P.A. COLLEGE OF ENGINEERING AND TECHNOLOGY

PALLADAM ROAD, POLLACHI - 642 002

DEPARTMENT OF MECHANICAL ENGINEERING

ME 2029

COMPOSITE MATERIALS

TWO MARK QUESTIONS WITH ANSWERS

ACADEMIC YEAR 2012 - 2013

Prepared By

Prof. T. Varun Kumar M.E., (Phd).

UNIT 1

INTRODUCTION

1. What is a composite?

A composite is structural materials that consist of two or more combined

constituents that are combined at a macroscopic level and are not soluble in each other.

One constituent is called the reinforcing phase and the one in which it is embedded is called

the matrix. The reinforcing phase material may be in the form of fibres, particles, or flakes.

The matrix phase materials are generally continuous. Examples of composite systems

include concrete reinforced with steel and epoxy reinforced with graphite fibres, etc.

2. What are advanced composites?

Advanced composite materials are manmade composite materials. They

include as resin, metal or ceramic matrix composite systems reinforced with high strength,

high modulus continuous or discontinuous forms (such as continuous fibres, chopped fibres,

whiskers or particles). The resultant material exhibits entirely distinct, but superior properties

which are not exhibited by the constituting elements. Advanced composites are composite

materials that are traditionally used in the aerospace industries. Theses composites have

high performance reinforcements of a thin diameter in a matrix material such as epoxy and

aluminum.

3. What are natural composites? Give examples.

The composite material that exists in nature is regarded as natural

composites. Examples of such composites include, wood (composed of cellulose and lignin),

human or animal body (composed of bones and tissues) or even rocks and materials. Bones

themselves are composites made of two components, the organic and inorganic. The

organic component consists largely of carbohydrates, fats and proteins imparting pliability to

the bones. The organic component is calcium phosphate which provides rigidity and strength

to the bones. Sea shells and elephant tusk are also natural layered composites.

4. What are the common types of composites?

The most typical types of composites consist of an addictive or reinforcement

such as fibres or particles embedded in a supporting material called matrix. Usually these

are structural materials but they can also be special materials such as electrical conductors.

Some composites have no matrix and are composed of one or more constituent forms

consisting of two or more different materials. Laminates, for example, are composed entirely

of layers which taken together, give the composite its form. Plywood with multiple plies with

different orientation of fibres is a typical laminate composite. Sandwiches which are

composed of a thick and light core material bonded on either side by a thin strong facing are

also regards as composites. Many felts and fabrics have no body matrix but consist entirely

of fibres of several compositions with or without bonding force.

5. What is a reinforced composite?

A reinforced composite is one in which the primary purpose of the insert is to

improve the mechanical properties of the composite. The insert is called reinforcement. The

composite is formed is called reinforced composite.

6. What is Filled composite? Give some examples of fillers commonly used in

composites.

A filled composite consists of an insert whose main function is to alter the

physical property other than the mechanical properties. Such an insert is called filler.

Addition of filler also reduces the cost of composite. Commonly used fillers include carbon

black, calcium carbonate, mica, talc, barium carbonate, calcium sulphate, china clay and

glass beads (also called spheres of glass).

7. What are the general requirements of all composite materials?

All the composite materials must meet the following requirement

The reinforcing phase must be distributed uniformly throughout the matrix

and the reinforced particles and fibres must not be in direct contact with

one another.

The reinforcing phase must not react with the matrix at the service

temperature as this will affect the interfacial bond with the matrix.

The reinforcing phase must not lose its strength and should be well

bonded to the matrix.

Matrix must have lower modulus of elasticity as compared to the

reinforcing phase.

8. List the various functions that a matrix phase performs in a composite material.

A matrix phase performs the following main functions:

It binds the reinforcement in place in the structure and protects it from

mechanical and chemical damage that might occur by abrasion of their

surface or by chemical attack or some extraneous source.

It separates the individual fibres and particles and deflects brittle crack

that otherwise can pass across the entire cross section of the composite.

The matrix takes the load and transfers it to the reinforcement in case of

fibre reinforced or particle reinforced composites.

The matrix also gives shape to the composite.

In some instances the matrix controls the electrical and chemical

properties of the composite.

9. What is the role of a coupling agent present in the interface between

reinforcement and matrix?

A matrix-reinforcement interface plays an important role in a composite as

the load is transferred from matrix to the fibres through this interface. The eventual

properties of a composite are greatly dependent on the nature of the interface. The desired

characteristics of the interface are controlled by a third material called the coupling agent or

compatilizer. The coupling agent can overcome the weak interaction between the matrix and

the reinforcement. The matrix and the reinforcement differ in their chemical nature and

surface characteristics but the addition of coupling agent results in their association, leading

to improve strength of the resultant composite.

10. Differentiate between dispersion strengthened and particle strengthened

composites.

Dispersion strengthened composites contain particles upto about 15% by

volume having size in the range 0.01µm to 0.1µm whereas the matrix of particle reinforced

composites contains particles in amounts from 15% to 40% by volume with size greater than

1.0µm. in special cases, particle content may be as high as 94%. In particulate composites

both the matrix and particles share the load equally whereas the matrix carries the major

load in dispersion strengthened composites.

11. What is the basic principle used in the fibre reinforced composites?

The basic principle used in fibre reinforced composites is that the materials

are generally stronger in fibre form than in bulk form. This is due to the alignment of

molecules along the fibre axis. This preferred alignment makes the strength and modulus of

both natural and synthetic fibres superior to those of the same material in the randomly

oriented bulk form. That is why major part of the load is carried by the fibres and not by the

bulk matrix. As fibres are to carry the major portion of the applied load they should be much

stronger and stiffer than the matrix. This property is also desirable for preventing the fibres

from buckling under compressive stresses.

12. What are ‘whiskers’? Name some whiskers that are currently available.

Whiskers are very fine single crystal materials with diameters of the order of

submicron. Their length may range from a fraction of a millimeter to several centimeters. The

aspect ratio (i.e. the length to diameter ratio) of whiskers generally ranges from 50 to 10000.

