Bonding carbon and aluminum for structural application ?

General Composites Discussions Composites Talk
#1

I have some questions regarding strength of secondary bond.

I’m planning a build and it involves a swing arm for a downhill mountain bike.

I would like to know if bonding the drop outs into a carbon swing arm would be strong enough?

how much overlap would be needed or surface area contact between the carbon of the swing arm and the aluminum (or Ti) of the drop out for bonding?

(or should I bolt it all instead?)

For example in this picture the drop outs may be similar but inserted and bonded into a suitable female portion of the carbon.

#2

Bolts are pretty fool proof. If you want to bond look at MMA glues, like araldite, plexus, loctite, etc. Commonly used to glue airplanes or cars together.

#3

Hi Susho, thanks for the reply,

Ya bolts are quicker but I thought there would be a real benefit to bonding. I’m not sure how thick the carbon area would need to be for strength while bonding may essentially provide a part that could be considered “monolithic” in properties. I would actually be worried that the bolted area would be more prone to failure.

#4

Yes they can be sufficiently strong as Ive done it in a few frames. Just take the principles of design of a bonded joint into account and you will be fine - ie, load in shear where possible, ensure good mechanical and chemical surface preparation. Cant quite see the detail in that pic sorry

#5

Thanks findhan, thanks for the input! This picture is just an example I found on the web, not the actual part I’m considering bonding. But it’s a very similar idea.

I’m learning some basic CAD first, then I will hopefully get to the machining.

#6

Here is a link to a bike with bonded parts: https://3dprint.com/136843/3d-printed-mountain-bike-frame/

Notice how the carbon tubes are bonded on the inside and outside of the tube within the titanium pieces. Probably can’t get a better joint than that given the geometry.

#7

Reliable aluminium bonding is no joke.

Aluminum undergoes extremely rapid surface oxidation. On the order of significant percentage of the surface in seconds. This isn’t much of an issue most of the time because the oxide is stable and protects the rest of the structure.

It becomes a problem when we want to create reliable structural bonds. Almost all structural adhesives actually bond really well to the aluminium oxide, and the bond between the oxide and the aluminium is fairly strong, as well. Similar in strength to the base aluminium alloy.

BUT, and this is a big one, the oxide adhesion to the underlying aluminium has essentially NO fatigue life!

So your structure will test extremely well and fail suddenly, with essentially no warning.

The most reliable method is to prepare the aluminium with the phosphoric acid anodize process. The original specification was spelled out in BAC 5555.

You might be able to find a shop somewhere in the Seattle area to do it for you.

Bolting to composites also has it’s share of issues, but they are predictable and can be compensated for in your design.

#8

Thanks rrobb, that is great info. I was planning on anodizing any ways due to protection against galvanic corrosion aswell. The anodized layer will also protect more than just the oxide surface.

My other thought was to use Ti instead of Al. The Ti also produces a stable oxide layer at sea level conditions, much like aluminum does, but is even more stable if I understand correctly. (I deal with dental implants and we also rely on the oxide layer to allow stable and lasting integration with bone) The added benefit I thought Ti would afford is more protection against galavanic corrosion from the oxide alone compared to aluminum.

Any further insight you can offer regarding Ti?

Any suggestions on the best way to bolt carbon to an alloy like I’m looking to do? (my hesitation with bolting is the added complexity when it comes to galvanic protection)

#9

that’s a beauty but WELL beyond what I can take on :smiley:

#10

You’re smart to be cautious about the galvanic issues and titanium will lessen the worry, but not eliminate it. Some adhesives are actually designed to provide isolation, but the default is a thin ply of glass. Works well and is cheap.

Not all anodizing processes improve bond reliability, the PAA process is the standard.

I know it’s not as lightweight, or efficient, but the mechanic in me really appreciates bolted joints.

Sorry, I don’t have much time, but here are some links that might help;

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.epotek.com/site/files/Techtips/pdfs/Tech_Tip_24_-_Bonding_to_AL.pdf&ved=0ahUKEwjn2_uLvMXRAhWIj1QKHbN2DYsQFgghMAI&usg=AFQjCNHZzhB1afteNS9IImzWebuAyeHhCA&sig2=yH8jRDAzIR-p_KnhQiWkTg

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.materialstoday.com/metal-finishing/features/anodizing-for-aerospace-101/&ved=0ahUKEwjn2_uLvMXRAhWIj1QKHbN2DYsQFggjMAM&usg=AFQjCNGuE1h18GHviD2KhoeiSY773G9ioA&sig2=H5hlKBQsEbQTc-PJHmcXGQ

https://www.google.com/url?sa=t&source=web&rct=j&url=https://ocw.tudelft.nl/wp-content/uploads/Lecture_14_-_Fittings_and_Design_Guidelines.pdf&ved=0ahUKEwju6b28ucXRAhVr7oMKHVXcDaoQFgghMAM&usg=AFQjCNGT5J6kV3jPKjoyYlOFLsR2e9hPag&sig2=bEQNfkTq7F7WYVhUBo1zGg