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DK8293_C001.fm Page 1 Wednesday, October 25, 2006 2:09 PM Asked 1 Frequently Questions about Composite Materials This chapter is intended to present basic concepts of composite materials in a simple, direct, reader-friendly question-and-answer format. Although self-generated, the questions represent the mindset of the uninitiated in that they are frequently asked questions about composite materials. Many topics discussed in the following presentation are discussed in detail in subsequent chapters of this book. Readers will also encounter many new terms related to composites, which are defined in the Glossary provided at the end of this book. What are composite materials? Composite materials (often referred to as composites) are man-made or natural materials that consist of at least two different constituent materials, the resulting composite material being different from the constituent materials. The term composite material is a generic term used to describe a judicious combination of two or more materials to yield a product that is more efficient from its constituents. One constituent is called the reinforcing or fiber phase (one that provides strength); the other in which the fibers are embedded is called the matrix phase. The matrix, such as a cured resin-like epoxy, acts as a binder and holds the fibers in the intended position, giving the composite material its structural integrity by providing shear transfer capability. The definitions of composite materials vary widely in technical literature. According to Rosato [1982], a composite is a combined material created by the synthetics assembly of two or more components a selected filler or reinforcing agent and a compatible matrix binder (i.e., a resin) in order to obtain specific characteristics and properties The components of a composite do not dissolve or otherwise merge into each other, but nevertheless do act in concert The properties of the composite cannot be achieved by any of the components acting alone. Chawla [1987] provides the operational definition of a composite material as one that satisfies the following conditions: 1. It is a manufactured material (not a naturally occurring material, such as wood). 1 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 2 Wednesday, October 25, 2006 2:09 PM 2 Reinforced Concrete Design with FRP Composites 2. It consists of two or more physically or chemically distinct, suitably arranged phases with an interface separating them. 3. It has characteristics that are not depicted by any of the constituents in isolation. The EUROCOMP Design Code [1996] provides a rather technical definition of a composite or composite material as a combination of high modulus, high strength and high aspect ratio reinforcing material encapsulated by and acting in concert with a polymeric material. In a practical sense, the term composite is thus limited to materials that are obtained by combining two or more different phases together in a controlled production process and in which the content of the dispersed phase in the matrix is substantially large. Portland cement concretes, and asphalt concretes are examples of man-made composite materials with which civil engineers are familiar. These clearly heterogeneous composites consist of a binding phase (Portland cement phase or asphalt) in which aggregates up to about 1 inch in size are dispersed. The composites used for construction include fiber-reinforced composites, in which fibers are randomly dispersed in cement or polymer matrix, and laminated composites made of a layered structure. You mentioned wood as being a naturally occurring composite material. Can you explain? Wood is a composite material that occurs in nature; it is not a manufactured material (hence not generally discussed as a composite). However, it is a composite material in that it consists of two distinct phases: cellulose fibers, and lignin that acts as a binder (matrix) of fibers. What is meant by plastic? According to the EUROCOMP Design Code, plastic is a material that contains one or more organic polymers of large molecular weight, is solid in its finished state, and can be shaped by flow at some state in its manufacturing, or processing into finished articles. From a chemistry standpoint, plastics are a class of materials formed from large molecules (called polymers), which are composed of a large number of repeating units (called monomers). The monomers react chemically with each other to form extended molecular chains containing several hundred to several thousand monomer units. Most monomers are organic compounds, and a typical polymer is characterized by a carbon chain backbone, which can be linear or branched. The molecular structure of the unit that makes up very large molecules controls the properties of the resulting material, the polymer or plastic. The rigidity of the chains, density and the regularity of packing (i.e., crystallinity) within the solids, and interaction between the molecular chains can be altered and thus change the bulk properties of the plastic. Do other composite materials occur naturally? Bone that supports the weight of body is an example of a naturally occurring composite material. Weight-bearing bone is composed of short and soft collagen fibers that are embedded in a mineral matrix called apatite. The human body (Figure 1.1) is an excellent example of a living structure made from composites. 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 3 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 3 FIGURE 1.1 The human body an example of a perfect composite structure. (Image by Dorling Kindersley.) What are some of the most commonly used fiber types? A variety of fibers are used in commercial and structural applications. Some common types are glass, carbon, aramid, boron, alumina, and silicon carbide (SiC). How are fibers arranged within a composite? The arrangement of fibers in a composite is governed by the structural requirement and the process used to fabricate the part. 