Carbon Fiber VS Fiber Glass VS Kevlar
Carbon Fiber vs. Fiber Glass vs. Kevlar
In talking to a lot of people about this, I have come to understand and realize that there are a lot of misconceptions and myths about these three materials, and most of the time people have it wrong. For example, people often argue that Carbon Fiber is stronger then Fiber Glass. That is a misconception. People often mistake strength for stiffness. I'll explain this a little bit more in detail.
The definition of strength is the capacity of an object or substance to withstand great force or pressure. Whereas stiffness is an indicator of the tendency for an element to return to its original form after being subjected to a force. Strength measures how much stress can be applied to an element before it deforms permanently or fractures. Hardness measures a material's resistance to surface deformation. This is actually the key to understanding the differences between the three different materials in question.
I am going to explain, once again, in a simpler way. If you take an object and apply pressure, pulling two ends of it in opposite directions, the point at which it breaks indicates its tensile strength. If you apply force that is larger than the strength of the material, the material would break. This is known as the ultimate tensile strength. Strength has nothing to do with how far the material can flex or deform. It is only defined by when it breaks. I hope this clears up any confusion around the concept of strength.
Stiffness is calculated by how far something stretches. The stiffness of a material is the ability of a material to resist deflection or deformation. It is important to understand that stiffness and strength are not related. You can has a very stiff material that is not very strong, or you can have a very strong material that is not very stiff. This brings us to elongation. The definition of elongation is the amount of extension an object has while under stress. This is usually expressed as a percentage of the original length. Elongation is basically how far the material will stretch before it breaks. So, if you have more elongation, you can essentially get a tougher material because it would stretch more before it breaks.
Imagine a plastic that is not very strong but stretches a long way. You hit it with a hammer, and the hammer would just bounce back or deform it. It's not going to snap it or shatter it. Now, imagine you do the same thing to the same size piece of glass. This may not be lot stronger than plastic, but it's a lot stiffer, so it doesn't deform as much. So, when you hit it with a hammer it will shatter in to thousand pieces.
Lastly, density is basically just how much a material weighs per unit area. Since we have established the basic definitions of what constitutes our material properties that we're interested, we can now actually start to have a look at the differences between the materials we are comparing.
Composite materials always have fibers sitting within a resin matrix. This basically means that we have a whole bunch of straight aligned fibers being held together by some sort of epoxy resin. We are not going to go much in depth on the epoxy resin for this article.
Let's take a look at the tensile strengths of different materials. First, let's bust some myths here by noticing that fiber glass is stronger then Carbon fiber. Ultimate tensile strength is defined by force over area, and glass is stronger than carbon fiber and Kevlar both. S-Glass is stronger than E-Glass, but it's a much more expensive grade of Fiber Glass.
In reference to the above graph, Carbon Fiber is stronger than High Strength Steel, but Kevlar and Fiber Glass are both stronger than Carbon Fiber.
If you check out this stiffness graph here, you will notice that Carbon Fiber is incredibly stiff, but don't confuse that with strength.
Now, let's look at this as horsepower and weight. A lot of horsepower and a lot of weight on a car doesn't help go fast. To go fast you need a healthy power to weight ratio. Similarly, what you need to understand is the strength/weight ratio and stiffness/weight ratio.
As we can see over here in terms of relative strength to weight, Kevlar is strongest in tension. It's an awesome material- it's very strong but it does have its drawbacks.
S-Glass is also stronger in strength to weight when compared to Carbon Fiber. S-Glass is a high quality material- it's very expensive, but it's not weaker than Carbon Fiber.
Generally, when people deal with E-Glass, people think that Carbon Fiber is stronger than Fiber Glass because they have E-Glass in mind. However, it's important to realize that the difference in terms of percentage is not all that different when looking at carbon fiber, E-Glass, and S-Glass. They are all pretty similar.
What I am trying to say here is that if your use or if your structure is only strength critical, then Fiber Glass might be the way to go as it's comparatively a lot cheaper. And, even if you go with E-Glass, it is not comparatively that much weaker.
That being said, the minute you put stiffness in the picture, it's a whole different story. If you see the same graph and look at the stiffness to weight ratio, there is a huge difference. Compared to S-Glass & E-Glass, Carbon Fiber is a lot stiffer. Compared to E-Glass, Carbon Fiber is five times stiffer. In fact, Carbon Fiber is stiffer than everything else by a lot more. Yes, even Kevlar!
If we were to reverse the direction of our loading and put the material in compression, all the other materials do quiet well except Kevlar.
When you look at this graph what you'll notice is that Kevlar looses by a considerable amount when you put it under compression.
So, if you are considering a material for a composite structure that might bare aerodynamic load, like a rear wing for a lot of downforce, then considering S-Glass or Carbon Fiber might be the best way to go. You might be able to get away with E-Glass depending on your particular structure and use, but you definitely should not risk it with Kevlar.
If you look at a stress/strain line graph you would understand it better. The stress/strain curve line means how much a material would move for a given amount of stress.
So, if you need a material with the best load bearing strength, S-Glass might be the way to go.
Kevlar is not bad either, coming in at second best place in the graph. It has really good abrasive qualities, and that is why it's used in bullet proof vests. It is a very good quality material for under body or skid plates of race cars.
So, the question is, what material body kit should you order for non competitive race or street cars. I would recommend Fiber Glass because it's the strongest, it has flex, and it's a seventh of the cost of Carbon Fiber. Additionally, it's not that much heavier.
Now, if you compare the weight of Carbon Fiber vs. Fiber Glass without Epoxy resin, Carbon Fiber is 30% lighter. But, once you make it into the structure, there is hardly a noticeable difference. This is because Fiber Glass needs a much thinner layer of Epoxy resin. In reality, it's only a 15% weight savings when compared to Fiber Glass.
The question you need to ask yourself while comparing Carbon Fiber and Fiber Glass is if a 15% weight difference is worth seven times the cost for your application. For most applications, it isn't worth it, and that money can be well spent elsewhere.
The reason why I am not addressing Kevlar here is because, for most applications, you would not need Kevlar considering Carbon Fiber and Fiber Glass would be the two best options for the most part. Fiber Glass is best used in areas where you need flex and strength. For structural parts, the underbody of the car, or tubing that requires strength, you don't want to have any flex. Carbon Fiber would be best for these areas.
I hope this article was useful. Leave a comment below, and let me know what you agree/disagree with, or if you would like me to write more informative stuff like this one.
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