Ad nauseum, life of composites

[rfryer]

Decades ago, as a teenager, I took part in the Three Peaks Cyclo Cross (I won't say "competed in" )
I was on a 531DB frame/fork with sprint rims and cyclo cross tubs. I hit something and came off quite spectacularly, once I'd worked out which way was up and found my bike I could see that both the frame and fork were bent to the point that the front tyre almost touched the downtube. The front wheel was undamaged.

If you hit something with that sort of force, whether you stove the wheel in or not, you're coming off, and your frame material won't matter

In general, I'm an apologist for CF, but I'd think that had the above frame been CF then any of the following outcomes might have been possible:

• The frame could have been essentially undamaged, and gone to a happy old age.
• The frame could have snapped, possibly impaling you on one of the ends of the shattered tubes
• The frame could have appeared undamaged, but then later failed at a much lower level of impact, possibly impaling...

This isn't enough to put me off riding CF, but I can fully appreciate the arguments against, and weigh up the risks accordingly.

[Vorpal]

I had a look at the test data on Sheldon Brown's site http://www.sheldonbrown.com/rinard/EFBe ... e_test.htm

The published analysis is actually pretty limited; they only discuss enough to make a conclusion. So, I decided to do my own analysis.

To start with, the test is extreme, and cannot be correlated to mileage. However, the manner in which the bikes are loaded seems to appropriately mimic usage. While it is difficult to say from an article such as this, it seems to be a reasoanbly designed accelerated life test. If that is the case, even if the results cannot be directly correlated to bicycle life, the comparisons are valid.

For the analysis, then....

In the analysis I did, I assume a Weibull distribution, which is typical of fatigue. The data are quite limted, so it is difficult to place much confidence in the results, but that is often the case for life data analysis from test results.

Despite a lack of confidence, I actually think that the results are quite interesting....

You can see that the unreliability increases with the number of cycles. This is what we would expect. However, the part that is (to me) interesting is that the welded steel frames and carbon frames exhibit a much more abrupt increase. What that means is that the time to failure is much more consistent than for the other frames.

In the case of the welded steel frames,

Classic fatigue failure with Fondriest: Both chain stays of the light-weight steel frame broke in the weld relatively early. The difference in rigidity between the thin, flexible tube and the substantial bottom bracket shell is probably excessive and provides for stress risers.

Given that the welded steel frames were different brands, it is surprising to me that the times to failure are so consistent to produce this very steep slope on the graph. It is perhaps less surprising for a carbon frame, given that the properties can be more easily controlled.

I didn't include the statistical confidence on the above graph because it made for a confusing mess of lines. In general, with an analysis like this, the only way you can really improve confidence in results is to test more samples. However, when results are very consistent, this also improves confidence. So, you can see here 70% confidence bounds on the results for lugged and welded steel frames...

To me, the only drawback is that which Brucey has described; that the user is less likely to have a warning of imminent failure with CF versus other materials.

However,

[[XAP]Bob]

To me, the only drawback is that which Brucey has described; that the user is less likely to have a warning of imminent failure with CF versus other materials.

That's pretty much the point though isn't it.
A minimal weight loss isn't sufficient for most purposes (Grand Tours excepted) to balance the risk of possible sudden failure.

The financial cost of CF frame has historically been a detriment, although that is less of an issue nowadays.

[Brucey]

IIRC that test was not carried out in a way that accurately simulated 'real' service conditions in that the loadings were all such that they would likely be above the fatigue limit of materials like steel and Ti. Nor did the test include any higher loadings than those applied. Real structures see a spectrum of different stresses and the effects don't always scale with stress in a predictable fashion, even any in one material.

in addition the effects of real-world loadings will vary with material. It is known that a few 'overload' stress cycles early on in structures made in some materials (steel etc) have a stress relief effect which then enhances the fatigue performance at lower loadings. This effect is comparable to that which is achieved in wheels when they are stress relieved. Oddly enough the effects on other materials of similar cycles are often completely different.

