What an ugly crack

[HeartCoppi]

" the weld gas aperture part. The head tube would not require weld apertures since the gas would be free to escape as there is no closed tubes involved."

 

The alu frames I've looked at closely have a hole cut into the head tube where the top tube and down tube meet the head tube.

 

Clearly the top tube and down tube do not fit into these holes for assembly. The holes are smaller than the diameter of the top or down tube.

 

I believe they are there for two reasons:

 

(1) when welding the saddle shaped weld around the outside to join the head tube to the top tube and the head tube to the down tube, there will that final instant where the weld closes onto the weld start point. By now there is already significant heat. The hot air inside the tube needs to escape as the weld closes and heat increases the pressure. It increases the risk of weld defect and reworking the weld because of pressure blowing out the melting pool. This is always more pronounced when working in smaller and lighter dimensions.

 

This is on the assumption that the other end of the top and down tube is closed.

 

It is possible that even Argon purging is taking place.

 

 

 

(2) another reason probably is to have an access point to shake out any debris that may get trapped during manufacturing. It really irritating when theres something making a noise in the tubes in a premium frame.

 

 

 

 

 

Just to be clear, I was not referring to welding cup flanges to the head tube.

 

...which brings me to another concern. The assumption is that: step1 weld cup flanges to top and bottom of head tube...step2 weld over those welds when doing the saddle welds to join the top tube and down tube to the head tube.

 

 

 

Sorry if I missed something.

 

Bit of guessing because not firsthand visual

[Johan Bornman]

" the weld gas aperture part. The head tube would not require weld apertures since the gas would be free to escape as there is no closed tubes involved."

The alu frames I've looked at closely have a hole cut into the head tube where the top tube and down tube meet the head tube.
Clearly the top tube and down tube do not fit into these holes for assembly. The holes are smaller than the diameter of the top or down tube.
I believe they are there for two reasons:
(1) when welding the saddle shaped weld around the outside to join the head tube to the top tube and the head tube to the down tube' date=' there will that final instant where the weld closes onto the weld start point. By now there is already significant heat. The hot air inside the tube needs to escape as the weld closes and heat increases the pressure. It increases the risk of weld defect and reworking the weld because of pressure blowing out the melting pool. This is always more pronounced when working in smaller and lighter dimensions.
This is on the assumption that the other end of the top and down tube is closed.
It is possible that even Argon purging is taking place.

(2) another reason probably is to have an access point to shake out any debris that may get trapped during manufacturing. It really irritating when theres something making a noise in the tubes in a premium frame.


Just to be clear, I was not referring to welding cup flanges to the head tube.
...which brings me to another concern. The assumption is that: step1 weld cup flanges to top and bottom of head tube...step2 weld over those welds when doing the saddle welds to join the top tube and down tube to the head tube.

Sorry if I missed something.
Bit of guessing because not firsthand visual[/quote']

 

You are right. I visualised the head tube manufacturing process in isolation and couldn't figure out why it would need escape holes. It does, once it is being welded to the frame.

 

Like these (for those still enduring this).

 

 

 

This is a view of the head tube taken from the inside of a sawn-off downtube.

 

I will see if I can get the owner to part from his frame, then I'll saw it up and we can have a field time looking at its construction method.

 

I still think this is a weld/heat treatment defect.

 

Edman - you are right. The cup is under compression but I still can't figure out of its crack was the cause of the head tube crack or vice versa. My hacksaw may reveal more.

 

 

 

 

[Capricorn]

i disagree with Coppi about the race cracking prior to the frame, and note: i base this solely on my assessment of the first two photos JB posted.

 

The headtube material seems to have displaced quite a bit, yet the race only has a crack in it with no obvious indications of displacement. ?However, because the race is the harder material, it might still have a fairly high yield strength, so it may still have displaced, but sprung back to it's original shape closing the defect to the point it appears as a mere crack.

So if the skirted portion of the headtube failed due to some welding during manufacture, it could have failed first, and residual stress, due to poor post weld treatment, could have caused the displacement which appears at odds with the width of the crack on the race. The race then takes up the stresses the rest of the headtube should have taken up and eventually cracks.

[Guest Big H]

I normally pump cracks full of bitumen using a hot lance and cover them with strips of BIDIM. Will not work here.....!!!!!!

 

For those who wants to upgrade from the two planks go and look here....

 

http://www.mindspring.com/~d.g1/headset.html

 

Viola...... an inexpensive home made headset press!!!!!!!!

