UNIVERSITY OF NIGERIA, NSUKKA

FACULTY OF ENGINEERINGDEPARTMENT OF METALLURGICAL AND

MATERIALS ENGINEERING

LECTURE NOTE ON MME 551(INTRODUCTION TO COMPOSITE

MATERIALS)3 UNITS

LECTURER: ENGR. I.C, EZEMA

COURSE CONTENT

1. General Overview: definition of matrices, reinforcements, classifications,

advantages, disadvantages.

2. Properties of matrices and types. Types and properties of reinforcements.

3. Fabrication processes of composites materials.

4. Factors affecting properties of composite materials.

5. Experimental characterization of composite materials.

6. Micromechanics of composites materials.

7. Application of composites.

8. Practical/Lab work on composite materials.

INTRODUCTION

WHAT IS A COMPOSITE?

Composite is a multiphase material having two or more physically and chemically

distinct components in which one of the components (reinforcements) has superior

mechanical properties.

In material science, the term composite is used to denote a material composed of a

matrix as a binder (continuous phase) containing a filler (fibrous or non-fibrous)

and reinforcements (discontinuous phase).

MATRIX = CONCRETE → SAND, CEMENT, WATER, GRAVEL.

With the addition of ROD it becomes REINFORCED CONCRETE.

There should be a definite interface between the matrix and the reinforcements

usually of zero thickness. The properties of the composite depends upon those of

the individual components and on their interfacial capability.

Modern composite materials evolve from the simplest mixture of two or more

materials to gain a property which was not there before. The reinforcement of

materials such as mud and clay by straws; concrete and concrete reinforced with

steel, granite consisting of quartz, mica, feldspar, wood (cellulose fiber in lignin

matrix) are typical examples.

Composite are used because the overall properties of the composites are superior to

the individual properties of the components; for example, polymer/ceramic

composite has a greater modulus than the polymer component but are not as brittle

as the ceramic. The following are some of the reasons why composites are selected

for certain applications.

Typical, reinforcing materials are strong with low densities while the matrix is

usually a ductile or tough material. If the composite is designed and fabricated

correctly, it combines the strength of the reinforcements with the toughness of the

matrix to achieve the desired properties not available in a conventional material.

The downside is that such composite are often much expensive than conventional

materials. The downside is that such composites are often much expensive than

conventional materials e.g. diesel piston, brake pads, tyres, the beech aircraft in

which 100% of the structural components are composite materials.

The strength of the composite depends primarily on the amount, arrangement and

type of the fiber reinforcements in the resin.

Typically, the higher the reinforcement content, the greater the strength. In some

cases, glass fibers are combined with other fibers such as aramid to create a hybrid

composite that combines the properties of more than one reinforcing material. In

addition, the composites is often formulated with fillers and additives that change

the processing or performance parameters.

Composites have been classified generally into:

i. Metal matrix composites (MMC)

ii. Ceramic matrix composite (CMC)

iii. Polymer matrix composite (PMC)

based on the matrix material that constitute the composite. Because of the low

processing temperature, the polymer matrix composites are more easier to fabricate

than the ceramic matrix composites. Several researchers all over the world

investigated the structures, properties and application of various composite

systems. Among the various composites fiber reinforced composites gain much

importance in various fields due to high strength to weight ratio (a very important

parameter in the design and fabrication of composites).

CLASSIFICATION OF COMPOSITES BASED ON MATRIX

Metal Matrix Composites (MMCs) include mixture of ceramics and metals

such as cemented carbides and other cermets, as well as Al or Mg reinforced

by strong high stiffness fibers.

Ceramic Matrix Composite (CMCs) include aluminium oxide and silicon

carbide embedded into ceramic matrix to improve properties especially in

high temperature applications.

In Polymer Matrix Composite (PMCs) thermoset resins are the most widely

used polymers in PMCs; epoxy, polyesters and phenolics are commonly are

mixed with fiber reinforcements.

CHARACTERISTICS OF COMPOSITES

ADVANTAGES

1. High mechanical properties.

2. Flexibility of design options.

3. Ease of fabrication

4. Light weight.

5. Corrosion resistance.

6. Impact resistance.

7. Excellent fatique strength.

MATRICES

POLYMER

-THERMOSET-THERMOPLASTIC

CERAMIC

-GLASS-CERMETS

-CEMENTS/CONCRETE

CARBON & GRAPHITEMETALS

-STEEL-ALUMINIUM

-TITANIUM-MAGNESIUM, ETC.