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 4 Wednesday, October 25, 2006 2:09 PM 4 Reinforced Concrete Design with FRP Composites What determines the mechanical and thermal properties of a composite? The mechanical and thermal properties of a composite depend on the properties of the fibers, the properties of the matrix, the amount and the orientation of fibers. What are some common types of matrix materials? A number of matrix materials are used by the industry (and their number is growing), the more frequently used matrix materials are: 1. Thermoplastic polymers: polyethylene, nylon, polypropylene, polystyrene, polyamids 2. Thermosetting polymers: polyesters, epoxy, phenolic, polymide 3. Ceramic and glass 4. Carbon 5. Metals: aluminum, magnesium, titanium What are some of the resins used in composites? Various polymers include epoxy, phenol, polyester, vinyl ester, silicone, alkyd, fluorocarbon, acrylic, ABS (acrylonitrile-butadiene-styrene) copolymer, polypropylene, urethane, polyamide, and polystyrene [Rosato 1982]. Are resins the same as matrix materials? Yes, but not all matrix materials are resins, as stated earlier. A matrix is a cured phase of resin. Fabrics made of fibers such as nylon, polyester, polypropylene, and so on are simple forms of cured resins that have different chemical structures. Do resins, epoxies, and polymers mean the same thing? A resin is a semisolid or pseudosolid organic material that has often high molecular weight, exhibits a tendency to flow when subjected to stress, and usually has a softening or melting range. In reinforced plastics, the resin is the material used to bind together the fibers. Generally speaking, the polymer with additives is called resin system during processing and matrix after the polymer has cured (solidified). Epoxies are a class of resins (or polymers) that are most commonly used. How are these resins classified? Resins (or polymers or plastics) can be classified in several different ways. The earliest distinction between types of polymers was made long before developing any in-depth understanding of their molecular structure; it was based on their reaction to heating and cooling. On this basis, resins or polymers are classified as thermoset (or thermosetting) and thermoplastic. From a chemical molecular chain standpoint, the main difference between the two is the nature of bonds between the molecular chains: secondary van der Waals in the thermoplastics and chemical crosslinks in thermosets [Young et al. 1998]. A thermoset resin (or polymer) is characterized by its ability to change into a substantially infusible and insoluble material when cured by the application of heat or by chemical means. Although upon heating, these polymers soften and can be made to flow under stress once, they will not do so reversibly i.e., heating causes them to undergo a curing reaction. Further heating of these polymers leads only to degradation, but it will not soften them. Bakelite is a good example of a thermo- 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 5 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 5 setting plastic (polymer or resin). Because Bakelite is a strong material and also a poor conductor of heat and electricity, it is used to make handles for toasters, pots, and pans; for molding common electrical goods, such as wall outlets and adapters; and for such diverse items as buttons and billiard balls. Resins similar to Bakelite are used in fiberboard and plywood. A thermoplastic resin (or polymer) is characterized by its ability to soften and harden repeatedly by increases and decreases (respectively) in temperature, with minimal change in properties or chemical composition [Mott 2002]. These polymers soften upon heating and can then be made to flow under pressure. Upon cooling, they will regain their solid or rubbery nature. Thermoplastics mimic fats in their response to heat; thermosets are more like eggs (boiled and hardened eggs cannot be transformed back into egg yolks; the change caused by heat is irreversible). Note that thermoplastics lose their properties dramatically after four or five cycles of heating and cooling. Plastics or polymers can also be classified on the basis of their molecular chains. When molecules are strung together like a string of paper clips, in one or three dimensions, the resulting compound is called a polymer or macromolecule. One class of polymers consists of linear chains, i.e., the chains extend only in one dimension. These polymers are linear polymers and are referred to as thermoplastic. They gradually soften with increasing temperature and finally melt because the molecular chains can move independently. An example is polyethylene, which softens at 85 C. The polymers in the other group have cross-links between chains, so that the material is really one three-dimensional giant molecule these are called crosslinked polymers and referred to as thermosetting [Selinger 1998]. Can you give some examples of thermoset and thermoplastic polymers? Examples of thermoplastics include ABS, acetals, acrylics, cellulose acetate, nylon, polyethylene, polypropylene, polystyrene, and vinyls. Examples of thermosetting polymers include alkyds, allyls, aminos, epoxies, phenolics, polyesters, silicones, and some types of urethanes. In addition to acting as binders, do resins serve any other functions? As mentioned earlier, resin systems used in composites act not only as binders of fibers but also add structural integrity and protection from environmental hazards such as moisture, corrosive agents in the environment, freeze-thaw cycles, and ultraviolet (UV) radiation. What is meant by resin system? Typically, resins used in composites are combined with several additives or modifiers (e.g., fillers, catalysts, and hardeners); the term resin system rather than resin is used as an all-inclusive term for a binder ready for use at the time of process or manufacturing of a composite. The purpose of these additives is to modify the properties of the resin to provide protection to fibers from moisture ingress and ultraviolet radiation, add color, enhance or reduce translucence, modify surface