COMPOSITE MATERIALS · 2017. 8. 24. · Module 5 10. Composite Materials Pruthvi Loy, Chiranth B....

Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru

10. COMPOSITE MATERIALS

Introduction

Classification of composites

Types of matrix materials and reinforcements

Production of FRP’s and MMC’s

Advantages and applications of composites

10.1 INTRODUCTION

A composite material or a composite may be defined as a materials system composed of two or

more physically or chemically distinct phases that are insoluble in each other and when

combined have improved properties over the individual materials.

Example: Wood (combination of cellulose fiber and lignin) – naturally occurring composite

Reinforced cement concrete – artificial composite

Components of a composite material:

Most of the composite materials are composed of just two phases; a continuous phase known as

matrix in which the second phase is dispersed without being soluble in it. The dispersed phase

acts as reinforcement and can be of different shape and sizes.

Primary phase (continuous) – matrix

Secondary phase (dispersed) – reinforcement

Figure 10.1: Composite

In reinforced cement concrete the steel bars along with the gravel and sand act as the

reinforcement which are bound together by cement matrix. Typically, reinforcing materials are

strong whereas the matrix is usually tough or ductile. Thus a composite material combines the

MODULE FIVE

COMPOSITE MATERIALS

Module 5 10. Composite Materials


strength of the reinforcement with the toughness of the matrix to achieve superior properties than

the individual components. Even though the high strength of the composite is largely due to the

reinforcement, the matrix provides support for the reinforcement and assists them in carrying the

loads.

The properties of the composite depend on the following factors;

The relative amount of the phases

Geometry of the dispersed phase (size, shape, distribution & orientation of reinforcement)

Bonding between the phases

Functions of matrix:

It holds the reinforcing material together and distributes the load evenly between them.

Protects the reinforcing phase from severe environment.

Helps to avoid the propagation of crack through the reinforcement by providing alternate

failure path along the interface.

Improves impact and fracture resistance of a composite.

It helps in alignment of reinforcement phase in a pre-determined direction.

Functions of reinforcement:

It gives strength and stiffness to the composite.

It helps in achieving directional properties in a composite.

Provides flexibility in designing a composite for specific applications.

10.2 CLASSIFICATION OF COMPOSITES

They are classified at two distinct levels.

I. Classification with respect to matrix constituent

a. Polymer matrix composites (PMC)

b. Metal matrix composites (MMC)

c. Ceramic matrix composites (CMC)

II. Classification based on reinforcement form

a. Fiber reinforced composites.

b. Particle reinforced composites.

c. Structural/laminar composites



10.3 MATRIX AND REINFORCEMENT MATERIALS

10.3.1 Matrix materials

The most commonly used matrix materials are metals, ceramics and polymers which are

discussed further.

10.3.1(a) Polymers

Polymer matrix material are generally called as resins and are classified as

a. Thermosetting material

b. Thermoplastic material

Thermosetting material

A thermosetting material is one which when cured by heat or chemical reaction changes into an

infusible and insoluble material. i.e., they undergo irreversible chemical reaction upon

application of heat

Example: polysters, epoxies, vinyl esters, phenolics etc

Thermoplastic material

Thermoplastic don’t undergo a chemical reaction upon application of heat, they simply melt on

application of heat and pressure to form a component. They can be repeatedly softened by

heating and hardened by cooling.

Example: polypylene, polymide etc

Table 10.1: Characteristics of commonly used polymers

SL No. Polymers Characteristics

1. Polyesters

Low cost

Good mechanical properties, electrical properties and heat

resistance

Curing temperature is 120 C

High shrinkage and low chemical resistance

2. Epoxies

Improved strength and stiffness

Excellent corrosion resistance

No by products during curing and low curing shrinkage

Curing temperature at 120 - 175 C

3. Vinyl ester

High toughness

Outstanding heat and chemical resistance

Corrosion and moisture resistance



4. Phenolics

Low cost

Excellent high temperature resistance

Curing temperature is 175 C

Good mechanical properties

Dimensional and thermal stability

High shrinkage and by products

Table 10.2: Comparison of thermosetting and thermoplastic polymers

Thermosets Thermoplastic

Resin cost is low Resin cost is slightly higher

High processing times for curing Fast and economic process

Most of them are toxic and requires storage in

refrigerator

Non-toxic as there are no reactive chemical

Cannot be reused Re melted and reused.