Due to verifying diameter they contain very few or no dislocations and their strength

approaches that of an ideal or perfect crystal (i.e. the theoretically achievable limit). Their

resistance against temperature and strain to failure are very much higher than high

performance fibres. The elastic strain to fracture may be as high as 5%. However, they

suffer from the problem of brittleness and are difficult process. Metals, oxides, carbides,

halides and organic compounds have been grown in whisker form. SiC, Si3N4, carbon, α-

alumina and K2O.6TiO2 whiskers are currently available. Among these, SiC whiskers seem

to be the best suited for metal-matrix composite reinforcement.

13. Define Flake Composites.

Flakes are often used in place of fibers as can be densely packed. Metal

flakes that are in close contact with each other in polymer matrices can conduct electricity or

heat, while mica flakes and glass can resist both. Flakes are not expensive to produce and

usually cost less than fibers. Flake composites have a higher theoretical modulus of

elasticity than fiber reinforced composites. They are relatively cheaper to produce and be

handled in small quantities.

14. Name the various grades of glass fibres used as reinforcement in glass fibre

reinforcement plastic (GRP).

There are three grades of glass fibres available for reinforcement in glass

fibres reinforced plastics. They are,

E-glass (or electrical glass),

C-glass and

S-glass.

15. List the types of fibres used as reinforcements in composites.

Organic fibres such as nylon and aramid; inorganic fibres of glass, graphite

and ceramics; and stainless steel fibres and wires of tungsten are some typical fibre

materials used as reinforcement in composites.

16. Give some of the characteristics of C-fibers.

C-glass (or chemical glass), is similar to E-glass in composition and provides

excellent resistance to chemical, especially acids. When combined with E-glass

reinforcement, this glass is very useful for surfacing mats for many applications that require

corrosion resistance.

17. What are the disadvantages of glass fibres?

The disadvantages of glass fibres are low modulus self abrasiveness which

frequently decreases the potential strength, relatively low fatigue resistance and poor

adhesion to matrix resins. This means that they do not impregnate the matrix well and

uniformly. Therefore, these fibres are never used for high performance composites. Through

this adhesively can be improved by sizing on the surface, this may provide a path for

absorption of moisture and create undesirable internal stresses.

18. Define Solid Microspheres.

Solid Microspheres have relatively low density, and therefore, influence the

commercial value and weight of the finished product. Studies have indicated that their

inherent strength is carried over to the finished molded part of which they form a constituent.

Solid glass Microspheres, manufactured from glass are most suitable for plastics. Solid glass

Microspheres are coated with a binding agent which bonds itself as well as the sphere‟s

surface to the resin. This increases the bonding strength and basically removes absorption

of liquids into the separations around the spheres.

19. What are the disadvantages of carbon fibres?

In spite of highly desirable characteristics, carbon fibres have certain

disadvantages. They have low impact toughness and high electrical conductivity. The later

property may cause „shortening‟ in unprotected electrical machinery. Another disadvantage

of carbon fibres is their high cost. Due to high cost they are sometimes used in hybrid

composites, along with relatively cheaper fibres such as glass.

20. Define Microspheres.

Microspheres are considered to be some of the most useful fillers. Their

specific gravity, stable particle size, strength and controlled density to modify products

without compromising on profitability or physical properties are it‟s their most-sought after

assets. They are of two types of microspheres they are solid microspheres and hollow

microspheres.

21. What are Kevlar fibres? What are the applications for which these fibres were

developed originally?

Kevlar is the trade name of aramid fibres developed at Du Pont Company.

The two varieties of Kevlar that are most common include „Kevlar 49‟ and „Kevlar 29‟. These

fibres were originally developed to replace steel wire used in radial tyres. Kevlar is about five

times stronger than steel on weight to weight basis.

22. Define Filled Composites.

Filled composites result from addition of filler materials to plastic matrices to

replace a portion of the matrix, enhance or change the properties of the composites. The

fillers also enhance strength and reduce weight. Fillers may be the main ingredient or an

additional one in a composite. The filler particles may be irregular structures, or have precise

geometrical shapes like polyhedrons, short fibers or spheres.

23. What is a carbon-carbon composites produced?

A carbon-carbon composite material consists of a carbonaceous matrix

reinforced with carbon fibres in the form of continuous filament yarns, cloth, chopped fibres

or three dimensional woven reinforcement.

24. What are aramid fibres?

Aramid is the generic term for fibres produced from „aromatic polyamides‟.

Polyamides are long chain polymers containing aromatic rings or so called as benzene rings.

These rings recur repeatedly in the structure of aramid fibres.

25. How is carbon fibres produced?

Carbon fibres are produced by pyrolysis of suitable carbonaceous products

such as polyacrylonitrile (PAN), pitch and staple rayon fibres called precursors. The process

involved three stages, namely, stabilization, carbonization and graphitization.

26. List some typical ceramic matrix composite systems.

Typical ceramic matrix composite systems include carbon-carbon composite,

molybdenum disulphide, Sic matrix/ carbon fibre composite, lithium alumino silicate matrix/

Nicalon (SiC) fibre, and ceramic matrix/ glass fibre composite.

27. What are the advantages of thermo plastic matrices?

Superior toughness.

Short moulding cycle.

Capability of fusion bonding.

Infinite Prepreg shelf life.

Easy repairability.

Reusability of scrap.

28. List the disadvantages of thermoplastic matrices.

Higher temperature of processing.

Relatively low heat resistance.

Sensitivity to solvents.

29. What are the most common advanced composites?

The most common advanced composites are polymer matrix composites

(PMCs) consisting of a polymer (e.g., epoxy, polyester, urethane) reinforced by thin diameter

fibres (e.g., graphite, aramid, boron). The reasons why they are the most common

composites include their low cost, high strength, and simple manufacturing principles.

30. What are hollow microspheres?

Hollow microspheres are essentially silicate based, made at controlled

specific gravity. They are larger than solid glass spheres used in polymers and commercially

supplied in a wider range of particle sizes. Commercially, silicate-based hollow microspheres

with different compositions using organic compounds are also available.