tension or wettability during the low-viscosity period before curing, and so on. Why are so many resin systems used in composites? Different resin systems offer different advantages and also have their own drawbacks. For example, polyester has a low cost and an ability to be made translucent. Its 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 6 Wednesday, October 25, 2006 2:09 PM 6 Reinforced Concrete Design with FRP Composites drawbacks include service temperatures below 77 C, brittleness, and a high shrinkage factor of as much as 8% during curing. Phenolics are low-cost resins that provide high mechanical strength but have the drawback of a high void content. Epoxies provide high mechanical strength and good adherence to metals and glasses, but high costs and processing difficulties are their drawbacks. Thus, each resin system offers some advantages but also has some limitations. The use of a particular resin system depends on the application. Are many kinds of composites used? How are they classified? Composites are classified generally in one of the two ways [Kaw 1997]: (1) by the geometry of reinforcement particulate, flake, and fibers; or (2) by the type of matrix polymer, metal, ceramic, and carbon. Particulate composites consist of particles immersed in matrices such as alloys and ceramics. Flake composites consist of flat fiber reinforcement of matrices. Typical flake materials are glass, mica, aluminum, and silver. These composites provide a high out-of-plane modulus, higher strength, and lower cost. Fiber composites consist of a matrix reinforced by short (discontinuous) or long (continuous) fibers and even fabrics. Fibers are generally anisotropic, such as carbon and aramid. What is meant by reinforced plastic? Reinforced plastic is simply a plastic (or polymer) with a strength greatly superior to those of the base resin as a result of the reinforcement embedded in the composition [Lubin 1982]. What are FRPs? FRP is an acronym for fiber reinforced polymers, which some also call fiber reinforced plastics, so called because of the fiber content in a polyester, vinyl ester, or other matrix. Three FRPs are commonly used (among others): composites containing glass fibers are called glass fiber reinforced polymers (GFRP); those containing carbon fibers are called carbon fiber reinforced polymers (CFRP); and those reinforced with aramid fibers are referred to as aramid fiber reinforced polymers (AFRP). You referred to FRP as fiber reinforced polymer. How is this different from fiber reinforced plastic? Actually, there is no difference. Plastics are composed of long chain-like molecules called polymers. The word polymer is a chemistry term (meaning a high molecular weight organic compound, natural or synthetic, containing repeating units) for which the word plastic is used as a common descriptive. The word polymer rather than plastic is preferred in the technical literature. However, the term fiber reinforced plastics continues to enjoy common usage because of the physical resemblance of the FRPs to commonly used plastics. The term FRP is most often used to denote glass fiber reinforced polymers (or plastics). The term advanced composites is usually used to denote high-performance carbon or aramid fiber-reinforced polymers (or plastics). Are FRPs different from regular plastics that are used for making common products such as plastic bottles, jugs, spoons, forks, and bags (i.e., grocery and trash bags)? What is the difference? FRPs are very different from ordinary plastics; the key phrase in FRP is fiber reinforced. Generally, plastic is a material that is capable of being shaped into any 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 7 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 7 FIGURE 1.2 Common household plastic products. form, a property that has made it a household name. Figure 1.2 shows a variety of unreinforced plastic products, such as a milk jug, coffee maker, bottles to contain medicine and liquids, compact disc, plastic bags, and other items found in common households. Literally hundreds of thousands of unreinforced plastic products are in use all over the world. These include our packing and wrapping materials, many of our containers and bottles, textiles, plumbing and building materials, furniture, flooring, paints, glues and adhesives, electrical insulation, automobile parts and bodies, television, stereo, and computer cabinets, medical equipment, cellular phones, compact discs, and personal items such as pens, razors, toothbrushes, hairsprays, and plastic bags of all kinds. In fact, we can say that we live in the Plastic Age (similar to the prehistoric Stone Age). Can you describe some of these plastics and their commercial uses? Table 1.1 lists several commercial and industrial applications of plastics along with desirable properties pertinent to those applications and suitable plastics. Some of these plastics are categorized as hard and tough they have high tensile strength and stretch considerably before breaking. Because of these superior properties, they are relatively expensive and have specialized applications. For example, consider polyacetals, which have high abrasion resistance and resist organic solvents and water. Therefore, they are used in plumbing to replace brass or zinc in showerheads, valves, and so on. Furniture castors, cigarette lighters, shavers, and pens are also often made from polyacetals as they give a nonstain as well as satin finish. 