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Comparison of Carbon Fiber, Kevlar® (Aramid) and E Glass used in Composites for Boatbuilding.

For more detailed description of the characteristics of Kevlar® and Carbon Fiber follow the links at the top.


Your can't talk about composite fibers without introducing epoxy. Although reinforcing fibers are used in many applications, if you are a boatbuilder, epoxy is the substrate you will be dealing with. Epoxy is used to bind whatever strengthening fiber is being used and to lock them in position so they can stay in position to do their job.

Fiber reinforced structures usually have the fibers running in specific directions to focus the reinforcement where it is needed and the epoxy keeps the fibers where they are needed. Although the main purpose of the epoxy matrix is to adhere to and transfer the loads to the fibers it is a strong material in its own right. It helps protect the fibers from damage and provides impact resistance.

Polyepoxide or as it is better known Epoxy, is a thermosetting network polymer which forms when an epoxyide resin reacts with a polyamine hardener. The resulting Epoxy polymer is heavily cross linked (networked), and thus strong, hard and rigid. It is also somewhat brittle.

Epoxy has found a wide range of applications, including fiber-reinforced plastic materials, general purpose adhesives and strong chemically resistant coatings and finishes.

Although epoxy bond mechanically if the surfaces are rough, the strongest bond is by far an ionic bond with its reinforcing fibers. That's why not all fibres are suitable as epoxy composite. The epoxy simply does not stick to everything.

Epoxy has proved to be a relatively safe material. The main risk associated with epoxy use is sensitization to the hardener which can result in serious allergic reactions, which can occur even after a few days. This can become asthma in sensitive individuals. Bisphenol A, which is used in epoxy resin, is a known endocrine disruptor.

It's always better to protect yourself by working with gloves and at the very least have a well ventilated area. Respirators are helpful if you have a tendency to react to chemicals.

Cured Epoxy is very resistant but will degrade if exposed to UV. The surface becomes chalky and looses strength. For this reason it needs to be coated with UV protection. Epoxy strength is degraded when exposed to higher temperatures. Over 350 °F (177 °C).

Tensile Strength

Tensile Strength

Tensile Strength is the maximum stress that a material can withstand, while being stretched, before if fails. Some non brittle material distorts before breaking, but Kevlar, Carbon Fiber and E glass are brittle and fail with almost no distortion. Tensile Strength is measured in Force per unit area: Pa or Pascals. Ultimate tensile strength (UTS) or ultimate strength are terms also used.

Stress is the force, strain is the deflection due to stress.

Comparing the Tensile Strength of
E Glass, Carbon Fiber and kevlar® (Aramid) MPa

Note: These figures are for comparison purpose. They can vary with the manufacturing process, composition of the epoxy, formulation of the aramid, precursor fibre for the Carbon fiber. Units are in MPa

  Fibre Strength Laminate Strength
E Glass 3450 1500
Carbon Fiber 4127 1600
kevlar® 2757 1430
Epoxy N/A 12-40

The tensile strength of Carbon Fiber, Kevlar® and Glass is similar. This comes as quite a surprise to many people.

When making comparisons it's important to note that there are differences in each of the materials in manufacturing processes, precursor materials, and after-treatments. These all influence the strength. The figures are given for comparison only, The figures given in this article mostly come from Wikipedia and from the spec sheets of manufacturing companies. Another source of data about materials is the various engineering pages about materials. MatWeb is an example of a site that provides Material Property Data.

In the end if Tensile Strength is your only concern, save your money and use e glass.

Density and Strength to Weight Ratio

Weight per Unit Volume or Density of Carbon fibre, Kevlar, and E Glass

The Denser a material is the heavier it will feel for an equal sized chunk.

When we compare the density of our 3 materials we see a significant difference. If you make up 3 samples exactly the same size and weight them you quickly see that Kevlar® fiber is much lighter, Carbon Fibre is next and the E Glass is the heaviest. So for the same weight of composite we get more strength.

In other words, any structure, where we require a given strength, can be smaller or thinner if made out of carbon fiber or Kevlar® composite than if made out of glass.

After making your samples and testing them, you would find that the Glass composite is almost twice as heavy as the Kevlar® or the Carbon Fiber Laminates. In other words it takes half as much Carbon or Aramid fiber to get the same strength as the glass sample. You can save a lot of weight using Kevlar® or Carbon Fibre. This property is called strength-to-weight ratio.

