Frame durability - testing with science

"Andrew Price" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Would be interested what the ng thinks of the frame durability tests recorded at -

efbe.deetour109.htmOpen ↗

Surprising, to a non engineer that the lighter frames did so well. Would be interested to know if
those with the technical knowledge consider the tests valid - one point strikes me that may have
been insufficient examples tested.

The article has been discussed a few times here. One criticism is that the test was unrealistic in
that it used a relatively few cycles of a too high load, which might generate misleading results.
That point taken, it still resulted with frames failing in classic manners. Both of the lugged steel
frames failed with cracks around the lower head tube lug, exactly the way my steel frame failed in
real use. While the test may not have been perfect, I think it did generate meaningful results, I
don't find the outcomes too surprising.

Quoted message said:

Would be interested what the ng thinks of the frame durability tests recorded at -

efbe.deetour109.htmOpen ↗

Surprising, to a non engineer that the lighter frames did so well. Would be interested to know if
those with the technical knowledge consider the tests valid - one point strikes me that may have
been insufficient examples tested.

Look forward to hearing , best Andrew

THese tests have been discussed here several times. The biggest problem I see with these tests are
the large loads and few number of loading cycles.

The load starts at 270 lbs for 100,000 cycles and then is increased to about 290 lbs for the next
100,000 cycles.

Rarely would a frame seem 270lbs in a cyclic manner, even with a large and relatively strong rider.
Then consider that the 200,000 cycles amounts to about 500-1000 miles of riding.

These are supposed to be fatigue tests and it would seem that by using a large load one can perform
the test more quickly.

However this is not true with fatigue tests. Fatigue is something that happens over millions and
millions of cycles and there is a definite relationship between the fatigue properties of a material
and the stress or load. Some materials have a endurance limit which means if the stress/load is kept
below there endurance limit, the fatigue crack does not grow. Steel and Titanium are such materials.

This means that if you increase the load to a load that is unusual, you may move past the endurance
limit and thus the fatigue crack will propagate.

Other materials like Aluminum have no fatigue limit so they require different design techniques to
deal with fatigue. In general they must be over built from a strengh standpoint to deal with the
fatigue issues.

When doing a fatigue test or other engineering test, it is important to understand the failure
criteria and the failure mechanisms and make sure that the test conditions are a good approximation
of the real world loading conditions so that the results are indeed valid for the real world.

Most any bike will last 1000 miles regardless of how it is ridden. THe real question is whether it
will last 5000 miles or 50,000 miles and that cannot be determined by tests such as this one.

On the other hand, if you are interested in knowing if Darren Hill or Marty Nostein could ride these
bikes in match sprint competition without failure, these tests might be appropriate.

The other issue is that in most cases, frame failure is due to manufacturing problems rather than
design problems, though in the case of some "stupid light" frames, they are designed to have a
limited life span.

jon isaacs

Quoted message said:

The article has been discussed a few times here. One criticism is that the test was unrealistic in
that it used a relatively few cycles of a too high load, which might generate misleading results.
That point taken, it still resulted with frames failing in classic manners. Both of the lugged
steel frames failed with cracks around the lower head tube lug, exactly the way my steel frame
failed in real use. While the test may not have been perfect, I think it did generate meaningful
results, I don't find the outcomes too surprising.

The fact that a frame suffers low cycle fatigue failure at a weak point is not surprising. However
whether a frame would sufferer similar failure given use within normal bounds is not within the
realm of this test to determine.

This is more of a strength test that a fatigue test and it is fatigue rather than strength that
causes frames to fail.

jon isaacs

Jon makes some good points regarding the test and part design. Like just about everything, though,
compromises for whatever reason are sometimes made.

It would be nice to have the luxury of time in order to complete millions of cycles during a test
like this on a large number of samples- speaking from personal experience, though, this simply is
not practical.

In my development of a stem durabiltiy evaluation protocol, I used a progressive load schedule.
The amount of cumulative damage (if _any_) that occured at the small loads just doesn't matter
near the end of the load schedule. In the future, the starting load I will use will be increased
in order to shorten the amount of time on the fixture (current protocol takes 10-14 days with it
operating 24/7).

The EFBE is a comparative test protocol (which is more than likely developed according to DIN
standards) that sheds some light on the frame durability subject. I would rather have this
information than no information at all.

--
=======================
Kraig Willett RBT Promotional rates: www.biketechreview.com/rbt.htm
=======================

Quoted message said:

However this is not true with fatigue tests. Fatigue is something that happens over millions and
millions of cycles and there is a definite relationship between the fatigue properties of a
material and the stress or load. Some materials have a endurance limit which means if the
stress/load is kept below there endurance limit, the fatigue crack does not grow. Steel and
Titanium are such materials.

Kinda sorta. Fatigue, by definition, is cyclic load. Whether that's 10 cycles or 10^10 cycles. True,
very low cycle fatigue has a different failure regime than high cycle fatigue, but once you get over
10^3, there's no dispute which regime dominates, and from then on, you're in well known territory.
On that basis, these tests are relevant.

