Terms and Definitions used in Composites and Reinforcing Fabrics such as Kevlar (Aramid) Fiberglass, and Carbon Fibre.
- Abrasion Resistance
Abrasion is the process of scratching, scuffing, marring, wearing down, or rubbing away. Some materials such as Kevlar resist abrasion exceptionally well and are used for protective gear.
Also known as Buckminsterfullerene are spherical carbon nanomaterials. It is a spherical form of carbon composed of interconnected pentagonal and hexagonal (5 - 6 sided) units. It has been compared to soccer balls. It occurs in nature and has been detected in outer space. It is the smallest spherical carbon structure known. There are much larger ones. The term buckyballs is also loosely used for any number of larger carbon spherical structures.
- Carbon Nano Tubes
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus. Most nanotubes have a diameter of a nanometer thus their name. Their length can be several million times longer than their width.
Nanotubes are made of hexagonal carbon crystals joined as a hollow tube. It looks like chicken wire in tube form, with no seam of course. Single and multi wall versions have been created.
Wikipedia has an interesting article on carbon nanotubes. If you want to buy a small quantity Play with Carbon Australia, has Carbon Nanotubes for sale.
- Numbering system for fiber size, equal to the mass in grams per 9000 meters of fiber. It is used in the textile industry. Nylon stockings are classified by their denier size
The mass density or density of a material is its mass per unit volume.
- Compressive Strength
When a material is loaded in such a way that it stretches it is in tension. If the material compresses and shortens it is said to be in compression. Some materials such as Kevlar have high tensile strength but low compression strength. Other materials such as epoxy and carbon fiber have similar or only slightly smaller compressive and tensile strengths.
- Electrical Conductivity
Measures a material's ability to conduct an electric current. Carbon fiber is an electrical conductor, fiberglass is not. In boat building, conductivity is significant because of galvanic corrosion.
- Fatigue Resistance
Fatigue is the localized and progressive structural damage that occurs when a material is subjected to cyclic loading. We are all familiar with bending metal back and forth until it breaks, this is fatigue. Carbon fiber is subject to fatigue whereas Kevlar is more resistant. Fatigue often results in catastrophic failure, where no obvious signs of damage are present when the material fails.
- HDT Heat Deformation Temperature
Maximum temps that a resin can withstand before the structure deforms from heat. Important to know when the application is close to a source of heat such as near an engine. Although the reinforcing fibres might be very heat resistant, the matrix, epoxy or polyester is more sensitive and likely to deform before the fibres are affected. Generally the higher the modulus (stiffness) the higher the resistance to heat deformation. It has been found that a part that is going to be subject to higher temperatures will be more resistant if it goes through a post cure cycle where it is heated to near the HDT and allowed to cool. This allows for maximum reaction in the epoxy/polyester components and increases the likelihood that all the chemical reactions have been completed.
Depending on the brand and formulation, epoxy can handle constant or repeated temperatures of about 160 -170 degrees F in dry conditions and about 140 - 160 degrees F in wet conditions.
High temperature epoxy can go as high as 300 or slightly more in dry conditions and 250 in wet conditions as long as it has had a post cure cycle.
- Hybrid Fabrics
Fabrics that have 2 or more types of fibers in their composition. Mixtures such as kevlar/carbon, glass/kevlar and glass carbon are all available. Metallic threads are sometimes also used. The advantage could be in overall improved characteristics, or in improved appearance, such as brightly dyed glass/carbon combinations.
- Kevlar and Twaron
Brand names for Aramid fibres. I have a page on Aramid properties.
- Prepreg / Pre Impregnated
This refers to a cloth often Carbon fiber, that has been coated with heat activated resin, usually epoxy. It is shipped to the customer, usually in refrigerated containers, cut and placed in position then subjected to pressure and heat to cure the epoxy. It is a clean and convenient way of working but has some drawbacks as well. The prepreg is usually kept in a refrigerator until used, and a source of heat, often steam is necessary. Since the 2 component parts of the epoxy are already mixed before they are applied tot he cloth the prepreg has a limited shelf life.
- Polyacrylonitrile, is the raw material (precursor) for a type of carbon fibers used as reinforcement. Pan carbon fiber is stronger in compression and tension than Pitch based fibers which are stiffer.
- A dry assembly that has been cut and shaped usually to fit into a specific mould or application. Preforms are produced when regular cloth and mat would be difficult to shape reliably by hand. It ensures that the part is suitably and reliably re-enforced. Pre forms are often custom made for a specific application and stitched or bonded with resins that will dissolve when embedded in matrix. It is often cheaper to produce a pre form than to take the time to carefully cut cloth and mat each time.
- Precursor Fibers
- Fibers that are the raw material for carbon fiber production. The precursor fibers are heated and carbonized in multiple steps to become carbon fiber. Generally there are 2 common precursor fibers. PAN (or polyacrylonitrile) and Pitch based Precursor fibres using tar or petroleum based fiber.
- Resin, Polyester
Thermosetting resin activated with a catalyst, often Methyl ethyl ketone peroxide. The resin is commonly used in boatbuilding with fiberglass reinforcement. It has a strong smell in use. The rate of setting can be modified by adding more or less catalyst, within limits.
Of the 3 resins commonly in use it has the lowest tensile strength, stiffness and resistance to fatigue.
Both polyester and vinyl ester resins are subject to water degradation due to the presence of hydrolysable ester groups in their molecular structures. It has lower adhesion than epoxy. It is a strong useful material but not the best. It has the advantage of being the least expensive and it has good UV resistance.
