Unit- 3 Composites MaterialsComposites materials are produced by combining two dissimilar materials into a new material tht ,may be better suited for a particular application than either of the Original material alone.Eg. Glass re-in forced plastic. Plastic is alone weak and has Low elastic Modulus, however it is stable chemically and constitutes an excellent matrix for the composites. The glass fibre provides the strength and stiffness, their modulus of elasticity is 50 times greater than that of Plastic. However GRP has much chemical resistant and have high modulus of elasiticity. Modern technologies require materials with unusual combination of properties that cannot be met by the conventional metal and alloys, ceramics and polymer materials.Materials can be combined in composites to produce a unique characteristics such as stiffness, toughness, and high temperature strength, that can be used for aerospace, underwater and transportation applications.Many composites materials are composed of just two phases, One is termed as matrix, which is continuous and Another surrounds the other phase, often called the dispersed phase. Classification of Composite materialsOne Simple Scheme for the Classification of the Composite materials is as follows,Composites1. Particle re-in forced

a. large particle

b. Dispersion strengthened

2. Fiber re-in forced

a. Continuous (aligned)

b. Discontinous (short)

3. Structural

a. Laminates

b. Sanwich panels

Particle reinforced Composites

The Dispersed phase for particle re-in forced composites is Equi-axised. Ie., particle dimensions are approx the same in all Direction.Types(based on re-in forcement and strengthening mechanism)

1.Large particle2. Dispersion Strengthened

Large Particle CompositesThe term large is used to indicate that particle matrix interactions cannot be treated on the atomic or molecular level; rather continuum mechanism is used.For most of the Composites the particulate phase is harder and stiffer than the matrix.

The re-in forcing particles tend to restrain movement of the matrix phase in the vicinity of each particle.

Matrix transfers some of the applied stress to the particle, which bears a fraction of Load.The degree of reinforcement or improvement of the mechanical behavior depends on strong bonding at the matrix particle re-in force.A filler improves the Properties of the materials and/or replace some of the polymer volume with a less expensive materials.Eg. Concrete - Cement (matrix), Sand and gravel (the particulates)o The particles should be approx the same dimension in all directions (equiaxed).o For effective re-in forcement the particles should be small and evenly distributed throughout the matrix. o The mechanical properties are enhanced with increasing particulate content.Large Particulates Composites are utilized with all three material types Metal Polymer Ceramics Eg. Cermets (Ceramic and metals) most common cermets are Cemented Carbide, which has extremely hard particles of a refractory carbide ceramics such as Ti C or W C, embedded in matrix of metal such as Cobalt and Nickel.These are used for extensively as cutting tools for hardened steels.Both elastomers and plastics are frequently re-inforced with various particulate materials.Use of Rubbers with Carbon Black, when added to Vulcanize rubber this extremely inexpensive materials enhances tensile strength, toughness and tear and abrasive resistance. Automobile tyres contain on the Order of 15 30 % vol of Carbon Black.Dispersion Strengthened CompositesMetals and metal alloys may be strengthened and hardened by the uniform dispersion of several volume per cent of fine particles of very hard and inert materials.Dispersion may beo Metallic - inert metalso metallic Oxides or Oxides material - inert Material

Strengthening mechanism involves interaction between the particles and Dislocation within the Matrix, as with precipitation hardening.

The Dispersion strengthening effect is not as pronounced as with precipitation hardening however the strengthening is retained at elevated temperatures and for extended time periods because particles are chosen to be unreactive with the matrix phase.

Eg. 1 The High temperature strength if Ni alloy may be enhanced significantly by addition of about 3 % vol of thoria (ThO2) as finely dispersed particles, this is known as thoria dispersed Nickel or TD Ni.Eg. 2 A very thin and adherent Alumina Coating is caused to form on the surface of extremely small flakes of Al, which are dispersed within al Al metal matrix, this material is termed as Sintered Al powder S Al P.Fiber Re-inforced CompositesFiber re-inforced composites areo High strength and/or stiffness on a Weight basis. o High Specific Strengtho High moduli that Utilizes low-density fiber and matrix materials.The arrangement or Orientation of the fibers relative to one another, the fiber concentration and distribution all have a Significant influence on the strength and other properties of fiber re-inforced composites.Some Critical length is necessary for the effective strengthening and stiffness of the composite material. The Critical length (lc ) is dependent ono the fiber diameter (d)o its ultimate or tensile strength (f) ando the fiber matrix bond strength or shear yield strength of the material (lc =

For number of glass and carbon fiber matrix Combination, this lc = 1 mm, which d = 20 150 times fiber diameter.Fiber for whichl>> lc( l > 15 lc ) are termed as Continuous

l lcare termed as Discontinuous or Short fibers.

