<
[email protected]> wrote in message
news:[email protected]...
> John Retchford writes:
>
Mark McMaster:> >> If I can pick nits, the description of how metal fatigue works is a
> >> common layman's mis-interpretation:
>
> >> "As a part of a spoke is bent beyond its elastic limit, the material will work-harden. If this
> >> continues it will become more brittle and weaker; a microscopic crack can then easily form."
>
> >> (As most readers here probably know, metal fatigue can occur below the elastic limit, and does
> >> not cause work-hardening or embrittlement).
>
Me:> > I do not think that this is the most appropriate forum in which to
> > debate the details of fatigue, but seeing you brought it up... I would not have made the
> > statement in the Sapim article, but I certainly would not make your statement either.
>
Jobst Brandt:> I think Mark's comments are appropriate and accurate in the context.
>
In the context of fatigue cracking, which is the only one introduced by Mark, I point out below that
his remark is somewhat misleading. He says he is picking nits. I am just suggesting we pick the
right nits.
Mf:> > I know of no instance of fatigue crack propagation in metals that
> > occurred without plastic deformation at the crack tip and that means that some part of the metal
> > has been strained beyond the elastic
> > limit. The plastic zone may be very small, but without it fatigue cracks in structural metals
> > do not propagate.
>
Jobst Brandt:> I think metallurgical terms, need be defined before making such
> claims.
Sorry. Which terms gave you difficulty?
Jobst Brandt:> On a microscopic scale, occurrence of yield is what leads to
> fatigue failures,
This is the point I make above. I am glad you agree with me.
Jobst Brandt continues:> but in the context of ultimate strength and yield of
> a specimen such as a spoke, stress refers to the load divided by the cross sectional area, rather
> than internal yield stresses caused by in-homogeneities in the metal. In the case of high quality
> spokes these locations are small enough that spokes fail primarily due to residual high surfaces
> macroscopic stresses from manufacture and installation that act in the way you describe.
>
I agree that the scale of the stress being considered (that is the area by which we divide the force
acting) needs to match the process being described. This is why I focussed on the microscopic scale.
Fatigue cracking involves only a relatively small number of atoms at a time. If we want to look at
things like ultimate strength (you introduced this) we need to consider the whole cross section
unless we want to consider the mechanism of the phenomonon. I am sure you are correct in pointing
out the importance of surface residual stress in influencing the fatigue life of spokes.
John Retchford