Metal and Composite detail design 3

[Gary70]

Hi,
Is there any design rule of thumb when designing a metallic (Aluminum or Titanium) and Composite detail part? For weight saving, if we change the metallic bracket (.063" Al. thick) to composite bracket assuming that the applied load is the same, what should the thickness of the composite part be?
Thanks.

 

[KENAT]

Well that's going to depend on various factors relating to the form of the bracket, direction & line of action of loading etc.

Posting guidelines faq731-376

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[GregLocock]

Practically how much thinner can you make a .063 part, from fibre and resin?

If you were serious you'd redesign the entire shape of the part.



Cheers

Greg Locock


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[rb1957]

how much weight are you going to save in a 0.063" brkt ?

remember Airbus's lesson (about being penny-wise ...)

Quando Omni Flunkus Moritati

 

[WKTaylor]

Maybe best to consider injection molded plastics. Certain reinforced plastics tend to be rigid/strong/tough, such as reinforced polyamide [nylons] or Polyamide/imide plastics for higher temps/toughness. For starters, check-out MIL-HDBK-700 [plastics] and MIL-HDBK-797 [nylon].

Typically, layed-up composite manufacturing practices are impractical for small parts like this. Results in voids/wrinkles/disbonds or wet/dry pockets... all bad.

I agree with rb1959... 0.063 thick metal bracket is hard to beat for simplicity/lightness; W/WO integral formed stiffener(s) and/or "lightening" [weakening] holes for weight savings.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[Gary70]

Supposed this part is a large Aluminum panel (hatch cover) 20" long X 6" wide X .063" thk , I think we could save more weight by changing it to thicker composite (fabric and tape plys) panel. In that case, what's the thickness ratio of the composite panel should be relative to the metallic panel assuming that the applied load is the same.
Thanks for your response.

 

[berkshire]

Now you are talking a different animal to a simple bracket.
The single skin aluminum panel will have more stiffness than a single skin advanced composite panel.
You can increase the stiffnes of the plastic composite part by separating the plys with a core.
Or you can do the same thing with the aluminum panel by reducing the skin thickness and adding a core, like aluminum honeycomb with a back skin glued together,
By the way this is also referred to as a composite panel, its just not an advanced composite.
You can also use chem etch to get a panel that is stiff and light, you put reinforcing ribs in, and etch out the stuff you do not want.
B.E.

 

[blakmax]

Berkshire

I have never understood why the terminology "composite" is applied to describe both the material form (i.e. a fibre composite) and composite to describe what is "composite construction". Frequently the same terminology is applied to mean adhesive bonded structures other than sandwich structure. This confusion extends to such documents as the FAA Advisory Circular AC 20-107B, where they even describe the certification methodology for "composites" (meaning both the materials and the method of construction on the basis of the one building block approach. Why? The methods for design and manufacture are entirely different as are the failure modes. The only similarity is that the materials start off sticky until the material is cured. While it is convenient that fibre composite and bonded structures can be certified in the same manner, there actually are better and far more economical methods for certifying bonded structures in a different manner to fibre composite structures.

One of the consequences is that the frequent poor performance of badly manufactured bonded structure is translated in a manner which denigrates the excellent structural advantages of fibre reinforced plastics.

I personally have adopted the terminology "composite" to mean ONLY the reinforced material, and describe honeycomb structures as sandwich structures.

Regards

Blakmax

 

[berkshire]

Max, the term predates plastic reinforced structures. A wood, steel tube, and fabric aircraft has been referred to by the CAA then later the FAA, as a composite aircraft since the 1920s. The original distinction was composite aircraft( made of more than one material.), and all metal aircraft. In fact my first repair station licence was for composite structures. I specalised in repairing fiberglass sailplanes, and it was not referring to them, it was referring to the wood and fabric aircraft I also repaired.
I always used the term "advanced composite" to describe a plastic reinforced fiberglas structure. Then Aramid fibers and carbon fibers came out and muddied that term some what, although I still use advanced composite to describe an item made from a resin with a reinforcing fiber of some sort added to it before the resin sets, either by chemical reaction, or temperature.
B.E.

 

[rb1957]

"Supposed this part is a large Aluminum panel (hatch cover) 20" long X 6" wide X .063" thk , I think we could save more weight by changing it to thicker composite (fabric and tape plys) panel. In that case, what's the thickness ratio of the composite panel should be relative to the metallic panel assuming that the applied load is the same."

sigh, we start talking about a brkt, only to find out you're talking about a hatch ... sigh

i don't think there's any rule of thumb for thickness, 'cause your ply lay-up changes everything. most important, is this secondary structure ? second, 20x6x0.063x0.1 = .75 lbs ... there still isn't much weight savings to be had. third, a composite panel will probbaly be much more expenseive than a s/m one.

one place where composites really have an advantage is if your panel has riveted stringers ... replacing with a sandwich panel saves a lot of manufacturing time.

Quando Omni Flunkus Moritati

 

[blakmax]

Jeez, Berkshire. You must be old! I thought I was the only person who worked on horse drawn zepplins! } (The } is my beard.)

But thanks for the explanation. I still think it is time we made the distinction clearer though. The penalty for certification of bonded structures as if they were "composite" structures is very significant.

Gary70, (no disrespect intended) essentially your questions amount to "If I change the hub-caps on my Ford, will it make it perform like a Chevy?"

My advice to you is to either undertake some self-study in composite design or else engage a knowledgeable specialist who can help you. Composite design is not just how thick or light the structure may be, it is also clouded by different failure modes, stiffness requirements, complexity associated with unbalanced or non-symetric laminate configurations and especially selection of fibre forms (unidirectional or woven) and production methods (vacuum bag or autoclave) and that is not even considering selection of resin types and cure temperatures.

As Berkshire already pointed out, even a change in design concept can result in a far better performance than just replacing a metal with black composite.

Regards

Blakmax

 

[berkshire]

Max,
I think I have a few years on you, I am 71 and have been repairing or constructing fiberglass/ advanced composite aircraft and their components since 1971 and wood and sheet metal ones since 1962.
B.E.

 

[Compositepro]

Blackmax, structural engineers have their own definition of composites.

http://en.wikipedia.org/wiki/Composite_construction

You will see this if you go to the structural engineering forum.

 

[SWComposites]

Gary70 - "Supposed this part is a large Aluminum panel (hatch cover) 20" long X 6" wide X .063" thk , I think we could save more weight by changing it to thicker composite (fabric and tape plys) panel. In that case, what's the thickness ratio of the composite panel should be relative to the metallic panel assuming that the applied load is the same. "

> you have provided no where near enough info to answer this question:
- what is the current aluminum alloy?
- what are the type of loads on the part? pressure, in-plane, combination, other?
- how it is attached?
- what are the design temperatures?
- what other design requirements are there: impact, lightning, sealing, etc?
- what composite material do you want to use? carbon fiber, fiberglass, other?

 

[WKTaylor]

Gary70...

In the second case You described, You MUST determine whether this panel is...

non-structural, IE: "covers a hole"; if missing/failed would NOT affect structural integrity of the aircraft.
(or)
structural, IE: carries substantial load thru it from surrounding structure; and the structure would be crippled if the panel was missing/failed; (or) has some other purpose that would be ccompromised if it was missing/failed.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.