UoE Composites lecture notes

1. Composites Materials
By
Dr. S. Kimutai

2. Course content
 Classification, general properties and applications of: iron and carbon steels,
copper, aluminum, zinc, magnesium, nickel and titanium alloys and new metals.
Cast iron: powder metallurgy.
 Corrosion- types and prevention.
 Plastics processing, classification, general properties, application of polymer
materials; polymerization mechanism. Copolymers. Elastomers,
 ceramics and
 composites and their applications in manufacturing

3. Introduction
 Metal, glass, plastics, ceramics
 require materials with unusual combinations of properties that cannot be met by the conventional metal alloys,
ceramics, and polymeric materials. This is especially true for materials that are needed for aerospace, underwater, and
transportation applications.
 For example, aircraft engineers are increasingly searching for structural materials that have low densities, are strong,
stiff, and abrasion and impact resistant, and are not easily corroded. This is a rather formidable combination of
characteristics.
 strong materials are relatively dense; also, increasing the strength or stiffness generally results in a decrease in impact
strength.
 classification distinguishes between (i) traditional and (ii) synthetic composites.

4. Classification of composites
 Metal Matrix Composites (MMCs) include mixtures of
ceramics and metals, such as cemented carbides and other
cermets, as well as aluminum or magnesium reinforced by
strong, high stiffness fibers.
 Ceramic Matrix Composites (CMCs) are the least common
category. Aluminum oxide and silicon carbide are materials
that can be imbedded with fibers for improved properties,
especially in high temperature applications.
 Polymer Matrix Composites (PMCs). Thermosetting resins
are the most widely used polymers in PMCs. Epoxy and
polyester are commonly mixed with fiber reinforcement,
and phenolic is mixed with powders. Thermoplastic
molding compounds are often reinforced, usually with
powders.
 classification of composite materials is shown in Figure
below, which consists of three main divisions: particle-
reinforced, fiber-reinforced, and structural composites;
 Composite = matrix (glue) + reinforcements (strength)
Fibers
• Various materials are used as fibers
in fiber-reinforced composites:
metals, ceramics, polymers,
carbon, and boron.
• The most important commercial
use of fibers is in polymer
composites. However, use of fiber-
reinforced metals and ceramics is
growing.

5. Concrete, Reinforced Concrete and Laminar composite
Concrete
 Concrete implies a composite material consisting of an aggregate of particles that are bound together in a solid body
by some type of binding medium, that is, cement.
Reinforced Concrete
 The strength of Portland cement concrete may be increased by additional reinforcement. This is usually accomplished
by means of steel rods, wires, bars (rebar), or mesh, which are embedded into the fresh and uncured concrete.
 Thus, the reinforcement renders the hardened structure capable of supporting greater tensile, compressive, and shear
stresses.
 Even if cracks develop in the concrete, considerable reinforcement is maintained. Steel serves as a suitable
reinforcement material because its coefficient of thermal expansion is nearly the same as that of concrete.
 In addition, steel is not rapidly corroded in the cement environment, and a relatively strong adhesive bond is formed
between it and the cured concrete.
Laminar composite
 A laminar composite structure consists of two or more layers bonded together to form an integral piece. The layers are
usually thick enough that this composite can be readily identified—not always the case with other composites. The
layers are often of different materials, but not necessarily.
 Plywood is such an example; the layers are of the same wood
 Examples of laminar composite structures: Automotive tires (rubber reinforced with carbon black), Windshield glass

6. Reinforced Concrete/Fiber reinforced plastics

7. A laminar composites

8. As a comparison between composites and metals, the
composites materials are some advantages as:
 Lightweight,
 High specific stiffness and strength,
 Easy moldable to complex forms,
 Easy bondable,
 Good dumping,
 Low electrical conductivity and thermal expansion,
 Good fatigue resistance,
 Part consolidation due to lower overall system costs,
 Low radar visibility,
 Internal energy storage and release.

