P
Peter Cole
Guest
jim beam wrote:
> Peter Cole wrote:
>> Ben C wrote:
>>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>>> [...]
>>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>>> bothers to respond.
>>>>
>>>> http://tinyurl.com/29v4u2
>>>
>>> OK I have a question.
>>>
>>> jim beam> i have ignored residual stress as a factor in these failures
>>> jim beam> because the majority of the fractures i've examined initiate
>>> jim beam> on the /inside/ of the spoke elbow bend, not the outside
>>> jim beam> [although i have examples of each]. residual stress in this
>>> jim beam> location is compressive so i'm just looking at the external
>>> jim beam> [+cyclic] load. Mike Prime> The inside of the spoke elbow
>>> will have TENSILE residual
>>> Mike Prime> stress, not compressive, because of the elastic springback
>>> Mike Prime> after bending. See below. That 0.5 Sy number is for a beam;
>>> Mike Prime> I'm too lazy to derive the number for a circular cross
>>> Mike Prime> section right now.
>>>
>>> Mike Prime> Since that location has tensile residual stress, tensile
>>> Mike Prime> applied mean stress from the spoke tension and bending,
>>> ^^^^^^^
>>>
>>> Is the _applied_ stress on the inside of the elbow from spoke tension
>>> and bending really tensile?
>>>
>>> I don't understand that. I thought when you bent a wire you got tensile
>>> stress on the outside of the bend and compressive on the inside?
>>>
>>> I know that the residual stress, after it springs back, is tensile on
>>> the inside.
>>
>> If the spoke has no bending moment (perfectly supported, perfect
>> path), the applied stress from spoke tension will be tensile (uniform)
>> across the cross section. Whatever bending force that is also present
>> will add to that. The bending force can be either way depending on
>> angular mismatch. If the spoke elbow is too long, another bending
>> force will act to open the spoke angle further, adding to the residual
>> (mfg) stress.
>
> but you're ignoring the actual residual stress profile. just alluding
> to residual stress being possible doesn't mean it's actually nucleating
> fatigue!
No, I'm not. You cited Luns in the context of stress profile, as far as
I could see he only confirmed Jobst's explanation and rejected yours.
I'm the only one on this NG (as far as I know) who actually tried to
measure residual stress, and found my results agreed with predictions:
http://tinyurl.com/356ru7
>
>>
>> The worst case would be an (initial spoke) angle too acute with elbow
>> too long. Both of those factors plus residual stress would all put
>> tension on the inside of the elbow.
>> If the spoke elbows are the right length, and the spoke angle is
>> corrected, the only significant stresses should be spoke tension and
>> residual.
>
> you're omitting bending. even if the spoke is resting against the hub
> flange, it can't protect against tensile stress on the inside of the
> elbow, only tensile stress on the /outside/, a place which, while
> fatigue does nucleate there from time to time, is not the commonest point.
I am not. You snipped it.
>
>> By stress relieving, the residual is reduced to non-fatiguing levels.
>
> presumptive straw clutching. plastic deformation subsequent to initial
> forming operations can /increase/ residual stress.
Of course, but stress relief doesn't cause bulk deformation. If wheel
building did, stress relief mitigates those residuals same as those from
manufacture.
>
>> But, if the spoke has (tensile) stress levels near yield in parts of
>> the cross section, those will be reduced as they are forced to yield
>> by the momentary overload
>
> and the regions of highest residual stress are on the /inside/ of the
> wire, not the inside or outside of the bend where fatigue nucleates.
See Luns Tee (again).
>> -- whatever the source. It's a "can't lose" proposition.
>
> eh? you need to read this:
> http://en.wikipedia.org/wiki/Fatigue_(material)#Miner.27s_rule
>
> particularly this:
> "In some circumstances, cycles of high stress followed by low stress
> cause more damage than would be predicted by the rule."
>
> sounds like our much-advocated "stress relief" scenario to me.
Sounds like you have no real understanding of Miner's rule.
Take the nominal -20% cyclical change in tension, add *1 cycle* of +50%
for stress relief and plug it into the formula.
