niterida

Well I am finishing my masters degree here in a few months and I figured I would pass on some very accurate information about one of the most unknown and and miss informed things in racing... Composites.
I will try to keep it very concise and expand if people have questions on particular subjects.
I will also focus on all the most common composite material carbon fiber/resin.

I will start first of all on the matrix material also know as the resin. All resins consist of 2 parts, 1. Resin and 2. Hardener
There are 3 common types of resin to choose from. Unless there is a compatibility issue with the mold and the resin system being used, the choice of resin is independent of any other decision in the manufacturing process. Things like working time, strength, and viscosity can drive the selection of a resin
1. Polyester based resin aka surfboard resin
2. Vinylester based resin
3. Epoxy based resins aka THE BEST MOST INCREDIBLE ONLY THE REAL RACERS USE STUFF

First I will deal with something that bothers me everyday reading stuff online. I'll call it the "Demonizing" of polyester and vinylester based resins. I swear if I read one more thing about how epoxy based resin is the ONLY resin that should be used I am gonna explode.
yes epoxy based resins are good, yes they have better strength properties, but unless the part being made is going on a plane or into outer space, you can get by with other resins especially on an amateur race car

Polyester based resins are considered inferior in every way when it comes to composites and I have actually read that they should NEVER be used with anything except cosmetic parts... BULL S**T, anyone here ever touched a surfboard? I have, in fact I own a few. they are made using fiberglass and polyester based resin, both infinitely inferior according to online sources. But I have had my board SLAM into rocks and my head and survive for years... how can this be, it isn't dry carbon/epoxy therefor it should fall apart if you look at it wrong, right?

ok rant over, what I recommend for the beginner is to use polyester based resins when learning composites, then move on to vinlyester based resins and so on. I would highly recommend using Ultra clear surfboard resin for a good beginning resin
http://www.tapplastics.com/shop/product.php?pid=38&


Now I will explain why the resin choice does not matter as much as online manufacturers lead on. The equation for stiffness and strength of a composites is:
Sc= Sf*Vf + Sr*Vr
where Sc is strength of composite,Sf strength of the fiber, Sr is strength of the resin, Vf is fiber volume fraction, Vr is resin volume fraction

ok lets do an AS4 uni carbon fiber fabric with an ProSet epoxy based resin vs polyester based resin using a 62% fiber volume in tension
AS4 carbon has a tensile strength of 550 ksi, the epoxy has tensile strength of 8000psi or 8ksi, and I'll act like an online distributor and say the 4 ksi (half the strength of the epoxy)
now lets compute the strength of both the composites
Sc=550*.62 + 8*.38= 344.04 ksi this is for the epoxy
Sc=550*.62 + 4*.38 =342.52 ksi this is for the polyester
HOW CAN THIS BE??????? they both have the same strength but I just paid more (double?) for epoxy based resin part online.
this is because the strength of a fiber composite is from the fiber, not the resin. you just fell victim to a business major and his marketing plan focusing on the resin selection.

niterida

Next I will talk about manufacturing methods of composites. This is where the previous equation comes into real play. The ability to control the resin content in the part will be the greatest influence on part strength. This means controlling the Vr of that equation, if you double the resin volume, you cut the strength in half.
there are 4 main methods to make composite parts
1. Hand wet lay up
2. Wet lay up with vacuum bag
3. Vacuum infusion
4. Pre-preg

here are the pros and cons of each method
1. Hand Lay Up (without Vacuum bag)
i. Drawbacks:
1. High resin to fiber ratio which leads to low strength to weight ratio
2. Hard to remove excess resin
3. Nothing holds fibers and resin in place while curing takes place
4. Very short working time ~20 minutes, makes doing large parts
5. difficult
6. Very messy
ii. Benefits:
1. Very simple and cheap
2. No excess equipment required (i.e. vacuum pumps, ovens…)
3. Can be done at room temperature
2. Wet Lay Up with Vacuum bag
i. Drawbacks:
1. Still possible to have excess resin
2. Short working time ~20 minutes
3. Hard to bag large items while mold is full of wet curing resin
4. More equipment needed (vacuum pump, piping, bagging material etc…)
ii. Benefits:
1. Constant pressure on part help hold it in place while curing takes place
2. Bag can squeeze excess resin into material to absorb excess resin
3. Higher strength to weight ratio
4. Can be done at room temperature
3. Vacuum Infusion
i. Drawbacks:
1. Very complicated, can take a while to prefect method
2. More equipment needed
ii. Benefits:
1. High strength to weight ratio, all excess resin is removed from part
2. Unlimited set up time for material and bagging equipment
3. Parts weigh the same each time, consistent resin/fiber ratio
4. Easy to work with, and easy to get correct fiber orientation
5. Very clean process, less VOC’s due to limited exposure to resin
6. Allows for thickness to be regulated
4. Pre-Preg with vacuum Bagging
i. Drawbacks:
1. Very expensive compared to other methods
2. More equipment needed than all other methods
3. Mold must be built to handle high temp and pressure of auto-clave
ii. Benefits
1. Very consistent strength and properties because fiber to resin ratio is pre-prescribed by manufacturer of material
2. Parts weigh the same each time
3. Easy to work with, and easy to get correct fiber orientation
4. Allows thickness to be regulated

