Structural Composite Material

Submitted By Albert 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 systems.) 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 into 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

–Melt thermoplastics

–Reduce viscosity

•Pressure is used to:

–Force the viscous resin-fiber material into a mold.

–Compact a laminate

–Squeeze out voids

•Vacuum is used to help pull out trapped air or other gasses that may

be produced during the chemical reaction.

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Manual Lay-up Methods for Composites:

•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

•Use rollers to spread resin, flatten fibers, squeeze out trapped air

•Repeat for 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-to-

diameter (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 continuous-

fiber 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, discontinuous-

fiber composites are generally much less costly than continuous-fiber 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, high-

temperature 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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Fibre Reinforced Composites:

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

Particle reinforced Composite:

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 provides stiffness and strength while the cement acts as the binder to hold the structure together.

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There are many different forms of particulate composites. The particulates can be very small particles (< 0.25 microns), chopped fibers (such as glass), platelets, hollow spheres, or new materials such as bucky balls or 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 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, computer housings, cell phone casings, office furniture, helmets, etc. are made from particulate reinforced plastics.

Structural Composite:

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 high-

strength 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

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course, lower than it would be if all the fibers were oriented in that direction.

One example of a relatively complex laminated structure is the modern ski.

The response of a particulate composite can be either anisotropic or

orthotropic.

Such composites are used for many applications in which strength is not a

significant

component of the design. A schematic of several types of particulate

composites

is shown in Figure:

Fig: Schematic representation of Laminate composites

In materials science, Composite laminates are assemblies of layers of fibrous

composite materials which can be joined to provide required engineering

properties, including in-plane stiffness, bending stiffness, strength, and

coefficient of thermal expansion.

Laminate. A laminate is a stack of lamina, as illustrated in Figure 1.6, oriented

in a specific manner to achieve a desired result. Individual lamina are bonded

together by a curing procedure that depends on the material system used. The

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mechanical response of a laminate is different from that of the individual

lamina that form it.

The laminate’s response depends on the properties of each lamina, as well as

the order in which the lamina are stacked.

Fig: Schematic of a laminuted composite.

The individual layers consist of high-modulus, high-strength fibers in a

polymeric, metallic, or ceramic matrix material. Typical fibers used include

graphite, glass, boron, and silicon carbide, and some matrix materials are

epoxies, polyimides, aluminium, titanium, and alumina.

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Fig: Laminated Sheet

Layers of different materials may be used, resulting in a hybrid laminate. The

individual layers generally are orthotropic (that is, with principal properties in

orthogonal directions) or transversely isotropic (with isotropic properties in

the transverse plane) with the laminate then exhibiting anisotropic (with

variable direction of principal properties), orthotropic, or quasi-isotropic

properties. Quasi-isotropic laminates exhibit isotropic (that is, independent of

direction) inplane response but are not restricted to isotropic out-of-plane

(bending) response. Depending upon the stacking sequence of the individual

layers, the laminate may exhibit coupling between inplane and out-of-plane

response. An example of bending-stretching coupling is the presence of

curvature developing as a result of in-plane loading.

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Fig: Lamina and Laminate

Lamina. A lamina is a flat (or sometimes curved) arrangement of unidirectional

(or woven) fibers suspended in a matrix material. A lamina is generally

assumed to be orthotropic, and its thickness depends on the material from

which it is made.

For example, a graphite/epoxy (graphite fibers suspended in an epoxy matrix)

lamina may be on the order of 0.005 in (0.127 mm) thick. For the purpose of

analysis, a lamina is typically modeled as having one layer of fibers through the

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thickness. This is only a model and not a true representation of fiber

arrangement.

Both unidirectional and woven lamina are schematically shown in Figure:

Hybrid Laminates :

Interply hybrid laminates

Intraply hybrid laminates

Interply-intraply laminates

Resin hybrid laminates

Composite laminates may be regarded as a type of plate[disambiguation needed] or thin-shell structure, and as such their stiffness properties may be found by integration of in-plane stress in the direction normal to the laminates surface. The broad majority of ply or lamina materials obey Hooke's law and hence all of their stresses and strains may be related by a system of linear equations. Laminates are assumed to deform by developing three strains of the mid-plane/surface and three changes in curvature.