31. What are the drawbacks of polymer matrix composites?

The main drawbacks of PMCs include low operating temperatures, high

coefficient of thermal and moisture expansion, and low elastic properties in certain

directions.

32. What are the principal fibers used in commercial applications?

The principal fibers in commercial use are various types of glass and carbons

as well as Kevlar 49.Other fibers, such as boron, silicon carbide, and aluminium oxide, are

used in limited quantities.

33. What are the influences of fiber in a composite laminates?

Density.

Tensile strength and modulus.

Compressive strength and modulus.

Fatigue strength as well as fatigue failure mechanisms.

Electrical and thermal conductivities.

34. What are the principal advantages of glass fibers?

Low cost.

High tensile strength.

High chemical resistance.

Excellent insulating properties.

35. Define the term ‘wettability’.

The adhesion between the reinforcing fiber and the matrix in composite

materials plays an important role in the final mechanical properties of the material. It is called

as wettability.

36. Define Cermets/Ceramal.

The Cermets is an abbreviation for the "'ceramic" and "metal." A Cermets is a

composite material composed of ceramic (Cer) and metallic (Met) materials. Cermets are

ideally designed to have the optimal properties of both a ceramic, such as high temperature

resistance and hardness, and those of a metal, such as the ability to undergo plastic

deformation. The metal is used as a binder for an oxide, boride, carbide, or alumina.

Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on

the physical structure of the material, cermets can also be metal matrix composites, but

cermets are usually less than 20% metal by volume. It is used in the manufacture of

resistors (especially potentiometers), capacitors, and other electronic components which

may experience high temperatures.

37. What are fiber metal laminates?

Fiber-reinforced polymer laminas can also be combined with thin aluminium

or other metallic sheets to form metal-composite hybrids, commonly known as fiber metal

laminates (FML).

38. What are coupling and coating agents?

Coupling agents and coatings are applied on the fibers to improve their

wetting with the matrix as well as to promote bonding across the fiber – matrix interface.

Both in turn promote a better load transfer between the fibers and the matrix.

39. What are the roles of matrix in FRP?

To keep the fibers in place.

To transfer stress between the fibers.

To provide a barrier against an adverse environment, such as chemicals

and moisture.

To protect the surface of the fibers from mechanical degradation.

40. What are the classifications of polymers?

Thermoplastics

Thermosets.

41. What will happen when load applied to FRP at glass transition temperature?

When an external load is applied, it exhibits an instantaneous elastic

deformation followed by a slow viscous deformation followed by a slow viscous

deformation. With increasing temperature, the polymer changes into a rubber like solid

capable of undergoing large, elastic deformations under external loads.

42. What is Interface?

The bonding between the fiber and the resin is called interface. It is a region

where coating and matrix diffused into each other‟s domain and form a flexible, three-

dimensional polymer network.

43. Define the terms filament, strand, yarn.

Filament is defined as any fibre whose aspect ratio (length to diameter ratio)

is for all practical purposes, infinity. Filament is thus a continuous fibre. The untwisted form

of glass fibers is called as strand. A group of 200 or more fibers are called as strand. Strand

is a primary bundle of continuous filaments combined in a single compact unit without twist.

Yarn is a generic term used for strands of fiber or filaments in a form suitable for weaving.

44. What is roving?

A roving is a group of untwisted parallel strands wound on a cylindrical

forming package. Roving‟s are used in continuous moulding operations, such as filament

winding and pultrusion.

UNIT 2

REINFORCEMENT AND MATRICES

1. Define the term fibres.

Polymers which can be drawn into long filament like materials having length

to dimension ratio (i.e. aspect ratio) of at least 100 are called fibres. Some of the examples

of fibres are nylon, Terylene, cellulose, Kevlar, etc.

2. Define the term resin.

The polymeric molecules in the form of flakes, granules pallets or viscous

liquid are termed as resins. Resins can be subsequently shaped into usable and product.

3. What are liquid resins?

If polymers are used as adhesives, potting compounds, sealants, etc. In liquid

form, they are preferred to as „liquid resin‟, e.g. polyester resins, epoxy resins, phenolic

resins, etc.

4. List various factors affecting polymer materials properties.

Additives.

Branching.

Copolymerization.

Cross-linking.

Degree of crystallinity.

Glass transition temperature.

Molecular weight and molecular weight distribution.

Plasticizers.

Polymer structure and shape.

5. List the kind of structures commonly formed in polymers.

Three types of structures are commonly formed in polymers. They are,

Linear.

Branched.

Network.

6. What do you mean by branching?

Branching is the process of bifurcation of long polymer chain into two

branches at some point along its length. The formation and extent of side chains depends on

the operating conditions of polymerization such as temperature, pressure and catalyst type.

7. Give examples of various filler materials used in plastics.

Wood flour, chopped fabrics, asbestos, carbon black, talc, mica, gypsum, and

milled glass are examples of fillers.

8. What is cross-linking?

Cross-linking is the process of introducing a primary covalent type link

between polymer chains or their segments using a chemical reagent (such as sulphur or

oxygen in vulcanization process). Physical means such as irradiation can also be used to

introduce cross-links.

9. What are plasticizers? What is the purpose of adding them in a polymer? Name

some common plasticizers.

Plasticizers are low molecular weight monomeric liquids that possess low

volatility. They are added to a polymer to impart plasticity and flexibility and thus improve

processibility. Vegetable oils, low molecular weight polyesters and various phthalates are

commonly used as plasticizers.

10. What are fillers and what function do they impart?

Fillers are materials used to provide bulk and modify certain properties such

as strength, impact resistance, heat resistance and other similar properties. They are

generally chemically inert and do not react with resin during processing.

11. What are additives?

Additives are substance added to a polymer to modify or control its properties

and reduce the cost. Various additives includes stabilizers, colorants, catalysts, lubricants,

flame retardants, etc. Sometimes fillers and plasticizers are also included in the category of

additives.

12. Define the term polymer.

Polymers are high molecular weight organic materials made up of long chain

molecules containing principally hydrogen and carbon and may also contain atoms of N, Cl,

O and S. These are complex and giant molecules with molecular weight in the range of 104-

107 and formed by many chemically bonded parts or units. Due to their high molecular

weight, these polymers are also called high polymers.