now for my next rant, If I read another online source say "dry" carbon I am gonna explode again. let me say this calmly but concisely
DRY CARBON IS A MYTH
dry carbon is the dry fabric without resin this is fabric on a roll. what most companies mean is pre-preg carbon. which is in my opinion bad for 90% of all applications.
pre-preg carbon must be cured at an elevated temperature. Torray has 250 F and 350 F
pre-preg carbon which is very good for aerospace applications. but for things like car parts it is bad, and here is why:
when the part cools down after curing, the 2 materials of the composite must shrink or expand do to the temperature difference. The thing is carbon has a negative thermal coeff of expansion, this means that as is warms up, it shrinks!! but the resin expands as it warms. this creates an internal stress already in your temperature cured part. And the zero stress state is at the curing temp.
to put this in perspective, I designed a part out of carbon/epoxy and with the 250 F resin it required 4 layers to take the load, but with the 350 F resin it needed 19 layers!!! this was due to the internal stress created by the post cure internal stress.
So those "dry carbon" parts out there are actually weaker at room temp due to internal stresses created by the curing process

niterida

Next I'll talk about properties you can expect out of most composites

Carbon is truly amazing in what it can do for how little it weighs. I'll list a popular fabric and epoxy based resin. These properties are done by me and they are very representative of what you will find out there.
first is an AS4 carbon fiber, 6k tow, 2X2 twill weave fabric with a ProSet epoxy resin
Also I will list E-glass and the same epoxy in a 1X1 plain weave 3.7 oz

first up, tensile strength. this is where the carbon makes other materials look like butter




This is an infused part cut into strips and pulled on a tension tester. The results are a tensile strength of 104ksi and and modulus of 10 Msi.
the piece in the picture is 6 layers and about .85 inches wide and it takes about 9000lbs to break samples of that size.
you are probably wandering why the 104ksi number is so much lower than the 340 ksi number I stated earlier. the difference is the weave and the tow. this is a twill weave while the numbers I stated before are for uni-directional fibers. also those numbers are in theory. actual numbers are slightly lower due to porosity in the part. the weave is lower because the fibers literally weave over and under the other fibers.

the fiberglass has a tensile strength of 40 ksi and a stiffness of 3.0 Msi

that's all for tonigh

Race Egr

Wow, I will be the first to say excellent write up and congrats on finishing your masters. Im assuming your getting your masters in materials engineering or something similar?

topdog

iTrader: (1)

WOW very great info here

subscribed

Race Egr

I put all of it in a pdf file that way h-t folks can download it and read it. I just thought some people might like to save it I hope you dont mind niterida. Here is the file:

http://www.filefactory.com/file/af66...nformation_pdf

niterida

Mechanical Engineering. My masters thesis is on the manufacturing and analysis of carbon wind turbine blades. basically I am making Cal Poly, SLO's first wind turbine.

I will write more stuff later, like how to make carbon wings, and other parts.

