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Tobias
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Hi there, I am pretty new to working with carbon, but have a good background in other materials. I am working on designing a strutted framework, fabricated out of 5mm pultruded carbon tube, and I thought it best to seek some advice about how best to make the joints. My plan is to build a jig which holds everything flat, and use a braided sleeve to form a reinforced skin. However since I also plan on painting the structure, I don't necessarily need the braid for aesthetics, however I also don't want 'spider knees' at each join. To simplify things, lets assume i am building a small chair, with some structural triangulation, which needs to hold about 10kg ( so in real world terms I guess it has to be over-engineered to not fail under 50kg, or something of the kind ). In my crude drawing I have 3 potential jointing processes, but I wonder if there is another I have not considered, or if I would be better off making a mould and a core, and using a cast component joined by tubes, rather than attempting to join struts together.
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Warren (Staff)
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The strongest of the 3 is the tube joiner in aluminium. Such inserts are commonly used to join carbon tubes. You need to ensure the insert extends a fair way into the tube to spread the load at the joint. Also with pultruded tube, the insert cannot be too tight otherwise it will split the tubes. You will find Roll wrapped tubes are stronger in practice for this kind of application due to the extra strenthening around the tube.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Tobias
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Hi Warren, So if you were to build such a structure, you would work with alloy inserts and a braided sleeve, or is there another approach that you would prefer? And should the inserts be glued in using the same epoxy resin as the wetting, or should they be made of a more flexible epoxy?
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Warren (Staff)
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They should be bonded in using a structural adhesive like our ET515Semi Flexible 15min Epoxy Adhesive The braiding you want to add will just add further strength but isnt essential if you use inserts.
Warren PenalverEasy Composites / Carbon Mods - Technical Support Assistant
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Tobias
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It would appear that the ET515 has no structural value at all, as it seems to neither bond to degreased aluminium nor to roughed carbon, as in I can scrape it off either with my fingernail after three days of curing. I only discovered this when I had a catastrophic failure of a rather beefy join. I kind of feel that the ET515 is no stronger than Araldite, no more nor less rigid, and in a similar manner is no more respective of its apparent 15 minute working time. Warren (Staff) (11/07/2013)
They should be bonded in using a structural adhesive like our ET515Semi Flexible 15min Epoxy Adhesive The braiding you want to add will just add further strength but isnt essential if you use inserts.
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carbonfibreworks
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Hello Tobias I have used the above adhesive on the top fairing mount on my classic racing motorcycle, i used it to join 10mm carbon tube to a piece of pre preg plate . I abraded the parts to be bonded with 180 grit paper and the part is under load and the joint has been 100% reliable. Regards Chris carbonfibreworks.co.uk
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Tobias
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Hmm, this was a 20mm carbon tube, and the join was to a strut coming in below, by means of an aluminium sleeve which wrapped around the tube like a clamp - made from 20x2mm stock.
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Shaneer22
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When bonding any metal I still use the wet sanding technique using 240 wet and dry sometimes 120 and sand metal with epoxy prior to bonding it really etches into the metal surface,belt and braces on my part.
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carbonfibreworks
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I will post an image of the part on Friday. Regards Chris carbonfibreworks.co.uk
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Warren (Staff)
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how big is the gap?? I have used ET515 personally for bonding inserts and its been a good bond. You need to key the alloy properly. Some people go as far as putting grooves in or even knurling the end to be inserted.
I guess you are using roll wrapped tubing?? Almost all roll wrapped tubes will come from the factory with a heavy coating of release agent still inside the tube, so you really need to ensure it is gone to get a good bond.
Some photos would be ideal.
If you're still having problems getting a good bond after that, then return us the glue and we will test it. It is theoretically possible to have a bad batch, although this has not happened to us yet.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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