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A sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff skins to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.

Open- and closed-cell-structured foams like polyvinylchloride, polyurethane,

polyethylene or polystyrene foams, balsa wood, syntactic foams, and honeycombs are commonly used core materials. Open- and closed-cell metal foam can also be used as core materials.

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Fig: Sandwitch Panel Composite

Laminates of glass or carbon fiber-reinforced thermoplastics or mainly thermoset polymers (unsaturated polyesters, epoxies...) are widely used as skin materials. Sheet metal is also used as skin material in some cases.

The core is bonded to the skins with an adhesive or with metal components by

brazing together.

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A summary of the important developments in sandwich structures is given below.

230 BC Archimedes describes the laws of levers and a way to calculate

density. 25 BC Vitruvius reports about the efficient use of materials in Roman truss

roof structures. 1493 Leonardo da Vinci discovers the neutral axis and load-deflection

relations in three-point bending. 1570 Palladio presents truss beam constructions with diagonal beams to

prevent shear deformations. 1638 Galileo Galilei describes the efficiency of tubes versus solid rods. 1652 Wendelin Schildknecht reports about sandwich beam structures with

curved wooden beam reinforcements. 1726 Jacob Leupold documents tubular bridges with compression-loaded

roofs. 1786 Victor Louis uses iron sandwich beams in the galleries of the Palais-

Royal in Paris. 1802 Jean-Baptiste Rondelet analyses and documents the sandwich effect

in a beam with spacers. 1820 Alphonse Duleau discovers and publishes the moment of inertia for

sandwich constructions. 1830 Robert Stephenson builds the Planet locomotive using a sandwich

beam frame made of wood plated with iron

The 1940 de Havilland Mosquito was built with sandwich composites; a balsa-wood core with plywood skins.

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Types of sandwich structures:

Metal composite material (MCM) is a type of sandwich formed from two thin

skins of metal bonded to a plastic core in a continuous process under controlled

pressure, heat, and tension.

Recycled paper is also now being used over a closed-cell recycled kraft

honeycomb core, creating a lightweight, strong, and fully repulpable composite

board. This material is being used for applications including point-of-purchase

displays, bulkheads, recyclable office furniture, exhibition stands, and wall

dividers.

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Fig: Different Types Of Sandwitch Composite

To fix different panels, among other solutions, a transition zone is normally used,

which is a gradual reduction of the core height, until the two fiber skins are in

touch. In this place, the fixation can be made by means of bolts, rivets, or

adhesive.

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Properties of sandwich structures:

The strength of the composite material is dependent largely on two factors:

1. The outer skins: If the sandwich is supported on both sides, and then

stressed by means of a force in the middle of the beam, then the bending

moment will introduce shear forces in the material. The shear forces result in the

bottom skin in tension and the top skin in compression. The core material spaces

these two skins apart. The thicker the core material the stronger the composite.

This principle works in much the same way as an I-beam does.

2. The interface between the core and the skin: Because the shear stresses in

the composite material change rapidly between the core and the skin, the

adhesive layer also sees some degree of shear force. If the adhesive bond

between the two layers is too weak, the most probable result will be

delamination.

Application of sandwich structures:

Sandwich structures can be widely used in sandwich panels, this kinds of panels

can be in different types such as FRP sandwich panel, aluminum composite panel

etc. FRP polyester reinforced composite honeycomb panel (sandwich panel) is

made of polyester reinforced plastic, multi-axial high-strength glass fiber and PP

honeycomb panel in special antiskid tread pattern mold through the process of

constant temperature vacuum adsorption & agglutination and solidification.

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Theory:

Sandwich theory describes the behaviour of a beam, plate, or shell which consists of three layers - two facesheets and one core. The most commonly used sandwich theory is linear and is an extension of first order beam theory. Linear sandwich theory is of importance for the design and analysis of sandwich panels, which are of use in building construction, vehicle construction, airplane construction and

refrigeration engineering.

