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Explained: What is the difference between fiberglass, kevlar, and carbon fiber – and what makes them so strong?

First off, when you see carbon fiber, Kevlar, and fiberglass (others include boron fiber and other polymers) , you are seeing basically a bundle of fibers similar to a rope or yarn. Now imagine taking that yarn and pulling on it (tension), it’s pretty strong (how hard you have to pull to break it) and stiff (how much it deforms under that force). Now, try pulling on it from the side, you instantly will pull the fibers from the bundle in that direction. If you push on it, the yarn or rope folds, providing no resistance. These are your fibers. These can be made from extruded graphite (carbon), extruded aramids (kevlar), or extruded glass and they will all give you different properties such as stiffness, strength, etc.

Now, to solve the problem of pulling or pushing on the fiber that we saw above, what you can do is set these fibers in a matrix, which is basically a glue that holds the fibers in place. Imagine taking your yarn, flattening it, and setting it in Elmer’s glue. If you did this, it would now have actual stiffness and strength in the two directions that previously provided none. This glue, is basically the matrix of a composite. This matrix can be a lot of things, such as thermosets (can not be separated from the fiber with heat) such as epoxy (most commonly used), phenolic, bmi, etc and thermoplastics (can be separated from the fiber with heat) Each of these matrices will have different properties themselves, but I won’t go into them here.

So, when you mix a fiber and matrix, you get a composite. The matrix and fiber both provide strength and stiffness based on their ratios, but in general, what fiber you use dictates most of the properties in the primary tensile direction and the matrix dictates the the other properties. This where it gets complicated. In general, your fiber will run in one direction (or two in the case of a bidirectional weave, but we will only consider unidirectional here). this will be the primary direction. This direction generally has properties on the order of 5 times better than the other directions, we call this being anisotropic or more specifically orthotropic. Here’s the beauty of composites- you can stack layers of this material to get properties in the direction you want. Therefore, you can customize the strength and stiffness based on the angle that you stack the layers (plies) and how many layers you have. This is why composites are so “strong”, but what is actually being referred to is it’s strength and stiffness to weight ratio (specific strength / stiffness). This means for less weight, you can have a stronger and stiffer object than if you made it out of a metal. This why They are special.

There’s a lot more to get into with composites, including applications and processing requirements, etc but it gets complicated fast. In general, fiberglass is relatively heavy but cheap and provides good impact resistance so you’ll see it used in large quantities for boat hull, as protective layers on other composite, and for generally cheaper applications. Carbon fiber is very strong, stiff, lightweight, but is very expensive and bad with puncture loads. It will be generally used where properties and weight matter such as in airplanes, bikes, high performance cars, etc though for a hefty cost. Kevlar has midrange properties, but it’s claim to fame is its energy absorption properties, specifically in ballistic puncture applications like bulletproof vests. There are other composites all around us – specifically steel reinforced concrete (steel rebarb fiber with concrete matrix), Adobe bricks (straw fiber and clay matrix), and even wood (organic fiber with an organic matrix).

Fiberglass is pieces of molten glass that are pulled into really long, thin strands. Then using resin, which for our purposes is a very strong glue, the strands are all bonded together in what is usually a haphazard crisscross way very similar to particle board. It’s strength comes from the fact that force upon it is distributed along the fibers and that because there are no seams or “grain” it lacks a structural weak point and can more or less maintain the same strength throughout.

Kevlar is a plant based fiber that has been given special treatments that make it super strong on its own, but most people think of the bullet stopping power and that comes from weaving the fibers to distribute the energy of the incoming bullet.

Carbon Fibers are built by alignment of graphite molecules in a special way that allows them to take advantage of the strong carbon crystal structure in a flexible fiber.

Fiberglass or composite boats are a great example of this. Once you create the molds you can easily make thousands of very precise copies of the original design very cheaply. They used to use plywood as a filler material for fiberglass boat bulkheads because it is rigid, bulky, and, relatively light. Now they mostly use foam because it is even lighter and while it is not as rigid, it doesn’t rot if it is exposed to water (this is a big problem with older fiberglass and ferrocement boats… a tiny puncture can let water into the filler material and wood filler will absorb water, becoming much heavier and will eventually rot to leave a void.) You can make incredibly strong bulkheads and structural components by sandwiching foam with many sheets of composite material. You can even add metal mesh, plates, and wire to armor areas that might experience trauma and to create hardpoints to mount equipment to your design like chain plates or motor mounts in a sailboat/motorboat. You can even add lead pellets or sand to the mix to balance your design or control its behaviour as forces are applied in different ways… extremely important for a sailboat or an airplane.

So, these materials can have fantastic strength and weight properties on their own, but when you add the ability to create intricate designs in ways that are easily replecateable and modifiable then you are able to build some really amazing things with these materials… you are almost building your design molecule by molecule through layering, shaping, and adding other materials and design elements. It’s a whole different world when you compare it to building something out of wood or metal where the macro characteristics of your materials (their shape mostly) broadly affect your design and construction methods. It’s truly a whole different way of building something.


they all have the same basic idea, which is bonding lots of fibres together with some form of plastic to create a material which is much stronger than the individual components. Fibreglass is one of many different types of GRP (glass reinforced plastic). Take a fibreglass canoe. If it was just the plastic ‘matrix’ material, it would be quite weak and would break easily, but is great for moulding and will take impacts much better than glass, which tends to shatter. By incorporating glass fibres, the material is made much stronger, but because the plastic is holding all the fibres together, the mixture doesn’t shatter as easily as glass.

It works with pretty much any fibre and plastic-like material. You even see the basic principle in steel reinforced concrete, where steel bars are incorporated into concrete to enhance its strength.

One Comment to Explained: What is the difference between fiberglass, kevlar, and carbon fiber – and what makes them so strong?

  1. I’m extremely impressed with your writing skills annd also with the layout on yur blog.
    Is this a paid themme or did you modify it yourself?
    Either way keep up the nice quality writing, it is rare to see a nice blog like this one today.

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