reinforcement of aluminum extrusion (strut like structure) with a composite.

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jamison1

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Greetings,

So I have an aluminum extrusion that is very much like a wing strut, although that actually isn't the application but the application of forces is very very similar. In any case this extrusion has been found as being too flexible and there is concern that it will crack. I would like to reinforce it with composite approach, ultimately weight is a minor concern vs strength for this part. I plan to fill the structure with urethane potting material for improved compressive strength and then strip the existing coating (2 part epoxy paint) prep with a wire brush followed by acid prep and then an epoxy adhesion promotor followed by a fiberglass or carbon fiber layup (maybe a combination of 1 layer of fiberglass and 1 layer of carbon).

What I am looking for advice on is the specific materials and techniques to use in making the layup. i.e. can you recommend an adhesion promotor, epoxy resin and laminating schedule. The existing structure is 4" x 1" in the shape of a naca foil with ~1/8" walls and 2 full length attachment bolts and is 6000 series aluminum.

There is a whole lot of information on the web about making composite structures with wood and other core materials but not much info when the core material is aluminum.

I should also say that this isn't part of a manned portion of an aircraft, it is part of a towed behind structure for a UAV experiment (research project) that will only be used over the ocean so failure won't put lives in jeopardy.
 

TFF

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This is just an opinion but why not make a fiberglass covered carbon spar put inside. 3/4”x 3/4”
With just covering the outside, any flex will probably make it peel. How much weight does this thing support? To me it sounds like it will need more layers than you think.
 

BoKu

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...I plan to fill the structure with urethane potting material for improved compressive strength...
Not a very good plan. Filling it will increase the stiffness of the local section, but will have negligible effect on column buckling, which is usually critical factor for a wing strut or similar structure.

...followed by a fiberglass or carbon fiber layup (maybe a combination of 1 layer of fiberglass and 1 layer of carbon)...
A single layer of carbon won't do anything for a 1/8" aluminum tube like that. And if you do apply enough carbon to make a difference, the aluminum is pretty much dead weight. You're probably better off starting with a foam core mandrel for the carbon.
 

jamison1

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Thanks for the replies. I am an EE by education so I haven't had any formal education on composites.

Part of the reason for filling the extrusion (and why an extrusion was chosen in the first place instead of a foam or other core material) is that there are several wires running through the center of the structure including 3 high voltage high current teflon jacketed wires. Filling the extrusion will stabilize these wires and also prevent water ingress. This structure is part of a central pontoon (in a sort of TRI pontoon arrangement) and supports the full weight of the craft (which is significant) durring takeoff and landing via a small hydrofoil and also has some sensors attached that are drilled through the leading edge and therefore weakening the overall structure.

Cost and timelines are also a factor. I am sort of committed to the idea of at least trying to reinforce the structure with a composite layup. If that doesn't work, no big deal. This is test vehicle.

What type of layup would I need to add significant strength to the structure? I was thinking 2-3 layers of 17 oz biaxial fiberglass.

Do you guys have any recommendations for an adhesion promotor to use on the acid prepped aluminum?
 

karmarepair

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Carbon fiber on an aluminum extrusion will rapidly dis-bond if there is any water about. This idea was tried with aluminum sailboat masts about 35 years ago. They were good for about a year before they would massively delaminate. Carbon fiber is SUPER "noble" on the galvanic scale.

Epoxy will stick pretty well to FRESHLY etched aluminum. Treat it with this stuff if you REALLY want it to stick 3M™ Aerospace AC-130-2 Clear BMS5-162 Type I, Form 2S Spec Surface Pre-Treatment - 50 mL Kit
Another trick is to wet sand the aluminum with "slow" catalyzed resin, and complete the rest of your layup before that bit cures, and you're faced with a secondary bond.

But you'll need to add enough laminate to make the aluminum irrelevant in any event. "load follows the stiffest path".
 

BJC

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Another trick is to wet sand the aluminum with "slow" catalyzed resin, and complete the rest of your layup before that bit cures,
That technique is new to me. Could you provide some examples of how and where it is used in HBA?