Moderate shrinkage Low shrinkage

Fracture toughness is low High fracture toughness

Relatively hard and brittle Soft and ductile

10.3.1(b) Metal matrix materials

Characteristics:

o Metal or alloys are used as matrix materials

o Generally used for structural materials at high temperature.

o They are advantages over PMC in applications requiring a long term resistance to serve

environment.

o Metal matrix can be plastically deformed and strengthened by various thermal and

mechanical treatments.

o But metals have high melting temperature and hence needs high processing temperature.

Examples:

Aluminium and Titanium composites reinforced with Silicon carbide (SiC), boron or carbon

fibers.



Applications:

For developing lighter MMC’s aluminium and titanium alloys are used as matrix materials for

aerospace and automobile applications (Aluminium alloy matrix composites are suited to

applications below the temperature of 400 C for high temperature titanium based alloys are used

for applications in temperature of 900 C )

o Missile body casings

o Wing structural elements

o Engine shaft

o Disk for turbine engine

o Hollow fan blades

10.3.1(c) Ceramic matrix materials

Ceramic materials are well known for their high temperature properties as well as their resistance

to oxidation, but they are very brittle which limits their application.

Ceramics make a better reinforcement material than the matrix. Most commonly used ceramic

materials are; Silicon nitride (Si3N4), Silicon carbide (SiC) and Alumina (Al2O3)

10.3.2 Reinforcement materials

10.3.2(a) Fiber reinforcement material

This is the most commonly used reinforcement in which the dispersed phase is in the form of a

fiber. They can be further grouped into three categories on the basis of fiber diameter;

o whiskers - extremely large length to diameter ratio (very small diameter)

o fibers - moderate length to diameter ratio (small diameter)

o wires – small length to diameter ratio (large diameter)

Examples:

Graphite, silicon carbide, aluminum oxide.

Polymers, glass, carbon, boron.

Steel, molybdenum, tungsten-automobile tyres

SiC reinforced Aluminium Composites

SiC reinforced Titanium Composites



Influence of fiber orientation on properties of composites:

There are two possible arrangements

a) Uniformly orientated (parallel alignment)

b) Randomly oriented

(a) Longitudinal loading (b) Transverse loading

Figure 10.2: Fiber orientation

The matrix phase of fibrous components may be metal, polymer or ceramic, in general metals

and polymers are used as matrix materials because some ductility is desirable. There must be

strong adhesive bonding forces between fiber and matrix to minimize fiber pull out.

10.3.2(b) Particle reinforced composites

Particles of different shape and size are dispersed in a matrix of second material to form a

particulate composite.

It can be sub classified as

o Large particle composites

o Fine particle / Dispersion strengthened composites

For large particle composites the particulate phase is harder and stiffer than matrix, for

dispersion strengthened composites, particles are normally much smaller (having diameter

between 10 to 100 nm). The strengthening in dispersion composites is due to particle-matrix

interaction, where major portion of the load is taken up by the matrix whereas the dispersion

only acts as barrier for dislocation motion.

10.3.2(c) Structural composites-laminates

A uniformly aligned fiber reinforced composites show anisotropic properties (i.e., properties

varies along different directions) but if layers of such composites are stacked and bonded

together in such a way that successive layers have their fibers aligned in different directions, the



composite on the whole will have high strength and uniform properties all direction, such

composites are called laminates.

Example: plywood.

Figure 10.3: Laminated composites

10.4 PRODUCTION OF COMPOSITES

The production of the following composites is discussed further;

Fiber Reinforced Plastics (FRP)

Metal Matrix Composites (MMC)

10.4.1 Production of FRP’s

FRP’s can be manufactured by several methods;

I. Open mould process:

a. Hand lay-up process

b. Spray-up process

c. Filament winding

d. Compression moulding

II. Closed mould process:

a. Pressure bag moulding

b. Vacuum bag moulding Bag moulding

c. Autoclave process

III. Continuous process:

a. Pultrusion process

b. Sheet moulding compound



10.4.1(a) Hand lay-up process:

This is the simplest and conventional method of producing fiber reinforced composites wherein

glass fibers are used as reinforcement and a polyester resin is used.

