B
Ben C
Guest
On 2008-01-20, [email protected] <[email protected]> wrote:
> On Jan 19, 7:17 pm, Ben C <[email protected]> wrote:
>> On 2008-01-19, [email protected] <[email protected]> wrote:
>>
>> > Ben, for those with background in mechanical design, most of what
>> > Jobst says regarding stress relief in spokes is easily understood,
>> > and pretty obviously correct.
>>
>> I understand the idea, and it is reasonable. And as you say later,
>> credit to Jobst for thinking of it. But I don't believe we can say more
>> than "it may be a factor in some failures."
>
> I think that statement is far too limited. And I think most people
> with extensive background in mechanical design principles would also
> think it's too limited.
If you're going to bring up the idea of extensive backgrounds in
mechanical design principles then I think you should try to indicate
what in particular that background teaches that alters the argument.
[...]
>> They are difficult to detect, but not impossible, and that doesn't
>> change the fact that do you have to actually detect them before you can
>> claim that they are a factor, rather than just a possible factor.
>
> Not at all. If:
>
> 1) you had a situation known to normally produce residual stresses in
> parts that routinely failed by fatigue, and
>
> 2) you applied techniques known to cause stress relief in those parts,
> and
>
> 3) you observed a significant improvement in fatigue life,
If. But that hasn't been done in any kind of even vaguely scientific
way. All we have is a couple of anecdotes.
> then it doesn't require the actual detection or measurement of those
> residual stresses to logically conclude they were an important factor,
> and are now lessened or eliminated.
Lack of evidence that you would expect to find if the hypothesis is true
is also a factor.
Jim Beam keeps asking: do spokes break more often in regions of high
tensile residual stress? I missed the post where someone explained
either that they do or that they don't or that we don't know or that it
doesn't make any difference.
> Yes, I understand there could be confounding factors. But IMO, what
> we see here are fairly desperate attempts to come up with _some_ way,
> _any_ way, to suggest that stress relief either doesn't exist, or
> doesn't work. In view of the logic behind it and the evidence in
> favor of it, neither the effort nor the motivation make much sense to
> me.
It comes down to exactly what the claim is that's being made. I'm sorry
but I think Jobst states it far too strongly. He is always saying:
residual stress breaks spokes, spokes break if you don't stress-relieve,
loose spokes break because they aren't properly stress-relieved, etc.
etc.
If I've misinterpreted his position, fine, but it is not willful
minterpretation and I don't have anything against Jobst personally.
But I can understand why people get angry with him, because failure to
distinguish clearly between hypothesis and fact is bad science, and that
is a serious matter. It might not be necessary to call him names, but it
is necessary not to let him off the hook.
> On Jan 19, 7:17 pm, Ben C <[email protected]> wrote:
>> On 2008-01-19, [email protected] <[email protected]> wrote:
>>
>> > Ben, for those with background in mechanical design, most of what
>> > Jobst says regarding stress relief in spokes is easily understood,
>> > and pretty obviously correct.
>>
>> I understand the idea, and it is reasonable. And as you say later,
>> credit to Jobst for thinking of it. But I don't believe we can say more
>> than "it may be a factor in some failures."
>
> I think that statement is far too limited. And I think most people
> with extensive background in mechanical design principles would also
> think it's too limited.
If you're going to bring up the idea of extensive backgrounds in
mechanical design principles then I think you should try to indicate
what in particular that background teaches that alters the argument.
[...]
>> They are difficult to detect, but not impossible, and that doesn't
>> change the fact that do you have to actually detect them before you can
>> claim that they are a factor, rather than just a possible factor.
>
> Not at all. If:
>
> 1) you had a situation known to normally produce residual stresses in
> parts that routinely failed by fatigue, and
>
> 2) you applied techniques known to cause stress relief in those parts,
> and
>
> 3) you observed a significant improvement in fatigue life,
If. But that hasn't been done in any kind of even vaguely scientific
way. All we have is a couple of anecdotes.
> then it doesn't require the actual detection or measurement of those
> residual stresses to logically conclude they were an important factor,
> and are now lessened or eliminated.
Lack of evidence that you would expect to find if the hypothesis is true
is also a factor.
Jim Beam keeps asking: do spokes break more often in regions of high
tensile residual stress? I missed the post where someone explained
either that they do or that they don't or that we don't know or that it
doesn't make any difference.
> Yes, I understand there could be confounding factors. But IMO, what
> we see here are fairly desperate attempts to come up with _some_ way,
> _any_ way, to suggest that stress relief either doesn't exist, or
> doesn't work. In view of the logic behind it and the evidence in
> favor of it, neither the effort nor the motivation make much sense to
> me.
It comes down to exactly what the claim is that's being made. I'm sorry
but I think Jobst states it far too strongly. He is always saying:
residual stress breaks spokes, spokes break if you don't stress-relieve,
loose spokes break because they aren't properly stress-relieved, etc.
etc.
If I've misinterpreted his position, fine, but it is not willful
minterpretation and I don't have anything against Jobst personally.
But I can understand why people get angry with him, because failure to
distinguish clearly between hypothesis and fact is bad science, and that
is a serious matter. It might not be necessary to call him names, but it
is necessary not to let him off the hook.
