Generally speaking, a composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realized.
2. Composite Materials
Composite Materials refers to all solid materials composed of more than one
substances.
A more narrow definition of composite materials is,
‘These are the solid materials composed of a matrix system which surrounds
and binds together the reinforcements’.
Generally speaking, a composite is considered to be any multiphase material
that exhibits a significant proportion of the properties of both constituent
phases such that a better combination of properties is realized.
According to this principle of combined action, better property combinations are
fashioned by the judicious combination of two or more distinct materials.
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3. Composite Materials
The other phase is called the dispersed phase or reinforcement phase, which
is surrounded by the matrix.
For most of composites, the reinforcement/dispersed phase is harder and
stiffer than the matrix
There is no universal definition of composite materials however from the scope of this
subject,
It serves the following purposes at least;
1) It consists of two or more physically distinct and mechanically separable materials.
2) It can be made by mixing the separate materials in such a way that the dispersion of
one material in the other can be done in a controlled way to achieve optimum properties.
3) The properties are superior and possibly unique in some specific respects, to the
properties of the individual components.
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4. Carbon/epoxy composite crutch. This
crutch is stronger than its aluminium
counterpart yet weighs 50% less, is
quieter, and is more aesthetically
High performance sporting goods
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6. What makes polymer reinforced composite a better choice
than metals?
Specific Modulus
and
Specific Strength
Higher specific modulus and specific strength of polymer
reinforced composites means that the weight of the
components can be reduced.
This is a factor of great concern where reductions in weight
result in greater efficiency and energy savings.
Composite Materials
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7. Composite Materials
Polymeric matrix composites (PMCs) are divided into two groups.
They differ principally in the type and length of the fiber reinforcement and in the type of
resin used.
1. Advanced composites:
It is characterized by very long and very high performance reinforcements.
The resin types have also superior thermal and mechanical properties.
Advanced composites are typically used for
Aerospace applications like Rocket motor, cases and airplane parts,
High performance sporting goods like golf clubs, tennis rackets etc.
2. Engineering composites:
These are characterized by shorter fibers and fibers with lower mechanical
properties.
The resins have also lower mechanical performance.
Products include;
Boat hulls, Tubs, Shower stalls, Fuel storage tanks etc.
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8. Any material can be used as a matrix, however, limited
number of materials are used depending on the following
Factors;
1) Ease of Fabrication
2) Compatibility with Fibers
3) Desired end properties
4) Cost
Composite Materials
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9. Function of Matrix Phase
The matrix phase serves several functions
1) It binds the fibers together and acts as the medium by which an externally
applied stress is transmitted and distributed to the fibers; only a very small
proportion of an applied load is sustained by the matrix phase.
2) The second function of the matrix is to protect the individual fibers from
surface damage as a result of mechanical abrasion or chemical reactions with
the environment. Such interactions may introduce surface flaws capable of forming
cracks, which may lead to failure at low tensile stress levels
3) The matrix separates the fibers, prevents the propagation of brittle cracks
from fiber to fiber, which could result in catastrophic failure; the matrix phase
serves as a barrier to crack propagation.
It is essential that adhesive bonding forces between fiber and matrix be high to
minimize fiber pull-out.
Bonding strength is an important consideration in the choice of the matrix–fiber
combination.
Adequate bonding is essential to maximize the stress transmittance from the
weak matrix to the strong fibers.
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12. Manufacturing of Fibers – Comparison
Five aspects of fiber properties are relevant to the use of fibers in composite materials;
1) Specific Properties
2) Thermal Stability
3) Compressive properties
4) Fiber fracture
5) Fiber flexibility
13. Composite Materials---Fiber-Matrix interface
The structure and properties of the
Fiber-matrix interface play a major role
in the mechanical and physical properties
of the composite materials.
The stresses acting on the matrix are
transmitted to the fibers across the interface.
The interface is a dominant factor.
Fibers and matrix have to be strongly
bonded together.
Composite materials with weak interfaces
have relatively low strength and stiffness
While
Strong interfaces give high strength and
stiffness
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14. Composite Materials---Fiber-Matrix interface
In a simple system, bonding at an interface is due to adhesion between fiber & matrix.
However, the fibers are often coated with a layer of material which forms a bond between
the fiber and matrix.
Adhesion can be attributed to five main mechanisms which can occur at the interface
either in isolation or in combination to produce the bond.
1)Adsorption and Wetting
2)Interdiffusion
3)Electrostatic attraction
4)Chemical Bonding
5)Mechanical adhesion
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15. Composite Materials--- Influence of fiber length
Case 1 Case 2
Case 3
Some critical fiber length is necessary for effective strengthening
and stiffening of the composite materials. The critical length is
dependent on the fiber dia.
Usually the critical length is on the order of 1mm which ranges
between 20 and 150 times the fiber dia.
load is
achieved
only at
the axial
center of
the fiber
there is
virtually no
stress
transference
and little
reinforcement
by the fiber