13. What for catalysts added to a polymer?

Catalysts are added to control the rate and extent of polymerization process

in the resin. Catalysts are not used up in the reaction of polymerization. A catalyst may serve

the following reactions:

Promote the polymerization.

Permit specific orientation reaction to occur.

Give the added benefits of initiating the reactions.

Catalysts also affect the shelf life of the polymer. Metallic as well as organic

compounds are used as catalysts. A well known catalyst system is Ziegler-Natta catalyst.

14. Why are lubricants added to polymer?

Many plastics when heated during processing they become sticky. To

overcome this problem lubricants are added to the plastics.

15. Define the term thermoplastics. Give some examples

Thermoplastics are the materials which can be reshaped or reformed

into a new form even after moulding. In words, thermoplastics can be subjected to

temperature and pressure again for forming a new design product. Some common examples

are:

Nylon.

Polyethylene (PE).

Polystyrene (PS).

Polypropylene (PP).

Polyvinyl chloride (PVC).

Polymethyl methacrylate (PMMA).

Polyetherketone (PEK), etc.

16. What are thermosetting plastics? Give some examples.

The plastics which are once moulded cannot be reformed or shaped into new

form are called thermosetting plastics or simply thermosets. Once moulded cannot be

reshaped even by subjected to heat and pressure due to the non-softening characteristic of

plastic. An example includes phenolic resin (Bakelite), epoxy resins, etc.

17. Which polymers are called naturally occurring polymers?

Polymers which are derived from plants and animals are called naturally

occurring polymers.

18. Name some of the naturally occurring and some synthetic polymers.

Cellulose.

Casein.

Silk.

Natural rubber.

Cotton.

Leather.

Resins.

Wood.

Tar.

Shellac are some naturally occurring polymers.

While plastics and elastomers (except natural rubber) are examples of

synthetic or man-made polymers.

19. Define the term plastics.

There is no standard definition for the term plastics since there is no general

agreement to any definition. In the simplest form plastics may be defined as a polymer which

can be shaped into hard and tough usable articles through the application of heat and

pressure. Examples of plastics are PVC, PE, PS, etc.

20. List the various advantages of plastics.

Articles of intricate shape can be fabricated with ease and can be

produced in large quantity at low cost.

They are lighter (low density) than metals.

They have good electric and thermal insulating properties.

Due to poor thermal conductivity they are very suitable for making

handles of cooking utensils, electric iron box, steering wheel, etc.

They are resistant to corrosion and for rust.

They can be obtained in variety of colours and shades.

They show good dampering properties.

21. List the disadvantages of plastics.

They are dimensionally unstable and tend to warp or creep.

They are also thermally unstable

Some plastics give unpleasant odour.

Most of the plastics deteriorate when exposed to air and sunlight.

Most of the plastics can be easily scratched.

Good plastic parts are expensive, particularly in small lots.

Non-biodegradable.

22. Name various categories of thermoplastics.

Commodity thermoplastics

Transition thermoplastics

Engineering thermoplastics

High performance thermoplastics

Thermoplastic elastomers.

23. What is polypropylene and what is its repeated structural unit.

If on every second carbon atom on the main chain of polyethylene (PP) a

hydrogen atom is replaced by a methyl group then the resulting polymer is called

polypropylene. The repeat structural unit is:

24. What is ‘PEEK’?

Polyether ether ketone (PEEK). It is a semi crystalline polymer with a

maximum achievable crystallinity of 48% when it is cooled slowly from its melt.

25. What is the advantage of cynate ester resin?

Cynate ester resin has a high glass transition temperature (Tg 265◦C), lower

moisture absorption than epoxies, good chemical resistance, and good dimensional stability.

Its mechanical properties are similar to those of epoxies.

26. What is APC?

Continuous carbon fiber-reinforced PEEK composites are known in the

industry as aromatic polymer composite or APC.

27. What is the function of fiber surface treatment?

The primary function of a fiber surface treatment is to improve the fiber

surface wettability with the matrix and to create a strong bond at the fiber-matrix interface.

28. Why the coupling agents are used with glass fiber?

Chemical coupling agents are used with glass fibers to (1) improve the fiber-

matrix interfacial strength through physical and chemical bonds and (2) protect the fiber

surface from moisture and reactive fluids.

29. Why silane is preferred as coupling agent with glass fibers?

The interfacial bond created by silanes allows a better load stress transfer

between fibers and matrix, which in turn improves the tensile strength as well as the

interlaminar shear strength of the composite.

30. Why the fillers are added to the polymeric materials?

Fillers are added to a polymer matrix for one or more of the following reasons:

Reduce cost.

Increase modulus.

Stiffness.

Reduce mold shrinkage.

Control viscosity.

Produce smoother surface.

31. Write down some filler materials?

The most filler for polyester and vinyl ester resins is calcium carbonate

(CaCo3), which is used to reduce cost as well as mold shrinkage. Examples of other fillers

are clay, mica, and glass micro spheres.

32. What is fiber architecture?

Fiber architecture is defined as the arrangement of fibers in a composite,

which not only influences the properties of the composite, but also its processing.

33. List various ceramic matrices.

Aluminium oxide (Al2O3).

Carbon, silicon carbide (SiC).

Silicon nitride (Si3N4).

34. Why coupling agents are added with composites?

Coupling agents are added to improve the wetting of fiber and promote

bonding.

35. What are acrylics?

Most acrylics are based on polymers of methyl methacrylate, that is, they are

Polymethyl methacrylate (PMMA) which may be modified by copolymerization or blending

with other monomers. Acrylics monomers MMA is a clear liquid made from acetone and

sodium cyanide. On polymerization, they normally become clear. They can also be obtained

by opaque varieties including fluorescent shades.

36. What are nylons and where does the name comes from?

The popular groups of Polyamides (PA) resins are known as nylons. They are

produced by the condensation reaction of diamine with dibasic acids or their derivatives.

Depending on the number of carbon atoms on the amines or the acid components, various

nylon designations are available. Examples are nylon 6, nylon 6/6, nylon6/10, etc.