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 8 Wednesday, October 25, 2006 2:09 PM 8 Reinforced Concrete Design with FRP Composites TABLE 1.1 Major Types of Commercial Plastics and Applications Type of Plastic Characteristics Typical Commercial Applications Low-density polyethylene (LDPE) Low melting; very flexible; soft; low density Poly(vinyl chloride), also known as PVC Tough; resistant to oils High-density polyethylene (HDPE) Higher melting, more rigid, stronger, and less flexible than low-density polyethylene Polypropylene Retains shape at temperatures well above room temperature Polystyrene Lightweight; can be converted to plastic foam Poly(ethylene terephthalate), also known as PET Phenol-formaldehyde resins Nylon, phenolics, tetrafluoroethylene (TFE)-filled acetals Easily drawn into strong thin filaments; forms an effective barrier to gases Strongly adhesive Bags for trash and consumer products; squeeze bottles; food wrappers; coatings for electrical wires and cables Garden hoses; inexpensive wallets, purses, keyholders; bottles for shampoos and foods; blister packs for various consumer products; plumbing, pipes, and other construction fixtures Sturdy bottles and jugs, especially for milk, water, liquid detergents, engine oil, antifreeze; shipping drums; gasoline tanks; half to two-thirds of all plastic bottles and jugs are made of this plastic Automobile trim; battery cases; food bottles and caps; carpet filaments and backing; toys Insulation; packing materials including plastic peanuts ; clear drinking glasses; thermal cups for coffee, tea, and cold drinks; inexpensive tableware and furniture; appliances; cabinets Synthetic fabrics; food packages; backing for magnetic tapes; soft-drink bottles Plywood; fiberboard; insulating materials Synthetic fabrics; fishing lines; gears and other machine parts Tetrafluoroethylene (TFE) fluorocarbons, nylons, acetals Acrylics, polystyrene, cellulose, acetate, vinyls Easily drawn into strong thin filaments; resistant to wear, high tensile and impact strength, stability at high temperatures, machinable Low coefficient of friction; resistance to abrasion, heat, corrosion Good light transmission in transparent and translucent colors, formability, shatter resistance High-strength components, gears, cams, rollers Light-transmission components Source: Adapted from Selinger, B., Chemistry in the Market Place, 5th ed., Marrickville, NSW, Australia: Harcourt Brace & Co. Australia, 1998; Snyder, C.H., The Extraordinary Chemistry of Ordinary Things, New York: John Wiley & Sons, 2003. 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 9 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 9 Polycarbonate is an example of a plastic that finds use in a wide variety of commercial applications. Polycarbonates are often used instead of glass because they are transparent, dimensionally stable, and impact resistant, even when subjected to a wide range of temperatures. Babies bottles, bus-shelter windows, plastic sheeting for roofing, and telephones are examples of polycarbonates. In sporting equipment, they are used in helmets for team players, motorcyclists, and snowmobilers. Because of their superior fire resistance, they are used in firemen s masks, interior moldings of aircraft, and in electronic equipment [Selinger 1998]. Nylon is yet another example of plastic that has excellent mechanical properties and resists solvents. As such, it is an ideal material for gears and bearings that cannot be lubricated. About 50% of molded nylon fittings go into cars in the form of small gears (for wipers), timing sprockets, and all sorts of clips and brackets. What are advanced composites and high-performance composites? Fiber reinforced composites are composed of fibers embedded in a matrix. The fibers can be short or long, continuous or discontinuous, and can be oriented in one or multiple directions. By changing the arrangements of fibers (which act as reinforcement), properties of a composite can be engineered to meet specific design or performance requirements. A wide variety of fibers, such as glass, carbon, graphite, aramid, and so on, are available for use in composites and their number continues to grow. An important design criterion for composites is the performance requirement. In low-performance composites, the reinforcements usually in the form of short or chopped fibers provide some stiffness but very little strength; the load is carried mainly by the matrix. Two parameters that are used to evaluate the highperformance qualities of fibers are specific strength and specific stiffness. Fibers having high specific strength and high specific stiffness are called advanced or highperformance fibers and were developed in the late 1950s for structural applications. Composites fabricated from advanced fibers of thin diameters, which are embedded in a matrix material such as epoxy and aluminum, are called advanced or highperformance composites. In these composites, continuous fibers provide the desirable stiffness and strength, whereas the matrix provides protection and support for the fibers and also helps redistribute the load from broken to adjacent intact fibers. These composites have been traditionally used in aerospace industries but are now being used in infrastructure and commercial applications. The advanced composites are distinguished from basic composites, which are used in high-volume applications such as automotive products, sporting goods, housewares, and many other commercial applications where the strength (from a structural standpoint) might not be a primary requirement. What is meant by specific strength and specific stiffness? What is their significance? Specific strength is defined as the ratio of the tensile strength of a material to its unit weight. Specific stiffness (also called specific modulus) is defined as the ratio of the modulus of a material to its unit weight. These properties are often cited as indicators of the structural efficiency of a material; they form very important and often critical design considerations for the many products for which composites offer unique advantages. 