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Read first, Composites FAQ

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autoreply

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This FAQ is maintained by Autoreply. Please respond in this topic with any proposed improvements. Take note, that – after your post has been taken into account – it will be deleted. Complete suggestions are highly appreciated. So not:
Yeah, I'd really like to see a link to a site, explaining this and that
But:
"This is better, compared to that in terms of beer, whiskey and genever
In the second post you can have a look at the "to-do-list" of this thread.

The aim is to give short, factual information about composites and provide the best links to read further on. Links in the text are explaining the mentioned term, articles below text link to internal (HBA) and external in-depth discussion.
Discussion should only be focused on improving and correcting the information and links in this post. This text is free of any copyrights, limitations or other nonsense. Use it, however you like. See attachment for a PDF of this post, though mind that it's not always the most recent one.

About composites


Composites in general consist of a combination in materials, usually the matrix (resin) and the fibers. Sometimes foam or honeycomb is used to increase the thickness at the price of a low weight gain, resulting in much more bending strength and stiffness. In-plane strength is barely affected, though it raises buckling strength enormously. We usually use sheets of woven/knitted fibers, but single-fiber-direction tape is also used where a high specific strength is required.
Flox (a mixture of cotton fibers and resin) is used as a glue-like substance, or to fill the corner of orthogonal surfaces. Microballoons (mixed with resin) are used as a filler, usually for painting.
All resins are sensitive to UV-radiation and thus need a 100% UV-block. Gel coat and PU-paint are the most common. For colors, other than white, special high-temperature resins are required, while foam should be able to withstand the higher temperatures too.
Fibreglast – Learning center
Fibreglast - General explanation about composites


Fibers

The most common fiber materials are glass fiber, carbon fiber and kevlar. Glass is relatively cheap and has strength, stiffness and weight, very roughly comparable to aluminium. Carbon fiber is roughly 3 times stronger, much stiffer and has almost the same density, about 20% lower fiber density, compared to glass. Because of it's stiffness it's more susceptible to impact damage. Prices are much higher, compared to glass, usually a factor of 5-10. Kevlar is close to carbon fiber in strength, price and weight, but notoriously hard to work with, especially to cut. It's mostly used in impact-carrying structures, like a safety cockpit.
Both strength and stiffness vary hugely, dependent on manufacturing method, fiber direction and fiber type. Manufacturers quotes can usually be considered as marketing and testing with the chosen manufacturing methods is required. Within a certain fiber, strength can vary as much as a factor of 3, with different manufacturing methods and fabric type.
The weave is usually defined by numbers, but their meaning may vary. Unidirectional (UNI) is strands of fibers in one direction, loosely tied together. This has very little strength, in the other direction. Bidirectional (BID) is woven with an equal amount of fibers in vertical and horizontal direction and much better at shear and distributed stresses. Satin is more drapable compared to BID. Knitted fabric has more resistance to draping over non-flat shapes, compared to UNI and BID.
Besides cloth, other forms of fibers are also used. Tape and rovings are (almost) unidirectional, mainly used in concentrated stress areas, like the landing gear, cockpit and spar. Graphlite is a special product, made from carbon rovings, and it's famous for it's high strength and stiffness.
HBA - Graphlite structural bending
Fiberglass - discussion of different weaves


Resins

There are 3 basic resins, in order of increasing strength: Polyester, Vinylester and Epoxy. Polyester is not used in aircraft construction, because of it's low strength, it's tendency to keep shrinking and it's deformation during aging. Epoxy is stronger, compared to Vinylester, but also more expensive. Many vinylesters are able to withstand very high temperatures without requiring a high-temp cure, like an oven. Epoxies don't have that advantage and usually require a high-temp cure to be able to withstand higher temperatures during use.
This makes the aircraft capable of withstanding much higher temperatures, so they don't need to be white and the structure can be used in hot spots like the cowling/exhaust. Vinylester seems to be a bit more resistant to vibrations and is much more resistant to many chemicals.
HBA - Laminating resin and resin prices
HBA - Composite fuel tanks and ethanol
Fibreglast – resins

Foam

Sometimes foam is used as a core material, placed between sheets or layers of laminate, for the purpose of increasing a particular structure's stiffness. For skin structures this added rigidity provides increased panel stability; in the case of beams in bending the larger cross section increases bending stiffness; and in the case of columnar structures, the thicker section increases the member's ability to carry higher loads before onset of buckling.
Many different foams are used. Polystyrene (hotwire-able, cannot withstand fuel or vinylester), extruded Polyurethane (can be hot wired, highly toxic and carcinogenicity during hotwiring, can withstand fuel and vinylester), expanding Polyurethane (will warp it's shape even after curing, otherwise like extruded PU), and Polyvinylchloride (PVC, very strong, can withstand fuel, vinylester, cannot be hot wired).
HBA - Last a Foam vs PVC
HBA - Jan Carlsson - comparing different foams

