Fibers are usually circular or nearly circular and are significantly stronger in the long direction because they are normally made by either drawing or pulling during the manufacturing process. Drawing orients the molecules so that tension loads on the fibers pull more against the molecular chains themselves than against a mere entanglement of chains. Due to the strength and stiffness advantages of fibers, they are the predominant reinforcement for advanced composites. Fibers may be continuous or discontinuous, depending on the application and manufacturing process.
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Before going over various types of fibers used as composite reinforcements, the major terminology used for fiber technology will be reviewed. Fibers are produced and sold in many forms
&#;&#;It is a benchmark specification to meet production manufacturing requirements. It is also a standard (one of many) upon which different materials are compared.
There are many different types of fibers that can be used to reinforce polymer matrix composites. The most common are carbon fibers (AS4, IM7, etc.) and fiberglass (S-glass, E-glass, etc.). As with the matrix, the fiber chosen will be determined by the end application.
Carbon fibers are conductive, have an excellent combination of high modulus and high tensile strength, have a very low (slightly negative) CTE, and offer good resistance to high temperatures.
Figure 1. Carbon Fiber Composites Examples
Carbon fibers are frequently categorized using tensile modulus. There are five categories of carbon fibers generally used in composites; low modulus, standard modulus, intermediate modulus, high modulus, and ultra-high modulus. The exact cut-off for these categories will vary depending on the reference consulted, but in general, low modulus fibers have a tensile modulus of less than 30Msi and ultra-high-modulus fibers have a tensile modulus greater than 75Msi. As a point of comparison, steel has a tensile modulus of 29Msi.
As the modulus increases, the fibers tend to get more brittle, more expensive, and harder to handle. Further, the tensile strength of the fibers generally increases as the modulus increases from low to intermediate but then tends to fall off in the high and ultra-high modulus fibers. I.e. the tensile strength of carbon fibers tends to be the greatest for the intermediate modulus fibers. For these reasons, standard and intermediate modulus fibers tend to give the best overall performance, unless the application is very stiffness oriented. This is illustrated even more clearly when fiber price and availability are also taken into consideration.
Fiberglass is, as its name implies, glass that has been spun into the form of fibers. Fiberglass is not as strong or stiff as carbon fibers, but it has characteristics that make it desirable in many applications. Fiberglass is non-conductive (i.e. an insulator) and it is generally invisible to most types of transmissions. This makes it a good choice when dealing with electrical or broadcast applications.
Figure 2. The application of fiberglass in infrastructure efficiency and sustainability (Source: CompositesWorld)
There are five major types of fiberglass.
E-glass and S-glass are, by far, the most common types found in composites. These types have good combinations of chemical resistance, mechanical properties, and insulating properties. Of the two, E-glass offers the more attractive economics, and S-glass offers better mechanical performance.
Natural fiber can be wood, sisal, hemp, coconut, cotton, kenaf, flax, jute, abaca, banana leaf fibers, bamboo, wheat straw, or other fibrous material. Natural fibers have low density, high specific properties, are biodegradable, are derived from renewable resources, have a small carbon footprint, and provide good thermal and acoustical insulation.
Figure 3. Natural Fiber Composites Application (Source: MDI)
A political/social advantage is that some products can be &#;farmed out&#; to semi-skilled indigenous workers. Replacement of fiberglass with natural fiber removes the concern about the potential of lung disease caused by the former and is a move toward sustainable development.
Aramid fibers are most commonly known as Kevlar, Nomex, and Technora. Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group (aramid); commonly this occurs when an aromatic polyamide is spun from a liquid concentration of sulfuric acid into a crystallized fiber. Fibers are then spun into larger threads in order to weave into large ropes or woven fabrics (Aramid). Aramid fibers are manufactured with varying grades based on varying qualities for strength and rigidity, so that the material can be somewhat tailored to specific design needs concerns, such as cutting the tough material during manufacture.
Each fiber mentioned above has many unique variables that must be taken into consideration when determining which to use for your project. This blog shows some examples of common products where each of the different fibers excels at.
Fibers are usually supplied in the form of rovings (glass fibers) or tows (carbon fibers). Rovings consist of straight continuous glass fiber strands or bundles of about 200 filaments; the number of strands depends on the end user, and these may be several kilometers long. Tows are likewise available in various configurations. The fibers are typically sized (coated) during production to promote wetting and adhesion, silane coupling agents being widely used for glass fibers. They may be woven into fabrics if required, including ad hoc fiber preforms for specific applications. Chopped fiber lengths range from less than 1 mm for injection to around 50 mm in randomly oriented mats for laminates.
Tow is the thread used to weave carbon fiber fabrics. As a standalone product, it can be used to make wound parts, pultrusion, or chopped for local reinforcement. A 24K tow (or strand) is composed of 24,000 individual filaments.