- Resin, Epoxy
Epoxy has the strongest tensile strength, stiffness and fatigue resistance of the commonly used resins. It is not as subject to water degradation and osmosis as polyester and vinyl ester. It is however more expensive and subject to UV degradation. Unlike vinylester and polyester which harden with the addition of a small amount of catalyst, epoxy comes in 2 parts that are integral to the structure and must be measured and mixed carefully. Many different formulations exist that set at different rates and different temperatures, as well as offer increased temperature resistance. Epoxy has better adhesion than polyester and vinylester. It sometimes has the disadvantage of creating a wax like amine blush that must be washed off before more layers are added. Some brands blush more than others. Wikipedia article on epoxy
- Resin, Vinylester
Vinylester characteristics lie between polyester and epoxy in strength, stiffness and fatigue resistance. It is also in the middle for adhesive properties and price. It is similar in use to polyester.
Resistance to bending. It is measured by the Young's modulus. Carbon fiber has high modulus.
- Young's Modulus
A measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial (stress in one direction) stress over the uniaxial strain (distortion in one direction) in the range of stress in which Hooke's Law holds. I have a page describing the Young's modulus of Carbon Fibre.
The higher the modulus the stiffer the material. Young's modulus applies to the material, stiffness applies to the finished part. Thus carbon fiber has a high modulus of elasticity, but a carbon fiber paddle is stiff.
- Tensile Strength
The maximum stress that a material can withstand while being stretched or pulled before necking or in the case of brittle materials failing
- Chopped Strand Mat
Chopped strand mat is very economical and offers good stiffness. However, for greater strength and a lower weight, biaxials and woven cloth fabrics are generally preferred. Chopped mat cloth is a sort of felted cloth. It offers the ability to stretch and fit into complex curves better than cloth. Small pieces can be torn and fit in nooks and crannies. It generally takes more resin to fill mat than woven cloth. It is often used as the first layer in a repair because it can conform to uneven surfaces better than woven cloth.
Chopped Strand Mat is available in several weights. The thinnest is a veil mat at less than .1 of an ounce. This is sometimes used as a finishing layer to help even out weave marks. Mat is usually embedded in Polyester resin and many of the mats are held together with a styrene binder that is dissolved by the polyester resin, improving bond. Fiberglass mat is a good way of getting bulk in a layup at the cost of increased weight because of the higher content or resin required to fill it.
- Plain Weave
The most common re-enforcement cloth weave. It is a one over one under pattern of weaving. The fibres run at 90 degrees to each other. it's the kind of woven fabric you will most likely run into when you buy fiberglass cloth at the hardware store. It is available in many weights.
- Biaxial or Bidirectional cloth
Cloth where the fibers run in 2 directions. +/- 45 degree is very common. This mean that the fibers cross at an angle of 45 degree. Most biaxial cloth are not woven but rather a series of threads hold the fibres together. Woven cloths are not as strong as when the layers are just laid on top of each other and fastened by non strength thread.
The edge of a woven cloth. There is often a thickening or different pattern at the very edge this is the selvage. These are sometimes seen in fiberglass tape where the edge is thickened.
- Twill Weave
A method of weaving where 2 the weft and warp are woven at 90 degrees to each other. Each fiber go over 2 fibers and under 2 fibers. Each row is offset by one. This creates a cloth that appears to have diagonal lines. Twill and Satin weaves are stronger than plan weave cloth. They also drape better and are easier to form around curves.
- Specific Strength or Strength to Weight Ratio
Strength divided by weight per unit volume. Materials that are light and strong have high strength to weight ratio. Carbon fiber and balsa wood both have high strength to weight ratios.
- Tow or Roving
An untwisted bundle of continuous filaments. Tows are designated by the number of fibers they contain. For example, a 12K tow contains about 12,000 fibres. Tow is used to wrap shapes or to provide dramatic increase in strength in a layup when applied in a specific direction. Some ultralight Skin on frame boats use Kevlar tow network to significantly increase stiffness of the framework.
Depending on the end use, tow or roving can be supplied to use in guns where the roving is automatically chopped, mixed with resin and sprayed onto a mould. Tow can be used to wrap shapes such as pipes or pressure vessels, it can be made as reinforcement for pulltrusion forms such as tent poles or tubing.
Both terms are in use. Some manufacturers use tow to describe the fibre bundles and Roving to describe coarse thick cloth. It is available as large spools.
Because of its strength and low stretch Kevlar Roving is used by some designers of Skin on Frame Ultralight boats as a way of making the framework stiffer. Kevlar roving is strung diagonally to the framework.
- Woven Roving
Is a high strength coarse fabric woven from tow or roving, used extensively in boat building to provide strength. Weights such as 18 ounces or more are common.
A cloth where majority of fibers are lined up in one direction. There is usually some threads sewn or woven in to keep the fabric together but they provide no significant strength.
The fibres that run lengthwise parallel to the edge of the weave. The Warp threads are as long as the roll is long.
These fibres run across the weave. They are only as long as the width of the fabric.
Composite reinforcing fibers such as glass, Kevlar, carbon are sold by weight per unit area. The lower the weight the lighter the fabric. I used 6 ounce cloth in my Skerry. This means that the fabric weighed 6 ounces per square yard. The weight of Glass, Kevlar and Carbon can also be expressed in grams per square metres.
Epoxy Works has a page on 'Determining Epoxy's Physical Properties'
email me if you find mistakes, I'll fix them and we'll all benefit: Christine