9. MATRIX CLASSIFICATION

10. Roles of matrix
In a composite material, the matrix material serves the following
functions:
 to give shape to the composite part,
 protect the reinforcements to the environment,
 transfer loads to reinforcements and toughness of material, together with
reinforcements.
 Holds the fibres together
 Protects the fibres from environment
 Distributes the loads evenly between fibres so that all fibres are subjected to the
same amount of strain
 Enhances transverse properties of a laminate
 Improves impact and fracture resistance of a component
 Helps to avoid propagation of crack growth through the fibres by providing
alternate failure path along the interface between the fibres and the matrix
 Carry interlaminar shear

11. Aims of reinforcements
to get strength, stiffness and other
mechanical properties, dominate other
properties as coefficient of thermal
extension, conductivity and thermal
transport.

12. Examples of composites

13. COMPOSITES COMMERCIAL APPLICATIONS
 Applications include aerospace, transportation, construction, marine goods,
sporting goods, and more recently infrastructure, with construction and
transportation being the largest. In general, high-performance but more costly
 Continuous-carbon-fiber composites are used where high strength and stiffness
along with light weight are required, and much lower-cost fiberglass composites
are used in less demanding applications where weight is not as critical.

14. Important Properties of composites
 Very strong and stiff, yet very light in weight, giving them strength to weight
and stiffness-to-weight ratios several times greater than steel or aluminum.
Improved torsional stiffness is also enhanced. These properties are highly
desirable in applications ranging from commercial aircraft to sports equipment.
 Fatigue properties are generally better than for the common engineering
metals. Toughness is often greater, too.
 Do not corrode like steel; this is important in automotive and other applications.
This include improved weatherability of composites in a marine environment
 possible to achieve combinations of properties not attainable with metals,
ceramics, or polymers alone.
 Better appearance and control of surface smoothness are possible with
certain composite materials. It is easier to achieve smooth aerodynamic
profiles for drag reduction also

15. RULE OF MIXTURE
 For particulate composites, the rule of mixtures predicts the density of the composite as well as
other properties (although other properties may vary depending on how the dispersed phase is
arranged)
 Density, ρc, is given as a fraction, f, as:
Fm + Ff = 1
f
f
m
m
c f
f 

 
 f
m f
f 
1
that
Note
f
f
m
m
c K
f
K
f
K 
 f
f
m
m
c f
f 

 

The rule of mixtures can also be used to predict the modulus of elasticity when the fibers are continuous and unidirecti
f
f
m
m
c E
f
E
f
E 

f
f
c E
f
E 
Since the matrix now contributes little to the stiffness, the modulus is approximated by:
Thermal and electrical energy can be transferred through the composite at a rate that is proportional to the
volume fraction, f of the conductive material

16. Longitudinal Loading, Am + Af = Ac

18. Exercise
 A uniaxial composite material is made into a circular rod with a 1.27-cm
diameter from 70 volume percent continuous carbon fibers and 30 volume
percent epoxy. The rod is subject to an axial force of 100,000 N. The composite
material in Example Problem 12.1 is to be replaced with a less expensive
composite made of 70 volume percent continuous E-glass fibers and 30 volume
percent epoxy. The elastic moduli are 5 GPa for the epoxy resin and 72.4 GPa
for the E-glass.
 (a) Compare the elastic modulus, composite strain, fiber and matrix stresses,
and density of this composite with the carbon epoxy composite. Use the density
of UHM carbon, and assume the density of the epoxy is 1.2 g/cm3. (b) Can
both the E-glass fiber and matrix withstand the applied force?

19. Solution

20. Assignment
 A continuous and aligned glass fiber-reinforced composite consists of 40 vol%
of glass fibers having a modulus of elasticity of 69 GPa and 60 vol% of a polyester
resin that, when hardened, displays a modulus of 3.4 GPa.
(i) Compute the modulus of elasticity of this composite in the longitudinal
direction. [30 GPa]
(ii) If the cross-sectional area is 250 mm2 (0.4 in.2) and a stress of 50 MPa is applied
in this longitudinal direction; compute the magnitude of the load carried by
each of the fiber and matrix phases. [11,640 N]
(iii) Determine the strain that is sustained by each phase when the stress in part (ii)
is applied.[m =f = 1.69 x 10-3]