8. Easy to machine.

DISADVANTAGES

1. Brittle failure mechanism.

2. High material cost.

3. High manufacturing cost.

4. Temperature limitations.

5. Actual mechanical properties not always as good as expected.

6. Mechanical properties very process dependent.

7. Not often environmentally friendly.

8. Low recyclability

9. It has an anisotropic property.

10. Low reusability.

11. Synthetic composites are not bio-degradable.

FUNCTIONS OF A MATRIX

In a composite material, the matrix material serves the following functions;

1. It holds the fibers together.

2. It protects the fibers from environment.

3. It distributes the load evenly between fibers, so that all fibers are subjected

to the same amount of strain.

4. It enhances the transverse properties of a laminate.

5. It improves impact and fracture resistance of a component.

6. It helps to avoid propagation of crack growths through the fiber by providing

alternate failure path along the interface between the fibers and the matrix.

7. It carries inter-laminar shear stress.

PROPERTIES OF A MATRIX

The following are the properties of a matrix;

1. Reduced moisture adsorption.

2. Low shrinkage.

3. Low coefficient of thermal expansion.

4. Good flow characteristics so that it penetrates fiber bundles completely and

eliminates voids during the compacting or curing process.

5. Reasonable strength, modulus and elongation (the elongation of the matrix

must be greater than that of the fiber).

6. The matrix must be elastic enough to transfer load to the fibers.

7. Strength at elevated temperature (depending on application)

8. Excellent chemical resistance (depending on application).

9. Low temperature capabilities (depending on application).

10. Should be easily processable into the final composite shape.

11. It must have dimensional stability.

TYPES OF MATRICES

1. POLYMER MATRIX:

Polymers make ideal materials as they can be processed easily, possesses

light weight and desirable mechanical properties. They are classified into

two main types; thermosets and thermoplastics e.g. for thermosets are

phenolics, polyester, polyimide, epoxy, etc. for thermoplastics we have

polyamide, polypropylene, polythene.

Advantages and Disadvantages of Thermosets and Thermoplastics

Thermosets Thermoplastics

1. The resin(matrix) cost is low The resin cost is slightly higher

2. Thermoset exhibit moderate

shrinkage.

The shrinkage for thermoplastics is low

3. Inter-laminar structure toughness

is low

Inter-laminar structure toughness is

high.

4. Good resistance to solvents Poor resistance to solvents.

2. METAL MATRIX:

Metallic matrices are essential constituents for fabrication of metal matrix

composites (MMC), which have potential for structure materials at high

temperature. Metal matrix has advantage over polymer matrix in application

requiring a long term resistance to a severe environment such as high

temperature.

Again, the yield strength and modulus of most metals are higher than those of

polymers which is an important consideration for application requiring high

transverse strength and modulus as well as the compressive strength of the

composite.

Another advantage of using metals is that they can be physically deformed and

strengthen by a variety of thermal and mechanical treatment. However, metals

have a number of disadvantages, namely high melting point (high processing

temperature), tendency towards corrosion at the fiber matrix interface, high

specific gravity.

Other matrices include ceramic matric, carbon matrix, and reinforced concrete.

REINFORCEMENTS (FIBERS OR PARTICLES)

Reinforcement for the composites can be fibers, particles or whiskers. Fibers

are essentially characterized by one very long axis with the other two axis often

circular or near circular.

Particles have no preferred orientation and so does their shape.

Whiskers have a preferred shape but are small both in diameter and length when

compared to fibers.

Fibers are the most important class of reinforcement as they satisfy the desired

condition and transfer strength to the matrix constituents influencing and

enhancing their properties as desired. Glass fibers are the earliest known fibres

used to reinforce materials. Ceramic and metal fibers were discovered later to

render composites stiffer and more resistant to heat. Metallic fibers are those

used in tyre reinforcements.

The performance of a fiber in a composite depends on its length, shape,

orientation, composition of the fiber and the mechanical properties of the fiber.

The two major roles of a fiber in a composite are;

1. To carry the applied load,

2. To transfer load.

Four major types of glass fibers are;

1. E-glass (good strength and electrical restivity).

2. S-glass (40% higher strength than E-glass. Better retention of properties at

elevated properties).

3. C-glass (better corrosion resistance than all other type).

4. Quartz (low dielectric properties).

FORMS OF GLASS FIBER

i. Rovings

ii. Roving woven

iii. Chopped strand mat

iv. Continuous strand mat

v. Surface veil

vi. Fabrics (glass fiber).

Other forms of fibres include;

i. Carbon fibers

ii. Organic fibers (natural fibers).

