James Annan said:Just getting my retaliation in first:-)
A strong tactic! I'm taking notes.
Quoted message said:"Proceed"? Since when has there been a discussion on this
topic that was other than "unnecessarily adversarial"?
Medium duly noted, and point accepted.
Quoted message said:Quoted message said:I'm waiting for my list of hypothetical coincidences.
Quoted message said:Sorry Sir, the dog ate my homework.
Don't let it happen again. Now go sit down.
Quoted message said:How about:
Quoted message said:1. That the spectrum of loads that a frame sees in normal use falls
entirely below the fatigue threshold of a steel frame. (I would
probably dispute that there _is_ a clear upper limit to the loads that
a frame experiences in normal use over an extended lifetime).
Quoted message said:2. That a wide range of frames made from different steels, tube
gauges, frame sizes and assembly methods can all have fatigue
thresholds of approximately the same value.
Quoted message said:3. That builders with a range of experience, training and skill level
can possibly design and build frames with endurance thresholds which
are reliably above the largest loads encountered, despite both of
these factors being not only unknown but effectively unknowable.
(Doesn't seem likely to me that the builders would have any particular
motivation for this even if they could do it - all that matters is
that the failure rate is adequately low in practice.)
Quoted message said:4. That every steel frame that does fail in real life was "defective"
in some way, or else "abused".
Quoted message said:5. That loads of a "reasonable" level (below the threshold of a steel
frame) will cause a detectable fatigue problem for well-built
aluminium frames that already survived 200,000 high-load cycles in the
test.
Quoted message said:6. That the S/N curves of Al frames are systematically steeper than
those of steel frames (hadn't come across this until you suggested
it).
Quoted message said:7. That the firm clamping of the head tube in the test leads to some
particular bias against the steel frames.
The statements fall into two broad groups. One group concerns the question:
does a practical fatigue threshold exist in steel frames? The second: are
the conditions of this test likely to reproduce the results seen on the
road?
First group: 1,2,3,4
Quoted message said:1. That the spectrum of loads that a frame sees in normal use falls
entirely below the fatigue threshold of a steel frame. (I would
probably dispute that there _is_ a clear upper limit to the loads that
a frame experiences in normal use over an extended lifetime).
"Entirely" is only a condition if you insist on _eternal_ life. I'd rather
not deal in absolutes. All that's necessary in practice is that a
statistically significant weighted proportion of the load experienced by
the frame falls below the endurance limit. As an upper load limit for a
given frame, I'd suggest the minimum load required to macroscopically yield
it (bend it).
Quoted message said:2. That a wide range of frames made from different steels, tube
gauges, frame sizes and assembly methods can all have fatigue
thresholds of approximately the same value.
I don't see why that should be a condition. Surely they only need to have
endurance limits above a certain significant minimum value.
Quoted message said:3. That builders with a range of experience, training and skill level
can possibly design and build frames with endurance thresholds which
are reliably above the largest loads encountered, despite both of
these factors being not only unknown but effectively unknowable.
(Doesn't seem likely to me that the builders would have any particular
motivation for this even if they could do it - all that matters is
that the failure rate is adequately low in practice.)
Again, this is a condition for eternal life. The same remarks as number 1.
"Unknowable" I disagree with: It's possible to determine both
experimentally. Your parenthetical point is realistic, of course.
Quoted message said:4. That every steel frame that does fail in real life was "defective"
in some way, or else "abused".
That's a tricky one. In a sense it's valid, because you can define "defect"
and "abuse" to span the entire set of failures. If you assume that a steel
frame should (effectively) never fail unless defective, then one that does
fail that isn't defective must have been abused (overloaded).
On the other hand, it's also a truism for any material. If your frame fails
through no obvious defect in a lifetime you consider inadequate, you should
probably have bought a sturdier frame.
Second group: 5,6,7
Quoted message said:5. That loads of a "reasonable" level (below the threshold of a
steel frame) will cause a detectable fatigue problem for well-built
aluminium frames that already survived 200,000 high-load cycles
in the test.
That depends entirely on the relative frequency of those lower loads, of
course. We need a load spectrum.
Quoted message said:6. That the S/N curves of Al frames are systematically steeper
than those of steel frames (hadn't come across this until you
suggested it).
I suggested this to account for the possibility that the test results might
still be misleading _in_the_hypothetical_absence_of_an_endurance_limit.
That is, to reorder the results with a lower test load, the S/N curves
_must_ cross - either as a result of the curves of the different frames
having markedly different gradients, or because the steel (and titanium)
frames have an endurance limit.
In practice, I think we agree that the curves must and do cross. The
interesting question is where.
Quoted message said:7. That the firm clamping of the head tube in the test leads to
some particular bias against the steel frames.
That's not impossible. Restraints are much harder to model than loads, and
the less stiff frames would suffer more from an unrealistically stiff
fixture. This comes back to the need for a strain calibration.
Quoted message said:That do for now? Of course, these are not entirely coincident but
partially overlapping. I have yet to see any evidence in support of
any of them. Have you?
See above.
Quoted message said:That's not to say there is no hint of truth behind any of them, of
course.
Of course.
James Thomson
