Broken Downtube in ISO test
 

Hi,

I recently had some samples made in Asia and they performed the ISO4210 test.
During the test the downtube broke. (at the end of the test)

The downtube is an Chromo 34.9 mm ovalised to 40x30 mm (40 mm wide).
We normally use a double butted tube (0.9-0.6-0.9) in our bikes but because of ovalising problems due to lack of a mould, a tube with a wall thickness of 1.0 was used (not butted)

How is it possible that this tube broke. It's quite an heavy downtube.
(We have never performed the test with the double butted downtube.)

Thanks,

Marc

testing:
There is a 460 Newton force at the front wheel axle moving forward and aft. (100.000 repetitions)

The lug edge concentrates the stress at the bottom of the downtube, in an area weakened by the brazing. It provides a fulcrum for the tube to bend at a single point with each cycle. I'd say this was not an unexpected failure and most lugged frames would fail this test in a similar manner.

I understand that the area of the crack has the most stress.

But shouldn't there be deformations?
First the stress goes over the Yield stress, then there is deformations, and then it cracks.
But on the picture the tube seems undeformed.

fatigue happens at a different scale than your pictures. There is some deformation, but you aren't going to see it from here. A typical fatigue crack will close right up and be barely visible for most of its life.

If you look into fatigue failures more, you will find that the far field stress does not have to exceed the yield stress. That's why fatigue failures are so common, people worry about exceeding yield stress, but don't think about the cycles that the structure will see. A simple representation of this is the Goodman diagram. It's not a perfect representation, but it does show the trade-off between cycles and stress leading to fatigue failure. Actually, the S/N curve is better, even with all its flaws. Only in the case of zero cycle failures will you see far field stress exceeding yield.

I have seen the argument made that thicker straight gauge will crack at the ends because the center of the tube doesn't deform as much. I have never really bought into this theory though. I suspect the ovalizing process or some other part of the process caused this to happen. And since you have never had the butted downtube tested, this doesn't really provide a data point either way. It's possible that your brazing process will not withstand this test.

Not familiar with frame stress testing, but I've an observation. The fixture that clamps the head tube has a sharp shoulder that could be contacting the downtube lug if flexed enough. The shoulder of the fixture could impart a localized stress that wouldn't be present in real world use.

that's a really interesting observation. Also, I would want to make sure that the lower head lug is held without any slop. If there is slop there, that would also magnify the loading. The setup looks suspiciously slapdash to me.

I had an unexpected failure on a fatigue test once that turned out to be due to loose bolts. It turns out that this has been studied. It results in impact loading, which accelerates fatigue crack initiation and growth by a significant factor. I need to research it again, because the subject keeps coming up.

Thanks for the responses, and I didn't really look in the Fatigue Stresses.

But is this test not a bit too much? If probably a lot of steel lugged frames will fail? Our bikes have been touring around the world for about 20 yrs without any frame failures.

And the test was performed in the category city&trekking bike. (since we built trekking bikes)
But the loads for the category MTB are three times higher. (1200N instead of 450N)

The observation that the testing setup is questionable is very helpful. We will do another test in the coming months. (with the double butted downtube)

it's a problem if the loads are excessive, because changing them is not going to happen. I have heard complaints that a lot of the loads are excessive.

My first thought was that the straight gauge tubing forces more of the stress to the end at the lug - as mentioned in the first response. Double butting helps alleviate this by allowing more flex in the middle away from the HAZ/stress risers and thereby lowering the stresses there. More simply, it spreads the flex across the whole tube.

At least that's my understanding of butted tubing vs straight gauge.

The flexing of a sg tube vs a butted one is an interesting theory and I'd love the see results of that test.
But, I think Lakerat nailed it- the testing fixture looks to be contacting the lug right at the shoreline and right where the tube cracked.

there is one builder that is pushing this theory hard, but the reason for butting is very simple, because the haz is weaker. So it needs to be thicker than the center of the tube. That was always the stated reason for it. I don't think that straight gauge pushes that much deflection to the end. I hate speculative engineering, particularly in cases that are not self evident and likely to submit to a good thought experiment. If someone can show me a good fea model of a frame that demonstrates this, then i would be happy. As it is, it's just speculation. And no, a diving board is not a good example.

I agree that the contact with the headset cone probably caused this.

However, I will stick by my hypothesis that if the middle is thinner, it would flex more than if it was thicker and thereby concentrate at least some flex there rather than force it all out into the next weaker area, the HAZ. I suppose I'm thinking of it like butted spokes - again, my understanding is that the butting reduces some of the stress on the nipple and bend. It seems like a fairly sound hypothesis and i'm sticking to it until someone proves me wrong with data! :-)

I never figured out how to get someone to pay me to do tests like that. My motto is, "no pay, no work"

That leads to the next question:

Are bike manufacturers obligated to have their frames tested? Or obligated by a insurance company?
I had a interesting conversation with a collegue bike manufakturer and he said that liability claims will force you to have everything tested with ISO "advanced" testing in your own country. (We're talking about Germany here)

Also the German testing institutes are already helping customers with liability lawsuits unless the bikes were tested at their Institute.

I don't think any of us here are involved in production scenarios. I thought the EU mandated testing, but I'm not sure. Certainly heard complaints about testing and changing designs as a result.

Last I saw, testing is fairly inexpensive. Less expensive than I would have done it for, that's for sure. It might be worth it to do it locally. But you have to find out the laws in your market.

There was talk about having small builders do testing here in the U.S., but it hasn't been instituted. The insurance company the most small builders use was talking about it. I am guessing they haven't had enough losses to require it.

ETA: I thought I had the European standards, but I can't find them.

I didn't dive too deeply, but the CEN standards apply to all of Europe, but aren't enforced in every country. Most countries allow self-certification, but CEN urges certified labs do the testing. ISO says that Germany is bound to implement the ISO standard. Maybe the ISO standard allows self-certification, I didn't see information about that

As the data and discussion from/over testing regimes accumulates it will become a baseline of interest in court cases. If the industry doesn't develop it's own standards then standards based on external opinions have a chance of being imposed or relied on. The US also has some interesting approaches in aviation which include the experimental designation.

Id guess it was heat effected but not cooled in an appropriate manner, due to production numbers requirement..

What's the appropriate cooling method? Does one actually exist?



This is the reason lug tips are often thinned with a file after brazing to decrease the stress riser.

The writings I've looked at (in over 40 years of playing with torches and tubes) all say pretty much the same thing concerning cooling. Draft free space. Open air with no water/oil bath.


I've always felt that it's the initial drop off from brazing temps that is most critical for the steel's retaining it's intended grain and characteristics. I've also been told that it's a differential of temp that produces the stresses that distort tubes. One reason that hearth brazing was considered best by some as it heats a large area with rather even temps through out the joint. Andy

Is the "heasdset" cone, cup of the fixture a typical equal to a headset press fit component?
As others have mentioned, the profile looks very close to the lower head lug.
Looks to me that there is some tube distortion 5-6 mm beyond the crack. Look at the reflections.

The test is pretty aggressive I think. Not to question it too much, but I wonder what the rational was.

pretty sure it was a standard test. Op gave up on us some number of months back though

when I dropped by the Gazelle company in 1988, they let me watch part of their 531 race bike frame production.

they use natural gas torch arrays to heat several lugs at once , then when hot, touched them with the brass filler wire.

quick , efficient.