1. Continuous Fiber

For Continuous fiber re-inforcement the strain in the matrix and the strain in the fiber under load are initially the same. At Low stress both deform elastically but increasing load, the matrix may deform plastically while the fiber still will be elastic.

to the fiber direction occur at the very low stresses. This appear to be the effect of stress concentration at the fiber reinforcement. To counter act this cross-pliedlaminates.

2.Discontinuous Fiber

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fiber is broken at the fiber end which are carrying stress than the middle part of the fiber. The strength of the Discontinuous fiber is always lower than the Continuous fibre structure.Eg. Chopped Glass fiber3. Random, Short and Discontinuous FiberApplication involves multi directional applied stresses, uses such fiber which are randomly oriented in the matrix material.Matrix PhaseFunctions1. The matrix phase binds the fiber together and acts as the medium by which an externally applied stress is transmitted and distributed to the fibers; only a small proportion of the applied load is sustained by the matrix phase.

The Matrix material should be

Ductile Elastic modulus of the fiber should be much higher than that of the matrix. 2. The matrix protects the individual fibers from the surface damage as a result of mechanical abrasion or chemical reactions with the environment. 3. The matrix separates the Fiber by virtue

Relative softness Plasticity Prevents the propagation of brittle cracks from fiber to fiber 4. It is essential that adhesive bonding forces between the fiber and matrix be high.

5. Metals such as Al, Cu and Commercial thermoplastic and thermosetting polymers are used as matrix materials.

Fibre Phase1. Fibers can be any Polymer, metal or ceramics that can be drawn into a thin long filament.

2. Important characteristics is that small diameter fiber is much stronger than the bulk material, because of the probability of a critical surface flaw that can lead to fracture diminishes, with decrease specimen volume and this is made as advantage.

3. On basis of Diameter and Character, fibers are grouped into three different classification

(1) Whiskers(2) fibers(3) Wires(1) Whiskers Very thin Single crystal that have extremely large length to diameter ratio. High degree of crystalline perfection hence flaw free High strength They are highly expensive and it is difficult to incorporate in matrix.Eg.Graphite, Si C, Si N, Al2O3(2) Fibers They are either polycrystalline or amorphous and have small diameter. They are either polymers or ceramics Eg. Aramids, glass, Carbon, Boron, Si C, Si N, Al2O3(3) Fine Wires Relatively Large Diameter Eg. Steel, Mo, W.Wires are utilized as Steel re-inforcement in Automobile tryes, in filament Wound rocket casing and in wire wound high pressure hoses.

Structural CompositesStructural Composites composed of both homogeneous and Composite materials the property of which depends not only on the properties of the constituents but also on the geometrical design of the various structural elements.Types1. Laminar composites

2. Sandwich Panels.

Laminar Composites1. Two dimensional Sheets that have a preferred high strength direction such as Wood and aligned Plastic re-inforced plastics.

2. The layers are stacked and Subsequently cemented together such that the orientation of the high-strength direction varies with each successive layer.

3. Lamination may be constructed fabric material such as cotton, plastic, paper or woven fiber embedded in a plastic.

Sandwich Panels1. Two strong outer Sheets or faces sperated by a layer of less dense material or core, which are lower stiffness and low strength.

2. Faces bear most of the in-plane load and also transverse bending stresses.

3. Function :

a. It separates the faces and resists deformation perpendicular to the face Plane

b. Provides a certain degree of shear rigidity along planes which are perpendicular to the faces.

Eg.Al Alloys, Fiber reinforced plastics, Ti, Steel and plywood.METAL MATRIX COMPOSITES

A metal matrix composite (MMC) is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as aceramic or organic compound. When at least three materials are present, it is called

