Carbon Fiber Basics: An easy to understand guide
Not all carbon fiber is created equal... That's right, there are several types of carbon fiber. There is low modulus, standard modulus, intermediate modulus, and high modulus. Then there are several tow size differences and a ton of different cloth patterns to choose from.
So what type of carbon fiber should you use for your next project? Well I couldn't tell you exactly but hopefully the following basic guide will help.
The most common way I've seen carbon fiber cloth listed is similar to the following two examples:
- T700- 12k, 7 oz. plain weave
- AS4- 6k, 11 oz. 4 harness satin weave
The "T700S and AS4" is the brand or type of carbon fiber.
The "12k and 6k" is the tow size. Number of filament fibers in a bundle.
The "7 oz. and 11 oz." is the weight of the cloth per yrd.- unless otherwise noted.
The "plain weave and 4 harness satin weave" is how the tow is woven into cloth.
Types of Carbon Fiber
In the example above we see T700S and AS4 listed as the brand or types of carbon fiber we are looking at. The T700S is made by Toray and after a quick lookup we find that Toray's T700S carbon fiber is a standard modulus carbon fiber with good tensile strength characteristics. Take a look at the technical data sheet: http://www.toraycfa.com/pdfs/T700SDataSheet.pdf
AS4 is made by HexTow, or Hexcel, and is also a standard modulus fiber with decent strength characteristics. Here is their technical data sheet: http://www.hexcel.com/resources/datasheets/carbon-fiber-data-sheets/as4.pdf
I'm not going to teach you how to read the data sheet-Besides I only understand about 1/3 of it anyway. However, what I believe are the most important things to understand are the tensile strength, modulus, and elongation/strain.
Tensile strength is how hard it is to break the carbon fiber by pulling it apart from end to end (imagine hanging on a rope until it breaks). The higher the tensile strength the harder it is to break, in other words it takes more weight (really it's force) to break the "rope."
Modulus is how stiff the fiber is- the higher the modulus the stiffer the carbon fiber is.
Note: There is give and take between tensile strength and modulus. In other words if you have a high modulus carbon fiber it will be stiffer but also weaker in tensile strength than a low or standard modulus fiber.
High Modulus Carbon Fiber = Stiffer but weaker and more brittle parts.
High Strength / Low Modulus Carbon Fiber = Stronger but more flexible parts.
So why would you use one over the other?
In bikes you see a lot of hype about "High Modulus" carbon fiber- So why would they use high modulus if it is actually weaker than a low modulus carbon fiber? Well, you can get a stiffer bike frame while using less material, therefore making it lighter. Also because the strength lost isn't enough to compromise the safety of the frame.
So can I use low modulus carbon fiber on a bike frame? Absolutely, however, in order to get a frame that is as stiff as a high modulus frame you will need more carbon fiber and your frame will be heavier. (Actually the weight difference isn't that much, but when you're pro everything matters.)
Elongation or Strain
How much the fiber can stretch before it breaks or reaches failure. Low modulus fibers stretch more than high modulus fibers before breaking.
Carbon Fiber Filament
A carbon fiber filament is the smallest part of the carbon fiber cloth. It is much smaller or skinnier than a human hair. The strands are then bundled together to create what's called "tow." The tow is what is used to create the cloth.
Carbon fiber filaments are created in different ways by different manufactures but we won't get into that in this guide. What you need to understand is that different types of filaments have their own set of structural values and properties- Like strength and stiffness- as we talked about above.
Note: It's common for distributors and retailers to leave out the brand and type of carbon fiber when selling cloth. If you're not sure and want to know, a phone call usually gets you the answer.
Carbon Fiber Tow or Filament Bundles
Tow is a bundle of individual fiber filaments or strands that create a larger strand- kind of like you seen in yarn except that it's not twisted (Twisting makes the bundles weaker). Tow is woven together into cloth or a weave.
There are several variations of tow sizes but the most common sizes I've seen are 3k, 6k, and 12k. The "k" stands for thousands so there are 3,000 individual filaments in a 3k tow, 6,000 strands in 6k, and so on.
Note: That larger tow bundles can be harder to shape around sharp corners and complex curves.
Cloth weight is usually shown in ounces per yard (in America). Tighter woven cloths using the same type of tow will weigh more than cloths using less tow in the same amount of space. I will often times use a heavier cloth in place of a lighter cloth to save on material cost since it is usually cheaper to buy a heavier cloth than buying a lighter cloth and doubling it.
There are several types of weaves out there; Twill, Satin, Plain, Uni-directional (Uni), triaxial, and so on.