The process details are as follows;

a. A gel coat is applied to the mould (wax, clay or wood) to facilitate easy removal of the

product from the mould.

b. A fiber glass reinforcement normally in the form of cloth or mat is placed on it.

c. The plastic resin mixed with catalyst and accelerator is applied on it by pouring.

d. A roller is used to thoroughly wet the reinforcement with resin and to remove the

entrapped air.

e. To increase the thickness of the composite more layers of fiberglass and resin may be

added.

Figure 10.4: Hand lay-up process

Advantages:

a. Simple and low cost process.

b. Ideal for large size components.

c. Design flexibility and longer fiber layup possible.

Disadvantages:

1. Only one side good surface finish.

2. Skilled labor is required for good quality.

3. Longer cure time is required.

Applications: Wind turbine blades, boats, etc.



10.4.1(b) Spray-up process:

In this process, continuous fibers are cut into desired length by a chopper, this is then mixed with

resin, catalyst and promoter as they are sprayed on to a gel coated mould through a spray gun.

The density of the composite can be increased by rolling to remove the entrapped air. Multiple

layers are added to produce composite of desired thickness.

Figure 10.5: Spray-up process

Advantages:

1. Low cost technique

2. Faster production as compared to hand layup.

Disadvantages:

1. Laminates tend to be resin rich and requires low viscosity resin to facilitate spraying.

2. Difficult to maintain uniform distribution of chopped fibers.

Applications: Bathtubs, shower trays, etc.

10.4.1(c) Filament winding:

In this process, strands of fiber reinforcement are fed through a plastic resin bath and then wound

on a suitable rotating mandrel in a pre-determined orientation. When sufficient layers have been

taken up on the mandrel it is cured at room temperature or at elevated temperature in an oven.

The molded composite is then stripped from the mandrel.



Figure 10.6: Filament winding

Advantages:

1. Excellent mechanical properties due to continuous fibers.

2. Fast and economical production.

Disadvantages:

1. Difficult to wind complex shapes.

2. Mandrel cost for larger components is high.

Applications: Tubular sections, chemical and fuel tanks, pressure vessels.

10.4.1(d) Compression moulding:

In this process, the part to be cured is placed between two matched steel dies under high pressure

and high temperature. The exertion of pressure eliminates the problem of development of voids.

Disadvantage:

1. High tooling cost

2. Need for large heated presses

Advantages:

1. Good dimensional accuracy.



Figure 10.7: Compression moulding

10.4.1(e) Bag moulding:

Bag moulding process is an improvement over most of the open mould processes (where

compaction is done using rollers) in that it further removes the entrapped air. The different bag

moulding techniques are pressure bag, vacuum bag and autotoclave process.

In all these techniques, plastic resins are first pored over a fiber reinforcement mat placed in a

mould. This mixture of fiber and resin is covered by a flexible sheet and then pressure is applied

so as to compact the composite. In case of pressure bag moulding pressurized air is used for

compaction whereas in vacuum bag moulding pressure is applied by vacuum principle.

Autoclave process is similar to vacuum bag process except that compaction is done in a furnace

at elevated temperature and higher pressures are applied.

Figure 10.8: Pressure bag moulding



Figure 10.9: Vacuum bag moulding

10.4.1(f) Pultrusion process:

Pultrusion is generally used for producing glass fiber reinforced polyester. In this process resin

impregnated fibers are first fed into a preforming box which gives appropriate orientation to the

fibers. Then it is passed through a heated die to get required cross section such as beams, tubes

and sections.

Figure 10.10: Pultrusion process

Advantages:

1. It is continuous and automated

2. Low material scrap

3. Dimensional stability

Disadvantages:

1. Heated die cost

2. Complex geometry



10.4.1(g) Sheet moulding compound:

In this process, plastic resin is first deposited over travelling polyethylene sheet with the help of

filler. On the top of this resin paste, continuous strand fiber glass roving cut to length of about 5

cm is deposited. Another layer of resin filler paste is added over this combination to form

continuous sandwich of fiber glass and resin. This sandwich is compacted using rollers.