Also welds in the 'as welded' condition are chock full of stress concentrations and residual stresses. Again the way the structure might respond to fatigue loads would vary enormously with the applied loading history.

So it is an interesting test but it isn't one that tells you for sure how those frames are likely to perform in the real world, not unless your riding exactly simulates the test conditions (some chance; you would drop dead trying probably).

The other conclusion you can draw is that any given set of test conditions may well be judged to favour one material over another. If you are charitable you could argue that the loading conditions in that test were chosen to make the tests of a shorter (more affordable) duration; if you are cynical, you might argue that the test conditions were chosen to show steel and Ti in a poor light vs Al and CF.

cheers

[pliptrot]

Oh, the irony: I found a crack in the weld behind the bottom bracket on my 853 frame at the weekend. I can thus return the result that in my life I have had 2 steel frames crack (one a dry joint in an expensive hand-built jobby) and now this. The 2 composite frames I have owned (admittedly much less used) remain crack free.........

[Bicycler]

And the thread currently above this one is: viewtopic.php?f=5&t=89190

Seriously, if this thread has contributed anything to this forum it's that individual anecdotes don't give much useful information about frame materials.

[MartinC]

Finding a crack in a frame is bad news. Finding one that can't be repaired is worse. What's even worse is not finding the crack before the frame fails whilst you're riding it.

[[XAP]Bob]

Finding a crack in a frame is bad news. Finding one that can't be repaired is worse. What's even worse is not finding the crack before the frame fails whilst you're riding it.

Yes - and this is where CF has its main weakness, steel will tend to signal imminent failure, CF doesn't tend to do the same.

Note the two important words in there: "tend"

[pliptrot]

Yes - and this is where CF has its main weakness, steel will tend to signal imminent failure, CF doesn't tend to do the same.

I only found this weekend's sad discovery because I went looking for it due to a persistent creak when standing up. The failure mode for composites is not likely to cause this, I imagine.

[Brucey]

Oh, the irony: I found a crack in the weld behind the bottom bracket on my 853 frame at the weekend. I can thus return the result that in my life I have had 2 steel frames crack (one a dry joint in an expensive hand-built jobby) and now this. The 2 composite frames I have owned (admittedly much less used) remain crack free.........

That is bad luck... but there is a reason why steel bicycle frames were not welded (TIG welded) together for many years; it is because it is difficult not to do it in such a way as a proportion of them will fail via fatigue at a relatively low mileage. Tiny lack of fusion defects or weld toe defects are usually responsible for cracks that appear in or near TIG welds in steel frames.

TIG welding was originally known as 'heliarc welding' (amongst other things) and was readily available to be used for the manufacture of bicycle frames etc (had anyone wanted to) from anytime after WW-II. Arguably it only started to be used in anger for making bike frames when the manufacturers realised that cyclists were now 'consumers' who might be happy to buy another frame or even another whole bike once every few years. This in turn probably only happened because of mountain bikes; lugs were not made in all the angles/tube sizes that might be required, and fillet brazing is difficult, so TIG (or even MIG) welded frames fitted the bill; who cares about fatigue when the thing (if it isn't crashed before it goes out of fashion...) isn't actually going to be ridden that much anyway....?

cheers

[NATURAL ANKLING]

Hi,
its not that my tig welded halfords MTB will not fail, (over 15 years old) its that it appears to have survived the punishment I have given it, its non suspension too.
I would expect that machine welded frames have pretty reliable welds, and if they were also gas shielded on the inside too, a human would not be able to exceed the quality and reliability also no tea break needed.

I have seen very small tubes 8mm maybe as small as 6mm machine welded (not sure the heat source but electrical arc / laser) destined for gas supply (not combustable gas) and the result is astounding. It was stainless steel and maybe steel will be more suceptable to oxidise when welded

One of the good things about braze / hard solder and of course lugged is stronger too like braze welding, no oxidation of the base metal by melting.
Not sure you can automate braze welding (fillet of weld looks like real steel welding)

Surely cost is the major factor od tig welded frames.