37. Write the applications of nylons.

Nylons are used great variety of household and industrial goods. Unbreakable

cups and saucers, bowls and tumblers are made in ivory luster nylons. Combs will not

become toothless age, zip fasteners that is unaffected by dry cleaning and doctors syringe

are made of nylons. The typical industrial applications include unlubricated gears, cams,

sleeve bearing, driving belts, valves, automobile speedometer, textile machinery, etc.

38. What are polyesters? In what way they can be compared and contrasted with

Polyamides?

Polyesters are the products obtained by the condensation of adipic acid and

bifunctional alcohols with removal of water. It is similar to polyamides in both condensation

products and has the common monomers „acid molecules‟. However the other monomer is

alcohol instead of amines. Like nylon, polyester can be made in the form of unbranched

thread like molecules (thermoplastics).

39. In what form is PET thermoplastic available?

PET thermoplastic is available as film, fibre and blow moulded products. The

processing to these forms leads to orientation or crystallization of molecular chain resulting

in high strength properties.

40. Give some uses of PET thermoplastics.

PET thermoplastic is widely used to make the popular polyester thread also

known as Terylene or Dacron used for sewing. Biaxially oriented film, also known as myler is

used as photo film, magnetic tapes, capacitors, cooking bags, food container packing and

coatings.

41. Write some applications of PEEK.

PEEK is used as a coating and insulating material for high performance

wiring, particularly required for aviation and computer industries. PEEK filaments may be

used for making cloths for hot filtration purposes.

42. What are ladder polymers?

Ladder polymers are double stranded polymer in which aromatic rings are

fused together or interconnected by links around which rotation cannot occurs except by

bond breaking. If the structure of ladder polymers is perfect, the chain could be broken only if

at least two bond on the same rings were broken. Thus, ladder polymers exhibits great

thermal stability.

43. List the three classes of commercial polyesters.

Saturated or thermoplastic polyesters

Unsaturated or thermosetting polyester resins

The liquid polyester resins called simply polyesters.

44. What are unsaturated polyesters?

Unsaturated polyesters are thermosets composed of linear polyester polymer

cross-linked with other monomers such as styrene. These are usually produced as solid

moulding compounds and are called alkyds.

45. What are liquid polyester resins?

Liquid polyester resins are converted into solid plastics by simply adding an

organic peroxide catalyst for polymerization or curing takes place. During the curing and

before hardening, fillers and reinforcements are added.

46. In what way epoxy are similar to unsaturated polyesters?

Epoxy resins are similar unsaturated polyesters in that they are mixed with a

cross linking agent which makes them set by forming a molecular network structure.

47. In what forms epoxy are available?

Epoxies are available in both liquid and solid forms. To produce solid forms,

the epoxy resins cured by using cross linking agents and catalyst or with hardeners

containing active hydrogen to develop the desired properties.

48. What properties make epoxies suitable for coatings?

Toughness.

Flexibility.

Adhesion.

Chemical resistance.

49. What are coatings?

The coatings are provided to the fiber surface to protect the fibers from

mechanical damage and environmental damage and also to enhance bonding of fiber to

matrix.

50. Give some applications of epoxies as coatings.

Process equipments,

tank and drum linings,

can coatings,

pipe linings,

protective coatings for the food industry

wire coatings

UNIT 3

MANUFACTURING OF ADVANCED COMPOSITES

1. Draw the flowchart for composite laminate part fabrication?

2. Define release film?

These films are used to prevent the composite parts from adhering to tool

surface. It is also placed between the bleeder and breather plies as a separator film. It is

used to prevent the resin flow into the breather piles, to which the vacuum system vents.

3. What is Bleeder plies?

Bleeder plies (used for bleed bagging system) often fiberglass cloth fabric,

are normally required to absorb excess resin and permit the escape of volatiles. Various

solvents and other volatile chemicals in the Prepreg that take part in chemical reactions

during layups must be vented, or an unacceptably porous structure will result.

4. What is RTM process?

RTM: Resin Transfer Molding Process. In RTM, several layers of dry

continuous strand mat, woven roving, or cloth are placed in the bottom half of a two-part

mold, the mold is closed, and a catalyzed liquid resin is injected into the mo ld via a centrally

located sprue. The resin injection point is usually at the lowest point of the mold cavity. The

injection pressure is in the range of 6 9–690 kPa (10–100 psi).

5. Define Breather plies.

Breather plies are usually fiberglass or synthetic fabric which is placed on top

of the release film to allow dispersion of vacuum pressure over the layup and removal of

entrapped air or volatiles during cure/consolidation. Coarse, open weave fabrics are used,

otherwise bridging and bag failure may occur.

6. Define Shrinkage.

Shrinkage is the reduction in volume or linear dimension s caused by curing

as well as thermal contraction. Curing shrinkage occurs because of the rearrangement of

polymer molecules into a more comp act mass as the cu ring reaction proceeds. The thermal

shrinkage occurs during the cooling period that follows the curing react ion and may take

place both inside and outside the mold.

7. What is resin flow?

The flow of resin through the fiber network is called resin flow. Proper flow of

resin through a dry fiber network (in liquid composite molding [LCM]) or a Prepreg layup (in

bag molding) is critical in producing void-free parts and good fiber wet-out.

8. Define Voids.

The most common cause for void formation is the inability of the resin to

displace air from the fiber surface during the time fibers are coated with the liquid resin. Void

s may also be caused by air bubbles and volatiles en trapped in the liquid resin.

9. What is the purpose of vacuum bag?

The vacuum bag provides the means of removing vapors, and encouraging

the required resin flow, so its design and implementation are important.

The vacuum is used to assist removal of trapped air or other volatiles.

The vacuum and pressure-temperature cycles are adjusted to permit

maximum removal of air with maximum resin flow.

10. What are the factors influencing the cure cycle?

It should be noted that the cure cycle depends on a number of factors,

including resin chemistry, catalyst reactivity, cure temperature, and the presence of inhibitors

or accelerators.

11. What is called Gel time?

The viscosity increases with increasing cure time and temperature. The rate

of viscosity increases is low at the early stage of curing. After a threshold degree of curve is

achieved, the resin viscosity increases at a very rapid rate. The time at which this occurs is

called the gel time.