Kevlar® (Aramid) and Carbon Fibers have a high strength-to-weight ratio when tested unidirectionally in direction of the fibers, while e glass has a lower strength-to-weight ratio. Glass is still quite high, just not as good as Kevlar® or Carbon. The units are kN.m/kg. N stands for Newton

  Fibre Strength Laminate Strength Density of Laminate grams/cc Strength-to-Weight
E Glass 3450 1500 2.66 564
Carbon Fiber 4127 1600 1.58 1013
Kevlar 2757 1430 1.44 993
Epoxy N/A 12-40 1-1.15 28

Compression Stregth Comparison of Kevlar, Carbon and Glass Fibers

Whereas Carbon and Glass are only slightly less strong and stiff in compression than in tension, Kevlar® is much less stiff and strong when compressed. In fact in some tests the Kevlar® was failing before the resin matrix. According to Researchers at Rowan University, pdf document, "The compressive strength of Kevlar® is 1/10 of its ultimate tensile strength". Again look at this figure as relative because there are many variations but it shows a significant characteristic of Kevlar® well known by Kayak and boat builders everywhere. Kevlar® is strong but does not like to be hit sideway which causes a compression strain, and often a crack.

Modulus of Elasticity

Stiffness or
Modulus of Elasticity

Young's modulus is a measure of the stiffness of an elastic material and is one of the ways used to describe materials. It is defined as the ratio of the uniaxial (in one direction) stress over the uniaxial strain (distortion in the same direction).

Youngs Modulus is = Stress / Strain

In other words materials with a high Youngs Modulus are stiffer than materials with lower Youngs Modulus.

A high modulus of elasticity is sought when bending or deflection is not wanted, while a low modulus of elasticity is required when flexibility is needed.

To be more absolutely accurate elastic modulus is not the same as stiffness. Elastic modulus is a property of the constituent material; stiffness is a property of a structure containing the various materials. In other words a material with high modulus of elasticity, will make a stiff structure.

Just as strength is not necessarily the same in all directions, Young's Modulus, or the tensile modulus of a material is not always the same in all directions. This is why reinforcing fibers are specifically aligned in one direction, to provide more strength and stiffness in that direction, in an epoxy build-up.

Comparing the Modulus of Elasticity
of E Glass, Carbon Fiber and Kevlar® (Aramid)

Again, don't look at the exact figures they can vary quite a lot. They are useful for comparison though. Stated in GPascals

Material Young's Modulus
E Glass 30-40
Carbon Fiber 125-181
Kevlar® 70.5-112.4
Epoxy 3

The stiffness of Carbon Fiber, Kevlar® and Glass are very different. Carbon Fiber is by far the stiffer of the composite materials.

The actual figures presented here are for comparison only. The important thing to take away is that Carbon Fiber is about twice as stiff as Kevlar, and about 5 times stiffer than glass.

There are many types of Kevlar, glass, and carbon fiber and they all differ. The exact figures here are almost meaningless what is important is the relative stiffness.

The down side of the exceptional stiffness of carbon fiber is the fact that it tends to be more brittle. When it fails it tends to fail without showing much strain or deformation. One speaks of catastrophic failure.

Flammability and Thermal Degradation

Both Kevlar® and Carbon Fibre are resistant to elevated temperatures. Neither have a melting point. Both Materials have been used for protective clothing and fabrics used near fire.

Glass eventually melts but is also highly resistant to high temperatures. For this reason fiberglass is sometimes used for curtains in areas where fire resistance is important. Matted glass fiber of course is used in buildings to improve fire resistance. Carbon and Kevlar® are used to make protective firefighting or welding blankets or clothing. Kevlar gloves are commonly used in the meat industry, to protect hands while using a knife.

Since the fibres are rarely used alone, the thermal resistance of the matrix, usually epoxy, is also important. Epoxy softens quickly when subjected to heat.

Electrical Conductivity

While carbon fibre is very definitely conductive, Kevlar® and Glass do not conduct electricity.

Kevlar® is used for guy lines in transmission towers. Although it is not conductive, it can absorb water and the water does conduct electricity (or rather minerals in the water make it conductive.) so in such applications a waterproof coating is applied to the Kevlar.

Because Carbon Fibre does conduct electricity, galvanic corrosion is a concern when it is in contact with other metallic parts.

Boaters, who have carbon fibre masts and spars, have learned to insulate their aluminium fasteners and connections to avoid corrosion.

UV Degradation

Aramid fibres will degrade in Sunlight and high UV environment.

Carbon Fibre or glass are not very sensitive to UV radiation.