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 10 Wednesday, October 25, 2006 2:09 PM 10 Reinforced Concrete Design with FRP Composites TABLE 1.2 Properties of Selected Composites and Steel Material Steel AISI 1020 HR AISI 5160 OQT 700 Aluminum 6061-T6 7075-T6 Titanium Ti-6A1-4V quenched and aged at 1000 F Glass/epoxy composite 60% fiber content Boron/epoxy composite 60% fiber content Graphite/epoxy composite 62% fiber content Graphite/epoxy composite Ultrahigh modulus Aramid/epoxy composite 60% fiber content Tensile Strength (ksi) Modulus of Elasticity ( 106 psi) Specific Weight, (lb/in3) Specific Strength ( 106 in) Specific Modulus ( 108 in) 55 263 30.0 30.0 0.283 0.283 0.194 0.929 1.06 1.06 45 83 10.0 10.0 0.098 0.101 0.459 0.822 1.02 0.99 160 16.5 0.160 1.00 1.03 114 4.0 0.061 1.87 0.66 270 30.0 0.075 3.60 4.00 278 19.7 0.057 4.86 3.45 160 48.0 0.058 2.76 8.28 200 11.0 0.050 4.00 2.20 Source: Adapted from Mott, R.L. Applied Strength of Materials, Upper Saddle River, NJ: Prentice Hall, 2002. Can you list a few important mechanical properties of composites and metals such as steel and aluminum to make a valid comparison and explain why composites are considered superior structural materials? Table 1.2 lists several key properties (tensile strength, modulus of elasticity, specific weight, specific strength, and specific modulus) of metals such as steel, aluminum, titanium alloys, and selected composites [Mott 2002]. It also lists the two important parameters specific strength and specific stiffness that are often cited as indicators of structural efficiency of a material. What does this information mean to a designer? The information provided in Table 1.2 is very important to a designer while selecting a material type for complex structural systems. Figure 1.3 gives a comparison of the specific strength and specific stiffness of selected composite materials [Mott 2002]. For example, consider a boron/epoxy composite having a specific weight of 0.075 and steel (AISI 5160 OQT 700) having a specific weight of 0.283; their strengths are comparable 270 ksi and 263 ksi, respectively. However, the specific strength of the boron/epoxy composite (3.60) is almost four times that of steel 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 11 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 11 9 8 Specific Strength Specific Stiffness Specific strength (x106 in.) Specific stiffness (x108 in.) 7 6 5 4 3 2 1 0 Graphite/epoxy Boron/epoxy (Ultrahigh modulus) Aramid/epoxy Glass/epoxy Titanium Ti6Al-4V Aluminum 6061-T6 Steel 5160 OQT700 FIGURE 1.3 Comparison of specific strength and specific stiffness of selected materials. (Adapted from Mott, R.L. Applied Strength of Materials, Upper Saddle River, NJ: Prentice Hall, 2002.) (0.929). Now consider the simple case of a rod designed to carry an axial tensile force. The cross-section of the boron/epoxy composite rod need only be one-fourth that of a steel rod. This reduction in cross-sectional area translates into reduced space requirements and also reduced material and energy costs. Figure 1.4 shows a plot of these data with specific strength on the vertical axis and specific modulus on the horizontal axis [Mott 2002]. Note that when weight is critical, the ideal material would be found in the upper right part of Figure 1.4. Can you describe the basic anatomy of composites? As stated earlier, the two main constituents of composites are fibers and resins. Additionally, they contain quantities of other substances known as fillers and additives, ranging from 15 to 20% of the total weight (and hence the term resin system). Fibers are the backbone of a composite. Their diameters are very thin much thinner than a human hair. For reasons explained earlier, their small diameter is the reason for their extreme strength. The tensile strength of a single glass filament is approximately 500 ksi. However, the thin diameter of a fiber is also a major disadvantage in that the compressive strength of a thin fiber is very small due to its vulnerability to buckling. To harness the strength of fibers, they are encased in a tough polymer matrix, which gives the composite its bulk. The matrix serves to hold fibers together in a structural unit and spread the imposed loads to many fibers within the composite, and to protect the fibers from environmental degradation attributed to moisture, 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 12 Wednesday, October 25, 2006 2:09 PM 12 Reinforced Concrete Design with FRP Composites 4.5 Aramid/epoxy composite Specific strength (x106 in.) 4 Boron/epoxy composite 3.5 3 Graphite/epoxy composite ultrahigh modulus 2.5 2 Glass/epoxy composite 1.5 Titanium Ti-6Al-4V 1 Steel AISI 5160 OQT 700 0.5 0 Aluminum 6061-T6 0 1 2 3 4 5 6 Specific modulus (x108 in.) 7 8 9 FIGURE 1.4 Specific strength vs. specific modulus for selected metals and composites (Adapted from Mott, R.L. Applied Strength of Materials, Upper Saddle River, NJ: Prentice Hall, 2002.) ultraviolet rays, corrosive chemicals, and to some extent, susceptibility to fire and from damage to fibers during handling. The function of the matrix is somewhat analogous to that of concrete in a conventional reinforced concrete member wherein the concrete surrounding reinforcing bars maintains the alignment and position of bars, spreads (or distributes) the imposed loads to all the reinforcing bars present in the member, and also provides environmental and fire protection to the bars. However, a major difference is noted. In a reinforced concrete member, concrete itself also shares the imposed loads (e.g., in beams and columns), whereas in a composite the matrix shares a negligible amount of load but helps transfer the load to fibers through interlaminar and in-plane shear; the entire imposed load is taken practically by the fibers alone. The third constituent fillers are particulate materials whose major function is not to improve the mechanical properties of the composite but rather to improve aspects such as extending the polymer and reducing the cost of the plastic compound (fillers are much less expensive than the matrix resin). One of the earliest examples of filler is wood flour (fine sawdust), long used in phenolics and other thermosets. Calcium carbonate is used in a variety of plastics. Polypropylene is often filled with talc [Rosen 1993]. Hollow glass spheres are used to reduce weight. Clay or mica particles are used to reduce cost. Carbon particles are used for protection against ultraviolet radiation. Alumina trihydrate is used for flame and smoke suppression [Katz 1978]. Fillers can also be used to improve certain properties of plastics. They almost all reduce 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 