 

Just a tip..... you can use washers and socket spanner heads for the bushings.

Big H2010-02-18 13:50:27

[HeartCoppi]

 

 

 

 

It is certain that there are various definitions of 'compression'

 

What is to be considered is the extent of compression.

 

 

 

There is a vast difference in the degree of compression between a press fit and an interference fit.

 

In an interference fit, there is a considered overlap of diameters, such that the shroud has to be heated to expand to slip over the inner diameter. On cooling, the shroud shrinks again and exerts high compression on the inner piece.

 

 

 

Aluminium is a soft and ductile material. The head tube is also very thin walled. The head set race is hard and strong and several times thicker.

 

On pressing the head set race into the aluminium flange cup, the flange presumable rides up over a broken sharp edge or champer on the entry face of the race.

 

The aluminium is slightly displaced outwards on full seating of the race. (Possibly measurable with a micrometer)

 

How much compression can the thin ductile aluminium ring really exert on a robust steel inner race?

 

Some, but not much !! very very little (compressive force)

 

 

 

Previous poster suggests that the aluminium failed first, eliminated critical compression and created a positioned relief for the failure of the race.

 

I maintain that there is no existence of any significant nor critical compression, considering the proportions and materials here.

 

 

 

Therefore the aluminium has virtually no influence on the mechanical life of the race. But the integrity of the race has huge force on the tensioned shroud.

 

 

 

There was a key statement in the paragraph above for Capricorn:

 

"The aluminium is slightly displaced outwards on full seating of the race. (Possibly measurable with a micrometer)"

 

This explains the burst appearance of the vertical crack in the head tube. Meaning that the aluminium circumference was experiencing stretch when it failed. This also implies that it is a tear and not a crack.

 

 

 

I would also like to refer to the most critical info given by JB in his opening post: the cracks coincide exactly in position.

 

When the steel race cracked first it did open and close slightly under continued use creating an immediately adjacent stress vector on the tensioned aluminium further stretching the aluminium sporadically till it fatigued and "tore". Because the circumference is stretched while the race is still inserted, the gap appears severe. There is a possibility that the aluminium may 'close' if the race is removed, but it will not return perfectly because it has a relative low elastic coefficient.

 

 

 

 

 

 

 

It would be interesting to see if the steel ring opens when removed.

 

ha...If you think it will, we have a problem, because it wont. It already could open with the existence of the tear and the total absence of compression, and wouldn't have to be removed for this to occur.

 

 

 

 

 

Finally the tear turns and follows the circumferential and vibrating plane of the race: as the forces from the road travel through the fork and crown and transverse through into the race and the aluminium it is in contact with.

 

NOTE: This would occur whether there was any welding, or none what so ever(as I propose) around the based of the head set flange cup form.

 

Failures are evidence or indicators of localized stress and do not unnecessarily arise or propagate to non stress zones, but continue to follow the cause.

 

 

 

Sorry if I missed something.

 

 

 

 

 

 

 

 

 

 

 

 

 

HeartCoppi2010-02-18 16:33:11

[HeartCoppi]

It would be great if you (JB) got hold of this frame again and were allowed to dissect it.

[Minion]

Maybe some information on the circumstances under which the crack

occurred may shed some light on the situation:

 

 

1. When did the owner notice the crack?

2. How long did it take the owner to notice the crack (i.e. time interval

   between last inspection and noticing it)?

3. Did the entire crack start and grow in a single incident or was it a

   gradual growth?

4. Did the crack just appear or was it brought on by some event like

   hitting a pothole?

5. How much mileage did the frame have before the crack appeared?

6. Did anything change or was any work done on the bike soon before the

   crack appeared?

7. Any other information?

[HeartCoppi]

Any other information?

 

...with regards the headset. What was the experience of the owner regarding the headset: was it very loose requiring regular tightening? was it over tightened? was there play in the headset i.e did the fork 'wiggle'? did the owner change forks?

 

(good question regarding the pot hole ( high vertical impact)) 

[Tieffels]

My current thinking: Compare the 2 scenarios of race vs head tube cracking first. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

1. Suppose head tube crack first: Would that lead to a cracked headset race? I think not, the race is still uniformly loaded and the cracked head tube will not cause a stress riser seen by the headset race

 

2. Suppose the headset race cracked first: That means all the forces seen by the headset will result in an opening in the race, placing a lot of concentrated stress on the headtube. Aluminium as you all know is not a fan of cyclic stress, and then the crack.

