foundations of materials science and engineering third edition · foundations of materials science...

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CHAPTER

12

Composite

Materials

1

Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi


Introduction

• Every material is composite at one or the other level.

• A composite material is a material system, a mixture or

combination of two or more micro or macroconstituents

that differ in form and composition and do not form a

solution.

• Properties of composite materials can be superior to its

individual components.

• Examples: Fiber reinforced plastics, concrete, asphalt,

wood etc.

2



Composite Survey

Large-

particle

Dispersion-

strengthened

Particle-reinforced

Continuous

(aligned)

Aligned Randomly

oriented

Discontinuous

(short)

Fiber-reinforced

Laminates Sandwich

panels

Structural

Composites



• Figure 9–18 Matrix and reinforcement options for polymer composites

9.3 Polymer Composites



Three main types of synthetic fibers for reinforce

plastic composite materials

1. Glass – fiber

2. Aramid – fiber

3. Carbon - fiber



Glass Fibers for Reinforced Plastic Composite Materials

Glass fiber reinforce plastic composite materials have high

strength-to-weight ratio, good dimensional stability, good

resistance to heat, cold, moisture and corrosion; good

electrical insulation properties; ease of fabrication; and

relatively low cost.

Properties

• Has lower tensile strength and modulus than carbon and

aramid fibers.

• Higher elongation

• Higher density than carbon and aramid fibers

• Low cost and hence commonly used.

6



Aramid Fibers for Reinforcing Plastic Resins

• Aramid = aromatic polyamide fibers.

• Trade name is Kevlar

Kevlar 29:- Low density, high strength, and used for ropes and

cables.

Kevlar 49:- Low density, high strength and modulus and used for

aerospace, marine, auto applications and other industrial

applications

• Used where resistance to fatigue, high strength and light

weight is important.

7



Carbon Fibers for Reinforced Plastics

• Light weight, very high strength and high stiffness.

• Application : aerospace (aircraft part)

• 7-10 micrometer in diameter.

• Tensile strength = 3.1-4.45 GPa

8



Comparison of Mechanical Properties

• Carbon fibers provide best combination of properties (high

strength, high stiffness, low density, lower elongation) .

• Due to favorable properties, carbon and aramid fiber

reinforced composites have replaced steel and aluminum in

aerospace applications.

9



Matrix Materials

• Polyester and epoxy resins are the two important matrix

materials.

• Polyester resins: Cheaper but not as strong as epoxy resins.

Applications: Boat hulls, auto and aircraft applications.

• Epoxy resins: Good strength, low shrinkage.

Commonly used matrix materials for carbon and

aramid-fiber composite.

10 Photomicrograph of a cross section fiber glass polyester

composite material



Polyester products

Polyester resin

Epoxy resin Working with epoxy



Fiber Reinforced-Plastic Composite Materials

• Fiberglass-reinforced polyester resins:

Higher the wt% of glass, stronger the reinforced plastic is.

Nonparallel alignment of glass fibers reduces strength.

• Carbon fiber reinforced epoxy resins:

Carbon fiber contributes to rigidity and strength while epoxy matrix contributes to impact strength.

Polyimides, polyphenylene sulfides are also used.

12



Fiberglass-reinforced polyester resins:

Storage tank

Piping



Carbon Fiber reinforced Epoxy resin

Car parts



Figure 12.19



• Figure 9–24 Polymer composite forming process

9.4 Composite Fabrication Techniques



Open Mold Process for Fiber Reinforced Plastics

• Hand lay-up process:

Gel coat is applied to open mold.

Fiberglass reinforcement is

placed in the mold.

Base resin mixed

with catalysts is

applied by pouring

brushing or spraying.

• Spray-up process: Continuous

strand roving is fed by chopper

and spray gun and chopped

roving and catalyst resin is

deposited in the mold.

17



Filament Winding

• Filament winding:

Fiber reinforcement is fed

through resin bath and

wound around suitable

mandrel.

Mandrel is cured and mold part is

stripped from mandrel.

18



Filament –Winding Process

Filament Winding Tank



Closed Mold Process

• Compression and injection molding:

Same as in polymers except that the fiber reinforcement

is mixed with resin.

• Sheet molding compound process:

Highly automated continuous molding process.

Continuous strand fiberglass

roving is chopped and deposited

on a layer of resin-filler paste.

Another layer of paste is

deposited on first layer.

Sandwich is compacted

and rolled into rolls. 20



Pultrusion Process

– The rolled up sheet is stored in a maturation room for 1-4

days.

– The sheets are cut into proper size and pressed in hot mold

(1490C) to form final product.

– Efficient, quick, good quality and uniformity.

• Continuous protrusion: Continuous strand fibers are

impregnated in resin bath, fed into heated die and drawn.

– Used to produce beams, channels, and pipes.

21



Concrete

• Flexible, economical, fire resistant, durable, fabricated on

site.

• Low tensile strength, less ductile and shrinkable.

• Concrete is a ceramic composite composed of coarse

granular material embedded in hard matrix of cement

paste.

• Concrete = 7-15% Portland cement, 14-21% water, ½ -

8% air, 24-30% fine aggregate and 31-51% coarse

aggregate.

23



Compressive Strength

• Compressive strength is higher than tensile strength and depends up on

settled time.

• High water content reduces compressive strength.

• Air entrainment improves workability and thus a lower water-to-cement

ratio can be used. It is done by adding air-entraining agents contain

surface –active agent that lower the surface tension at the air-water

interface so that extremely small air bubbles form (less than 100 micro

m)

24

Air

Bubbles



Proportioning Concrete Mixture

• Facts to be considered:

Workability

Strength and durability

Economy of

production

• Water to cement ratio

determines compressive

strength.

• Steel reinforcements are

used to improve tensile

properties as in bending.

25



Reinforced concrete

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