Discussion in 'Composites' started by autoreply, Dec 10, 2010.
Can anybody recommend a US company that can do composites testing to get mechanical properties?
P-C, A composites Engineer can probably give you what you need, or a Company that employs theses engineers.
Hey, total noob question about fiberglassing and foam. I was thinking of a more "automated" way to fabricate parts. I read a post somewhere that said the foam core is just used to "form" the part, not really an element of strength. That said, could one take the BID and UNI fabric, do a layup without the foam core and then have a 3d router cut out multiple pieces out of a really big, say 4'x8' layup?
If you are talking about a sandwich panel (with a fiberglass and epoxy layup on each side of a foam core) then the foam core is definitely not there "just to form the part". The foam is critical to the stiffness of the part.
Yes, you could use a router to make flat parts from a 4x8 sandwich panel. The edges would need to be closed out.
Do some more reading on the function of the cores. Two types of cores are common:
Massive foam cores, usually hot wire cut from blue extruded styrene foam, provide the shape of the part and support the laminated fiber-resin skin against buckling. This is sometimes described as sandwich core construction, where the fiber-resin facings are the outermost part of the structure and the core bridges from skin to skin. A wing, stabilizer, or control surface made this way has one continuous very low density rib that covers the entire span of the part;
Sandwich cored skins, with fiber-resin facings on both sides of a core. These are made on some sort of a mold. The mold can be anything from a flat table (caul plate) to a flexible plate that is shaped to suit for each part, to a dedicated mold for a specific skin. These tend to be skins - that is the part being made is one side of a larger, "hollow" part. The skin is made up of fiber-resin laminate, core, then another fiber-resin laminate. The laminates, the core, and the internal ribbing of the larger part all interact, and must be considered together in design.
Fiberglass laminates without cores are only rarely used in little airplanes. Primary loads are easily carried with pretty thin laminates. Typical 160 -220 mph airplanes use 21-22 oz/yd fiberglass fabrics, about 0.025" thick when laminated. Sailplanes using carbon fiber use one ply of 6 oz fabric on each side of 1/4 cores. With that level of fiber-resin on the outside, a foam core, either of massive type or of sandwich type are used. No core with just the thin outer facing will usually require either frequent ribs, frames, etc all to prevent buckling (large scale buckling, wrinkling, and crippling) or they require much thicker laminations. Either way the light way to structures in our size of airplanes is cored composites of some sort. WEIGHT IS THE ENEMY, so we do things the lightest way that is both practical and strong enough.
For things such as cowlings, some folks have laminated 4 plies of BID cloth with no cores, but they still require substantial internal bracing and reinforcement. Some kit aircraft use cored skin cowls much like the rest of the structures in composite airplanes - these are the lightest. The factories for certified aircraft have used laminate cowlings with much more thickness and significant other structures internal to the laminations.
You will generally not see foam glassed on only one side. It is hardly any stiffer than with no foam. I have seen people propose structures where most of the part has a core, but areas have no core where the piece is bent. This can work for making a control surface or a fuselage (look up Vision aircraft's "fold-a-plane").
This is done to manufacture many pieces inside an airplane. My wings are "hollow" type as there are equipment bays and fuel storage in them. There are ribs at each end of each panel, to close out fuel tankage, to give places to anchor equipment, etc. There are also longitudenal structures to assist in tying upper and lower skins together and to close out the aft end of the wing. These many pieces were all made by laminating large sheets of foam with fiber-resin and vacuum bagged to minimize weight, then sawn out and bonded in with dry micro and fiber-resin tapes. Some places sell laminated sheets of foam reinforced with fiber-resin facesheets for this purpose.
As the saying goes, "there is nothing new under the sun". I have been on the patent committees of a couple large companies. Lots of "proud parents" presenting "babies" that are closer to being "senior citizens" in that piece of my work.
Correct about the stiffness double the thickness and you get 4 times the stiffness, however Foam also transfers loads between inner and outer skins. Last-A-Foam (LAF) has a very good density strength but poor surface adhesive strength because of it's crumbly nature - but good for sanding, whereas European Type foams like Divinycell and Kleggecell are heaps stronger than LAF, and on paper H60 is way better than LAF but in reality, it doesn't have the density strength as LAF, but the next grade up H80 does. There is a small penalty in weight for the H80, but I tried H60 and was very unimpressed.
Now in Kleggcell there is an intermediate Grade, I can't remember exactly but perhaps 70 it has good density strength, the problem with Kleggecell is that it outgases at a lower temperature. Not something you want for post curing.
Both Brands are owned by Diab now.
Good grief, how did you all learn so much about this stuff?
Hey another question. What if one could get a 5d milling machine (a really big one) to cut the entire "skeleton" out of foam as 4 quarters and get the longerons all formed from the same foam as the skin. Then you glass each piece with the different fabric like bid and uni in their prescribed directions and quantity, that should reduce build time considerably right?
The mechanics of solid, plates, shells, and composites are all taught in engineering schools in departs with title like Mechanical Engineering and Aerospace Engineering. There are textbooks and other books out there on the topic. I have three engineering degrees, and can go from concepts to detailed calculations on these topics on a number of these topics. Engineering is real and allows us everything from space probes and satellites to CNC machine tools and additive manufacturing to our computers and cell phones. Have some respect for the effort that went in the training to do this stuff.
In a world where many folks select "easy" paths and engineering students are looked down upon as geeks or worse, we still seem to be living in a world full of the creations of those same people who chose to do things that were "hard", like math and engineering, not to mention folks who learned skilled trades or otherwise do the hard things that need to be done.
Just yesterday, I chided a member briefly on how poorly formed inquiries rarely excite volunteers to put big significant effort into responses. This is such a case... Please consider thinking your way through the ideas you have and put in an amount of effort in the formulation of the inquiry that is similar to what you expect of the response.
So, what if? Responding to a minimal effort inquiry with a similar level of response detail - The machines exist; Your scheme is most likely to require more total build time and cost than conventional methods; and it is unlikely to succeed in making parts to correct dimensions and shapes.
Your homework is to handle some of the foams we use or at least view some videos of folks working with it, then view some videos on foam fabrication and laminating with cloth and resin. There are videos out there from the Rutan Aircraft Factory and Mike Arnold on building his AR5 that can be found pretty easily. I suspect that once you see these, you will see why your proposed scheme is so unworkable, but we can talk about it more once you have looked into the conventional processes and understand them.
Design of an aircraft is very difficult, design of a composite aircraft is harder than from wood,aluminum or steel. Designing a cost effective new process to build aircraft adds even more difficulty. Burt Rutan and Van have succeeded in doing that so if you are in their league skills wise I think there might be an opportunity to make a contribution.
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