is laced with an elbow angle that is too large, there will be a bending stress in operation (load stress) that will put the outside skin in tension. If the angle is too small, the load stress will be tension on the inside skin. If the load path for a spoke is straight from the hub to the rim, there will be no moment (bending stress), only uniform tension across the cross section and shear stress. By overloading the spoke, any existing notch conditions (small cracks, threads) yield in tension and after unloading have residual compressive stress which retards crack growth (see reference). The important factor is that the static load plus overload plus residual totals to greater than yield, if only in very local spots where stresses become naturally concentrated. As for the claim that spokes always crack from the outside of the elbow (which doesn't agree with my limited experience), it's a certainty that cold forming a ~90 degree bend will leave micro cracks on the outside skin. Stress relief will yield these and generate beneficial (compressive) residual stress in the immediate vicinity (see reference). It does not matter if the residual skin stress from forming was compressive, the stress relief will mitigate the fatigue effect of surface flaws and provide additional benefit. As Jobst has frequently pointed out, these effects are at the microscopic level, the source I cited explains the mechanism. Stress relief by brief overload before a part is put into service is a well established method for improving fatigue life. The only requirement is that the overload be applied in the same direction as the service load. The literature abounds with examples, I just cited one source. This can only be controversial via willful ignorance. There are other requirements than direction of applied proof load, namely ductility and defect size. For a large defect in a ductile material, you will in fact get plasticity and residual compression when you release the load. If the defect is very small, or the material is brittle, proof loading may form a crack. There will be some residual compression at the crack tip, but not enough to make the part stronger than it was before it was cracked. |
#10
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Residual stress, fatigue and stress relief
On Apr 23, 11:06 am, Ben C wrote:
On 2008-04-23, Peter Cole wrote: Ben C wrote: The evidence we would expect to see for residual stress being a factor just isn't there. The point I made by posting the source was that overloading was a recognized technique to manipulate residual stress -- either to reduce or increase it depending on the desired outcome. Interesting point and thank you for posting it. The idea of creating residual compressive stress at a notch as you describe is not something I've heard before. If a spoke is laced with an elbow angle that is too large, there will be a bending stress in operation (load stress) that will put the outside skin in tension. If the angle is too small, the load stress will be tension on the inside skin. If the load path for a spoke is straight from the hub to the rim, there will be no moment (bending stress), only uniform tension across the cross section and shear stress. By overloading the spoke, any existing notch conditions (small cracks, threads) yield in tension and after unloading have residual compressive stress which retards crack growth (see reference). The important factor is that the static load plus overload plus residual totals to greater than yield, if only in very local spots where stresses become naturally concentrated. As for the claim that spokes always crack from the outside of the elbow (which doesn't agree with my limited experience) I don't know who's claiming that. My understanding was that if residual stress were a factor, we would expect to see the majority of outbound spoke failures starting from the inside and the majority of inbound spokes failures starting from the outside. Let me just check I got that the right way round... Yes I think so since residual stress is tensile on the outside of the bend for a spoke whose angle you made less acute (inbound), and the other way round for the other ones. The highest residual stresses I think jim beam has been saying are in the interior of the spoke and not on the skin at all. But, we don't see any particular pattern of whether failure starts on the outside or inside, or on the exterior or in the interior, for outbound or inbound spokes one way or the other. 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. But as I said we don't have much evidence that there isn't such a pattern either, since most people don't look at their broken spokes in such detail. It's a pity Jobst didn't since he reports experiencing a big change in number of broken spokes after he started stress-relieving. Examination of the broken spokes might have helped confirm the theory that residual stress was a significant factor in why they broke. But I think you're saying with this new link that fatigue would be mitigated at notches on either side of either kind of spoke anyway. , it's a certainty that cold forming a ~90 degree bend will leave micro cracks on the outside skin. Stress relief will yield these and generate beneficial (compressive) residual stress in the immediate vicinity (see reference). It does not matter if the residual skin stress from forming was compressive, the stress relief will mitigate the fatigue effect of surface flaws and provide additional benefit. As Jobst has frequently pointed out, these effects are at the microscopic level, the source I cited explains the mechanism. I don't remember Jobst mentioning anything about this mechanism of notches resulting in compressive residual stress but never mind. Stress relief by brief overload before a part is put into service is a well established method for improving fatigue life. The only requirement is that the overload be applied in the same direction as the service load. The literature abounds with examples, I just cited one source. This can only be controversial via willful ignorance. The controversy here is not that brief overload relieves stress or that stress relief improves fatigue life. 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. |
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