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PostPosted: July 21, 2014, 10:04 pm 
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I've read somewhere that the semicircular (fishmouthed) welds in round tubes are much stronger than those straight line ones found in square tubes.

Is that true?
If so, Why? Aren't square tubes supposed to have more weld length? Does the curved weld shape helps?
Is there a big difference in weld strength?

Please only focus on weld strength, not wishing to restart the round vs square tube debate.

Thanks guys


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PostPosted: July 21, 2014, 11:43 pm 
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My guess, and it's only a guess, is that because of the extra length of the weld and the fact it wraps around would make the fishmouth stronger

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PostPosted: July 22, 2014, 12:42 am 
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I would think it's stronger due to the fact it's tridimensional. But I'm tired and may have brain fade...


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PostPosted: July 22, 2014, 2:16 am 
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bigtime wrote:
Is there a big difference in weld strength?




No, nonsense.

In the cases of both tubes, relative to what we do, the tube is going to fail before the weld so the point is moot regardless of any strictly technical answer.


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PostPosted: July 22, 2014, 8:03 am 
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It is not the weld that is stronger, but the actual designed of the attachment. The fish mouth design spreads the heat affected zone over a larger area, so it is not all in just one plane. It also helps relative to the stress loads applied thru the attachment so you do not a concentrated load at one attachment point. If you have option to package a taper or fishmouth design, that should be your first choice. It can easily reduce the stress concentration by 200 or 300%. Dave W


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PostPosted: July 22, 2014, 10:44 am 
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I can't see how a round fish-mouthed weld could be stronger than a square tubing weld simply because of the weld length. That is, 1" square tubing has a circumference of 4", while a 1" round tube has a circumference of 3.14", or 21.5% less weld.

I also can't see how a round fish-mouthed weld distributes the heat any different, other than inducing less heat distortion due to the weld length being less than the square tube by the above amount.

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PostPosted: July 22, 2014, 11:24 am 
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The welds on square tubing are all tension. Fishmouthing puts some of that into shear. Plus like Dave said there are no sharp corners so the stress concentrations due to the shape are lower.

Now I'm not sure how it actually plays out in the real world but I don't find it hard to believe.

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PostPosted: July 22, 2014, 9:00 pm 
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KB58 wrote:
I can't see how a round fish-mouthed weld could be stronger than a square tubing weld simply because of the weld length. That is, 1" square tubing has a circumference of 4", while a 1" round tube has a circumference of 3.14", or 21.5% less weld.

I also can't see how a round fish-mouthed weld distributes the heat any different, other than inducing less heat distortion due to the weld length being less than the square tube by the above amount.



That math isn't quite right; the circumference is not 3.14 for a fishmouth. You are thinking a circle cut straight across. A fish mouth is a much more complex geometric shape. It can be described as a sine wave, and using some calculus to find the arc length you'd get to a 1" round pipe fishmouthed at 90 degrees is ~7.64" which is a significant improvement over any non-fishmouthed joint.


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PostPosted: July 23, 2014, 7:44 pm 
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Well that math isn't quite right either. The amplitude of the wave is only about 3/8" depending on tube thickness, so the total length of the weld couldn't possibly exceed 4.5", and that's with a square wave. A sine wave would probably bring it down to 4". In any case the difference in strength isn't significant when you consider:
1) A good weld on a square tube will beat a mediocre weld on a round tube, and it's easier to make a good weld on a square tube.
2) Welds on Locost tubes just aren't stressed that much. As already mentioned, the tubes will bend first.


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PostPosted: July 23, 2014, 9:33 pm 
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That's true, I was using a theoretically perfect intersection of two cylinders. In real life, they get trimmed a bit short on 2 sides leading to an uneven sine wave. Point still stands that there is more linear weld length than a simple squared off weld, and as discussed it can lead to stronger joints. The realistics among us have brought up the point that well executed welds are strong enough regardless of specific geometry for our space frames. We don't want to be building in the realm where redundancy is so low that individual welds become absolutely crucial to structural integrity.


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PostPosted: July 23, 2014, 11:15 pm 
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esp42089 wrote:
. The realistics among us have brought up the point that well executed welds are strong enough regardless of specific geometry for our space frames. We don't want to be building in the realm where redundancy is so low that individual welds become absolutely crucial to structural integrity.


Give this man a cigar.

Any weld of such magnitude fails before the tube itself was most likely done by a 3 year old drunk and blindfolded.

This thread that has no bearing on anything this side of NASA.

*Warning The Surgeon General has determined that cigarette smoking leads to lung cancer. and giving alcohol to 3 year olds is illegal in 47 States*


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PostPosted: July 23, 2014, 11:58 pm 
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This thread that has no bearing on anything this side of NASA.

Sometimes people like to discuss topics and further knowledge for the simple reason of better understanding things. It's a philosophical pursuit that leads to questions like why not use research grade argon while welding (which has no bearing on anything this side of NASA), but makes a great thinking exercise about why NASA does this?

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Any weld of such magnitude fails before the tube itself was most likely done by a 3 year old drunk and blindfolded.

Every weld should fail before the tube; it should fail right in the HAZ. But I'm probably arguing semantics with you on that point.

Back on topic:
An interesting thing to think about: If we take a cross-section slice of the tube, and move towards the weld joint taking new slices, what would we see?
With a square-cut joint we would see unaltered metal, then suddenly ~100% large-grain, heat effected metal, and then finally alloyed material made of the parent steels and the weld metal. There is a section of very bad metal that makes up the entire cross section with a square cut joint that represents the most likely failure point.

With a fishmouth joint we would see the heat affected zone start at two opposing sides and slowly split and move around to rejoin at 90 degrees from where they started, never taking more than say ~30% of the total cross section. This means you have more good metal in any cross-section slice with a fishmouth than with a straight cut joint and this leads to greater strength.


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PostPosted: July 24, 2014, 12:41 am 
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esp42089 wrote:
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Any weld of such magnitude fails before the tube itself was most likely done by a 3 year old drunk and blindfolded.

Every weld should fail before the tube; it should fail right in the HAZ. But I'm probably arguing semantics with you on that point.


I've seen pictures of plenty of awful welds on home built cars that will fail long before the tube. It doesn't seem to be that unusual.

If the weld fails before the tube then you won't be able to use the entire strength of your structure. Its like using a single tiny screw to bolt each piece of roll cage tubing together. In an impact all of the tiny screws will break and you'll be left with a heap of tubes; pretty useless in terms of safety.

The weld should be the strongest part and its minimum KSI should be greater than that of the tube. When something fails it should be the tube right next to the weld's HAZ.

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PostPosted: July 24, 2014, 9:25 am 
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[quote="a.moore] The weld should be the strongest part and its minimum KSI should be greater than that of the tube. When something fails it should be the tube right next to the weld's HAZ.[/quote]

^^^^^ This is spot on for s good weld.

If it's a bad weld, either too hot or too cold, the weld shape and strength of the tube or strength of the filler don't matter.

FWIW in like sized parts, a 1" round vs a 1" square, the square tube is stiffer.

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PostPosted: July 24, 2014, 10:46 am 
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"Every weld should fail before the tube"

I must question that. The lab that I worked in before retirement had a materials section which, among other things, helped support the certification procedures for welders in the naval dockyard. Any welder who produced a weld that failed before the base material would have lost his ticket. To take an extreme case of process certification, the certification of submarine pressure hull welds used a 'bulge test' wherein an explosive charge deformed a hull plate weldment about 1/4 the diameter of the aperture in the anvil: again, if there was any failure in the weld (or the HAZ) the procedure failed.

If the weld fails you probably need more practice or are using an inappropriate process.

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