Dogginator

Here are some possible topics:
1) woven sheets vs unidirectional sheets
2) applications where carbon fiber is simply over kill compared to e-glass
3) Carbon-carbon. There was also a recent thread discussing carbon fiber reinforced epoxy wheels where people were confusing that with carbon-carbon.
4) Ply directions for shear loads.
5) Void effects on interlaminar shear strength.

niterida

let me explain how properties for composites are defined. In metals it really doesn't matter how you pull on it, it will break at the same stress in all directions. but composites are defined by the 1,2,3 direction.
the 1 direction or longitudinal direction (1,L) is the fiber direction. This applies mainly for uni-directional fibers.
the 2 direction or transverse direction (2,T) is normal to the fiber direction. this is basically the properties of the resin in unidirectional fabrics
the 3 direction is same as the 2 direction.

in this test the 1 and 2 directions both have fibers because it is a woven fabric. and the 3 direction is normal to the weave. Composites are non-isotropic materials. meaning they have different properties in different directions. so if you pull in the fiber direction, you will get greatly different properties than if you pull normal to the fiber direction. This is what makes composites so great, you can have strength only in the direction that your load path takes while not wasting material in directions that has no load associated with them.

here is a picture of a +/- 45 sample failure, the sample is about 1/10 as strong and 1/20 as stiff if you pull in this direction vs the fiber direction


Here is a more comprehensive list of properties for non pre-preg materials
This is an AS4 carbon fiber 2X2 twill weave 6k tow with ProSet 117LV/226 epoxy resin

Ultimate tensile strength, σ1,2tu(ksi): 104 ksi
Ultimate shear strength, τ12 (psi) [45 tensile test] 9673 psi
Apparent shear strength, SH (psi) [ILSS test] 6929 psi
Tangent Modulus of Elasticity, Es (ksi) [short beam test] 93.186 ksi
Modulus of Elasticity, E1,2t(Msi) [tensile test] 10.0 Msi
Shear Modulus, G12 .535 Msi
Poison's ratio .0775

next is the most common type of fiberglass with the same resin

Ultimate tensile strength, σ1,2tu(ksi): 40.8 ksi
Ultimate shear strength, τ12 (psi) [45 tensile test] 10500 psi
Apparent shear strength, SH (psi) [ILSS test] 4670 psi
Tangent Modulus of Elasticity, Es (ksi) [short beam test] 23.78 ksi
Modulus of Elasticity, E1,2t(Msi) [tensile test] 3.0 Msi
Shear Modulus, G12 .521 Msi
Poison's ratio .0156

ok, lets spend a few minutes thinking about these numbers. the fiberglass is indeed weaker in the fiber direction at 40 ksi, vs 104 ksi for carbon, it is also less stiff at 3 Msi while carbon is 10 Msi.
the tangent modulus of elasticity is also know as bending stiffness, so you can see carbon is about 4 times better in bending.
things like shear strength and shear modulus are very matrix dominated so they do not vary much for carbon and fiberglass. this is where an epoxy resin would help, but if the designer of the part put it in pure shear, well they should be fired.

niterida

I'll get to all those in just a little

akandok

Hey, I have had this one question that I cant find the answer to.


Where do you find the vacuum bags?

niterida

http://www.fibreglast.com/showproduc...plies-106.html
I recommend the vinyl bag and yellow sealent tape
they are not the cheapest, but this is a good place to start

thawley

It's about damn time. Go Mustangs!

akandok

Good stuff


Last time I found some bags that were like $200+ a piece.. I must of been looking at the wrong stuff.

akandok

btw, Im working on this on and off...

http://www.lotustalk.com/forums/f101...-repair-56314/

niterida

ok this is a good question. True composites are the unidirectional type, but even those can have their drawbacks. The advantage that I have stated before is that with fiber composites, you can have good properties in the direction of the load path and waste no excess material for strength in other directions.
A strut is a good example where a uni fiber is good. you know the load is going right through the strut and you can design for that. you can have an extremely light and stiff strut if designed correctly. But what happens if you hit the strut from the side? if there are no fibers in that direction, it will be relying on just the resin in the transverse direction to take the unexpected transverse load. This is where a weave would be a good idea.

Take the front fork of a road bike, you'll notice that they all have a weave layup. Even though the load should be going straight up fork, sometimes they get a side load and need fibers in the transverse direction to take unexpected loads.

Both a uni and weave layup have advantages, uni can be very tedious to do because it is generally very thin and you need 2 layers for every one you would need of weave. Weave fabrics are weaker though. Uni composites are capable of over 375 ksi in tension, while a weave will max out at about 140-150 ksi (for a satin harness areo grade pre-preg)

So to wrap it up, uni is great if you know EXACTLY where the load is going. but you can still put uni in different angles as additional layers for unexpected loads. weave is better for manufactuing because it is easy to work with and builds thickness quickly

Adam West

great write-up. SUBSCRIBED!!!