Tapes consist of unidirectionally aligned fiber tows pre-impregnated, prepregged for short, with a thermoset or thermoplastic resin. The resins most commonly used in aerospace and other high-performance applications are high-performance 2 part thermoset epoxies, and the following high-performance thermoplastics: polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), polyetherimide (PEI), and polyphenylene sulfide (PPS)
Two-dimensional woven products are usually offered as a 0º, 90º construction. However, bias weaves (45º, -45º) can be made by twisting the basic 0º, 90º construction. Weaves are made on a loom by interlacing two orthogonal (mutually perpendicular) sets of yarns (warp and fill). The warp direction is parallel to the length of the roll, while the fill, weft, or woof direction is perpendicular to the length of the roll. Textile looms (Fig. 2.18) produce woven cloth by separation of the warp yarns and insertion of the fill yarns. Most weaves contain similar numbers of fibers and use the same material in both the warp and fill directions.
Reinforced mats are made of either chopped strands or continuous strands lay down in a swirl pattern. Chopped strand mats are just that, mats made up of strands of glass fiber that have been chopped to shorter lengths, while continuous strand mats consist of fibers that run the full length of the mat. Mats are generally held together by resinous binders. They are used for medium-strength parts having uniform cross-sections. Both chopped and continuous-strand reinforcing mats are available in weights varying from 240 to g/m2 and in various widths. Surfacing mats, or veils, are thin, lightweight materials used in conjunction with reinforcing mats and fabrics to provide a good surface finish.
Prepregs are composite materials in which a reinforcement fiber is pre-impregnated with a thermoplastic or thermoset resin matrix in a specific ratio. Prepregs are most commonly found in conjunction with carbon fiber, as they have unique properties stemming from the fact that they have specific heat and pressure requirements. These materials are more widely used in aerospace and other high-end industries.
Addcomposites is the provider of the Automated Fiber Placement (AFP) ecosystem - including the Fiber Placement System (AFP-XS), 3D Simulation and Programming Software (AddPath), and Robotic Cells (AddCell). With the leasing program for the AFP system (AFPnext), composites manufacturers can work with thermosets, thermoplastics, dry fiber placement, or in combination with 3D Printers on a monthly basis.
Introduction to composites materials by Tri-Dung Ngo
The fiber reinforcement provides the structural performance required of the final part. The fibers or filaments come in many chemical types and forms and are the primary contributor to the stiffness, strength and other properties of the composite. The dominant chemical types of commercially available fibers are: fiberglass, aramid, carbon, polyester and vectran. Other fiber types may be suitable for special applications. The dominant forms that fibers are sold include: Strands (or roving or tow or yarn. Many fibers or filaments stranded together in a bundle, wound in a spool or reel,) woven fabrics (flattened strands of filaments woven in a variety of weaves to a type of fabric or cloth,) unidirectional (strands laid side by side and stitched or held together by other means, forming a kind of fabric that bares reinforcement only in the fill direction,) multiaxials (unidirectional woven fabrics stitched together in a combination of orientations,) and chopped strand mat (chopped strands held together with some kind of glue or &#;binder&#; in the form of a non-woven fabric.) All fibers designated for use in composites undergo chemical treatments and are coated with some kind of &#;sizing&#;. Sizing is a chemical that binds the filaments together, reduces abrasion, facilitates impregnation and acts like a coupling agent that enhances compatibility with one or more types of resin.
Selection of reinforcement type greatly influences costs. This is not only because of the quality and cost of the material itself, but also because higher cost material usually requires highly skilled personnel, more sophisticated production facilities and often more labour hours.
Fiberglass
Fiberglass
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was first discovered in and was firstly made commercially available in as insulation material. It became popular in the &#;s, when some of the health hazards associated with asbestos were becoming apparent. Due to the similarity in shape between the fiberglass and the asbestos fibers, fiberglass was able to effectively replace asbestos in many applications such as electrical, thermal, and acoustic insulation and structural reinforcement. Today it is the dominant reinforcement fiber in composite construction, accounting for over 90% of worldwide consumption. This is simply because it has good strength to weight characteristics, can be processed easily and sells at a low price. Glass filaments are made relatively easily by extruding molten glass. The diameters of the fibers produced range from 5 to 25 microns. Many different types and qualities of glass sell for significantly different cost. The most widespread quality is &#;E-glass&#;, &#;E&#; from the word Electric implies that it is an electrical insulator. It is low cost product, used mostly in the marine industry. Others are &#;S-glass&#; and &#;S2-glass&#;, the letter &#;S&#; comes from the word Strength implying that it has improved mechanical properties. These types are much more expensive and used mostly in armor applications. &#;S&#; is certified as for the production parameters. &#;AR glass&#; is resistant to alkali chemical attack. &#;C&#; or &#;T-glass&#; is resistant to acid and corrosion. &#;A-glass&#; is glass with more alkali content similar to window glass, and costs a bit less. Generally, when cost is a major driving force in the selection of a reinforcing material, fiberglass is usually preferred.
