Talk Composites - The Forum for Advanced Composites

Best way to joint fabricated carbon strut structure.

http://www.talkcomposites.com/Topic7003.aspx

By Tobias - 7/8/2013 4:03:37 PM

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. 

Thanks,
T
By Tobias - 1/14/2014 7:37:25 PM

For future reference, I've just had confirmation from customer service at Permabond that the correct adhesive for structural bonds between, for example, metal and composites is their ES550 and ES558. They are described as:                
         
            
               
                  "single-part epoxy pastewhich flows like solder when heated during curing.The adhesive is toughened for maximum impactresistance, along with excellent peel and shearstrength. ES558 is ideal for bonding a wide range ofmaterials including metals, ferrites, ceramics andcomposites."

ES550 is a non-flowing adhesive, whereas the 558 will flow through a capillary joint.

ET515 is described as:

"ET515 is a semi-flexible toughenedadhesive with good adhesion to a variety ofsubstrates such as wood, metal, ceramics and someplastics and composites. It’s a relatively fast curingepoxy; reaching handling strength in 15 minutes. It isideal for bonding different materials wheredifferential thermal expansion is anticipated."

This confirms my conclusion that the ET515 is best suited to situations where a bond between large surfaces which have differing tensile qualities, for example adhering a wood veneer to a piece of spring steel (if you can find an application for such a purpose). Comparing the shear strengths between the adhesives also confirms my findings: ET515 Mild steel 8 - 12 N/mm2, ES558/550 Steel 27 - 41 N/mm2.

Thankfully I don't think I had any failures due to leftover release agents. Most of the structure was built from pultruded tube, and all of the failure points were between the composite and metal components of the structure. My revised version of the structures will use different components and adhesives for sure.