It is fairly irrelevant however because neither Kevlar, glass nor carbon fibre are often used on their own in boatbuilding applications. They are embedded in a matrix that often degrades in UV light. This is the case of epoxy resin which will go chalky and lose strength if allowed to remain in sunlight. Polyester and Vinylester resins are more resistant to UV exposure but are weaker than epoxy.

If you anticipate using these composites outside you need to protect them from UV. Some UV resistant epoxy resins such as this one from Merton's or this epoxy glaze from Key Resin are available.

Fatigue Resistance

If a part is made to bend and straighten repeatedly it eventually fails due to fatigue. Whereas Carbon fibre is somewhat sensitive to fatigue and tends to fail catastrophically without showing many signs of distress, Kevlar® is more resistant to fatigue. Glass is somewhere in between and can be quite fatigue resistant depending on the type of glass and the setup.

Abrasion Resistance

Kevlar® has a strong abrasion resistance. This makes it difficult to cut. Suppliers often sell special shears and extra strong scissors for Kevlar® cloth. One of the common uses of Kevlar® is as a protective glove for use in areas where the hand might be cut by glass, or while using sharp blades. Carbon fibre and Glass are less resistant.

Chemical Resistance

Aramids are sensitive to strong acids, bases, and some oxidizers, like chlorine bleach (sodium hypochlorite). These cause degradation of the fiber. Regular chlorine bleach (e.g. Clorox®) and hydrogen peroxide cannot be used with Kevlar®, oxygen bleaches such as sodium perborate (e.g. OxiClean®) can be used without damaging the Aramid fiber.

Carbon Fibre is very stable and is not sensitive to chemical degradation. The epoxy matrix can degrade however.

Adhesion to Matrix

In order for carbon, Kevlar® and Glass to give their best performance, they must be kept in position by the matrix, usually epoxy. Therefore the ability of epoxy and the various fibers to stick together is essential.

Both Carbon and Glass have no trouble sticking to epoxy however the aramid-epoxy bond is not as strong as we would like. This reduced adhesion allows water penetration to occur.

As a result Aramids tends to absorb water. This combined with its not ideal adhesion to epoxy means that if the surface of a kevlar® composite is damaged (such as with a sharp blow) and water can get in, then it is possible that the Kevlar® will absorb water along the fibers and weaken the composite.

If there is water penetration and the structure is subject to freezing and thawing further damage can occur.

Colour and Weave

Aramid in its natural state is a light golden colour. It can be coloured and many wonderful hues are now available. Glass has also been made in coloured versions. Several Carbon Fibre Kevlar® or Glass are available if you need carbon stiffness and panache and colours. supplies Carbon fiber in fabulous weave patterns.

Carbon Fiber is always black. It can be blended with coloured aramid, but by itself it cannot be coloured.

What does this all mean to the boatbuilder?

The short answer is I'm not sure. Or rather the choice of materials involves compromises.

If strength is the only consideration, then stronger varieties of Kevlar® or Glass are best. If stiffness, strength AND light weight are THE important factors then Carbon fiber is your man, if price is the deciding factor then go for glass.

There are several mixtures available. In particular Kevlar® has been successfully mixed with carbon fiber and with glass to get the good traits of each and reduce the less desirable ones.

Comparison chart of Glass, Aramid and Carbon Fibre

E=Excellent, G=Good, P=Poor, F=fair
  Glass Aramid Carbon Fibre
Cost E F P
Weight to Strength Ratio P E E
Tensile Strength E E E
Compressive Strength G P E
Stiffness F G E
Fatigue Resistance G-E E G
Abrasion Resistance F E F
Sanding/Machining E P E
Conductivity P P E
Heat Resistance E F E
Moisture Resistance G F G
Resin Adhesion E F E
Chemical Resistance E F E
Remember these ratings are relative to each other. Not to all materials.

I try to be accurate and check my figures, but mistakes happen. Check the suitability of any material against the technical information provided by the manufacturer.

Test results can vary considerably from sample to sample, material manufacturing, or environmental conditions. The tables I provide are for comparison, not for planning critical builds. I believe the figures are as accurate as I can find but I did not do the tests myself.

Many of the strength figures I quote come from Wikipedia so are secondary sources. I try to check with the the actual source or manufacturers when I can. I also get useful information from published research. I sometimes make mistakes (!!?!) in transcribing the data.

There are several different varieties of aramid fibers which go by trade names such as Kevlar®, Twaron® and Nomex®. Although they are all in the general class of Aramids their properties vary within a range. Aramids are also being produced in China under other names.

Depending on what the precursor fiber used to make Carbon Fibre, the tensile strength will be higher or lower as will the stiffness.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine