Structural Composite Material
Submitted By Albert Halder Halder
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Introduction A typical composite material is a system of materials composing of two or more materials (mixed and bonded) on a macroscopic scale. For example, concrete is made up of cement, sand, stones, and water. If the composition occurs on a microscopic scale (molecular level), the new material is then called an alloy for metals or a polymer for plastics.
Generally, a composite material is composed of reinforcement (fibers, particles, flakes, and/or fillers) embedded in a matrix (polymers, metals, or seramics). The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. When designed properly, the new combined material exhibits better strength than would each individual material. The numerous features of composite materials have led to the widespread adoption and use through many different industries. It is because of these unique features of composites that people benefit. Below are some of the most important features of composites, and the benefits they provide Composites are incredibly lightweight, especially in comparison to materials like concrete, metal, and wood. Often a composite structure will weigh 1/4 that of a steel structure with the same strength. That means, a car made from composites can weigh 1/4 that of a car made from steel. This equates to serious fuel savings.
The advantages demonstrated by composites, in addition to high stiffness, high strength, and low density, include corrosion resistance, long fatigue lives, tailorable properties (including thermal expansion, critical to satellite structures), and the ability to form complex shapes. (This advantage was demonstrated in the ability to create “low observable,” or stealth, structures for military sy stems.) An
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example of recent OMC application is the next-generation U.S. tactical fighter aircraft, the F-22. Over 24% of the F-22 structure is OMCs. The B-2 bomber, shown in Fig. 2, is constructed using an even higher percentage of composites, as are current helicopter and vertical lift designs. For example, the tiltrotor V-22 Osprey is over 41% composite materials. The upper-use temperature of PMCs has also increased dramatically: early epoxies were considered useable (for extended periods) up to 121°C (250°F).
Current generation polymers, such as bismaleimides, have increased that limit to around 204°C (400°F), and the use of polyimide- matrix composites has extended the range to 288°C (550°F). Once considered premium materials only to be used if their high costs could be justified by increased performance, OMCs can now often “buy their way onto” new applications. This is due not only to a dramatic drop in
materials costs, but also in advances in the ability to fabricate large, complex parts requiring far less hand labor to manually assemble. A recent example of this is the addition of large composite structures in the tail and landing gear pods on the C-17 cargo aircraft. Clearly, the applications, technology, confidence, and other considerations of high-performance OMCs have expanded dramatically since the 1980s. Perhaps the most dramatic example of this is the growing use of high-performance OMCs in the commodity market of infrastructure.
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COMPOSITE MATERIAL
Composite materials (also called composition materials or shortened to composites) are materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure. The new material may be preferred for many reasons: common examples include materials which are stronger, lighter or less expensive when compared to traditional materials. A composite material is made by combining two or more materials – often ones that have very different properties. The two materials work together to give the composite unique properties. However, within the composite you can easily tell the different materials apart as they do notdissolve or blend in to each other. A composite material can be defined as a combination of two or more materials that results in better properties than those of the individual components used alone. In contrast to metallic alloys, each material retains its separate chemical, physical, and mechanical properties.
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Fig: Composite Material
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Natural composites exist in both animals and plants. Wood is a composite – it is made from long cellulose fibres (a polymer) held together by a much weaker substance called lignin. Cellulose is also found in cotton, but without the lignin to bind it together it is much weaker. The two weak substances – lignin and cellulose – together form a much stronger one.
The bone in your body is also a composite. It is made from a hard but brittle material called hydroxyapatite (which is mainly calcium phosphate) and a soft and flexible material called collagen (which is a protein). Collagen is also found in hair and finger nails. On its own it would not be much use in the skeleton but it can combine with hydroxyapatite to give bone the properties that are needed to support the body.
People have been making composites for many thousands of years. One early example is mud bricks. Mud can be dried out into a brick shape to give a building material. It is strong if you try to squash it (it has good compressive strength) but it breaks quite easily if you try to bend it (it has poor tensile strength). Straw seems very strong if you try to stretch it, but you can crumple it up easily. By mixing mud and straw together it is possible to make bricks that are resistant to both squeezing and tearing and make excellent building blocks. Another ancient composite is concrete. Concrete is a mix of aggregate (small stones or gravel), cement and sand. It has good compressive strength (it resists squashing). In more recent times it has been found that adding metal rods or wires to the concrete can increase its tensile (bending) strength. Concrete containing such rods or wires is called reinforced concrete.
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Typical engineered composite materials include: Composite building materials such as cements, concrete Reinforced plastics such as fiber-reinforced polymer Metal Composites Ceramic Composites (composite ceramic and metal matrices)
Modern examples : The first modern composite material was fibreglass. It is still widely used today for boat hulls, sports equipment, building panels and many car bodies. The matrix is a plastic and the reinforcement is glass that has been made into fine threads and often woven into a sort of cloth. On its own the glass is very strong but brittle and it will break if bent sharply. The plastic matrix holds the glass fibres together and also protects them from damage by sharing out the forces acting on them. Some advanced composites are now made using carbon fibres instead of glass. These materials are lighter and stronger than fibreglass but more expensive to produce. They are used in aircraft structures and expensive sports equipment such as golf clubs. Carbon nanotubes have also been used successfully to make new composites. These are even lighter and stronger than composites made with ordinary carbon fibres but they are still extremely expensive. They do, however, offer possibilities
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for making lighter cars and aircraft (which will use less fuel than the heavier vehicles we have now). The new Airbus A380, the world’s largest passenger airliner, makes use of modern
composites in its design. More than 20 % of the A380 is made of composite materials, mainly plastic reinforced with carbon fibres. The design is the first large-scale use of glass-fibre-reinforced aluminium, a new composite that is 25 % stronger than conventional airframe aluminium but 20 % lighter.
Why use composites? The biggest advantage of modern composite materials is that they are light as well as strong. By choosing an appropriate combination of matrix and reinforcement material, a new material can be made that exactly meets the requirements of a particular application. Composites also provide design flexibility because many of them can be moulded into complex shapes. The downside is often the cost. Although the resulting product is more efficient, the raw materials are often expensive.
Composite materials are generally used for buildings, bridges and structures such as boat hulls, swimming pool panels, race car bodies, shower stalls, bathtubs, and storage tanks, imitation granite and cultured marble sinks and countertops. The most advanced examples perform routinely on spacecraft in demanding environments. Composite materials are becoming more important in the construction of aerospace structures. Aircraft parts made from composite materials, such as fairings, spoilers, and flight controls, were developed during the 1960s for their weight savings over aluminum parts. New generation large aircraft are designed with all composite fuselage and wing structures, and the repair of these advanced composite materials requires an in-depth knowledge of composite structures, materials, and tooling. The primary advantages of composite materials are their high strength, relatively low weight, and corrosion resistance.
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Making composites : Most composites are made of just two materials. One is the matrix or binder. It surrounds and binds together fibres or fragments of the other material, which is called the reinforcement.
Manufacturing Methods for Composite Materials: •Manual Lay-up or Spray-up •Vacuum Bagging •Autoclave Processing •Filament Winding •Pultrusion •Matched Die Molding (SMC) •Resin Transfer Molding
All of these methods are tailored for the specific materials that are being processed. Polymer chemistryplays an important role in selecting the appropriate resin for a given fabrication method.
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Composites Processing Summary: •The processing usually involves a cycle (or multiple cycles) of applied
temperature, pressure, and vacuum.
•Elevated temperature is used to : Initiate and sustain chemical reaction in thermosetresins Initiate
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Melt Melt thermoplastics
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Reduce Reduce viscosity
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•Pressure is used to: Force the viscous resin-fiber material into a mold. Force
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Compact Compact a laminate
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Squeeze Squeeze out voids
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Vacuum is used to help pull out trapped air or other gasses that may
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be produced during the chemical reaction.
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•Begin with a mold – Apply mold release agent •Apply a thin layer of catalyzed resin to form a gel coat– Protects from blistering,
stains, weather, etc. •Apply layer of fabric or mat reinforcing •Pour, brush, or spray resin onto fiber reinforcement reinforcement
•Use rollers to spread resin, flatten fibers, squeeze out trapped air •Repeat for additional additional reinforcement layers •Let cure
Fig: Conventional Hand Layup
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Reinforcement:
The reinforcing phase provides the strength and stiffness. In most cases, the reinforcement is harder, stronger, and stiffer than the matrix. The reinforcement is usually a fiber or a particulate. Particulate composites have dimensions that are approximately equal in all directions. They may be spherical, platelets, or any other regular or irregular geometry. Particulate composites tend to be much weaker and less stiff than continuousfiber composites, but they are usually much less
expensive.
Particulate
reinforced
composites
usually
contain
less
reinforcement (up to 40 to 50 volume percent) due to processing difficulties and brittleness. A fiber has a length that is much greater than its diameter. The length-todiameter (l/d) ratio is known as the aspect ratio and can vary greatly. Continuous fibers have long aspect ratios, while discontinuous fibers have short aspect ratios. Continuous-fiber composites normally have a preferred orientation, while discontinuous fibers generally have a random orientation. Examples of continuous reinforcements include unidirectional, woven cloth, and helical winding (Fig. 1.1a), while examples of discontinuous reinforcements are chopped fibers and random mat (Fig. 1.1b). Continuous-fiber composites are often made into laminates by stacking single sheets of continuous fibers in different orientations to obtain the desired strength and stiffness properties with fiber volumes as high as 60 to 70 percent. Fibers produce high-strength composites because of their small diameter; they contain far fewer defects (normally surface defects) compared to the material produced in bulk. As a general rule, the smaller the diameter of the fiber, the higher its strength, but often the cost increases as the diameter becomes smaller. In addition, smaller-diameter high-strength fibers have greater flexibility and are more amenable to fabrication processes such as
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weaving or forming over radii. Typical fibers include glass, aramid, and carbon, which may be continuous or discontinuous. The principal purpose of the reinforcement is to provide superior levels of strength and stiffness to the composite. In a continuous fiber-reinforced composite, the fibers provide virtually all of the strength and stiffness. Even in particle reinforced composites, significant improvements are obtained. For example, the addition of 20% SiC to 6061 aluminum provides an increase in strength of over 50% and an increase in stiffness of over 40%. As mentioned earlier, typical reinforcing materials (graphite, glass, SiC, alumina) may also provide thermal and electrical conductivity, controlled thermal expansion, and wear resistance in addition to structural properties.
Matrix :
The continuous phase is the matrix, which is a polymer, metal, or ceramic. Polymers have low strength and stiffness, metals have intermediate strength and stiffness but high ductility, and ceramics have high strength and stiffness but are brittle. The matrix (continuous phase) performs several critical functions, including maintaining the fibers in the proper orientation and spacing and protecting them from abrasion and the environment. In polymer and metal matrix composites that form a strong bond between the fiber and the matrix, the matrix transmits loads from the matrix to the fibers through shear loading at the interface. In ceramic matrix composites, the objective is often to increase the toughness rather than the strength and stiffness; therefore, a low interfacial strength bond is desirable. The type and quantity of the reinforcement determine the final properties. Figure 1.2 shows that the highest strength and modulus are obtained with continuousfiber composites. There is a practical limit of about 70 volume percent reinforcement that can be added to form a composite. At higher percentages,
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there is too little matrix to support the fibers effectively. The theoretical strength of discontinuous-fiber composites can approach that of continuous-fiber composites if their aspect ratios are great enough and they are aligned, but it is difficult in practice to maintain good alignment with discontinuous fibers. Discontinuous-fiber composites are normally somewhat random in alignment, which dramatically reduces their strength and modulus. However, discontinuousfiber composites are generally much less costly than continuous-fiber composites. composites. Therefore, continuous-fiber composites are used where higher strength and stiffness are required (but at a higher cost), and discontinuous-fiber composites are used where cost is the main driver and strength and stiffness are less important. The purpose of the matrix is to bind the reinforcements together by virtue of its cohesive and adhesive characteristics, to transfer load to and between reinforcements, and to protect the reinforcements from environments and handling. The matrix also provides a solid form to the composite, which aids handling during manufacture and is typically required in a finished part. This is particularly necessary in discontinuously reinforced composites, because the reinforcements are not of sufficient length to provide a handleable form. Because the reinforcements are typically stronger and stiffer, the matrix is often the “weak link” in the composite, from a structural perspective. As a continuous phase, the
matrix therefore controls the transverse properties, inter laminar strength, and elevated-temperature strength of the composite. However, the matrix allows the strength of the reinforcements to be used to their full potential by providing effective load transfer from external forces to the reinforcement. The matrix holds reinforcing fibers in the proper orientation and position so that they can carry the intended loads and distributes the loads more or less evenly among the reinforcements. Further, the matrix provides a vital inelastic response so that stress concentrations are reduced dramatically and internal stresses are redistributed from broken reinforcements. In organic matrices, this inelastic response is often obtained by micro cracking; in metals, plastic deformation yields the needed compliance. Debonding, often properly considered as an interfacial phenomenon, is an important mechanism that adds to load redistribution and
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blunting of stress concentrations. A broad overview of important matrices is provided subsequently.
Classification Of Composite Material: ● The First level is based on Matrix phase: The matrix is the monolithic material into which the reinforcement is embedded, and is completely continuous. This means that there is a path through the matrix to any point in the material, unlike two materials sandwiched together. In structural applications, the matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement. In high temperature applications, cobalt and cobalt-nickel alloy matrices are common. The composite materials are commonly classified based on matrix constituent. The major composite classes include Organic Matrix Composites (OMCs), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs). The term organic matrix composite is generally assumed to include two classes of composites, namely Polymer Matrix Composites (PMCs) and carbon matrix composites commonly referred to as carbon-carbon composites. These three types of matrixes produce three common types of composites. » Polymer matrix composites (PMCs), of which GRP is the best-known example, use ceramic fibers in a plastic matrix. » Metal-matrix composites (MMCs) typically use silicon carbide fibers embedded in a matrix made from an alloy of aluminum and magnesium, but other matrix materials such as titanium, copper, and iron are increasingly being used. Typical applications of MMCs include bicycles, golf clubs, and missile guidance systems; an MMC made from silicon-carbide fibers in a titanium matrix is currently being
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developed for use as the skin (fuselage material) of the US National Aerospace Plane. » Ceramic-matrix composites (CMCs) are the third major type and examples include silicon carbide fibers fixed in a matrix made from a borosilicate glass. The ceramic matrix makes them particularly suitable for use in lightweight, hightemperature components, such as parts for airplane jet engines
● The second level of classification refers to the reinforcement form which include: -Particle Reinforced -Fiber Reinforced -Structural
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Structural Composites also classified into two sub group namely
» Laminates » Sandwitch Panels
There are also more other Classification :
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F i br bre e Re Reii n f or orce ced d Compos Composii te tes s: Technologically, the most important composites are those in which the dispersed phase is in the form of a fiber . Design goals of fiber-reinforced composites often include hig strength and stiffness on a weight basis. These characteristics are expressed in terms of specific strength and specific modulus parameters, which correspond, respectively, to the ratios of tensile strength to specific gravity and modulus of elasticity to specific gravity. Fiber-reinforced composites with exceptionally high specific strengths and modulli have been produced that utilize low density fiber and matrix materials.
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A fiber-reinforced composite (FRC) is a composite building material that consists of three components: (i) the fibers as the discontinuous or dispersed phase, (ii) the matrix as the continuous phase, and (iii) the fine interphase region, also known as the interface. This is a type of advanced composite group, which makes use of rice husk, rice hull, and plastic as ingredients. This technology involves a method of refining, blending, and compounding natural fibers from cellulosic waste streams to form a high-strength fiber composite material in a polymer matrix. The designated waste or base raw materials used in this instance are those of waste thermoplastics and various categories of cellulosic waste including rice husk and saw dust.
FRC is high-performance fiber composite achieved and made possible by cross-linking cellulosic fiber molecules with resins in the FRC material matrix through a proprietary molecular re-engineering process, yielding a product of exceptional structural properties. Through this feat of molecular re-engineering selected physical and structural properties of wood are successfully cloned and vested in the FRC product, in
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addition to other critical attributes to yield performance properties superior to contemporary wood. This material, unlike other composites, can be recycled up to 20 times, allowing scrap FRC to be reused again and again. The failure mechanisms in FRC materials include delamination, intralaminar matrix cracking, longitudinal matrix splitting, fiber/matrix debonding, fiber pull-out, and fiber fracture. Fiber-reinforced composites are subclassified by fiber length.For short fiber, the fibers are too short to produce a significant improvement in strength. Fibre Reinforced Composites are composed of fibres embedded in matrix material. Such a composite is considered to be a discontinuous fibre or short fibre composite if its properties vary with fibre length. On the other hand, when the length of the fibre is such that any further increase in length does not further increase, the elastic modulus of the composite, the composite is considered to be continuous fibre reinforced. Fibres are small in diameter and when pushed axially, they bend easily although they have very good tensile properties. These fibres must be supported to keep individual fibres from bending and buckling.
Fig: Fiber Composite
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Application: There are also applications in the market, which utilize only waste materials. Its most widespread use is in outdoor deck floors, but it is also used for railings, fences, landscaping timbers, cladding and siding, park benches, molding and trim, window and door frames, and indoor furniture. See for example the work of Waste for Life, which collaborates with garbage scavenging cooperatives to create fiber-reinforced building materials and domestic problems from the waste their members collect
Parti cle re r ei nf or orce ced d Compos Composi te te: : Particle Reinforced Composites are composed of particles distributed or embedded in a matrobody. The particles may be flakes or in powder form Concrete and wood particle boards are examples of this category.
Composites refer to a material consisting of two or more individual constituents. The reinforcing constituent is embedded in a matrix to form the composite. One form of composites is particulate reinforced composites with concrete being a good example. The aggregate of coarse rock or gravel is embedded in a matrix of cement. The aggregate aggregate provides stiffness stiffness and strength while the cement acts acts as the binder to hold the structure together.
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There are many different forms of particulate particulate composites. composites. The particulates can be very small particles (< 0.25 microns), chopped fibers (such as glass), platelets, hollow spheres, or new new materials such such as bucky balls or carbon carbon nano-tubes. In each case, the particulates provide desirable material properties and the matrix acts as binding medium necessary for structural applications.
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Particulate composites offer several advantages. They provide reinforcement to the matrix material thereby strengthening the material. The combination of reinforcement and matrix can provide for very specific material properties. For example, the inclusion of conductive reinforcements in a plastic can produce plastics that are somewhat conductive. Particulate composites can often use more traditional manufacturing methods such as injection molding which reduces cost. Large-particle and Dispersion-Strenghened Composites are the two subclassification subclassification of particle –reinforced composites. The distinction between these is based upon reinforcement composites. The term “large” is used to indicate that particle-matrix interactions cannot be treated on the atomic or molecular level ; rather, continuum mechanics is used. For most of these composites, the particulate phase is harder and stiffer than the matrix. These reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle. In essence, the matrix transfers some of the applied stress to the particles, which bear a fraction of the load. The degree of Reinforcement or improvement of mechanical behavior depends on strong bonding at the matrix-particle interface.
For dispersion-strengthened composites, particles are normally much smaller, with diameters between 0.01 and 0.1 µm (10 and 100nm). Particle-matrix interaction that lead to strengthening occur on the atomic or molecular level. The mechanism that lead to strengthening occur on the atomic or molecular level. The mechanism of strengthening is similar to that for precipitation hardening . Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations. Thus, plastic deformation is restricted such that yield and tensile strengths, as well as hardness, improve.
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Fig: Particulate composite Application:
The most common particulate composite materials are reinforced plastics which are used in a variety of industries. Automotive Glass reinforced plastics are used in many automotive applications including body panels, bumpers, dashboards, and intake manifolds. Brakes are made of particulate composite composed of carbon or ceramics particulates. Consumer Products Many of the plastic components we use in daily life are reinforced in some
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way. Appliances, toys, electrical products, products, computer housings, cell phone casings, casings, office furniture, helmets, etc. are made from particulate reinforced plastics .
Str uctu ucturr al Composi Composi te te: : A Structural Composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of constituent materials but also on the geometrical design of the various structural elements. Laminar composites and sandwitch panels are two of the most common structural composites; only a relatively superficial examination is offered here for them.
Fig: Structural Composite
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A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction such as is found in wood and continuous and aligned fiber-reinforced plastics. The layers are stacked and subsequently cemented together such that the orientation of the highstrength direction varies with each successive layer. For example, adjacent wood sheets in plywood areallinged with the grain direction at right angles to each other. Laminations may also be constructed using fabric material such as cotton,paper,or woven glass fibers embedded in a plastic matrix. Thus a laminar composite has relatively high strength in a number of directions in the two-dimensional plane; however, the strength in any given direction is, of