a hybrid composite. An MMC is complementary to a cermetMMCs are made by dispersing a reinforcing material into a metal matrix. The reinforcement surface can be coated to prevent a chemical reaction with the matrix. For example, carbon fibers are commonly used in aluminum matrix to synthesize composites showing low density and high strength. However, carbon reacts with aluminum to generate a brittle and water-soluble compoundAl 4C3 on the surface of the fiber. To prevent this reaction, the carbon fibers are coated with nickel or titanium boride.MatrixThe 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.ReinforcementThe reinforcement material is embedded into the matrix. The reinforcement does not always serve a purely structural task (reinforcing the compound), but is also used to change physical properties such as wear resistance, friction coefficient, or thermal conductivity. The reinforcement can be either continuous, or discontinuous. Discontinuous MMCs can be isotropic, and can be worked with standard metalworking techniques, such as extrusion, forging or rolling. In addition, they may be machined using conventional techniques, but commonly would need the use of polycrystaline diamond tooling (PCD).Continuous reinforcement uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are embedded into the matrix in a certain direction, the result is ananisotropic structure in which the alignment of the material affects its strength. One of the first MMCs used boron filament as reinforcement. Discontinuous reinforcement uses "whiskers", short fibers, or particles. The most common reinforcing materials in this category are alumina and silicon carbide. [1]MMC manufacturing can be broken into three types: solid, liquid, and vapor.Solid state methods Powder blending and consolidation (powder metallurgy): Powdered metal and discontinuous reinforcement are mixed and then bonded through a process of compaction, degassing, and thermo-mechanical treatment (possibly via hot isostatic pressing (HIP) or extrusion). Foil diffusion bonding: Layers of metal foil are sandwiched with long fibers, and then pressed through to form a matrix. Liquid state methods Electroplating / Electroforming: A solution containing metal ions loaded with reinforcing particles is co-deposited forming a composite material. Squeeze casting: Molten metal is injected into a form with fibers preplaced inside it. Spray deposition: Molten metal is sprayed onto a continuous fiber substrate. Reactive processing: A chemical reaction occurs, with one of the reactants forming the matrix and the other the reinforcement. Vapor deposition Physical vapor deposition: The fiber is passed through a thick cloud of vaporized metal, coating it. CONCRETE

Concrete is a building and structural material obtained by mixing

1. Cement,

2. Mineral aggregates (sand and gravels)

3. Water

In suitable proportion so that the result is a Plastic and workable mass that can be molded into any shape.

The Quality of Concrete depends on the properties of the materials used, the methods of batching and mixing the methods of Construction Mixes with Cement-sand-gravel or crushed stone volume proportions to1:1.5:3

1:2:4

1:3:5

Exceptfor cement rich mixtures now-a-days the following proportion are used

1:2:3.5

1:2.5:4

1:3:5

Water addition is for the required workability for any application.ASPHALT CONCRETEAsphalt concrete is competitive with cements concrete for use in construction of highways, roadways and other surfaces.

It is made up of by mixing a suitable graded hot aggregate produced from crushed rock with asphalt cements

ASPHALT = asphaltic bitumen and bitumen is generally synonymous.Asphalt is a black to dark brown solid or semisolid material consisting predominantly of mixtures of hydrocarbon that are completely soluble disulfide. Thermoplastic material and it softens on heating and hardens on cooling. Resistance to non-oxidizing acids and corrosive salts but it is attacked by H2SO4 and many Organic Solvents. soild or semisolid products and require heating to convert them into a fluid state before application. Asphalt are processed to Asphalt cements Liquid asphalt products known as Cutbacks Asphalt emulsion Process : Asphalt cements is used as the hot milk of asphalt cements with suitable grade minerals aggregates spread and compacted on rolling a temperature between 140 and 80 C.It cures immediately on cooling. Depending on the size of the graded aggregates asphalt concrete can contain 5 8 % of pure asphalt cements

Asphalt vs. Concrete DrivewayAsphalt DrivewayConcrete Driveway

Cost

$2.50 - $4.00 /sq.ft.$4.00 - $6.00+ /sq. ft.

Durability

Less durableMore durable

Weather Issues

Softens in high heatCracks in freezing temperatures

Longevity

20 years+40 years+

MaintenanceResealing every 3-5 yearsSealing optional; occasional degreasing

Finishes

NoneMultiple types

Colors

Primarily blackMultiple color options

ResurfacingYesNo

Repairs

EasyDifficult

REINFORCED CONCRETE

Cements concrete is a mixture of cements, water, sand, crushed stone/gravel etc.,Cements acts as a binder for sand and stone chips / gravels.

Concrete is a heterogeneous material characterized by a high compressive strength (28 4000 psi) but low tensile strength (2 350 psi).

When a concrete is bend the failure occur in the tension side of the member, resulting in crack in the concrete mass. This is overcome with Steel in the form of rods, wires, bars or fabric embedded in the fresh concrete.If the tensile stress in steel bar exceeds the value of Compressive strength of concrete, the concrete will develop a tensile crack. But this crack will not be detrimental, since the bond between steel and concrete prevents the width of cracks from becoming significant. If the Width is larger, then the concrete will become permeable to moisture and gases, then corrosion of the steel reinforcement may occur.The thermal expansion of steel is so close to that of concrete.Adhesive bond between Steel-Cements is very strong and the steel is protected from corrosion by the highly alkaline environment of the cement paste.Adhesion can be improved by imparting special surface patterns that provide interlocking between the cement paste and surface of the Steel.