Honeycomb Usage:

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Materials

Summary—Advantages and Di

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A detailed chemical composition analysis and properties of composite

materials are outlined here. It is also resistant at high temperature. The

environmental impact and future research trend of the existing composite

material use for oilfield applications are presented as guideline. The researchers

should now think how to develop an environment-friendly, sustainable composite

material. The indication of sustainable composite material research is also an

important issue for future research area.

Strong developmental activities focusing primarily on products & processes

need to be pursued in India. Towards such an objective, a multi-agency approach

involving the

industry,Government,academia,researchlaboratory,certification/standardization

and user agencies would be required for a quantum jump in composite

technology in the country.

Thus, the key thrust areas may be summarized as hereunder :

• Short & long term R&D

• Application development

• Fabrication & testing support

• Availability & pricing of raw materials

• Manpower training

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• Technical support services for materials & process selection, process

optimization & design, product quality improvement etc.

• Standardization

Adoptation of automated technologies (such as RTM, pultrusion) along with

proper technical/training support would help achieving the improved quality &

quantity of composite products. The biggest advantage of composite processing is

that unlike with metals, it is not capital intensive and a smaller volume production

can be justified.

"The future is in Structural composites" is the realization of many decades

of high-technology progress toward different materials and parts assembled and

combined as monolithic units that would provide a combination of versatility,

strength and other properties beyond the possibilities of conventional materials

like metal, wood or concrete.

Assessing the importance of structural composites as an advanced

performance material the Advanced Composites Mission programme was

launched by the Department of Science & Technology (DST), Govt. of India. The

Mission-mode activities are being implemented by the Technology Information,

Forecasting & Assessment Council (TIFAC), an autonomous organization under

DST.

The Advanced Structural Composites Mission aims to improve upon the

laboratory-industry linkages towards application development &

commercialization. The Mission has been successful in launching 26 projects

across the country in active collaboration with the industry and national

laboratories.

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The Mission has been catalytic in bridging the knowledge gaps and bringing

together the industries & the users. Such an objective oriented, demand driven

and time bound programme on composite technology with the involvement of

stake holders would go a long way in developing innovative structural composite

applications meeting international quality and wider acceptance thus contributing

to the growth of knowledge-based business in India.

An efficient mechanism such as the Advanced Structural Composites

Mission can help in synergising the users & industry thus reaching the products to

the market with a shorter gestation period.

In India, the indigenous achievements are very scattered compared to the

large geographical area. There is an urgent need to launch very directed,

concerted & planned efforts for developing & demonstrating novel composite

products. This would call for an improved awareness, technology adaptation,

technology sourcing & subsequent transfer etc. all at one place.

While the national centres of excellence in India possess very rich expertise

in Structural composite technology, the knowledge flow to the industry has not

come up to the desired level. It is high time to bring the technology sourcing

avenues and industries together on one platform for technology development,

transfer & subsequent commercialization.

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Reference:

Callister’s Material Science and Engineering

Wikipedia, the free encyclopedia

Chapter 7: Advanced Composite Material – FAA

Composite Materials for Aerospace Structures By Dr. Douglas S.

Cairns, Lysle A. Wood Distinguished Professor

Ashby, M. F. “Technology in the 1990s: Advanced Materials and

Predictive

Design,” Philosophical Transactions of the Royal Society of London,

A3222. Reinhart, T. J., ed. Engineered Materials Handbook Volume 1,

Composites

LAMINATED COMPOSITE PLATES By David Roylance

Composite Material - Dr. M. Medraj Mech. Eng. Dept. - Concordia

University

THE ADVANTAGES OF COMPOSITE MATERIAL IN MARINE

RENEWABLE ENERGY STRUCTURES By M Mohan, Gurit, UK

Composite materials – RSC Advancing the Chemical Sciences

Structured composite - Wikipedia, the free encyclopedia