13 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 13 mold shrinkage and thermal expansion coefficients, and also reduce warpage in molded parts. Mica and asbestos increase heat resistance. In addition to the above described three constituents, coupling agents are used to improve the fiber surface wettability with the matrix and create a strong bond at the fiber-matrix interface. For example, coupling agents are used with glass fibers to improve the fiber-matrix interfacial strength through physical and chemical bonds and to protect them from moisture and reactive fluids [Mallick 1993]. The most common coupling agents are silanes. For maximum fiber efficiency, stress must be efficiently transferred from the polymer to the reinforcing agents. Most inorganics have hydrophilic surfaces (i.e., they have an affinity for absorbing, wetting smoothly with, tending to combine with, or capable of dissolving in water), while the polymers are hydrophobic (i.e., incapable of dissolving in water), which results in a poor interfacial adhesion. This problem is exacerbated by the tendency of many inorganic fibers particularly glass to absorb water, which further degrades adhesion [Rosen 1993]. Carbon fiber surfaces are chemically inactive and must be treated to develop good interfacial bonding with the matrix. Similarly, Kevlar 49 fibers also suffer from weak interfacial adhesion with most matrixes [Mallick 1993]. Other constituents are added to composites in minute quantities for various important reasons. Most polymers are susceptible to one or more forms of degradation, usually as a result of environmental exposure to oxygen or ultraviolet radiation, or to high temperatures during processing. Stabilizers are added to inhibit degradation of polymers. Plastics are often colored by the addition of pigments, which are finely powdered solids. If the polymer is itself transparent, a pigment imparts opacity. A common pigment is titanium dioxide, which is used where a brilliant, opaque white is desired. Sometimes pigments perform other functions as well. An example is calcium carbonate, which acts both as a filler and a pigment in many plastics. Black carbon is another example that acts both as a stabilizer and a pigment. Dyes are another constituent used in minute quantities in producing plastics. Dyes are colored organic chemicals that dissolve in the polymer to produce a transparent compound, assuming that the polymer is transparent to begin with [Rosen 1993]. Flammability of polymers is a serious concern when designing with composites. Being composed of carbon and hydrogen, most synthetic polymers are flammable. Flame-retardants are added to polymers to reduce their flammability. The most common flame-retarding additives for plastics contain large proportions of chlorine or bromine [Rosen 1993]; however nonhalogenated flame retardants (other than chlorine or bromine) are being researched and implemented actively. What are the major considerations when designing with reinforcing fibers? Composites are engineered materials for which fibers and resins are selected based on their intended function. Selection of appropriate fibers and resins are two major engineering decisions to be made in designing composites. Generally speaking, three considerations must be met when designing with fiber reinforcement: 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 14 Wednesday, October 25, 2006 2:09 PM 14 Reinforced Concrete Design with FRP Composites 1. Fiber type: glass, carbon, aramid, or others 2. Fiber form: roving, tow, mat, woven fabrics, or others 3. Fiber architecture, i.e., orientation of fibers The fiber architecture or fiber orientation refers to the position of the fiber relative to the axes of the element. Fibers can be oriented along the longitudinal axis of the element (at 0 to the longitudinal axis), transverse to the longitudinal axis (at 90 to the longitudinal axis), or in any other direction at the designer s discretion to achieve optimum product efficiency. This customization flexibility is unique to the fabrication of composites, which gives them versatility in applications. Although fiber orientation in a composite can be so varied that the resulting product is virtually an isotropic material with equal strength in all directions, in most cases composite structural elements are designed with the greatest strength in the direction of the greatest load. For example, for composite reinforcing elements such as bars and tendons, fibers are oriented longitudinally (i.e., in the direction of the applied or anticipated tensile force). Once the fiber type and orientation are determined, an appropriate resin and the fiber-resin volume ratio are selected. The strength of a composite depends on the fiber-resin volume ratio the higher the ratio, the stronger and lighter the resulting composite. Of course, higher fiber content results in increased product cost, especially for composites containing carbon and aramid fibers, including process difficulties. Production of composites is amenable to a variety of processes, which can be fully automated or manual. Automated processes involve production of composites completely in a factory. Manually, the fibers and resin can be combined and cured on site. Pultruded products (so called because they are produced through a mechanical process called pultrusion) such as various structural shapes (e.g., beams, channels, tubes, bars) are examples of composites that are produced in a factory in their entirety and the finished products shipped to sites for the end use. Other automated processes for producing composites for construction applications include filament winding and molding. Filament winding can take place at a plant facility or at a construction site. Molding processes (several kinds exist) are also used in a plant facility. Alternatively, for low-volume applications such as structural repair and retrofit, fibers and resins can be mixed and cured on site, a manual process referred to as hand- or wet-lay up systems. In all cases, the fiber reinforcing material must be completely saturated