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Massimiliano
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Hi, I have the structure below which, under solicitations, is bending a bit towards the direction of the red arrows. Pipes are 40mm diameter. I was thinking about moulding two half pipes composed mostly of several layers of unidirectional carbon tape and then glue with mma the half pipe to the existing pipes (along the orange lines). The layup I am thinking is 10x layers of 50mm wide 650gm2 carbon unidirectional tape, alternated with some 50mm wide plain carbon tape. What do you think about the idea and the layup schedule? Also, maybe adding a 2 mm pvc core in between would help? I am just worried that the shear force could cause the two carbon frames to slip on the pvc core, so maybe a not cored layup would be better? 
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Lester Populaire
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+xHi, I have the structure below which, under solicitations, is bending a bit towards the direction of the red arrows. Pipes are 40mm diameter. I was thinking about moulding two half pipes composed mostly of several layers of unidirectional carbon tape and then glue with mma the half pipe to the existing pipes (along the orange lines). The layup I am thinking is 10x layers of 50mm wide 650gm2 carbon unidirectional tape, alternated with some 50mm wide plain carbon tape. What do you think about the idea and the layup schedule? Also, maybe adding a 2 mm pvc core in between would help? I am just worried that the shear force could cause the two carbon frames to slip on the pvc core, so maybe a not cored layup would be better? I think a much better approach would be to have a shear web in-between at least two of the tubes. either made from carbon with a plus minus 45° layup or arguably just from steel welded in. This way the hole structure becomes an I-beam like structure instead of acting like single tubes which would massively increase the fore-aft stiffness. wouldn't do much about torsion (neither would your approach tho). ciao
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Hanaldo
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Yeh it looks like a terrible design for rigidity in that plane, it's got 0 bracing for it. Must be quite strong in the opposite direction!
But yes, the solution to this is more about bracing the structure properly. It would be quite basic maths to work out what the approximate bending loads must be to be causing 40mm steel tubing to deflect, and then from there it won't be hard to work out that to make it sufficiently strong with carbon shells is extremely inefficient. What it would cost to make those shells vs what it would cost to weld in another steel bar, its a pretty obvious choice.
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Massimiliano
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Hi, I should have specified that it is not stainless steel unfortunately but aluminum, this is one of the reasons it is bending and also why I thought about a thick carbon shell on the existing pipes.
Dismounting the structure to weld other aluminum is almost undoable because all of the wirings and structures attached to jt
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Hanaldo
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Doesn't make a great deal of difference, the issue is still the geometry of the structure. It needs horizontal bracing or gussets. Really the big problem with it is the way they have placed bent tubes together, they're not using the tensile strength of the tubes very well. If that outer tube on the right had continued straight down rather than being bent back and joining to the middle tube at a horrendous angle, and then they had added a horizontal brace or gusset to connect them then they would be utilising the tensile strength of the tubes much better and it would have been far more rigid. As it is, the main tubes on the left are working in tension, and the other tubes are really doing more work in compression which is no good.
The process for welding vs the process for structural bonding isn't hugely different, so I still feel like it would be close to the same amount of work for either method. If you can't disassemble the thing or remove the wiring, how are you going to prep it properly for adhesive? Much easier to prep a 50mm area on each tube for where the new tube will be welded on than to prep the entire tube in order to bond a new shell on.
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Massimiliano
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Group: Forum Members
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+xDoesn't make a great deal of difference, the issue is still the geometry of the structure. It needs horizontal bracing or gussets. Really the big problem with it is the way they have placed bent tubes together, they're not using the tensile strength of the tubes very well. If that outer tube on the right had continued straight down rather than being bent back and joining to the middle tube at a horrendous angle, and then they had added a horizontal brace or gusset to connect them then they would be utilising the tensile strength of the tubes much better and it would have been far more rigid. As it is, the main tubes on the left are working in tension, and the other tubes are really doing more work in compression which is no good. The process for welding vs the process for structural bonding isn't hugely different, so I still feel like it would be close to the same amount of work for either method. If you can't disassemble the thing or remove the wiring, how are you going to prep it properly for adhesive? Much easier to prep a 50mm area on each tube for where the new tube will be welded on than to prep the entire tube in order to bond a new shell on. Totally agree on the bad design but unfortunately I bought it already made. The aluminum is powder coated, I read that mma bonds very well to powder coated so I thought to just do a proper cleaning. I will also analyze the points of welding, if no wiring could be affected by the heat that could be feasible. Thanks for everyone's usual kind advices!
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oekmont
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If possible you might consider to add some plates to the sides, at least around the knot section in the middle. They would take the shear forces and would change the stiffness of the structure drastically. no need for welding, a few bolts would be enough.
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Massimiliano
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+xIf possible you might consider to add some plates to the sides, at least around the knot section in the middle. They would take the shear forces and would change the stiffness of the structure drastically. no need for welding, a few bolts would be enough. This could be doable too, thank you
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Massimiliano
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I contacted the guy who originally made it and he is suggesting this addition (highlighted in red) made out of ss and bolted to the existing structure. What do you think? 
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Lester Populaire
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the plates on the sides was exactly what i was referring to with the shear webs, which would be far superior to the proposed tube that is bolted on.
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