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Composite Material Defined
• A materials system composed of two or more physically
distinct phases whose combination produces aggregate
properties that are different from those of its constituents
Advantages
• 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

Components in a Composite Material Nearly all
composite materials consist of two phases:
1. Primary phase - forms the matrix within which the
secondary phase is imbedded
2. Secondary phase - imbedded phase sometimes
referred to as a reinforcing agent, because it usually
serves to strengthen the composite
The reinforcing phase may be in the form of
fibers,particles, or various other geometries

Classification Scheme for Composite Materials
1. Metal Matrix Composites (MMC) - mixtures of
ceramics and metals, such as cemented carbides and
other cermets
2. Ceramic Matrix Composites (CMC) – Al2O3 and SiC
imbedded with fibers to improve properties,
especially in high temperature applications
The least common composite matrix
3. Polymer Matrix Composites (PMC) - thermosetting
resins are widely used in PMC Examples: epoxy and
polyester with fiber reinforcement,and phenolic with
powders

Functions of the Matrix Material(Primary Phase)
 Provides the bulk form of the part or product made
of the composite material
 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, in some cases deforming so
that the stress is essentially born by the reinforcing
agent

The Reinforcing Phase (Secondary Phase)
• Function is to reinforce the primary phase
• Imbedded phase is most commonly one of the
following shapes:
Fibers
Particles
Flakes
• In addition, the secondary phase can take the form of
an infiltrated phase in a skeletal or porous matrix
Example: a powder metallurgy part infiltrated with
polymer

Fibers
Filaments of reinforcing material, usually circular incross-
section
 Diameters range from less than 0.0025 mm to about 0.13
mm, depending on material
 Filaments provide greatest opportunity for strength
enhancement of composites
 The filament form of most materials is significantly
stronger than the bulk form
 As diameter is reduced, the material becomes oriented in
the fiber axis direction and probability of defects in the
structure decreases significantly

Continuous vs. Discontinuous Fibers
 Continuous fibers - very long; in theory, they offer a
continuous path by which a load can be carried by the
composite part
 Discontinuous fibers (chopped sections of continuous
fibers) short lengths (L/D = roughly 100)
Important type of discontinuous fiber are whiskers –
hair like single crystals with diameters down to about
0.001 mm (0.00004 in.) with very high strength

Fiber Orientation – Three Cases
 One-dimensional reinforcement, in which maximum
strength and stiffness are obtained in the direction of
the fiber
 Planar reinforcement, in some cases in the form of a
two-dimensional woven fabric
 Random or three-dimensional in which the composite
material tends to possess isotropic properties

Particles and Flakes
 A second common shape of imbedded phase is
particulate, ranging in size from microscopic to
macroscopic
 Flakes are basically two-dimensional particles - small
flat platelets
 The distribution of particles in the composite matrix
is random, and therefore strength and other
properties of the composite material are usually
isotropic
 Strengthening mechanism depends on particle size

The Interface
 There is always an interface between constituent
phases in a composite material
 For the composite to operate effectively, the phases
must bond where they join at the interface

Interphase
 In some cases, a third ingredient must be added to
achieve bonding of primary and secondary phases
 Called an interphase, this third ingredient can be
thought of as an adhesive

Properties are Determined by Three Factors:
1. The materials used as component phases in the
composite
2. The geometric shapes of the constituents and
resulting structure of the composite system
3. The manner in which the phases interact with one
another

Computing composite properties
Determine the mechanical property of Continuous and
Aligned Fiber composites
Longitudinal direction
Fc =Fm + Ff
The assumption of an isostrain state :
The ratio of the load carried by the fibers to that carried by the matrix is

Transverse Loading
The assumption of an isostress state :

Determine the mechanical property of Discontinuous
and Aligned Fiber Composites
If the fiber length (l) is less than critical(lc), the longitudinal
strength
If (l >lc), the longitudinal strength
Where

Determine the mechanical property of Discontinuous
and Randomly Oriented Fiber Composites
Where K = fiber efficiency parameter

= 0.1 -0.6.

EXAMPLE PROBLEM
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.
(a) Compute the modulus of elasticity of this composite in the
longitudinaldirection.
(b) If the cross-sectional area is 250 mm2
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.
(c) Determine the strain that is sustained by each phase when
the stress inpart (b) is applied.

Polymer Matrix Composites (PMCs)
A polymer primary phase in which a secondary phase is
imbedded as fibers, particles, or flakes
Commercially, PMCs are more important than MMCs
or CMCs
Examples: most plastic molding compounds, rubber
reinforced with carbon black, and fiber-reinforced
polymers (FRPs)

Fiber-Reinforced Polymers (FRPs)
A PMC consisting of a polymer matrix imbedded with high-
strength fibers
Polymer matrix materials:
Usually a thermosetting (TS) plastic such as unsaturated
polyester or epoxy
Can also be thermoplastic (TP), such as nylons (polyamides),
polycarbonate, polystyrene, and polyvinylchloride
Fiber reinforcement is widely used in rubber products such
as tires and conveyor belts

Fibers in PMCs
Various forms: discontinuous (chopped), continuous, or
woven as a fabric
Principal fiber materials in FRPs are glass, carbon, and
Kevlar 49
Less common fibers include boron, SiC, and Al2O3, and
steel
Glass (in particular E-glass) is the most common fiber
material in today's FRPs;

Metal Matrix Composites (MMCs)
A metal matrix reinforced by a second phase
Reinforcing phases:
1. Particles of ceramic (these MMCs are commonly called
cermets)
2. Fibers of various materials: other metals, ceramics,
carbon, and boron

Cemented Carbides
One or more carbide compounds bonded in a metallic
matrix
Common cemented carbides are based on tungsten
carbide(WC), titanium carbide (TiC), and chromium
carbide(Cr3C2)
Tantalum carbide (TaC) and others are less common
Metallic binders: usually cobalt (Co) or nickel (Ni)

Ceramic Matrix Composites (CMCs)
A ceramic primary phase imbedded with a secondary
phase, which usually consists of fibers
Attractive properties of ceramics: high stiffness,
hardness, hot hardness, and compressive strength;
and relatively low density
Weaknesses of ceramics: low toughness and bulk
tensilestrength, susceptibility to thermal cracking
CMCs represent an attempt to retain the desirable
properties of ceramics while compensating for their
weaknesses

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