Figure 10.11: Sheet moulding compound

Advantages:

1. High volume production.

2. Minimum material scarp.

10.4.2 Production of MMC’s

MMC’s can be manufactured by the following two techniques;

Liquid state processing – Stir casting

Solid state processing – Powder metallurgy

10.4.2(a) Stir casting

A high ductility metal that forms the matrix phase is first melted in a crucible placed in a

furnace; the reinforcement phase is then added and is mixed thoroughly by a mechanical stirrer.

This composite mixture is then poured into a sand mould of desired shape and size and allowed

to solidify.



10.4.2(b) Powder metallurgy

Metal matrix composites are extensively produced by powder metallurgy techniques. Metallic

materials such as copper, nickel, aluminium, cobalt and steel are used in their powder form as

matrix material. The metal matrix in the form of powders are mixed with fibers, whiskers or

particles and then fed into a mould of desired shape. Pressure is then applied to compact the

powder which is followed by sintering in order to strongly bind the particles.

10.5 ADVANTAGES AND APPLICATIONS OF COMPOSITES

10.5.1 Advantages

The advantages of composites are;

Light Weight - Composites are light in weight, compared to most woods and metals.

Their lightness is important in automobiles and aircraft, to enhance the fuel efficiency.

High Strength - Composites can be designed to be far stronger than aluminum or steel.

Metals are equally strong in all directions. But composites can be engineered and

designed to be strong in a specific direction.

High Strength to Weight ratio - Composite materials can be designed to be both strong

and light; they have highest strength-to-weight ratios in structures today.

Good Corrosion Resistance - Composites resist damage from the weather and from harsh

chemicals that can eat away at other materials. Composites are good choices where

chemicals are handled or stored. Outdoors, they stand up to severe weather and wide

changes in temperature.

High-Impact Strength - Composites can be made to absorb impacts—the sudden force of

a bullet, for instance, or the blast from an explosion. Because of this property, composites

are used in bulletproof vests and panels, and to shield airplanes, buildings, and military

vehicles from explosions.

Design Flexibility - Composites can be molded into complicated shapes more easily than

most other materials. This gives designers the freedom to create almost any shape or

form.

Dimensional Stability - Composites retain their shape and size when they are hot or cool,

wet or dry. Wood, on the other hand, swells and shrinks as the humidity changes.

Composites can be a better choice in situations demanding tight fits that do not vary.

Nonconductive - Composites are nonconductive, meaning they do not conduct electricity.

This property makes them suitable for such items as electrical utility poles and the circuit

boards in electronics. If electrical conductivity is needed, it is possible to make some

composites conductive.



Low Thermal Conductivity - Composites are good insulators—they do not easily conduct

heat or cold. They are used in buildings for doors, panels, and windows where extra

protection is needed from severe weather.

Durable - Structures made of composites have a long life and need little maintenance.

10.5.2 Applications

The various application areas of composites are;

Aircraft/Military - Commercial, pleasure and military aircrafts, including components for

aerospace and related applications.

Appliance/Business - Composite applications for the household and office including

appliances, power tools, business equipment, etc.

Automotive/Transportation - The largest of the markets, products include parts for

automobiles, trucks, rail and farm applications.

Civil Infrastructure - A relatively new market for composites, these applications include

the repair and replacement of civil infrastructure including buildings, roads, bridges,

piling, etc.

Construction - Includes materials for the building of homes, offices, and architectural

components. Products include swimming pools, bathroom fixtures, wall panels, roofing,

architectural cladding.

Consumer - Products include sports and recreational equipment such as golf clubs, tennis

rackets, snowmobiles, mobile campers, furniture, microwave cookware.

Corrosion-resistant equipment - Products for chemical-resistant service such as tanks,

ducts and hoods, pumps, fans, grating, chemical processing, pulp and paper, oil and gas,

and water/wastewater treatment markets.

Electrical - This encompassing market includes components for both electrical and

electronic applications such as pole line hardware, substation equipment, microwave

antennas, printed wiring boards, etc.

Marine - Products for commercial, pleasure and naval boats and ships.

References:

1. Fundamentals of Materials Science & Engineering – William D. Callister

2. Material Science and Metallurgy – K.R.Phaneesh

3. Material Science and Metallurgy – Kesthoor Praveen

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