-Bast (stem) e.g. flax, jute, hemp, kenaf, banana, plantain.

-leaf e.g. sisals, banana, palm.

-seed e.g. cotton, kapor.

-fruit e.g. palm, coconut.

-wood fiber e.g. sawdust.

-stalk e.g. rice, bamboo, wheat, corn, etc.

Other additives relevant in composite production include

1. FILLERS: fillers are inert substances added to reduce the resin cost or

improve its physical properties such as hardness, stiffness, impact strength.

Commonly used fillers are calcium-carbonate, hydrated alumina, clay

(kaolin), calcium-sulphate, mica, feldspar, silica, talc, flake glass, milled

glass, etc.

2. COLORANTS: colorants are often used in composites to add color

throughout the part. Colorants can be mixed as part of the resin or applied as

part of the moulding process (as a gel coat). Also a wide range of coatings

can be applied after moulding.

3. RELEASE AGENTS: it is an agent that facilitates the removal of part from

the mold. This products can be added to the resin, applied to the mold or

bath. Correct selection of release agents can optimize not only cycle time but

also consistency of surface finish, minimizing post mold operations prior to

painting or bonding. Zinc Stearate is a popular type of release agents that is

mixed inside the resin for compression molding. Waxes, silicon oil, and

other release agents can be used at the lowest possible concentration

(because heating the mold too much will cause cracking). Use of excessive

amounts can reduce mechanical strength and affect adhesion characteristics

or may cause cracking of the composites.

CATALYSTS, ACCELERATORS/PROMOTERS AND INHIBITORS

In polyester for instance, the most important additive is catalyst or initiator.

Typical peroxide such as MEKP-Methyl Ethyl Ketone Peroxide is used for room

temperature curing processes or Benzoyl Peroxide is used for heat curing

processes.

PROMOTERS

In polyester, Cobalt Napthenate is used to cause the unsaturated resin to react

(croos-link) and become solid. Some additives such as TBC-Tertiary Butyl

Catechol are used to slow the rate of reaction and are called inhibitors. They are

used in polyester resin to extend their shelf life (the time during which the

polyester can still be useful). Accelerators such as DMA-Dimethyl Aniline speed

up curing process. It does the same work as cobalt naphthanate.

MAUFACTURING METHODS FOR COMPOSITES

Composites can be made using the following methods;

1. Hand lay-up method.

2. Spray lay-up method.

3. Resin transfer molding.

4. Compression molding.

5. Injection.

6. Extrusion.

7. Pultrusion.

8. Filament winding.

All these methods are tailored for the specific material to be processed. Polymer

chemistry plays an important role in selecting the appropriate resin for the given

fabrication method.

ASSIGNMENT 1

Write short notes on the composite manufacturing methods.

MICROMECHANICS & MACROMECHANICS OF COMPOSITES

Micromechanics is the study of material behavior on small length scales

where the interplay of the individual component (fiber and matrix) as part of the

definition of the behavior of the heterogeneous material is investigated.

Macromechanics is the study of the material behavior on a large scale

whereby the material is considered a homogeneous continuum or structural

elements made from the material are considered or investigated.

VOLUME FRACTION OF COMPOSITES

Vf + Vm + Vv = 1

Where;

Vf = volume fraction of fibre

Vm = volume fraction of matrix

Vv = volume fraction of void content

But Vv = 0

Therefore, Vf + Vm = 1 or V m=1−V f

Vf + Vm = Vc

Where VC = volume fraction of composite

Volume fraction of fiber (in percentage)

Vf = V mV c

Volume fraction of matrix (in percentage)

Vm = V mV c

Mass fraction

Mf + Mm = 1

Mf + Mm = Mc

Mass fraction of fiber

Mf = M fM c

Mass fraction of matrix

Mm = M mM c

RULE OF MIXTURE EQUATION (ROM)

1. PARALLEL MODEL (UNIDIRECTIONAL DIRECTION)

F

a) Strength, σ c = σ fVf + σ mVm

= σ fVf + σ m (1-Vf)

b) Youngs modulus, Ec = EfVf + EmVm

Ec = EfVf + Em (1-Vf)

c) Density, ρc = ρfVf + ρmVm

ρc = ρfVf + ρm (1-Vf)

2. SERIES MODEL (TRANSVERSE DIRECTION)

Strength, 1σ c

=V fσ f

+ Vmσ m

σ c = σ f σ mσ mVf +σ fVm

Modulus, 1Ec

=VfEf

+ VmEm

Ec = EfEmEmVf + EfVm

FF