What the Carbon? Weaves (Part 2 of 5)

We are going to talk about different types of fabric weaves, what they do, and what applications they are useful in. Everything will be discussed in a high-level manner. It’s easy to get into the technical details, but that may only be interesting to composite engineers. If you haven’t see part one, then you can check it out here. So let’s dive in and look at the different type of carbon weaves. 

There are a bunch of different types of weaves, but we will only be looking at the following:

  • Unidirectional and tow
  • Bidirectional (plain and twill)
  • Braided

Unidirectional and Tow:

photo credt: Bikerumour.com

Tow is the most basic of carbon fibre materials. Tow is just a group of carbon strands, all aligned in the same direction. You can sort of think of it like your hair. If you’ve ever pulled on one of your hairs, its pretty easy to break, but pull on all of them, it’s actually quite strong. Similarly, tow is made up of lots and lots of individual carbon strands. For example, a 12k dry carbon tow is made up of 12,000 strands of carbon fibre!

  • Where are these weaves used? Tow is perfect for lashing carbon tubes together. For example, you might have seen it used on early carbon fibre bicycles. It can also be used as thin unidirectional reinforcements in your laminations.

Unidirectional fabric is similar to tow, but it generally comes in much wider sections. For example, a tow strand might be only 1 inch wide, whereas unidirectional fabric can come in standard 48, 54, or 60 inch fabrics.  This fabric has stitches running perpendicular to the unidirectional fibres, just so that it doesn’t fall apart.

  • Where are these weaves used? Unidirectional fabric has some very interesting properties. It is quite strong and stiff along the length of the fibre, but weak across the fibres. When laminated, the carbon sheet can bend and flex. It’s perfect for panels that might need to bend around gentle curves. Of  course, there are uses for it. You can buy unidirectional carbon tubes. These tubes can withstand compression pretty well, and don’t typically bend either. These tubes are often used as crash structures.

Bidirectional:

When you think of carbon fibre, you’re probably thinking of bidirectional fabrics (specifically twill, which we will get to in a minute). Bidirectional is simply unidirectional tow woven together.

In the case of plain weave, it means that the tow is woven one over, one under (check out the image). All bidirectional fabrics have benefit of strength in two directions. There are different types of plain weave as well. One version that we are starting to see is from Textreme. It is an extremely thin fabric. This allows for more layering (more strength) and can be used to make very lightweight parts. You have probably seen this on some F1 cars.

  • Where are these weaves used? Plain weave is great for creating large flat pieces. The one major downside of plain weave is that it does not form around compound 3D curves. When attempting to use plain weave for compound 3D curves, you will experience severed de-lamination which will weaken your carbon parts.

Twill is what you have been seeing everywhere on cars. It’s generally a 2×2 twill (which means two strands of tow go over 2, under 2. This makes for the zig-zag pattern we are all used to seeing. The biggest benefit of twill is its ability to conform to 3D compound curves.

DSC08163
2x2 Twill
Plain Weave

Braided:

Lastly, is braided carbon fibre. You might have seen this on an old Top Gear clip. The process of making these parts are very complex and requires specialized machinery. This makes these parts quite expensive. Because of the types of weave, braided parts are strong in pretty much every direction: compression, tension, and torsional.

  • Where are these weaves used? That expense is worth it if you’re using these parts for the right applications. Sail boat masts, drive shafts and steering columns are a few examples.  The benefit is definitely the weight savings. In the case of a driveshaft, a carbon driveshaft will spin up fast, creating for better throttle responses. For carbon sail masts, there is less mass high up, which helps significantly lower the centre of gravity.

In the next article we will start to discuss resin systems and laminations!

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