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Adhesive replacing rivets or bolts

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12notes

Well-Known Member
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Aug 27, 2014
Messages
1,286
Location
Louisville, KY
I'm doing initial calculations on a "black aluminum" approach to my turtledeck and canopy, and have made some approximate calculations for replacing rivets and/or bolts with adhesive. This is probably completely unnecessary, and I would be better off just using the same number of rivets on the composite, but now it will bug me if I don't know.

What I've read, and I can't help but feel is incomplete, indicates that, given even distributions of the n rivets/bolts, if the tensile, shear, and peel strengths of the n * rivets and n* bolts equals the same respective strengths X bond area of the adhesive, then the bond should be equivalent. Is this accurate, or at least good enough to be within, say 10%? Are there other factors that need to be taken into account in the calculations?

I'm also assuming the adhesive is appropriate for the materials, conditions, and exposure of the joint. 3M DP420 was what I had in mind specifically, but I'm really curious about this in a general sense rather than specific calculations based on that. I'm not asking anyone to do my math homework for me.

I'm only looking for a good approximation, so factors which change the strength 1% ish, while they are a welcome addition to the post, are essentially statistical noise in this phase of my design. This is a first pass of answering "Can it be done with a FOS of 3 with an acceptable weight gain?" rather than "What's the optimum way to do it for a FOS of 1.5?" If it passes this sniff test, then I'll move to the second question. The turtledeck redesign is mainly for looks and to add a lip in front to easier seal the canopy opening - I do not expect any weight savings or strength advantages here. Ultimately, I'm more likely to use rivets, but am interested in this as an alternative.

The other item I couldn't easily find information on how the differing rates of thermal expansion of different materials affect the whole, specifically an adhesive joint as opposed to one that is riveted/bolted. Let's say I put a carbon fiber turtledeck on an aluminum skin fuselage. Would the ever cause a noticeable problem on a small plane, or is this not a significant factor? The number of planes that seem to just slap one on would indicate it's not significant, but maybe there's way more math than it appears behind some of these modifications, or there's an internal hat section longeron on the bottom I can't see. The plane I'm building is only 13 feet long, turtle deck about 4', my gut says that it wouldn't move enough to matter, but actual experience beats my gut every time.

Would an adhesive be better or worse for thermal expansion than a line of rivets? It seems that the rivets would concentrate the load on their shafts and try to open their own holes up, but the adhesive would distribute the load better, especially if it has a little bit of flex in it.

EDIT: I forgot to mention that a layer of kevlar/fiberglass would be at the bottom of the carbon to prevent galvanic corrosion.
 
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