Composite materials

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• Classification
• Reinforcing Phase
• Properties
• Other Structures
• Metal Matrix Composites
• Ceramic Matrix Composites
• Polymer Matrix Composites

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 Definition
• A materials system composed of two or more
physically distinct phases whose combination
produces aggregate properties that are different
from those of its constituents
• Examples:
– Cemented carbides (WC with Co binder)
– Plastic molding compounds containing fillers
– Rubber mixed with carbon black
– Wood (a natural composite as distinguished from a
synthesized composite)

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 Why Composites are Important
• Composites can be very strong and stiff, yet very light in
weight, so ratios of strength‑to‑weight and
stiffness‑to‑weight are several times greater than steel
or aluminum
• Fatigue properties are generally better than for common
engineering metals
• Toughness is often greater too
• Composites can be designed that do not corrode like
steel
• Possible to achieve combinations of properties not
attainable with metals, ceramics, or polymers alone

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Disadvantages and Limitations of Composite
Materials
• Properties of many important composites are
anisotropic ‑ the properties differ depending on the
direction in which they are measured – this may be
an advantage or a disadvantage
• Many of the polymer based composites are subject
to attack by chemicals or solvents, just as the
polymers themselves are susceptible to attack
• Composite materials are generally expensive
• Manufacturing methods for shaping composite
materials are often slow and costly
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 One Possible Classification of
Composite Materials
• Traditional composites – composite materials that
occur in nature or have been produced by
civilizations for many years
– Examples: wood, concrete, asphalt
• Synthetic composites - modern material systems
normally associated with the manufacturing
industries, in which the components are first
produced separately and then combined in a
controlled way to achieve the desired structure,
properties, and part geometry
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Classification

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 Classification
Primary Phase, Matrix

Secondary Phase,
Reinforcement

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 Functions of the Matrix Material
(Primary Phase)
• Protect phases from environment
• Transfer Stresses to phases
• Holds the imbedded phase in place, usually
enclosing and often concealing it
• When a load is applied, the matrix shares
the load with the secondary phase

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V= volume fraction
Sigma= stress
E= modulus
L=length
A=cross sectional area
Epsilon= strain
Subscript C,M,P= composite ,material,particle

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Influence of fiber length:

Where,
Lc= critical length
Sigma f=fracture stress
D= diameter
τc =shear strength or bond strength
between matrix and fiber

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Influence of direction:

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Isostrain conditions: Consider an idealized lamellar composite
test Sample with alternate layers of continuous fibers and
matrix material, as in Fig. In this case the stress on the
material causes uniform strain on all composite layers. This
type of loading on the composite sample is called Isostrain
condition.
First, the load on the composite structure is equal to the sum
of the load on the fiber layers plus the load on the matrix
layers, Pc = Pf + Pm
σcAc = σfAf + σmAm
Since the lengths of the layers of matrix and fiber are equal,
the areas Ac, Af and Am can be replaced by the volume
fractions Vc, Vf and Vm
σcVc = σfVf + σmVm____________ (I)

Since the volume fraction of the total composite is 1, then Vc

For isostrain conditions εc = εf = εm _________________ (II)

Ec = EfVf + EmVm
This equation enables a value for the elastic modulus of a composite to be calculated knowing
the elastic moduli of the fiber and matrix and their volume percentages.

Equations for the loads on the fiber and matrix regions of a lmellar composite structure loaded
under isostrain conditions:

Pc = Pf + Pm
Isostress conditions: Consider the case of an idealized lamellar
composite structure consisting of layers of fiber and matrix in
which the layers are perpendicular to the applied stress as in
Fig. The stress on the composite structure produces an equal
stress condition on all the layers.
We shall begin with an equation that states that the stress on
the total composite structure is equal to the stress on the fiber
layers and stress on the matrix layers. Thus
σc = σf = σm
The total strain for the composite in the directions of the
stresses is thus equal to the sum of the strains in the fiber and
matrix layers,
εc = εf + εm ___________ (I)
εcVc = εfVf + εmVm
since Vc = 1
εc = εfVf + εmVm
Equation (I) becomes
σ = σVf + σVm
Ec Ef Em
 1 = Vf + Vm Ec Ef
Em

1 = Vf Em + Vm Ef Ec Ef
Em

Ec = Ef Em Vf Em + Vm Ef
Q1.

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Q. For a continuous and oriented fiber-reinforced composite, the moduli of elasticity in
the longitudinal and transverse directions are 19.7 and 3.66 GPa (2.8 × 106 and 5.3 × 105
psi), respectively. If the volume fraction of fibers is 0.25, determine the moduli of elasticity
of fiber and matrix phases.

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Structural Composites:
A structural composite is normally composed of both
homogeneous and composite materials, the properties
of which depend not only on the properties of the
constituent materials but also on the geometrical design
of the various structural elements.
Laminar composites and sandwich panels are two of the
most common structural composites.

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Laminar composite:
A laminar composite is composed of two-dimensional sheets or panels that
have a preferred high-strength direction, such as is found in wood and
continuous and aligned fiber–reinforced plastics. The layers are stacked and
subsequently cemented together such that the orientation of the high-strength
direction varies with each successive layer.
For example, adjacent wood sheets in plywood are aligned with the grain
direction at right angles to each other. Laminations may also be constructed
using fabric material such as cotton, paper, or woven glass fibers embedded in
a plastic matrix. Thus a laminar composite has relatively high strength in a
number of directions in the two-dimensional plane; however, the strength in
any given direction is, of course, lower than it would be if all the fibers were
oriented in that direction.

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Sandwich panel:
Sandwich panels, considered to be a class of structural composites, are
designed to be lightweight beams or panels having relatively high stiffness
and strengths. A sandwich panel consists of two outer sheets, or faces,
that are separated by and adhesively bonded to a thicker core (Figure
16.17). The outer sheets are made of a relatively stiff and strong material,
typically aluminum alloys, fiber-reinforced plastics, titanium, steel, or
plywood; they impart high stiffness and strength to the structure
and must be thick enough to withstand tensile and compressive stresses
that result from loading. The core material is lightweight and normally has
a low modulus of elasticity. Core materials typically fall within three
categories: rigid polymeric foams (i.e., phenolics, epoxy, polyurethanes),
wood (i.e., balsa wood), and honeycombs.
Sandwich panels are used in a wide variety of applications including
roofs, floors, and walls of buildings and in aerospace and aircraft (i.e., for
wings, fuselage, and tailplane skins).

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and elastic modulus of an aramid fiber-reinforced polycarbonate
composite. (a) The longitudinal tensile strength is determined using
Equation 16.17 as

σcl ∗ = σm ' (1−Vf ) + σ f ∗Vf

= (45 MPa)(0.70) + (3600)(0.30)

= 1100 MPa (160,000 psi)

(b) The longitudinal elastic modulus is computed using Equation

Ecl = EmVm + EfVf

= (2.4 GPa)(0.70) + (131 GPa)(0.30)

= 41 GPa (5.95 × 106 psi) 40