Stinkycheezmonky

Hmm...nice turn-around for here. Please continue, this looks good. As long as you don't mind niterida, can I request Race egr continue updating the PDF? It hasn't happened in a few weeks here, but we had a problem with losing information.

BillB

Great posts keep em coming!

jdm civic 2000

Yes more!

get RIGHT

Great post.

Ti3d in

Just wondering what are the % elongation values of the samples?

niterida

good numbers to use are 1% for carbon and 4% for e-glass as max strain values for designing.

I will write some more tonight, any more suggestions of what people want to know?

Mr.E.G.

Yes, I have a suggestion. I really appreciate the information, but I was wondering if you could provide some more specific production info based on the approach you would take to make specific parts.

For example, if you were to make a single element wing which is to be suspended between the end plates with no other support system, with the expected max aerodynamic load of 400 lbs, would you use a weave, a unidirectional carbon fiber, etc.?

Naturally, the production methods can vary based on the desired shape of the part, so I am not necessarily asking for information regarding how you would make the mold and all of that. Really, I am asking for a "these are the materials I would use and why" type of explanation.

To get the ball rolling, here are some parts that I would love an expert opinion on, materials wise:

*a flat front splitter (meaning no upward connector pieces or integrated airdam, it is simply a 2D plate that is hung from the frame rails via a set o aluminum brackets and the bumper sort of sits flush against the topside of the splitter) with a thickness of 1/4". An example of a question that I would love to have answered related to this part; would you simply lay multiple layers of fiberglass, or would you do a layer of some lesser composites to build it up and encase it in a carbon fiber wrap? How would you go about building the thickness?

*a 3D front splitter with a converging front edge and maybe even a little built-in contouring (such as the front splitter of an Aston Martin DR9 ALMS GT1 car). This splitter is relatively thin at the leading edge and gets thicker as it goes back toward the mounting points/ body of the car.

* a single element flat underbody panel of constant thickness, say 1/4". This piece would be suspended from the bottom of the car and must withstand negative pressure (assuming you've designed an effective under body system) of potentially several hundred pounds. Think of the single under body panel on a Ferrari f430.

*simple ducting such as the type you can find on the radiator ducting of an BMW motorsports E46 M3 (I chose this reference car for it's abundance of available engine bay pics)

*a hood, door skin, or other body panel that does not need to support a particularly high load

*a composite racing seat which, naturally, must withstand a variety of loads in different directions.

*a load bearing tubular triangulation piece such as strut tower bar, or even something more fancy like the composite engine bay cradle found on a Lamborghini Mercelago

*an intercooler charge pipe. Using composites for such a task may be less than ideal, but I am just curious to know how you would build such a part to withstand 30 psi, for example.

*an upper or lower wishbone such as what you would find on a formula car

*a carbon fiber monocoque such as the base chassis of a Ferrari Enzo or the passenger compartment portion of a Mercedes SLR Mclaren. Don't worry, this is just a matter of curiosity. I am not planning on attempting this. I am sure this is probably even beyond your understanding in its entirety. I am just curious as to the starting point.

The main piece I am interested is the under body panel. I have been working on designing just such a part for some time, and I am building in most of the part's rigidity and strength by creating an initial flat panel, once that layer is cured I lay some strategically placed strips of cardboard (think of that as my light weight sandwich material. sort of a poor mans honey comb) and then lay up another layer of fabric across the cardboard which causes the fabric to "bridge" over the cardboard, which results in an effect similar to bead rolling a flat piece of sheet metal. In other words, by doing this, I make a part that is less likely to flex because of the structure built in by the second layer. The problem with this method is that produces a part that is not uniform in thickness, is slightly heavier, and is not entirely flat on one side. It works, but has some drawbacks. I can't help but think that if I knew more about the materials and was able to arrange various weaves or even various different fabrics in an ideal layout, that I would ultimately have a piece with similar or better mechanical properties, only without the drawbacks. You see what I'm saying? How would YOU go about making this part armed with your superior understanding of composites?

Forgive me if my request is not what you had in mind. Any comments on the above mentioned would make for some great conversation.

It would be especially helpful for me, as someone who dabbles heavily with composites, but is often relying on trial and error than prior knowledge. Thank a million for this thread. You are a true asset to the forum.

RacerZook

iTrader: (1)

You said it right there. Honeycomb. Carbon Honeycomb sandwhiched by a carbon weave. Stack some unidirectional in a few small spots to make hardpoints so you can drill and bolt whatever you want to it.