12. What is the significance of finding Gel time?

The gel time is an important molding parameter, since the flow of resin in the

mold becomes increasingly difficult at the end of this time period.

13. What are the types of open mould process?

Spray lay up

Hand lay up

Filament winding

Sheet moulding compound

14. What are the types of closed mould process?

Vacuum bag moulding

Pressure bag moulding

Compression moulding

Autoclave

Injection moulding.

15. List out the preparation of moulding compound ingredients.

Resins

Plasticizers

Fillers

Lubricants

Catalysts

Stabilizers

Coloring materials.

16. Define Viscosity of Fluid.

Viscosity of a fluid is a measure of its resistance to flow under shear stresses.

Low-molecular-weight fluids, such as water and motor oil, have low viscosities and flow

readily. High- molecular- weight fluid s, such as polymer melts, has high viscosities and flow

only under high stresses. The two most important fact ors determining the viscosity of a fluid

are the temperature and shear rate.

17. Write short notes on filament winding?

In a filament-winding process, a band of continuous resin-impregnated

roving‟s or monofilaments is wrapped around a rotating mandrel and cured to produce

axisymmetric hollow parts.

18. What is compression moulding?

Compression molding is used for transforming sheet-molding compounds

(SMC) into finished products in matched molds. The principal advantage of compression

molding is its ability to produce parts of complex geometry in short periods of time. Non

uniform thickness, ribs, bosses, flanges, holes, and shoulders, for example, can be

incorporated during the compression-molding process.

19. Give names of various polymers used in advanced polymer composites.

Epoxy

Phenolics

Acrylic

Amino acids

Urethane

Polyamide.

20. What are the types of filament winding patterns?

There are three types of patterns, they are

Hoop or circumferential winding

Helical winding

Polar winding

21. Define epoxy.

Epoxy resins are the most commonly used resins. They are low molecular

weight organic liquids containing epoxide groups. Epoxide has three members in its ring:

one oxygen and two carbon atoms.

22. What are prepregs?

Prepregs are a ready-made tape composed of fibers in a polymer matrix.

They are available in standard widths from 3 to 50 in. (76 to 1270 mm). Depending on

whether the polymer matrix is thermosets or thermoplastic, the tape is stored in a refrigerator

or at room temperature, respectively. One can lay these tapes manually or mechanically at

various orientations to make a composite structure.

23. Name the materials used in hand layup process.

Materials: - Resins: Epoxy, Polyester, Vinyl ester, Phenolic etc.

Fibers: Glass, Carbon, Aramid or natural fibers.

24. What is the application of Bag molding process?

The Bag- molding process is used predominantly in the aerospace industry

where high production rate is not an important consideration.

25. How is the maximum temperature inside the layup determined?

The maximum temperature inside the layup depends on (a) the maximum

cure temperature, (b) the heating rate, and(c) the initial layup thickness.

26. Define blisters.

Blisters are interlaminar cracks formed at the end of molding due to excessive

gas pressure in the interior region of the molded part.

27. What are the advantages of compression molding process?

The principal advantage of compression molding process is its ability to

produce parts of complex geometry in short periods of time.

28. What are the various defects present in the compression molded SMC part?

The various defects present in the compression moulded SMC parts are,

Blister

Surface pin holes

Voids

knit line

Fiber buckling

Insert

Sink mark etc.,

29. What is warpage?

Warpage is critical in thin- section moldings and is caused by variations in

cooling rate between sections of different thickness or different fiber orientations.

30. Give some examples for pultruded parts?

The common pultruded products are solid rods, hollow tubes, flat sheets, and

beams of a variety of cross sections, including angles, channels, hat sections, and wide-

flanged sections.

31. What are the factors affecting the quality of composites?

The factors affecting the quality of composites are the

Cure time

Temperature

32. What is meant by pressure vessel forming?

Pressure-vessel forming is a method in which a pressure vessel is combined

with integrally-heated tooling. Pressure vessel forming has the following advantages:

Since a pressure vessel is a less expensive piece of equipment, its size is

not limited by cost and the production of large parts is easily feasible.

Dry or Prepreg material forms.

Matched die metal tools can be used (to control both part surfaces).

33. What is meant by match die forming?

Matched-die forming is a most widely used forming system for thermoplastics

because it is available and vary from small simply operated hand processes to large

computer controlled presses. (The dies used are generally made of metal which can be

internally heated and/or cooled. The dies are designed to fixed gap of close tolerance.

34. What are the factors considered in match die forming?

High forming pressure required for good solidation.

High fabrication costs.

Friction at die interface.

Long heating and cooling times.

Non-uniform deformation/pressure, if thickness mismatch exists.

35. What are the types of graphite crystals arrangement in carbon fibers?

Arrangement of graphite crystals in a direction to the fiber axis:

Circumferential.

Radial.

Random.

Radial-circumferential.

Random-circumferential.

36. What are the two types from which the carbon fibers are manufactured?

Carbon fibers are manufactured from two types of precursors (starting

materials), namely, textile precursors and pitch precursors.

37. Write short notes on autoclave molding.

Autoclave composite molding is a similar to pressure- bag and vacuum- bag

molding but uses a high- pressure chamber instead. The advanced composite process

produces denser, void free moldings because higher heat and pressure are used during

curing.

UNIT 4

MECHANICS OF LAMINATED COMPOSITES

1. What are two basic approaches to the micromechanics of composite materials?

The two basic approaches to the micromechanics of composite materials are

Mechanics of materials

Elasticity.

2. What are the approaches performed in elasticity?

The elasticity approach actually is at least three approaches

Bounding principles.

Exact solutions.

Approximate solutions.

3. Define lamina.

Initially stress free.

Linearly elastic.

Macroscopically homogeneous.

Macroscopically orthotropic.

4. Define stress.

Stress is a measure of the internal forces acting within a deformable body.

Quantitatively, it is a measure of the average force per unit area of a surface within the body

on which internal forces act

5. What are the basic assumptions made in lamination theory?

Fibers are uniformly distributed throughout the matrix.

