On 2007-09-09,
[email protected] <
[email protected]> wrote:
> Ben C? writes:
[...]
>> Isn't it the other way round? The residual stresses are compressive
>> on the exterior on outside of the bend (I thought) and tensile on
>> the exterior of the inside. So it's the elbows that become more
>> obtuse for which the building-bend adds to the manufacturing
>> residual stress?
>
> When the bend is made, the main stress is tension on the outside of
> the bend
Yes but that bend springs back, leaving residual compressive stress on
the exterior on the outside?
> and that bend is increased when the outbound spokes get their
> spoke line corrected, either by spoke tension or manually. In any
> event, the outside of the spoke elbows are in tensile stress on those
> spokes.
Yes, the applied stress is on the outside of the bend on the exterior on
the outbound spokes.
[...]
>> Yes, but as I said before, if the spoke is near yield under normal
>> spoke tension _there must be a moment_. I don't see how you can
>> have one without the other.
>
> A spoke that is fully supported in the aluminum flange is loaded
> purely in tension as it wraps around a curved bore that matches its
> inner radius. The spoke head should rest solidly on the entrance to
> the flange and the spoke pulled around the corner. Such a spoke will
> last a long time. The straight pull spoke came along because people
> were not seeing the effect of reasonable flange bores with reasonable
> flange thickness and stress relieving.
>
>> That's why I think the moment is key: it's where big stresses at the
>> elbow, on the exterior, come from.
>
> There is no moment in a fully supported elbow and if you look at a
> flange bore after removing the spoke you should see a smooth curve
> that matches the spoke elbow shape.
That's what one would hope for in a good wheel, certainly.
But if there's no moment there's no stress there from the spoke line
correction bend. Because there's nothing there to hold that bend in
place.
The last time we went through this it went slightly differently.
Outbound spoke
--------------
1. You said we bend the elbow to correct the line, it can't spring back,
so the bend is held there, and so the stress remains high on the
outside. I call this "retained build stress" to avoid confusion with
manufacturing residual stress.
2. I said for spoke tension to hold the bend in place needs sufficient
moment. If the spoke is flush to the flange you don't have that
moment.
3. We repeated (1) and (2) to each other approximately 300 times.
4. Eventually you said yes but what do you mean by "flush"? It's never
100% completely totally and utterly flush. We get a little
question-mark shaped hook, the details of which are complicated.
5. I said OK so there might be a moment, retained stress might be there,
and stress-relief might get rid of it.
But now you're saying "there's no moment". In that case there is no
retained build stress on the outside of the bend. 100kgf or so _with no
moment_ is not enough to hold a bend in a spoke.
Really they're the same thing. Residual stress (the kind that remains
inside a wire after it's been bent and sprung back) requires knowledge
of history. But retained build stress is just applied stress. It doesn't
matter how it got there: if spoke tension is 100kgf and there's a
too-big radius, you have a moment, and too-high stress on the elbow
outside. No moment, nothing to worry about. If you stress-relieve and it
makes the stress go away then it necessarily also made the moment go
away.
So saying stress-relief is a process of helping things gain intimate
conformity of some kind (as jim beam has said) and saying that it yields
the bits of the cross section that are still close to yield from spoke
line correction (as you have said) pretty much amount to the same thing.
As for residual manufacturing stress-- it sounds like that might add a
bit to the tensile skin stress on the inside of the bend of the inbound
spokes. So it may have a contribution to failures of inbound spokes that
begin on the inside. But there's no reason to believe it's significant:
Even if we accept the evidence that stress-relief practice reduces
failures we don't know how much of that is to do with reducing retained
build stress as opposed to reducing manufacturing residual stress.
Since we're bending the elbow again anyway I'm inclined to think
manufacturing residual stress is basically ancient history by the the
time we've finished the wheel.