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CHAPTER
12
Composite
Materials
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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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.
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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Composite Survey
Large-
particle
Dispersion-
strengthened
Particle-reinforced
Continuous
(aligned)
Aligned Randomly
oriented
Discontinuous
(short)
Fiber-reinforced
Laminates Sandwich
panels
Structural
Composites
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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• Figure 9–18 Matrix and reinforcement options for polymer composites
9.3 Polymer Composites
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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Three main types of synthetic fibers for reinforce
plastic composite materials
1. Glass – fiber
2. Aramid – fiber
3. Carbon - fiber
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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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.
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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.
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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
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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.
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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
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Polyester products
Polyester resin
Epoxy resin Working with epoxy
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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.
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Fiberglass-reinforced polyester resins:
Storage tank
Piping
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Carbon Fiber reinforced Epoxy resin
Car parts
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Figure 12.19
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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• Figure 9–24 Polymer composite forming process
9.4 Composite Fabrication Techniques
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Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi
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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.
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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.
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Filament –Winding Process
Filament Winding Tank
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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
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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.
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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.
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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)
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Air
Bubbles
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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.
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Reinforced concrete