Advantage of RCC1. It is economical

2. Verry ductile and fire resistant

3. Maintanance of RCC Structures is negligible.

4. It is impermeable to mopisture penetration.

5. Different shapes can be obtained easily

6. RCC are not affected by vermins, fungus or other such insects.

7. RCC materials are easily available and abundance.

PRESTRESSED CONCRTERCC crack in the tensile regions when subjected to a relative small fraction of working load. To make a better use for structural, prestressed concrete has been designed.The PSC involves introduction of an internal compressive stress into a structure. Thus cracking in tensile region is no longer inevitable in concrete structure and this make the materials more resistant to chemical attack.Prestressed concrete is designed so that the tensile cracks may be developed only at some load greater than the working load.Before the tensile stress occurs in the pre-stressed concrete, the preapplied compressive stresses first must be counteracted or wiped out.The precompression is usually obtained by two main methods1. Pretensioning

2. Post tensioning

1. Rolls of post-tensioning cables

2. Pulling anchors for post-tensioning cables3. Pulling anchors for post-tensioning cables

4. Pulling anchors for post-tensioning cables

5. Post-tensioning cables stripped for placement in pulling anchors

6. Positioned post-tensioning cables

7. Post-tensioning cable ends extending from freshly poured concrete8. Post-tensioning cable ends extending from concrete slab

9. Hydraulic jack for tensioning cables

10. Cable conduits in formworkIn pre-tensioning, the prestress force is applied by means of high strength steel wire called tendons, which arranged end to end between two fixed anchorages. Tendons are then stretched to a high state and molds are filled with fresh concrete. After the concrete has hardened, the tendons are released from the anchorages and concrete is then put into compression because these tendons contract. The stress in tendon is transferred as Bonding stress.Prestressed concrete is widely used in highways and railways bridges, pretension units for span upto 15 m, for long span posttension construction is used.Vessel and tanks are being prestressed by passing the tendons around them and there by applying a uniform precompression to the vessel walls.

CONCRETE POLYMER COMPOSITESIntroduction of polymer meterials into concrete is rapidly expanding.Following are the main clasess of CPC1. Polymer impregnated concrete (CPC)PCP, mortor and ceramics can be result in major improvement in mechanical strength, elastic modules and impact resistance.PCP involves incorporating of a suitable monomers into the pore structure of preformed cured concrete or mortar and subsequently in-situ polymerize either by free radical mechanism or by heat and pressure.Step 1. Impregnating of mortars with monomers involves preparation of mortar by mixing cements with sand in 1: 3 and using high water:cement ratio of 0.7 which after curing gives a much porous materials that favors penetration of monomers.Step 2. Then the material is dried and heated in under pressure to produce the polymer impregnation.Polymers may be incorporated in concrete in the following ways:1. By adding a polmerizable monomers to fresh concrete or motor mix and then curing concrete and monomers

2. By adding latex into fresh cements or cement mix and then curing the composition in presence of polymers

3. By impregnating a crude mortar or concrete with a monomers using thermal or radiation catalyst.

2. Pre-cast Polymer (PCP)

A pure quality silica sand of size 30-40 mesh is mixed with a polymeric binder such as epoxy, methyl methacrylate and polyester.

Usually these resins are used as monomers which after mixing the sand and casting are polymersized either by heat or by radiation.The product has much improved properties compared to Portland cements combination but the price is high.Advantages of polymer concrete include: Rapid curing at ambient temperatures High tensile, flexural, and compressive strengths Good adhesion to most surfaces Good long-term durability with respect to freeze and thaw cycles Low permeability to water and aggressive solutions Good chemical resistance Good resistance against corrosion Lighter weight (only somewhat less dense than traditional concrete, depending on the resin content of the mix) May be vibrated to fill voids in forms Allows use of regular form-release agents (in some applications) Dielectric Disadvantage Product hard to manipulate with conventional tools such as drills and presses due to its strength and density. FIBER REINFORCED CEMENTS Cements offers great potential for fiber strengthening dur to its low modulus. It shows significant strengthening and increased work of fracture by utilizing a veriety of reinforcement asuch as metal wires, glass fiber, asbestors, carbin fibers and Al fibers. The fiber limit the length of crack during the early stages of initiation from pre-exisitng flaws to fibre ahead of crack tip opposing elastic displacement of the matix. Polypropylene and Nylon fibers can: Improve mix cohesion, improving pumpability over long distances Improve freeze-thaw resistance Improve resistance to explosive spalling in case of a severe fire Improve impact resistance Increase resistance to plastic shrinkage during curing Steel fibers can: Improve structural strength Reduce steel reinforcement requirements Improve ductility Reduce crack widths and control the crack widths tightly, thus improving durability Improve impact and abrasionresistance