Perfect bonding exists between the fibers and the matrix.

The matrix is free of voids.

Both fibers and matrix behave as linearly elastic materials.

6. Describe the maximum principal stress theory?

Maximum Principal Stress Theory - Yield occurs when the largest principal

stress exceeds the uniaxial tensile yield strength. Although this criterion allows for a quick

and easy comparison with experimental data it is rarely suitable for design purposes.

According to the maximum stress in the principal material directions is equal

to or greater than the corresponding ultimate strength.

7. What are the uses of lamination theory?

Use of lamination theory: lamination theory is useful in calculating stresses

and strains in each lamina of a thin laminated structure.

8. Describe the maximum principal strain theory?

Maximum Principal Strain Theory - Yield occurs when the maximum

principal strain reaches the strain corresponding to the yield point during a simple tensile

test. In terms of the principal stresses this is determined by the equation:

9. What are isotropic materials?

An isotropic material is one which looks the same in every direction. We

cannot define any special direction using the material properties. In other words, none of the

properties depend the orientation; it is perfectly rotationally symmetric. Note that in order to

be isotropic the material must be homogenous on the length scale of interest, (i.e.,) the

same at every point in the material. For instance, rubber is a very isotropic material. Take a

rubber ball, and it will feel the same and bounce the same however you rotate it. On the

other hand, wood is an anisotropic material: hit it with an axe and it will take more force to

break of you are cutting across the grain than along it.

10. What are anisotropic materials?

An anisotropic material is a material which does not behave the same way in

all directions. Take wood for example. Wood is very strong along the grain. Against the

grain, however, it will easily break. An anisotropic material, properties are different in all

directions so that the material property symmetry.

11. What are orthotropic materials?

By definition, an orthotropic material has at least 2 orthogonal planes of

symmetry, where material properties are independent of direction within each plane. Such

materials require 9 independent variables (i.e. elastic constants) in their constitutive

matrices. In contrast, a material without any planes of symmetry is fully anisotropic and

requires 21 elastic constants, whereas a material with an infinite number of symmetry planes

(i.e. every plane is a plane of symmetry) is isotropic, and requires only 2 elastic constants.

12. What are principal material directions?

For material directions 1, 2, and 3, you observe or specify any of these

values, depending on the model type and material orientation:

The axes of the referenced coordinate system

One of the solid directions

For 3D material orientations, you select one of the following six combinations

of the current coordinate system directions for the three principal material directions:

13. What is called fiber pullout?

Fiber pull-out is one of the failure mechanisms in fiber-reinforced composite

materials. Other forms of failure include delamination, interlaminar matrix cracking,

longitudinal matrix splitting, fiber/matrix debonding, and fiber fracture. The cause of fiber pull-

out and delamination is weak bonding. Fiber pullout is the opening of the matrix crack may

cause broken fibers to pull out from the surrounding matrix. This is resisted by the friction at

the fiber-matrix interface.

14. Name any two FEA software’s used in composite?

FEA software: MSC-NASTRAN, ANSYS, ABAQUS, LS-DYNA.

15. What are the failure modes observed during longitudinal tensile loading?

Failure modes:

a) Fiber breakage.

b) Fiber pullout.

c) Matrix bridging.

16. What is important function of matrix in FRP?

An important function of the matrix in a fiber-reinforced composite material is

to provide lateral support and stability for fibers under longitudinal compressive loading.

17. What are the two failure modes present in compression?

Two different localized bucking modes are

Elastic micro buckling

Fibre kinking.

18. What are the assumptions made for macro mechanics of laminated composites?

The matrix is homogeneous, isotropic, and linear elastic.

The fiber is homogeneous, isotropic, linear elastic, continuous, regularly

spaced, and perfectly aligned.

The lamina (single layer) is macroscopically homogeneous,

macroscopically orthotropic, linear elastic, initially stress-free, void-free,

and perfectly bonded.

The laminate is composed of two or more perfectly bonded laminae to act

as an integrated structural element.

19. List few special types of laminates?

Unidirectional laminate

Angle-ply laminate

Cross-ply laminate

Symmetric laminate.

20. What is the failure modes observed during longitudinal compressive loading?

Extensional mode

Shear mode.

21. What are quasi-isotropic laminate?

Quasi-isotropic laminate: these laminates are made of three or more laminas

of identical thickness and material with equal angles between each adjacent lamina.

22. Why the mechanics of FRP is more complicated?

Fiber-reinforced composites are microscopically in homogeneous and non-

isotropic. As a result, the mechanical of fiber-reinforced composites are far more complex

than that of conventional materials.

23. What are assumptions made in FRP during analysis?

Both fibers and matrix are linearly elastic isotropic materials.

Fibers are uniformly distributed in the matrix.

Fibers are perfectly aligned in the direction.

There is perfect bonding between fiber and matrix.

The composite lamina is free of voids.

24. What is the difference between homogeneous and isotropic material?

For a homogeneous material, properties do not depend on the location, and

for an isotropic material the properties depends on location.

25. What are the types of problems associated in interlaminar stresses?

Three types of interlaminar stress problems associated with three types of laminates:

1. (±q) Angle-ply laminates

2. 0/90 Cross-ply laminates

3. Combination of angle and cross ply laminates.

26. Where the interlaminar stresses will be high?

Interlaminar stresses can be significantly high over a region equal to the

laminate thickness near the free edges of a laminate. The free edges may be at the

boundaries of a laminated plate, around a cutout or hole, or at the ends of a laminated tube.

27. Define rule of mixture.

The Rule-of-Mixtures model is used to describe three-dimensional solids

having an arbitrary number of material phases with arbitrary orientations and volume

fractions. Orientations are defined for each phase using a triad of space-fixed rotation

angles in a 3-2-1 sequence. These angles rotate the composite material frame to the

phase frame. The orientation of each phase is defined by starting with the phase frame

aligned with the composite frame and rotating the phase material frame degrees about the

3-axis of the composite material frame, then rotating the phase frame degrees about the

2-axis of the composite frame, and finally rotating the phase frame degrees about the 1-

axis of the composite frame. Rule-of-Mixtures composites are, in general, fully anisotropic.

