If you have ever wondered why one piece of carbon fiber might look different from another piece of carbon fiber, you’re not alone. Carbon fiber comes in many different weaves and each one serves a different purpose, and it isn't just cosmetic.
Carbon fibers are made from precursors such as polyacrylonitrile (PAN) and rayon. The precursor fibers are chemically treated, heated and stretched, then carbonized, to create high-strength fibers. These fibers, or filaments, are then bundled together in tows which are identified by the number of carbon filaments they contain. Common tow ratings are 3k, 6k, 12k and 15k. The “k” refers to a thousand, so a 3k tow is made of 3,000 carbon filaments. A standard 3k tow is typically .125” wide, so that is a lot of fiber packed into a small space. A 6k tow has 6,000 carbon filaments, a 12k has 12,000 filaments, and so on. This large number of high-strength fibers bundled together is what makes carbon fiber such a strong material.
Woven Carbon Fiber
Carbon fiber typically comes in the form of a woven fabric, which makes it easier to work with and can give additional structural strength depending on the application. Because of this there are many different weaves used for carbon fiber fabric. The most common are Plain, Twill and Harness Satin, and we will go into more detail for each.
A plain weave carbon fiber sheet looks symmetrical with a small checkerboard style appearance. In this weave the tows are woven in an over/under pattern. The short space between interlaces give the plain weave a high level of stability. Fabrics stability is the ability for a fabric to maintain its weave angle and fiber orientation. Due to this high level of stability, plain is not well suited for layups with complex contours, it will not be as pliable as some of the other weaves. Generally, plain weave fabrics are suited for flat sheets, tubes and 2D curves.
One drawback to this weave pattern is the harsh crimp (the angle the fiber makes when woven, see below) in the tows due to the short distance between interlaces. The harsh crimp can create stress concentrations which can weaken the part over time.
Twill serves as a bridge between a plain weave and the satin weaves we will discuss next. Twill has good pliability and can form to complex contours, and it is better at maintaining its fabric stability than a harness satin weave, but not as good as plain weave. If you follow a tow strand in a twill weave it passes over a set number of tows and then under the same number of tows. The over/under pattern creates a diagonal arrowhead look, known as a “twill line”. The longer distance between tow interlaces means fewer crimps compared to a plain weave and less potential stress concentrations.
2x2 Twill is likely the most recognizable carbon fiber weave in the industry. It is used in many cosmetic and decorative applications, but also has great functionality, it has both moderate formability and moderate stability. As the 2x2 name implies, each tow will pass over 2 tows then under two tows. Similarly, 4x4 Twill will pass over 4 tows then under 4 tows. It has slightly more formability that 2x2 Twill, since the weave is not as tight, but it will also have less stability.
Harness Satin Weaves
The satin weave was designed thousands of years ago for making silk fabrics with excellent draping qualities, while also looking smooth and seamless. For composites, this drapability means it can easily form and wrap around complex contours. Because the fabric is so formable, it, expectantly, has low stability. Common harness satin weaves are 4 harness satin (4HS), 5 harness satin (5HS) and 8 harness satin (8HS). As you increase the number of the satin weave, formability will increase and fabric stability will decrease.
The number in the Harness Satin names indicates the total number of tows passed over then under. For 4HS, it will pass over 3 tows then under 1. For 5HS, it will pass over 4 then under 1, and 8HS will pass over 7 and under 1.
Spread Tow vs. Standard Tow
Spread tow material can be a good compromise between using uni-directional material and standard woven material. As a fiber tow weaves up and down to create a fabric, the strength is reduced due to the crimp in the tow. As you increase the number of filaments in a standard tow, from 3k to 6k for example, the tow becomes bigger (thicker) and the crimp angle becomes harsher. One way to avoid this is to spread the filaments out into a wider tow, this is called a spread tow and there are a couple of benefits gained by doing this.
The spread tow offers a smaller crimp angle than a standard tow weave and can decrease the crossover defects by increasing the smoothness. A lower crimp angle will result in higher strength. Spread tow material is also easier to work with than uni-directional material and still has reasonably good fiber pull up prevention.
Spread Tow Plain Weave
Spread Tow Twill Weave
As the name implies, uni, meaning one, all of the fibers are oriented in the same direction. This gives uni-directional (UD) fabric some high strength benefits. UD fabric is not woven, there aren’t any interlacing fibers with crimping that can weaken the structure. Rather, there are continuous fibers that increase the strength and stiffness. Another benefit is the ability to tailor the layup with better control for performance characteristics. A bicycles frame is a good example of how UD fabric can be used to tune the performance. The frame must be stiff and rigid in the bottom bracket area to transfer the rider’s power to the wheels, but the frame also needs to have some compliance and flex to not beat up the rider. With UD material you can pick the precise direction of the fibers to get the strength you need.
One major drawback with UD though is its workability. UD tends to fall apart quite easily during the layup process since it has no interlaced fibers to hold it together. If the fibers are placed incorrectly it can be almost impossible to correctly reorient them all again. Machining parts made with UD fabric can also cause issues. If there is any fiber pull up where the features were cut, those loose fibers can pull up all the way across the part. Typically, if UD material is chosen for a layup, a layer of woven material is used for the first and last layer to assist with machineability and part durability. This is what is done for hobbyist drone frames all the way up to production rocket parts.