Aramid
Aramid
was invented by DuPont in &#;s (&#;Kevlar&#; is the registered trade name of Dupont aramid) as a result of research on nylon (polyamide) fibers. It was firstly introduced in the market in the &#;s as tire reinforcement and like fiberglass, as an asbestos substitute. The chemical structure of aramid shows the aromatic benzene rings along the polymeric backbone. The word aramid is a contraction of its chemical description&#;aromatic polyamides. Strength and modulus of aramid are very good, density very low, UV resistance low and compression and shear strength are similar to E glass. Its value comes from excellent toughness and resistance to impact, damage, abrasion and heat (up to 500 oC.)
The superior toughness of aramid is an outcome of the energy consuming failure mechanism of its fibers. This energy absorbing failure mechanism makes it ideal for use in armor, military and ballistic applications, like helmets and bullet-proof vests. Among many other very important uses, it is used for firefighting protection, on the underside of airplanes (protection against stone hits during take off and landing) and the under side of race cars. It is used generally in important structures whenever impact, abrasion and/or heat is anticipated. It also blends very well and works together with other fibers, like glass and carbon. Carbon-aramid &#;hybrid&#; constructions bare the high strength and stiffness of carbon and the impact protection of aramid. Processing aramid in composite fabrication is somewhat more difficult than fiberglass and carbon. Toughness makes fabrics difficult to cut with conventional methods. Fiber wetout is more difficult than fiberglass and carbon. Orthophthalic polyester resin will not adhere well to aramid (isophthalic polyester is much better). Post fabricating aramid components, e.g. trimming off the edges is also difficult. Quality cutting tools are recommended.
Carbon
Carbon
has the highest strength and highest price of all reinforcement fibers used in composites today. It was invented in the UK in early s at the Royal Aircraft Establishment at Farnborough, Hampshire. The most common method of making carbon fiber is the oxidation and thermal pyrolysis of an organic precursor, polyacrylonitrile (a polymer fiber based on acrylonitrile.) When heated in the correct conditions, the non-carbon constituents evaporate away. The resulting fiber is 93&#;95% carbon. Instead of PAN as a precursor, carbon fiber can also be manufactured from pitch or rayon. The size, or thickness of Carbon tows is measured in &#;k&#; or thousands of filaments. A 3k tow has 3,000 filaments and a 12k has 12,000. Carbon fibers exhibit: substantially better strength and stiffness values than all the others, outstanding temperature performance, high electrical and low thermal conductivity. Impact or damage tolerance of pure carbon composite products can be from relatively low to very poor, and greatly depends on processing method. Despite that, when weight on a composite product is important, carbon fiber is the reinforcement to use.
Hybrid
When different types of reinforcing fibers are combined, usually woven in a fabric, the result is a hybrid fiber reinforcement.
Hybrid fabrics
are a weaved mixture of two or more different types of fiber yarns. This mixing is generally utilized in order to take advantage of the good properties and characteristics of each reinforcement type, while at the same time mitigating the effects of their less preferable properties (synergy effect). A similar synergy result could be obtained by two or more layers of the different materials, however on the other hand, hybrid fabrics also prevent or reduce the possibility of delamination. This happens because on hybrid fabrics the different types of yarns are woven into each other, thus making the whole thickness of the laminate skin material more uniform, (avoiding layers of different materials, that have different mechanical properties, with higher concentration of stress and mechanical loads at the point of surface contact of different material layers).
High modulus polyester - Diolen
HMP and
Diolen
are high strength polyester fibers. Such fibers have been used in industrial applications for many decades, in a wide field of applications like tire reinforcement, safety belts, ropes and nets. Recognition of their value in composite applications for aerospace, defense, marine and transportation has come in the late years. They are used both as primary reinforcement and in hybrid arrangements with fiberglass and aramid. Toughness makes them ideal for use on the outside layers of composite construction for impact protection. Placed on the outside layers they also protect aramid and glass from moisture (and blistering) and being smooth, they reduce print-through. At the same time, their very low density also makes them ideal for use in the middle layers, where working as a core material, they add stiffness to the structure. Other attractive features include good impact and fatigue resistance, and potential for vibration damping. Generally, properties of Diolen resemble the properties of aramid at lower level, but its reasonable price and reliability make it attractive. Diolen performance at elevated temperature falls off. It should not be used in above 200o F applications. Processing is easier than this of aramid, but not as easy as fiberglass and carbon.
Vectran
Vectran
is the outcome of 30 years research on liquid crystal polymer (LCP) fibers. 100% of world production is now owned by one company, Kuraray Co., Ltd. Annual production is only 600 tons in , but is growing at a rate of about 10% a year. This fiber is formed by melt extrusion of LCP through fine diameter capillaries. It is used in aerospace composites and many other high value industry applications. It exhibits high strength and modulus, excellent creep resistance, high abrasion resistance, excellent flex/fold characteristics, minimal moisture absorption, excellent chemical resistance, high dielectric strength, outstanding cut resistance, high impact resistance, outstanding vibration damping characteristics, very good property retention at high/low temperatures (melting point of 330 °C, with progressive strength loss from 220 °C.) They have gold color, similar to aramid, but are usually painted in other colors for cosmetic purposes. Vectran also adheres to resins better than aramid. Among others, it should be used in applications where weight and impact resistance are important.
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