Al Stem - internal fatigue/creep cracks

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Valy
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by Valy

Was changing bars, decided to take a peek inside the stem completely on a whim just before installation.

Image



They don't look deep, but there is no way I'm using that. Specified torque was used and I had a stem of the same brand (Giant SL) with around 20k km without such cracks. Same torque wrench used on both. This cracked stem has about 3000km max on it, the only thing it's been installed on the bike around 4.5 years.

Just wanted to share, as I'm guilty of taking these components for granted and as a reminder to actually check every now and then. ImageImageImageImage

pocari123
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by pocari123

Did you check the stem when you bought it? Might’ve been there already


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by Weenie


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Valy
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by Valy

No. It was bought new. Since it appears anodised, I'd guess the cracks are not expected to be there after manufacturing.

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TonyM
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by TonyM

What is the brand and model?

MichaelB
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by MichaelB

:shock:

Will check my stem shortly ....

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Valy
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by Valy

Giant Connect SL OD2. Image

bm0p700f
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by bm0p700f

Those "cracks" may not be cracks. You cant see fatigue. If the stem was cracked visbably you would notice flex and then snap. The inside of the stem is not finished like the outside. The anodising is not uniform and the forging process affects the microstructure which shows up in the depth of anodisiing. I suspect those "cracks" are not cracks at all.

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Valy
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by Valy

bm0p700f wrote:Those "cracks" may not be cracks. You cant see fatigue. If the stem was cracked visbably you would notice flex and then snap. The inside of the stem is not finished like the outside. The anodising is not uniform and the forging process affects the microstructure which shows up in the depth of anodisiing. I suspect those "cracks" are not cracks at all.
You can see the "cracks" which could be a result of fatigue. Either way the "cracks" appearing in a high stress area is just too much of a coincidence for me to ignore.

The inside is rougher yes, all the more reason to be concerned.

MikeD
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by MikeD

I don't know if those are actual cracks or not, but stems are relatively cheap and I'd replace it just for peace of mind.

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Valy
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by Valy

MikeD wrote:I don't know if those are actual cracks or not, but stems are relatively cheap and I'd replace it just for peace of mind.
Image - done

I'm generally interested in the reason of these cracks. Although it's 3D forged it looks like it was made in different steps with 3D forging as one of the last steps.

I am not clear if it's a design or manufacturing flaw. If anyone knows about the manufacturing process, input would be highly welcomed. Image

bm0p700f
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by bm0p700f

The thing they are not cracks. Anodising leaves those lines due to differences in microstructure. ople

If the stem had cracks it would fail. so you dont have fatigue cracks, youd know very rapidly if you did.

There is alot of assumptions made by people looking at something they dont understand and the applying what they think is the most reasonable explanation.

As I have not condtucted NDT testing (X-ray) to find out if there are cracks are not I cannot say with absolute certainty those lines are not cracks but neither can you valy say they are cracks.

You are unlikely to have taken the faceplate off just at the right time to catch the cracks before the stem fails. People never manage that with rim before a spoke pulls through. That fatigue by the way. Fatigue failure just happen. They can be spoted via NDT testing hense why aricraft fuleages get x rayed. you cannot see the crack until it is too late. Once you see them failure (catasthropic) is about to happen. So it would have happend the last ime you rode the bike.

spud
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by spud

general roughness of the interior surfaces, and multitude of the "cracks" leads me to agree with the above - they don't finish the interior of the stem, and what you are seeing is leftover forge marks with accumulated anodizing/deposits. BTW, the stem is unliekly to crack transversly across the bottom on a road bike - that portion of the stem is under compression, you'd be more likely to see cracks on the upper part of the tube, which is under tension.

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Valy
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by Valy

Yeah good points bm. When I posted deep down I knew it was not likely to be a through crack to the metal, at least nothing immediately catastrophic but I would over react all over again if I saw something similar. I've been pestering a few people and looked at some papers but did not find anything that I understood was clearly talking about anodization cracking due to different grain sizes/structure. Seeing some NDT tests, allowance for cracks was mentioned - I did not understand if it was for a crack depths vs depth of material or number of cracked areas per plane. It is possible to have cracks in metal without immediate failure though.

Looking inside, apart form the cracks, there is a groove that goes all the way around the inner circumference of the stem - which I can only assume is a weld seam, except I have no firm idea of how the stem is manufactured. I imagine if it's a one piece forged process, then the seam would be convex?

As for cracks - yes I agree but also there is pulling on the bars as well as the weight and shocks from terrain. Also uneven load on bars from being out of the saddle. So overall - it's option:

1) There was some flex in the stem, cracking anodising around the groove bolt holes - probably some of the highest stressed points.
2) Anodising was still facilitated by flex in the stem, but cracked due to poor bonding - ie was some loose material from forging that was not integral part of the stem?
3 A mix of the two?

TheKaiser
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by TheKaiser

Valy wrote: ↑
Wed May 22, 2019 5:21 pm
MikeD wrote:I don't know if those are actual cracks or not, but stems are relatively cheap and I'd replace it just for peace of mind.
Image - done

I'm generally interested in the reason of these cracks. Although it's 3D forged it looks like it was made in different steps with 3D forging as one of the last steps.

I am not clear if it's a design or manufacturing flaw. If anyone knows about the manufacturing process, input would be highly welcomed.
This may answer some of your questions: http://www.peterverdone.com/the-stem-pr ... gineering/

There is both the blog post, plus an embedded email reply from someone at Salsa, so be sure to read, or at least scan, the whole post.

by Weenie


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Valy
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by Valy

TK - That was really cool to read, thanks a lot for posting.

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