28. What are hygroscopic strains?

The strain caused due to the swelling and contraction of moisture is called

hygroscopic strains

29. How a laminate is constructed?

A laminate is constructed by stacking a number of laminas in the thickness (z)

direction.

30. How the tensile load applied to the discontinuous fibers transferred to them?

Tensile load applied to a discontinuous fiber lamina is transferred to the fibers

by a shearing mechanism between fiber and matrix.

31. Define iso-stress and iso-strain.

Iso-strain and Iso-stress of a composite material, in an isostrain each

component has a uniform deformation, while in isostress each material has a uniform stress.

The stress and strain, for isostrain and isostress, respectively, are in general “additives” but

depend on the moduli and volume fraction of each component.

32. Define Micromechanics.

The study of composite material behaviour wherein the interaction of the

constituent material is examined in detail as part of the definition of the behaviour of the

heterogeneous composite materials.

33. What are the advantages of Tsai-Hill failure criterion?

The advantages of Tsai-Hill failure criterion are:

The variation of strength with angle of lamina orientation is smooth rather than

having cusps that are not seen in experimental results.

The strength continuously decreases θ grows from 00 rather than the rise in

uniaxial strength that is characteristic of both the maximum stress and the

maximum strain criteria.

The maximum stress and strain criteria are incorrect by 100% at 300.

Considerable interaction exists between the failure strengths X, Y, S in the Tsai-

Hill criterion, but none exists in the previous criteria where axial, transverse, and

shear failures are presumed to occur independently.

UNIT 5

COMPOSITE STRUCTURES

1. What are fatigue properties of composite material?

The fatigue properties of a material represent its response to cyclic loading, which is

a common occurrence in many applications.

2. What are the variables influencing the cycle to failure?

The cycle to failure depends on a number of variables, such as Stress level, stress

state, mode of cycling, process history, material Composition and environmental

conditions.

3. What is S-N diagram?

Fatigue behavior of a material is usually characterized by an S-N diagram, which

shows the relationship between the stress amplitude or maximum stress and number

of cycles to failure on a semi logarithmic scale.

4. When will the number of cycles to failure increase?

The number of cycles to failure increases continually as the stress level is reduced.

5. What is the common practice of specifying the fatigue strength of the material?

It is common practice to specify the fatigue strength of the material at very high

cycles, say, 100 or 107 cycles.

6. Write the ASTM code for tension-tension fatigue cycling test.

The tension-tension fatigue cycling test procedure is described in ASTM D3479.

7. Name the two types of fatigue test on composites.

Stress-controlled and strain-controlled tests.

8. Write the formula for finding the maximum fatigue stress.

The maximum fatigue stress can be estimated using the following relation,

S= mlog (N) + b

Where S = Maximum fatigue stress

N = Number of cycles to fail

a,b = constants.

9. What will be the effect of lower fatigue strength in flexure?

The lower fatigue strength in flexure is attributed to the weakness of composites on

the compression side.

10. What is the effect of notches on fatigue strength?

The fatigue strength of a fiber-reinforced polymer decreases with increasing notch

depth as well as increasing notch tip sharpness. Stacking sequence also plays an

important role in the notch effect in fiber-reinforced polymers.

11. What is the damage mechanism during fatigue failure?

(a) Fiber breakage at high stress levels and (b) Matrix micro cracks followed by

debonding at low stress levels.

12. Why strain-controlled test is expected produce a higher fatigue life than stress –

controlled test?

Because of less damage development in a continuously reducing stress field, a

strain-controlled test is expected to produce a higher fatigue life than a stress-

controlled test.

13. What are the factors influencing strength of bonded joints?

Factors (bonded joints): the ratio of lap length to substrate thickness, modulus of

adhesives.

14. Write any two advantages of bonded composites.

1. Distribute the load over a larger area than mechanical joints.

2. Require no holes.

3. Add very little weight to the structure.

15. Why failure theories are not applicable to composite materials?

Fiber-reinforced polymers are not isotropic, nor do they exhibit gross yielding. Thus,

failure theories developed for metals or other isotropic materials are not applicable to

composite materials.

16. Write down the maximum strain theory.

According to the maximum strain theory, failure occurs when any strain in the

principal material directions is equal to or greater than the corresponding ultimate

strain.

17. What are the basic joints of composites?

For composite laminates, the basic joints are either mechanical or bonded.

18. Write any two advantages of mechanical joints.

1. Permit quick and repeated disassembly for repairs or replacements without

destroying the substrates.

2. Require little or no surface preparation.

3. Are easy to inspect for joint quality.

19. Write any two disadvantages of mechanical joints.

1. Require machining of holes that interrupt the fiber continuity.

2. May reduce the strength of the substrate laminates.

20. Write any two advantages of bonded joints.

1. Require no holes.

2. Add very little weight to the structure.

21. Write any two disadvantages of bonded joints.

1. Are difficult to disassemble without either destroying or damaging substrates.

2. Are difficult to inspect for joint quality.

3. Need surface preparation (cleaning, pretreatment, etc...

22. What are the basic failure modes in bolted joints?

Basic failure modes in bolted laminates: (a) shear-out, (b) net tension failure,

(c) cleavage, (d) bearing failure. Compositions of these failure modes are possible.

22. Name the most widely used bonded joint.

The simplest and most widely used bonded joint is a single-lap joint.

23. What are the basic bonded joints used in composite laminates?

Basic bonded joint configurations: (a) single-lap joint, (b) double-lap joint, (c) single

and double-strap joints, (d) stepped lap joint, and (e) scarf joint.

24. Write two important design considerations of bonded joints.

1. Increasing the ratio of lap length to substrate thickness h improves the joint

strength significantly at small L/h ratios, the improvement is marginal.

2. Tapering the substrate ends at the ends of the overlap reduces the high normal

stresses at these locations.

PREPARED AND PUBLISHED BY:

Prof. T. Varun Kumar M.E., (Phd).

Assistant Professor,

Department of Mechanical Engineering,

P.A.College of Engineering and Technology,

Pollachi, Coimbatore - 642 002.