Regarding endurance limits, a lot of carbon steels and Ti alloys exhibit endurance limits, but this
rarely applies to the high-strength alloys because endurance, iirc, is due to strain aging. High
alloy content tends to remove the strain aging mechanism. As modern frames are usually made of the
high-strength alloys, it's too simplistic to state that steel & Ti won't fatigue with lower loads.

Jim

Jon Isaacs said:

The load starts at 270 lbs for 100,000 cycles and then is increased to about 290 lbs for the next
100,000 cycles.

Rarely would a frame seem 270lbs in a cyclic manner, even with a large and relatively strong
rider. Then consider that the 200,000 cycles amounts to about 500-1000 miles of riding.

Only if the test is supposed to model a bike frame eventually failing purely as a result of the
forces imposed by steady pedaling. Frankly I'd expect any reasonable road frame to last almost
forever if only ridden in such a manner (say for hour rides on a smooth track), even if used by a
strong rider. And they should certainly last a lifetime under the lower pedalling forces that I can
exert - nevertheless, I have had two frames fail in normal use.

OTOH, a road frame in normal use will occasionally see forces in the large range used in the test
when hitting bumps or when the rider momentarily exerts himself very strongly. These events may only
happen once or twice a mile, so the 200,000 cycles of the test would correspond to 100,000 or more
miles of riding.

Quoted message said:

These are supposed to be fatigue tests and it would seem that by using a large load one can
perform the test more quickly.

However this is not true with fatigue tests. Fatigue is something that happens over millions and
millions of cycles and there is a definite relationship between the fatigue properties of a
material and the stress or load.

Fatigue shows a strong relationship between the size of the stress and the number of cycles that
can be endured before failure. asme.orgfatigue2.htmlOpen ↗ shows a
sample fatigue curve - on this curve, an increase of only about 20% in the applied stress results
in the number of cycles to failure decreasing by more than a factor of 10, and a doubling of the
stress drops the number of cycles by over 1000. The result is that even a relatively rare stress
will dominate in the creation of fatigue failure if it is significantly larger than the more normal
stresses encountered. The rider who puts a normal pedaling load on the frame 250 times per mile
would see fatigue failure just as soon from the bumps that he hits only once every four miles, if
those bumps result in twice the stress load of a normal pedal stroke. I'd also note that fatigue
tests are frequently carried out in the range of 10s of thousands of cycles up to millions of
cycles as shown in the figure cited above. The frame test using 200,000 cycles is not atypical of
industry testing.

(BTW, I am currently doing real-world fatigue testing on a 60 cm Cannondale frame that is 14 years
old. At about 6000 miles per year it's doing fine so far. As long as I don't hit more than one
significant bump per mile it should last for many more years.)

Just to add my two cents,

It may just be the anal retentive engineer in me but testing one frame only (which as far as I can
tell is what this company did) tells us absolutely nothing. I could build a frame that would outlast
any of those in the tests, but come production you'll have variance in the quality. That would be
why those of us involved with quality assurance sample MANY products on a line before we draw
conlusions. IMHO the method of loading is the least of the problems with this test.

Jon

"Andrew Price" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Would be interested what the ng thinks of the frame durability tests recorded at -

efbe.deetour109.htmOpen ↗

Surprising, to a non engineer that the lighter frames did so well. Would

be

Quoted message said:

interested to know if those with the technical knowledge consider the

tests

Quoted message said:

valid - one point strikes me that may have been insufficient examples tested.

Look forward to hearing , best Andrew

Jon Isaacs said:

Quoted message said:

The EFBE is a comparative test protocol (which is more than likely developed according to DIN
standards) that sheds some light on the frame durability subject. I would rather have this
information than no information at all.

One just has to be careful interpreting this data, IMHO.

True.

Quoted message said:

It is often used to say that Aluminum frames are more fatigue resistant than steel frames when
what it really says is that aluminum frames must be stronger to achieve the same real world
fatigue life of a steel frame.

Given that the test was of a limited number of frames of different construction methods and with
only one sample of each frame, I don't see how any general conclusions can be drawn about properties
of any frame material.

I certainly don't see anything in the results that would make me draw the conclusion you do above,
there are far too many variables and no controls.

Quoted message said:

This page states the endurance limit of 4340 is about 450 MPa.

This reinforces the point about low-alloy steel being unrepresentative does it not?

Quoted message said:

3. Tests which use unrealistically large loads move to different parts of S-N curves which makes
comparison tricky and prediction difficult unless the actual fatigue life is "backed out" of
the tests using some modeling assumptions.

The whole point of mapping an S-N surve is that it /does/ allow you to extrapolate. Reliably. Unless
the material exhibits an endurance limit, most metals give a good approximation of a straight line
when plotting S vs. log N.

Quoted message said:

One obviously cannot draw any real conclusions from these tests because the actual geometries and
therefore stresses are unknown.