What you need to understand about weaves is that the more under and over lapping within the weave the weaker it will be. For example a plain weave will be more prone to breaking than a 4 harness satin weave using the same type of tow. Why? Because as the cloth begins to come under tension the fibers try to straighten out but can't because there is another tow bundle under them restricting them from becoming straight. Therefore, there is a shear force applied to the straightening tow and it snaps. In other words as the tow straightens out it presses on the tow that is running under or over it going the opposite / cross direction.
The less overlapping or "bumpy" the weave is over a certain area the stronger it will be. For example a 4 harness satin weave has fewer overlapping bumps than a 2x2 twill weave, which has less overlapping than a plain weave. The strongest cloths are the non woven types like uni-directional cloth because it doesn't experience the shear stress caused by the over and under lapping fiber bundles. It's better in both compression and tension than any other cloth- But it's usually not as pretty.
"Prepreg" is short for Pre-impregnated cloth- it just means that the cloth whether carbon fiber, fiber glass, Kevlar, etc is pre wetted out with epoxy. Here a couple of types of prepreg cloth.
Wet Prepreg Cloth- You can make your own wet prepreg cloth by pouring and spreading epoxy on the cloth before adding it to the mold or structure.
Dry Prepreg Cloth- Like the wet prepreg, a dry prepreg is just carbon fiber that's been pre-impregnated with epoxy. You'll find it usually referred to as "Dry Carbon" cloth. This is because the epoxy used on the cloth is not wet like a regular layup epoxy. However, the "dry carbon" epoxy is usually a bit sticky when it's at room temperature. You can make your own, but most manufacturers just buy it already made. Basically, This type of prepreg usually requires an oven because the epoxy doesn't start the curing process until it's reached a certain temp like 250 deg Ferinheight. However, most often the carbon fiber is stored it a freezer to keep it from kicking off and curing prematurely.
Cloth fiber Orientation
Imagine you have a roll of cloth that is 35" wide and 100 yrds long.
The seller will often times tell you the fiber orientation of the cloth. This is important to know in case you need a piece of cloth cut at a certain length. For example if you need a piece of uni cloth that is longer than 35" do you buy the roll with fibers running on the 0 deg, 45 deg, or 90 deg axis?
0 degrees means the tow runs the length of the roll- so in the example above, the fiber will run the length of the roll or 100 yrds. So if you needed a piece of cloth over 35" long you would want this roll of cloth.
90 deg means that the tow runs across the roll- so in the example above, the fiber will run the width of the roll or 35" long.
45 deg means that the tow runs 45 deg to the 0 and 90 deg axis. I've only seen this type of cloth in bi-directional , triaxial, and quadaxial configurations.
Uni or unidirectional cloth
Uni cloth has all of its tow bundles running in the same direction with some type of binder holding it together so it doesn't fray or come apart. The binder could be made of several different things including small strands of carbon fiber, fiberglass, Kevlar, epoxy, and etc.
Uni cloth is good for projects that required strength in one direction more than the other direction. For example on a longboard you need more strength running length-wise on the board than you do running across the board. Therefore if you used bidirectional cloth for the whole board you would be adding unnecessary weight to the board because you don't need all of the excess material running across the board. So using more uni cloth and less bidirectional cloth adds more strength over the span of the board without adding the extra weight caused by the excessive cross fibers that comes with using the bidirectional cloth.
Another example of a common use for unidirectional cloth is on airplane wing spars where you need strength and stiffness running along the length of the wing.
Bidirectional cloth is the most common and it is usually least expensive. It is great for projects where you need strength in multiple directions. For example bidirectional cloth works well on car hoods, instrument panels, computer cases, and etc. In my opinion it is also the prettiest. On bikes you will often see bidirectional cloth on the outside of the frame which is usually mostly cosmetic since the other layers are usually unidirectional cloth.
You usually see triaxial cloth on skis and snow boards. It usually has its fibers running in the 0, +45, and -45 dreg fiber orientations. The +45 and -45 deg orientation of the cloth helps strengthen and stiffen the ski and snow boards' torsional flex- In other words it helps to keep the ski or snowboard from twisting.
This cloth is less common but is used when you need strength running in all directions equally across the part. The fiber orientation is usually 0, +45, -45, and 90 deg.
It is important to note that you can achieve any of these fiber orientations by cutting you cloth appropriately and layering it accordingly. For example you can take unidirectional cloth and by layering it with one layer at 0 deg, another at 90 deg, another at +45 deg, and another at -45 deg, you just created a quadaxial cloth. However, buying a quadaxial cloth is usually cheaper than layering several layers of uni.
Another note is that the cloths above can be woven or layered using a binder. Just remember that woven clothes will not be as strong as non-woven cloths- However, the strength difference may be negligible for your project.