Bending moments transfer through adhesive bonded joint

[adampar]

Hi respected engineers!
I face a design issue in which i would like assistance due to my lack of experience. I want to connect a CFRP tube with metal inserts at each edge (the metal fittings are of smaller diameter than the CFRP tube) and the proposed joint is an adhesive bonded one. After searching on the net, I found that these joints are mostly used to transfer axial-torsional loads. Some preliminary calculations done, have shown that the peel and shear stresses are too high to sustain a bending load, as well as the out-of-plane strength of the CFRP tube is also low. Just to provide more info, the tube will act as a cantilver beam of approximately 600mm, and a max moment of 60Nm must be tolerated. What type of joint would you advise me to use to transfer this moment? Could this be done with some alternation of the design and still use adhesive a the connnection or is it mandatory to use a mechanical type connection?

 

[rapt]

You probably have not received a response as no respected engineer would allow a connection to rely on adhesive bond, whether it ne direct tension or bending!

 

[OldBldgGuy]

Rapt better not talk to anyone in the wood-I, SIP, EWP, or OSB business...
Yes, their materials are more conducive to adhesive bonding than adampar's problem but there are plenty of guys whose lives are spent developing adhesives to do all sorts of engineering applications. I would suggest the wrong people have looked at the post so far.

 

[rapt]

OldBldgGuy,

I will change that to "in my experience in concrete structures". And yes there are plenty of people pushing connection solutions in concrete structures as well that I would never accept as an engineer who respects himself!

 

[SWComposites]

to OP:
1) this forum is mostly populated by civil engineers; suggest you delete this post and repost in the Composite engineering forum
2) you should first size the tube to take the bending load, then work on the end fittings
3) depending on the application, type of loads - static or fatigue (# cycles), size of tube, and bunch of other things, an adhesive joint might work
4) but you should plan on fabricating some parts and testing them to get experience with the materials and failure modes

 

[rapt]

Can you aerospace guys let us poor structurals know which commercial planes use adhesives in this way so that we know which ones not to fly on please!

 

[racookpe1978]

Adhesives have been used in aircraft composite construction since WWII's mosquito bomber was laid up with plies of laminated wood and cloth. The hexagon honeycomb material long used in aero applications since the mid-50's is another composite, as are the Kevlar overlays used in aircraft, spacecraft, and the high-end cars for lightweight but strong parts. Expensive too, but that's why such composites are not used for buildings.

Standard steel and concrete are simply cheaper, easier to fabricate without clean rooms and climate-controlled heat, humidity, and wind limits.

For this, I agree: Size the cyliner for the loads, then insert the end pieces into the hollow cylinder. Do not take "credit" for the resistance to bending of the two inserts, but check that the bending resistance at the end of each insert does not cause a "kink" (stress riser) in the tube. (You'll have a stiff end, then a sudden transition to the more flexible middle section. )

 

[adampar]

Thank you all for your suggestions. The discussion has been transfered to the Composites Forum (

http://www.eng-tips.com/viewthread.cfm?qid=408316).

Please let's continue our discussion there. I will delete this post soon.