Design


Composites behave orthotropically; they don't have equal strength or stiffness in varying directions. Classical lamination theory describes it's behavior. This can be approximated by assuming mechanics of materials and varying the strength and Young's modules, dependent of direction.
Clearing the mold, resistance to fuels and water vapor and stress concentrations are major concerns. Galvanic corrosion needs to be accounted for, when using carbon. Ethanol in mogas is another major concern.
An error often made is using a different material to “reinforce” a given design. This usually leads to high stress concentrations in unexpected places.
HBA - Spar placement, design considerations
HBA - Spar-less wing construction
Composites world - Rough overview of composites mechanical properties
Composite world - Discussion of several material property databases
Alexander Schleicher - Spar construction and layout
NIAR - AGATE material properties database

Production

There are many techniques for composite production. 2 steps are important. The way the composite is “shaped” and the way the lay-up is made. The layup can be shaped via various techniques.

Molds

  1. HBA - Best plug making technique
  2. HBA - Mold making thread
  3. HBA - Homemade CNC-router
  4. HBA - Billski on female mold construction
  5. Canard - pictures of a home-made oven
  6. Palatov - description of making an oven
  7. Fibreglast - mold making
  8. AN2 - Designing and building a float plug
  9. Jet Aerospace - mold making gallery
Mold-less

  1. Fram.nl – moldless flat construction.
  2. HBA - Plug and mold vs moldless construction
Lay-up methods
Vacuum infusion/pressure assisted

  1. Vacuum infusion/pressure assisted
  2. Fram.nl – vacuum infusion
  3. Easy-composites video tutorial on vacuum infused parts making
Hand lay-up

  1. Hand lay-up
  2. DG Flugzeugbau - Sailplane manufacturing
  3. Wet lay-up tube making followed by wet-bagging
Pre-pregs

  1. Homemade pre-pregs

Surface finish

Wayne hicks – composite finishing
George Sychrovsky – finishing a composite airplane
HBA - Gelcoat pinhole problems
HBA - Laminar surface finish
DG Flugzeugbau - Care and protection of Polyester Gel coats
Temperature of painted composites

Video
Building JS1 sailplane (In Afrikaans)
Building DG sailplanes (In German)
Building Schempp Hirth sailplanes (In English)
Building Diamond aircraft (In English)
Building a template-based car mold
Automated hot-wiring foam
Making a mold from a plug (In English)
Discovery channel - Vacuum injection moulding (In English)
How to make a composite bonnet via molds (English), part 2, part 3

Project websites
Building the HP24 composite sailplane, very detailed
Peter Garrison - Melmoth
Mike Arnold AR5/6
Tony Pileggi - Composite Corsair 82%
RB Aerospace build blog
Jonkers sailplane building

Suppliers
USA
CA, OR, WA - Fiberlay composites - delivers to small customers, competitive prices
Composites one, all over the US, wide product assortment
CA, GA, ON - Aircraft spruce, composite section
OH - Fibreglast, very wide assortment
IL - Wicks aircraft, composite section
IN - Applied Vehicle composites
WA, Fiberglasssupply
MI, Resin services product specifications

Europe
Germany BW - R&G Faserverbundwerkstoffe
Germany - LTB Antwerpen
Netherlands - Polyservice, no apparant aerospace grade materials
Netherlands, no 1, no2, no3, no4, no5
Czech republic and most of former Eastern Europe - Havel composites

Literature
HBA - Orion gives a good overview of available literature and their strong and weaker points
Online - NIAR - AGATE material properties database
Online - R&G handbook of composites, excellent start (German and English)
Online - (NIAR) Guide for Low Cost Design and Manufacturing of Composite General Aviation Aircraft
Online - Very thorough manual about building the KR2S composite aircraft from Scratch
Online - OpenEz builders guide
Book - Alex Strojnik – Low power composite aircraft structures
Book - Burt Rutan - Moldless Composite Sandwich Aircraft Construction
Book - Robert M. Jones - Mechanics of composite materials
Book - Michael Chun-Yu Niu - Composite Airframe Structures (Niu)
Book - Hong T. Hahn, Stephen W. Tsai - Introduction to Composite Materials
Book - Robert M. Jones - Mechanics of Composite Materials
 

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