with resin, compacted to squeeze out excess resin and entrapped air bubbles, and fully cured prior to applying loads. A variation of wet-lay up system is prepreg (short for pre-impregnated), which consists of unidirectional fiber sheets or fabrics that are pre-impregnated (i.e., precoated) with a resin system and ready for application on site. Machine applied systems are also available but are not commonly used because of the complexities of field applications. You alluded to the term pultrusion. What does this mean? The term pultrusion refers to a continuous, mechanical process (see Figure 1.5) for manufacturing composites that have uniform cross-sectional shapes such as I, 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 15 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 15 Uni-directional glass Guides Resin bath Continuous strand mat Surfacing veil Preformer Cut-o saw Heated die The puller FIGURE 1.5 Schematic of pultrusion process. (Courtesy of Strongwell Corporation, Bristol, VA.) FIGURE 1.6 Fiberglass composite structural elements formed by pultrusion. (Courtesy of Strongwell Corporation, Bristol, VA.) L, T, rectangular, and circular sections and hollow rectangular and circular tubes (similar to steel shapes) as shown in Figure 1.6 The process is automated; it involves pulling a fiber-reinforcing material through a resin bath and then through a heated (shaping) die where the resin is cured. Pultrusion is a cost-effective production process and is the dominant manufacturing process used for producing structural shapes, reinforcing bars, and prestressing tendons. What are the main types of fibers used for producing composites and on what basis are they selected? Composites used for civil engineering applications are produced typically from three types of fibers: glass, carbon, and aramid. The selection of fiber type depends on 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 16 Wednesday, October 25, 2006 2:09 PM 16 Reinforced Concrete Design with FRP Composites the specific needs for a particular structural application. Various factors important for a composite design include the required strength, stiffness, corrosion resistance, durability, and cost. Cost is a major consideration and often plays a pivotal role in the selection process. Glass fibers are the least expensive. Carbon fibers are much more expensive than glass fibers, and aramid fibers are the most expensive. Typically, these fibers can cost several dollars per pound; some cost as high as $30 per pound in the year 2005. By comparison, structural steel in the year 2005 cost approximately $0.50 to $1.00 per pound. Considerable cost differences are found in terms of composite types, i.e., glass composite vs. carbon composite vs. aramid composite. Can you describe various types of fibers? Let us briefly discuss three types of commonly used fibers: glass, carbon, and aramid. The quality of these fibers in terms of greater strength and corrosion resistance continues to improve as new technologies evolve. Glass fibers are produced from silica-based glass compounds that contain several metal oxides. A variety of glass fibers are produced to suit specific needs. The E-glass (E stands for electrical), so called because its chemical composition gives it excellent insulation properties, is one of the most commonly used glass fibers because it is the most economical. S-glass (S stands for structural) offers greater strength (typically 40% greater at room temperature) and also greater corrosion resistance than provided by E-glass. The corrosion-resistant E-CR glass provides even better resistance to corrosive materials such as acids and bases. Carbon (and graphite, the two terms are often used interchangeably) fibers are produced from synthetic fibers through heating and stretching. Polyacrylonitrile (PAN), pitch (a by-product of the petroleum distillation process), and rayon are the three most common precursors (raw materials) used for producing carbon fibers. Some of these fibers have high strength-to-weight and modulus-to-weight ratios, high fatigue strength, and low coefficient of thermal expansion, and even negative coefficient of thermal expansion. Aramid fiber is an aromatic polyamide that provides exceptional flexibility and high tensile strength. It is an excellent choice as a structural material for resisting high stresses and vibration. What is Kevlar? Kevlar is the trademarked name for the aramid fiber produced by the DuPont Company. In what forms are various fibers commercially available? An individual fiber of indefinite length used in tows, yarns, or rovings is called filament. Because of their small diameters, filaments are extremely fragile, the primary reason for which they are sold in bundles. The industry uses different terminology for describing bundles of filaments that is based on the fiber type. For example, glass and aramid fibers are called strands, rovings, or yarns. The term strands refers to a collection of continuous glass or aramid filaments, whereas the term rovings refers to a collection of untwisted strands. Untwisted carbon strands are called tows. The term yarns refers to a collection of filaments or strands that are twisted together. Carbon fiber is commercially available as tow, i.e., a bundle of untwisted fiber filaments. For example, 12K tow has 12,000 filaments and is com- 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 17 Wednesday, October 25, 2006 2:09 PM Frequently Asked Questions about Composite Materials 17 monly sold in a variety of modulus categories: standard or low (33 to 35 msi), intermediate (40 to 50 msi), high (50 to 70 msi), and ultra-high (70 to 140 msi). Fibers for infrastructure applications are most commonly supplied in the form of rovings, tows, and fabrics. Both tows and rovings can be used to produce a wide variety of reinforcing materials such as mats, woven fabrics, braids, knitted fabrics, preforms, and hybrid fabrics. Mats are nonwoven fabrics that provide equal strength in all directions. They are available in two forms: chopped and continuous strand. Chopped-strand mats are characterized