Thanks,


BJC
 

Hot Wings

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In any case this extrusion has been found as being too flexible and there is concern that it will crack. I would like to reinforce it with composite approach, ultimately weight is a minor concern vs strength for this part.
Is the concern strength or stiffness? Have cracks already been observed? If the existing part has been found to be strong enough, and the flexibility hasn't created any secondary problems, maybe cracking isn't really a problem?
Identify the highest stress area and then look at the SN curve to estimate the service life. It might be simpler to just have a spare available and inspect for cracks after each voyage/use.

Pot the wires in a seconday structure, like PVC tube, so the wires can be swapped into the new extrusion as a unit.
 
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wsimpso1

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Cracking? Harrumph. Just saying you are worried about cracking is not saying much. Is the tube seeing cyclic loading in tension, in compression, in both? Torsion? Is it seeing any external bending loads, or is the bending moment due to column loading alone? This is ME stuff and you really have to get into how it is loaded, number of cycles etc. For it to be loaded similar to a wing strut, it would have to see both tension and compression, and be defined by it compressive buckling loads. Is it? Look up Euler buckling of columns.

As to laminating composites over aluminum, that is just plain bad. Home shop level bonding of aluminum to anything is practically guaranteed to delaminate over time - aluminum oxidizes at the free surface, and migrates into the bond line, peeling it apart. There is no metallic aluminum in nature for good reason... Also, graphite fiber is conductive and will cause galvanic corrosion at the interface with metals, so the graphite fiber must be electrically isolated from metals. Then there is the whole issue of a composite thick enough to carry load and substantially strengthen the structure, the aluminum has become a heavy core. Oh, and just about anything polymeric placed inside is close to useless from a bending stiffness or bending strength standpoint- it might have other uses, but as a strength or stiffness improver, you might as well drop it from the analysis.

Better options are to design a composite tube manufactured over any manner of removable mandrels. There are discussions on this topic on here available by using advanced search tool. Everything from polished and waxed mandrels removed with big mechanical devices to plastic or glass tubes left in to dissolution methods have been used. Yeah, make it over an aluminum tube, and remove the tube by dissolving it with acid. Include my handle and "composite tube"in the search.

As to how thick, you are entering the realm of ME and AE, and composite mechanics is a senior/graduate level course, usually cross listed in ME and AE. We really learned stuff over there in ME and AE classes. I suspect you need an ME on your team. You have to define how much stiffness and/or strength you need, how long your column is, and then learn about column loads and buckling (Euler buckling). We can help with that, check your work, etc, but we are unlikely to do the work for you. We will however point you to relevant texts. I like Timoshenko and Gere for mechanics and Shigley for design.

One of the simplest methods is to learn how to specify Stiffness needed and the up gauge your aluminum tube to meet those needs. But recommend a laminate schedule over an undefined tube for carrying an undefined load? Would you specify cable if I did not specify currents, voltage, frequencies and max losses?

Billski
 

karmarepair

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That technique is new to me. Could you provide some examples of how and where it is used in HBA?

Thanks,


BJC
I learned about it in a boatbuilding context, and used it once to bond some hardware. The idea is to simultaneously mechanically roughen the surface, get the oxide layer off, and seal the bare aluminum before a new oxide layer forms.
 

wsimpso1

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That technique is new to me. Could you provide some examples of how and where it is used in HBA?

Thanks,


BJC
West System actually has a kit for doing the surface prep and bonding. Gougeon Brothers also recommend it in the book on wooden boat building, but there, IIRC, they also recommend backing it up with bolting, implying that the bonding is mostly to bed the device so shifting and water intrusion is eliminated or greatly reduced.

i won't do straight bonding, but I will bed and bolt where immersion is rare.

Billski
 

wsimpso1

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Oh, and the reason we can be so quick to tell you that your suggested scheme is not so good is because we have talked about this before. Feel free to use the advanced search function to find our previous threads on this topic.
 

opcod

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The West and boater used this, but with nothing compare of an airplane load. It's more an alu bushing is imbedded in epoxy and by surface contact it get a mass of epoxy over it and you cover fiber over it. Disbond will be and especially with carbon. So the main disbeleive is usually as: no, it will not happen after a 2 weeks period. But anyway, all fine people here told it's a bad idea with explanation. I guess the answer..
 
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