> Peter Cole wrote:
>> Ben C wrote:
>>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>>> [...]
>>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>>> bothers to respond.
>>>>
>>>> http://tinyurl.com/29v4u2
>>>
>>> OK I have a question.
>>>
>>> jim beam> i have ignored residual stress as a factor in these failures
>>> jim beam> because the majority of the fractures i've examined initiate
>>> jim beam> on the /inside/ of the spoke elbow bend, not the outside
>>> jim beam> [although i have examples of each]. residual stress in this
>>> jim beam> location is compressive so i'm just looking at the external
>>> jim beam> [+cyclic] load. Mike Prime> The inside of the spoke elbow
>>> will have TENSILE residual
>>> Mike Prime> stress, not compressive, because of the elastic springback
>>> Mike Prime> after bending. See below. That 0.5 Sy number is for a beam;
>>> Mike Prime> I'm too lazy to derive the number for a circular cross
>>> Mike Prime> section right now.
>>>
>>> Mike Prime> Since that location has tensile residual stress, tensile
>>> Mike Prime> applied mean stress from the spoke tension and bending,
>>> ^^^^^^^
>>>
>>> Is the _applied_ stress on the inside of the elbow from spoke tension
>>> and bending really tensile?
>>>
>>> I don't understand that. I thought when you bent a wire you got tensile
>>> stress on the outside of the bend and compressive on the inside?
>>>
>>> I know that the residual stress, after it springs back, is tensile on
>>> the inside.
>>
>> If the spoke has no bending moment (perfectly supported, perfect
>> path), the applied stress from spoke tension will be tensile (uniform)
>> across the cross section. Whatever bending force that is also present
>> will add to that. The bending force can be either way depending on
>> angular mismatch. If the spoke elbow is too long, another bending
>> force will act to open the spoke angle further, adding to the residual
>> (mfg) stress.
>
> but you're ignoring the actual residual stress profile. just alluding
> to residual stress being possible doesn't mean it's actually nucleating
> fatigue!
No, I'm not. You cited Luns in the context of stress profile, as far as
I could see he only confirmed Jobst's explanation and rejected yours.
I'm the only one on this NG (as far as I know) who actually tried to
measure residual stress, and found my results agreed with predictions:
http://tinyurl.com/356ru7
>
>>
>> The worst case would be an (initial spoke) angle too acute with elbow
>> too long. Both of those factors plus residual stress would all put
>> tension on the inside of the elbow.
>> If the spoke elbows are the right length, and the spoke angle is
>> corrected, the only significant stresses should be spoke tension and
>> residual.
>
> you're omitting bending. even if the spoke is resting against the hub
> flange, it can't protect against tensile stress on the inside of the
> elbow, only tensile stress on the /outside/, a place which, while
> fatigue does nucleate there from time to time, is not the commonest point.
I am not. You snipped it.
>
>> By stress relieving, the residual is reduced to non-fatiguing levels.
>
> presumptive straw clutching. plastic deformation subsequent to initial
> forming operations can /increase/ residual stress.
Of course, but stress relief doesn't cause bulk deformation. If wheel
building did, stress relief mitigates those residuals same as those from
manufacture.
>
>> But, if the spoke has (tensile) stress levels near yield in parts of
>> the cross section, those will be reduced as they are forced to yield
>> by the momentary overload
>
> and the regions of highest residual stress are on the /inside/ of the
> wire, not the inside or outside of the bend where fatigue nucleates.
See Luns Tee (again).
>> -- whatever the source. It's a "can't lose" proposition.
>
> eh? you need to read this:
> http://en.wikipedia.org/wiki/Fatigue_(material)#Miner.27s_rule
>
> particularly this:
> "In some circumstances, cycles of high stress followed by low stress
> cause more damage than would be predicted by the rule."
>
> sounds like our much-advocated "stress relief" scenario to me.
Sounds like you have no real understanding of Miner's rule.
Take the nominal -20% cyclical change in tension, add *1 cycle* of +50%
for stress relief and plug it into the formula.