What is it?

Carbon fiber is very fine strands of carbon woven together in a pattern. It is a composite material, meaning it utilizes the properties of multiple separate materials to make one better material. In this case it uses carbon and a resin.

How is it made?

Carbon fiber starts off as an organic material (which, by nature, includes carbon). The material is stretched into very thin strands. These strands are then heated to extremely high temperatures, which shakes all of the non-carbon atoms off of the strands, leaving them as pure carbon. These strands are then bundled up into rovings, which are woven together to create carbon fiber cloths. The rovings are named according to how many strands of carbon fiber are present in each "bundle". Common ones you will see are 1k, 3k, 6k and 12k.

Carbon fiber patterns

Plain Weave

This weave pattern looks like a checkerboard with evenly spaced squares.

Pros

The stiffest and most stable carbon fiber weave due to the short space in between weaves

Cons

Due to the frequency of weaves, this weave has a frequently occurring harsh crimp angle - the angle formed as a result of a strand starting above another strand then going underneath. A stress concentration can build up in these angled parts which can weaken the part over time.
Due to the stiffness of the sheets, it is the most difficult to form into rounded or bent shapes. As such it is best suited for flat plates.

Twill Weave

This weave pattern looks like a series of arrows travelling in diagonal lines. Twill weaves are named according to the number of other strands a single strand will pass over before passing under the same number of strands. As such, a plain weave could technically be considered a 1x1 Twill Weave.

2x2 Twill

4x4 Twill

Pros

Compared to plain weave, the decreased stability means the weave is more flexible and can more easily conform to a desired shape.
Decreased frequency of weaves means crimp angles occur less frequently, which can improve the life cycle of the part.

Cons

Twill weave is an intermediate between plain weave and satin weave. As such it has decent stability. It will come apart a little more easily than plain weave will, but not as easily as satin weave.

Harness Satin Weave

In a satin weave, a strand will go over many other strands before going underneath only one strand. Satin weaves are named according to the total number of strands they travel past in one “cycle”. For example, in 4HS, a single strand would travel over 3 strands before travelling underneath 1.

4HS

8HS

Pros

The most flexible weave
Longer part life due to far fewer crimp angles

Cons

Due to being the most flexible, it is the least stable

Unidirectional

As the name implies, unidirectional carbon fiber runs in only one direction. It does not have a weave pattern.

Pros

The strongest form of carbon fiber in its respective direction

Cons

Falls apart easily in all directions except for its intended direction, so is typically used alongside sheets of other weaves

Carbon Fiber Part Fabrication

In general, carbon fiber parts start off as individual cloths of various patterns. These cloths are stacked on top of each other and bonded together using a resin. The number of cloths you stack depends on the desired thickness of your part.

650g 2x2 Twill 12k Carbon Fibre Cloth 1m - Easy Composites

For plates, the process is very straightforward: a piece of cloth is laid out and covered in resin. Then another cloth is placed on top and also covered in resin. The process is continued until the desired thickness is reached. Once all the layers are applied, the stack of cloths is placed in an airtight bag and a vacuum is used to suck all the air out of the bag and compress the sheets together. The stack is allowed to cure, and the sheet is ready.

For tubes, the process is very similar, except instead of laying the cloths flat, they are wrapped around a base tube (of some other material, like aluminum) one at a time, with resin in between each layer. The part is similarly vacuumed, then once it has cured, the base tube is removed.

For more complex shapes, molds are created. Layers are laid along the inside of a mold so that they cure in that position.

Also note that an alternative to applying layers and then applying resin in between, there are pre-impregnated sheets of carbon fiber cloth that have resin stuck to them alread. They have a protective sheet that preserves the resin until it is used and can be peeled off, similar to the ones you would have on the sticky side of a sticker. These sheets need to be oven-cured, but they can be advantageous because they stick together automatically without the need to manually supply resin.

Mechanical Properties

Compared to typically used metals, carbon fiber is very brittle. This is something to keep in mind when designing, because unlike metals which will deform slightly as they approach their critical stresses, carbon fibers will fail much more abruptly.

Here is a comparison between carbon fiber and some commonly used metals.

Name	Tensile Strength (MPa)	Young's Modulus (GPA)	Density (g/cm³)	Notes
T300	3530	230	1.76	A standard modulus carbon fiber from Toray Composite Materials
T1000G	6370	294	1.80	An intermediate modulus carbon fiber from Toray
M60J	3820	588	1.93	A high modulus carbon fiber from Toray
AISI 1018	370	205	7.87	Low carbon steel, a commonly used steel
6061 - T6	260	70	2.70	A commonly used aluminum alloy

Safety Concerns

Carbon fiber particles can cause irritation both on your skin as well as internally. As such it is important that when machining parts out of carbon fiber, no skin is exposed, and proper respiration equipment is worn.