 

As pointed out earlier, just because it is such a neat crack (the headset) does not indicate that it happened later. Even with constant expanding and closing it will stay pretty much perfect.

 

My money still on 2.

[Johan Bornman]

Maybe some information on the circumstances under which the crack occurred may shed some light on the situation:

When did the owner notice the crack? Only once he felt a clack-clack feedback when he braked.

How long did it take the owner to notice the crack (i.e. time interval between last inspection and noticing it)?  He doesn't wash his bike often enough to notice a starting crack and then track its progress. It was a one-shot affair. As far as he sees it' date=' it was just there one fine day.

Did the entire crack start and grow in a single incident or was it a gradual growth? I have no doubt that it was gradual growth. That is how these things work. He doesn't know.

Did the crack just appear or was it brought on by some event like hitting a pothole?  No event.

How much mileage did the frame have before the crack appeared? No-one knows but he rides every single day of his life. It was a second-hand frame only ridden by a little old lady who used it to go to church on. He bought it two years ago and rode it a lot.

Did anything change or was any work done on the bike soon before the crack appeared? No work done. The headset is kaput, so that hasn't been replaced in years.

Any other information?  Sorry, not.

Coppiheart:

...with regards the headset. What was the experience of the owner regarding the headset: was it very loose requiring regular tightening? was it over tightened? was there play in the headset i.e did the fork 'wiggle'? did the owner change forks? Last time it was tightened was way back in 1970 something.

 

(good question regarding the pot hole ( high vertical impact)) 

I doubt vertical impact will have any effect on that weld. I think it is a classic fatigue crack, not impact crack.
An impact of that nature will also crack all your molars.[/quote']

[Mackie]

I normally pump cracks full of bitumen using a hot lance and cover them with strips of BIDIM. Will not work here.....!!!!!!

 

For those who wants to upgrade from the two planks go and look here....

 

http://www.mindspring.com/~d.g1/headset.html

 

Viola...... an inexpensive home made headset press!!!!!!!!

 

Just a tip..... you can use washers and socket spanner heads for the bushings.

I like, a boer maak n plan.

[HeartCoppi]

My current thinking: Compare the 2 scenarios of race vs head tube cracking first. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

1. Suppose head tube crack first: Would that lead to a cracked headset race? I think not, the race is still uniformly loaded and the cracked head tube will not cause a stress riser seen by the headset race

 

2. Suppose the headset race cracked first: That means all the forces seen by the headset will result in an opening in the race, placing a lot of concentrated stress on the headtube. Aluminium as you all know is not a fan of cyclic stress, and then the crack.

 

As pointed out earlier, just because it is such a neat crack (the headset) does not indicate that it happened later. Even with constant expanding and closing it will stay pretty much perfect.

 

My money still on 2.

[/quote']

 

Perfect. Very well explained. Your money is safe

 

In addition if the crack originated on the circumferential plane of the ?weld? then it would have continued all the way around in the HAZ possibly deviating only through the embossed groove to hold the headbadge, splitting the whole flange cup off the tube. It would follow (in physics laws): 'the path of least resistance' and NOT suddenly turn 90deg across a supported wall.

HeartCoppi2010-02-19 04:49:21

[Capricorn]

 

least resistance is governed by the material and it's defects and grain structure, not line of sight or logic derived from a cursory visual assessment. Fracture mechanics is derived from the law of physics. dont be so easily dismissive of seemingly illogical paths for crack propagation.

 

It' all down to probabilities as the failure progression was not tracked at some other time.

My supposition is based on the assumption that the headtube wall is the support for the race to help propagate stresses through the rest of the frame. So once the vertical crack appears, it very quickly could have caused the horizontal crack to propagate. The race in turn takes more of the vertical load as it's support structure has been weakened.

Since it's a harder material, it is able to take more fatigue loading which over time results in the crack we see. Once the race weakened, it exacerbated the tearing load on the already weakened alu structure above the race fault.

Again, without further evidence, it's all down to probabilities. I merely offer another hypothesis. No law of physics was deliberately or unintentionally violated. I'm sure your conjecture attempts the same

Capricorn2010-02-19 05:58:43

[jollypoo]

My supposition is based on the assumption that the headtube wall is the support for the race to help propagate stresses through the rest of the frame.

 

 

 

 

 

 

[Schwynn]

?Have I got the wrong topic?

 

 

 

That is exactly what popped into my minds eye when I read the topic header!