Discussion in 'Composites' started by autoreply, Dec 10, 2010.

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1. Dec 10, 2010

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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:
But:
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.

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

Mold-less

Lay-up methods
Vacuum infusion/pressure assisted

Hand lay-up

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

#### Attached Files:

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• ###### CompositesV5_1.doc
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Last edited: Jun 18, 2012
wanttobuild, thump, blane.c and 3 others like this.
2. Dec 11, 2010

#### Moderator

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I try to only mention the issues that are still open/need action.
From my point of view, all other posts (except the first one ) can be deleted, so we keep a clean and clear topic.
Especially links (both internal and external) are highly appreciated. Several are mentioned in the first post and below and it'd be great when you've found those threads you'd be willing to post them here.

???Orion, discussing stress concentrations because of “strengthening” carbon strips.
???Orion, discussing his prepreg experience
???KR2 – Making a C-spar in a mold

Last edited: Feb 2, 2012
3. Dec 22, 2011

### Voyeurger

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Hi Auto,
I am finally jumping into composite experimentation. One thing that seems ignored by the resins/epoxy vendors (everywhere) is which core material is compatible or best suited to a particular resin. For example, I bought a gallon of clear polyester layup resin from U.S. Composites. NOWHERE on the product or the company website does it say whether I have orthopthalic or isopthalic resin which wouldn't matter if I new the range of core materials I can choose that will not melt when I apply this resin.
It would be a blessing to those of us starting out to have a simple spreadsheet with resin types and the foams that may safely be used with them. The U.S. Composites website simply states the polyester resin may be safely used with any urethane foam. Aircraft Spruce has a single polyurethane foam offering at $118.00 per 4X8 panel. Certainly there's other/cheaper products my resin won't melt. Surfboard foam for instance. What names do they go by? What are the other common uses of the compatible foams (to aid in sourcing)? Just my$.02.
Thanks Auto. Lovely job you've done here.
Gary

4. Feb 2, 2012

### Aircar

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A fantastic compilation on composites Autoreply --just scanned for the first time (I opened the Schempp Hirth 'building ' link and took a trip down memory lane ..with updates ) It occurrs to me that the work of JohnHart-Smith would fit here very nicely -John has developed a number of simplified design and analysis procedures that could be of much value ( I will see if I can contact him (if he is in the country tight now) and if he is able and amenable to some way to post here . Googling 'John Hart-Smith' should turn up some links and maybe papers (his work for Boeing possibly)

5. Feb 2, 2012

### craig saxon

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6. Feb 2, 2012

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Thanks Aircar. I can say that I'm rather proud of what we achieved here. We, because while I might be the keyboard-monkey, most of the information comes from many of the other users of HBA and it truly is a group effort.

Having said that, there are still a lot of matters "unsolved", most of that is in the second post, so input from anyone on that is highly appreciated.

I've also trashed some of the older comments and I've added the "unanswered questions" to the second post.

7. Feb 26, 2012

### bwilson4web

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Having bought a foam-and-fiberglass airplane, N19WT, I was new to 'tap testing.' I've already started and so far, feel comfortable with the wing. However, a Utube or MPEGs would help:
• tap testing and finding a delamination
• delamination repair
Once I understood the principles of delamination repair, I was less concerned with buying N19WT but I didn't find it until the night before inspecting the plane.

BTW, I did find one delamination tap-test video showing the process and audio of the changes. Helpful, it didn't cover things like marking the boundary. So far, no Utube/MPEG videos showing repair of a delaminated area.

Just a suggestion,
Bob Wilson

8. Mar 7, 2012

### harrisonaero

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thump likes this.
9. Aug 3, 2012

### AVI

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"SuppliersUSACA, OR, WA - Fiberlay composites - delivers to small customers, competitive pricesComposites one, all over the US, wide product assortmentCA, GA, ON - Aircraft spruce, composite sectionOH - Fibreglast, very wide assortmentIL - Wicks aircraft, composite sectionIN - Applied Vehicle compositesWA, FiberglasssupplyMI, Resin services product specifications..."Autoreply, You've got AC Spruce listed above in CA, GA, and ON under USA Suppliers. FYI, Ontario (ON), last time we checked, is still a province in Canada, and not part of the USA. History tells us that the US invasion force was repellled by a vastly outnumbered Canadian force during the War of 1812. Canada remains Canadian. - On a more serious note, do you have more listings for Canadian suppliers?

Last edited: Aug 5, 2012
10. Sep 28, 2014

### tunnels

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The easiest way to find a void is using your fingernails and scratching the surface and listening for the sound change !!
second and much quicker way is pouring boiling water over the surface !! quick, simple and finds them all regardless of size and shape !they stand up instantly so don't over do it ! find ,mark and move on !!
These are voids not delamination !!! two completely different things !!!.

Void is just a air bubble Under the glass layer sometimes can be seen sometimes not depending on the resin and how clear it is when its hardened !!
What is the biggest cause of voids ?? lack of resin!!! laying glass on a dry surface and then trying to force the resin down through the glass !!!
wet the surface with a generous coating of resin first then lay the glass into the resin and wet from the top as well !! Leave the glass to soak and let nature help you for a few minutes before using the squeegee !!
resin does not penetrate the bundles of fibres immediately !!! It needs time !!! The gel time is really important when using any resin !!35 to 45 minutes is best for Vinylester resins !! you should know the maximum and minimum catalyst ratio for the resin you are using !!! and work within that range !

Delamination is just what the word means ! the glass has pulled away from the surface and if its foam then the glass will have pulled part of the foam with it , The foam being the weaker of the 3 !!! so the void it has made is actual in the foam !!! ! the peelablity of foams is abysmal and really quite scary.
Don't believe me ?? do a sample of foam, resined to glass and see for yourself how easy It will peel off when its hard !

Last edited: Sep 28, 2014
11. Oct 3, 2014

### tunnels

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Oh forgot also if you are using a strong torch light don't shine straight at the surface in front of you or you wont see anything !!!!hold the light at arms length away from you and point the beam back to wards you but aimed at the surface of the glass !! its makes a prismatic shadow and very easy to see and spot everything even tiny bubbles size of a pencil lead can be seen !! Easy aah !!

12. Feb 1, 2017

### lr27

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You wrote:
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.
I think you should mention that kevlar isn't very good in compression.
a reference, though I've read that it's weak in compression in many places:

13. Mar 26, 2017

### proppastie

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Paper on flaws....might be good for our "hand layups". Gives some good data on strength even if it is in "metric".

if it is already here somewhere just delete the post.

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• ###### composite paper AIAA_2004_0174.pdf
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14. Oct 26, 2018

### Tantrum1

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None of the links worked for me...

15. Nov 23, 2018

### Dusan

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The link in the first post are still not working...

Rconc and Kyle Boatright like this.
16. Nov 28, 2018

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The problem is that the forum software has changed such that all internal links have changed. If you the first line below into the format of the second, you are still able to refer to posts/topics on HBA.

Changing them all in the OP is a considerable amount of work...

17. Nov 29, 2018

### proppastie

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That needs a "sticky" every forum

18. Apr 17, 2019

### BJC

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19. May 28, 2019

### KeithO

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Could anyone advise which is the least expensive supplier for bulk rolls of 7781 bid and Uni fabric in the US ? There seem to be huge differences in price. Also who sells the least expensive aerospace grade epoxy resin ? Many suppliers now asking \$130+ per gallon for epoxy ???

20. May 28, 2019

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