If the intent is to trash this test, the only basis on which to do it, in my opinion, is the small
sample size. The testing methods however are sound.

Jim.

Quoted message said:

I personally think that most frame failures are the result of

manufacturing

Quoted message said:

defects or later damage.

While it's probably true that "most" frame failures are the result of manufacturing defects or
later damage, there's a trend lately towards building at the edge of what's possible with a
given material, with the result being frames that fail simply because there's no margin of error
in the design.

Our headlong pursuit of ultimate lightness, ignoring that different materials have differing
strengths and weaknesses, is idiotic. Just because you can safely build a carbon frame at under 2.5
lbs doesn't mean that a benchmark has been set that requires all materials be used to create frames
of similar weight, and yet that's the way things have been heading. It's little surprise that we see
such frames fail.

--Mike-- Chain Reaction Bicycles chainreactionbicycles.comchainreactionbicycles.comOpen ↗

"Jon Isaacs" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Quoted message said:

Only if the test is supposed to model a bike frame eventually failing purely as a result of the
forces imposed by steady pedaling. Frankly I'd expect any reasonable road frame to last almost
forever if only ridden in such a manner (say for hour rides on a smooth track), even if

used

Quoted message said:

by a strong rider. And they should certainly last a lifetime under

Quoted message said:

the lower pedalling forces that I can exert - nevertheless, I have had two frames fail in
normal use.

As far as I can see and remember, this test simulates fatigue failure from pedalling loads. So
that is really the scope of this test, how long will

the

Quoted message said:

fame handle large pedalling loads.

Quoted message said:

OTOH, a road frame in normal use will occasionally see forces in the large range used in the test
when hitting bumps or when the rider momentarily exerts himself very strongly. These events may
only happen once or twice a mile, so the 200,000 cycles of the test would correspond to 100,000
or more miles of riding.

This test did not simulate bump loadings, so I suggest that without

further

Quoted message said:

analysis, it is not possible to draw any conclusions about the fatigue

life

Quoted message said:

from bumps.

Quoted message said:

The rider who puts a normal pedaling load on the frame 250 times per mile would see fatigue
failure just as soon from the bumps that he hits only once every four miles, if those bumps
result in twice the stress load of

a

Quoted message said:

normal pedal stroke.

I agree but IMMSMW, these tests did not address the loads imposed by

anything

Quoted message said:

other than cyclic pedalling.

I personally think that most frame failures are the result of

manufacturing

Quoted message said:

defects or later damage.

Jon Isaacs

Jon Isaacs said:

Quoted message said:

Only if the test is supposed to model a bike frame eventually failing purely as a result of the
forces imposed by steady pedaling. Frankly I'd expect any reasonable road frame to last almost
forever if only ridden in such a manner (say for hour rides on a smooth track), even if used

by a strong rider. And they should certainly last a lifetime under

Quoted message said:

the lower pedalling forces that I can exert - nevertheless, I have had two frames fail in
normal use.

As far as I can see and remember, this test simulates fatigue failure from pedalling loads.

The test setup imposed a set of 100,000 forces of ~270 lbs and 100,000 forces of ~290 lbs on the end
of the crank. Whether this is from a normal pedal stroke, an unusually strong pedal stroke say for
starting a sprint, or from a rider having the force of his pedal stroke amplified by hitting a bump
doesn't matter as long as the result is this number of cycles with the tested forces. Given how
quickly the number of 'cycles to failure' decreases with increasing force, I'd guess the dominant
contributor to fatigue failure in real-world cycling would be the relatively small number of pedal
strokes that have a much larger than average amount of force rather than the majority of pedal
strokes with a normal amount of force. The test setup was designed to closely simulate the force at
the start of a sprint with the rider standing, leaning the bike appreciably, and pushing very hard
on the pedals for a few strokes. So it does simulate the heaviest loads that will be encountered on
only a small fraction of the total number of pedal strokes. As such, both the forces and the number
of cycles seem within the range that a frame may see during real riding use over a life of around
100000 miles although it will obviously vary greatly depending on the rider and type of riding.

Quoted message said:

So that is really the scope of this test, how long will the fame handle large pedalling loads.

Quoted message said:

OTOH, a road frame in normal use will occasionally see forces in the large range used in the test
when hitting bumps or when the rider momentarily exerts himself very strongly. These events may
only happen once or twice a mile, so the 200,000 cycles of the test would correspond to 100,000 or
more miles of riding.

This test did not simulate bump loadings, so I suggest that without further analysis, it is not
possible to draw any conclusions about the fatigue life from bumps.

Quoted message said:

The rider who puts a normal pedaling load on the frame 250 times per mile would see fatigue
failure just as soon from the bumps that he hits only once every four miles, if those bumps result
in twice the stress load of a

normal pedal stroke.

I agree but IMMSMW, these tests did not address the loads imposed by anything other than cyclic
pedalling.

I personally think that most frame failures are the result of manufacturing defects or
later damage.

Jon Isaacs