by randomly distributed fibers that are cut to lengths ranging from 1.5 to 2.5 in. Continuous-strand mats are formed by swirling continuous-strand fiber onto a moving belt and finished with a chemical binder that serves to hold fibers in place. Continuous-strand mat is stronger than the choppedstrand mat. Because of its higher strength, continuous-strand mat is used in molding and pultrusion processes. Chopped-strand mat is relatively weaker than the continuous-strand mat but is relatively cheaper. Fabricated on looms, woven fabrics are available in a variety of weaves, widths, and weights. Bidirectional woven fabrics provide good strength in 0 and 90 directions and are suitable for fast fabrication. A disadvantage with woven fabrics is that fibers get crimped as they pass over and under each other during weaving, which results in a lower tensile strength of the fabric. Hybrid fabrics are manufactured with different fiber types. Braided materials are produced by a complex manufacturing process. Although they are more efficient and stronger than woven fabrics, they cost more. Braided materials derive their higher strength from three or more yarns intertwined with one another without twisting any tow yarns around each other. Braids are continuously woven on the bias and have at least one axial yarn that is not crimped in the weaving process. Braided materials can be flat or tubular. Flat braids are used primarily for selective reinforcement (e.g., strengthening specific areas in pultruded parts), whereas tubular braids are used to produce hollow cross-sections for structural tubes. Knitted fabrics permit placement of fibers exactly where they are needed. They are formed by stitching layers of yarn together, which permits greater flexibility in yarn alignment as the yarns can be oriented in any desired direction by putting them atop one another in practically any arrangement. A major advantage of knitted fabrics is the absence of crimping in the yarns as they lay over one another rather than crossing over and under one another (as in the case of woven fabrics). The absence of yarns crimping results in utilization of their inherent strength, and helps create a fabric that is more pliable than woven fabrics. What is so special about composites? What is wrong with using the conventional materials such as steel, concrete, aluminum, and wood? Nothing is wrong with using the conventional building materials such as steel, concrete, aluminum, and wood. However, for certain applications, composite materials offer an attractive, and often the preferred, alternative because of the many properties that are superior to those of conventional building materials. Composites evolved as more efficient structural materials because of their many superior properties: ultra-high strength, corrosion resistance, lightweight, high fatigue resistance, nonmagnetic, high impact resistance, and durability. Because composites are man- 2007 by Taylor and Francis Group, LLC DK8293_C001.fm Page 18 Wednesday, October 25, 2006 2:09 PM 18 Reinforced Concrete Design with FRP Composites made materials, they can be engineered (i.e., their shapes or profiles can be produced at designer s discretion) to meet the needs of specific applications. Structures built with composites also have low life-cycle costs. REFERENCES AND SELECTED BIBLIOGRAPHY Ball, P., Designing the Molecular World, Princeton, NJ: Princeton University Press, 1994. Benjamin, B.S., Structural Design with Plastics, New York: Van Nostrand Reinhold Co., 1982. CFI, Composites for Infrastructure, Wheatridge, CO: Ray Publishing, 1998. Chawla, K.K., Composite Materials: Science and Engineering, New York: Springer-Verlag, 1987. Emsley, J., Molecules at an Exhibition, Oxford: Oxford University Press, 1998. EUROCOMP, Structural design of polymer composites, in EUROCOMP Design Code and Handbook, J.L. Clarke (Ed.), London: E & FN Spon, 1996. Gay, D., Hoa, S.V., and Tsai, S.W., Composite Materials: Design and Applications, Boca Raton, FL: CRC Press, 2003. Harper, C.A. and Petrie, E.M., Plastic Materials and Processes: A Concise Encyclopedia, New York: Wiley Interscience, 2003. Herakovich, C.T., Mechanics of Fibrous Composites, New York: John Wiley & Sons, 1998. Hollaway, L.C. and Leeming, M.B., Strengthening of Reinforced Concrete Structures Using Externally-Bonded FRP Composites in Structural and Civil Engineering, Boca Raton, FL: CRC Press, 1999. Katz, H.S. and Milewski, J.V. (Eds.), Handbook of Fillers and Reinforcements for Plastics, New York: Van Nostrand Reinhold Co., 1978. Kaw, A.K., Mechanics of Composites Materials, Boca Raton, FL: CRC Press, 1997. Lubin, G. (Ed.), Handbook of Composites, New York: Van Nostrand Reinhold Co., 1982. Mallick, P.K., Fiber Reinforced Composite Materials: Manufacturing and Design, New York: Marcel Dekker, 1993. Marshall, A., Composite Basics Seven, Walnut Creek, CA: Marshall Consulting Co., 2005. McCrum, N.G., Buckley, C.P., and Bucknall, C.P., Principles of Polymer Engineering, New York: Oxford University Press, 1988. Miller, T.E. (Ed.), Introduction to Composites, 4th ed., New York: Composites Institute of the Society of Plastics Industry, 1997. Mott, R.L., Applied Strength of Materials, Upper Saddle River, NJ: Prentice Hall, 2002. Potter, K., An Introduction to Composite Products, New York: Chapman & Hall, 1997. Rosato, D.V., An overview of composites, in Handbook of Composites, G. Lubin (Ed.), New York: Van Nostrand Reinhold Co., 1982. Rosen, S., Fundamental Principles of Polymeric Materials, New York: John Wiley & Sons, 1993. Schwartz, M., Composites Materials Handbook, 2nd ed., New York: McGraw-Hill Co., 1992. Selinger, B., Chemistry in the Market Place, 5th ed., Marrickville, NSW, Australia: Harcourt Brace & Co. Australia, 1998. Snyder, C.H., The Extraordinary Chemistry of Ordinary Things, New York: John Wiley & Sons, 2003. Young, J.F., Mindess, S., Gray, R.J., and Bentur, A., The Science and Technology of Civil Engineering Materials, Upper Saddle River, NJ: Prentice Hall, 1998. 2007 by Taylor and Francis Group, LLC

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