Classification of Composite
Since the reinforcement material is of primary importance in the
strengthening mechanism of a composite, it is convenient to classify
composites according to the characteristics of the reinforcement.
The following three categories are commonly used.
- Fiber Reinforced – In this group of composites, the
fiber is the primary load-bearing component.
- Dispersion Strengthened – In this group, the matrix
is the major load-bearing component.
- Particle Reinforced – In this group, the load is shared
by the matrix and the particles.
Fiber Reinforced Composites
Fiberglass is likely the best know fiber reinforced composite
but carbon-epoxy and other advanced composites all fall into this
category. The fibers can be in the form of long continuous fibers,
or they can be discontinuous fibers, particles, whiskers and even
weaved sheets. Fibers are usually combined with ductile matrix
materials, such as metals and polymers, to make them stiffer,
while fibers are added to brittle matrix materials like ceramics
to increase toughness. The length-to diameter ratio of the fiber,
the strength of the bond between the fiber and the matrix, and
the amount of fiber are variables that affect the mechanical properties.
It is important to have a high length-to-diameter aspect ratio
so that the applied load is effectively transferred form the matrix
to the fiber.
Fiber materials include:
Glass – glass is the most common and
inexpensive fiber and is usually use for the reinforcement of
polymer matrices. Glass has a high tensile strength and fairly
low density (2.5 g/cc).
Carbon-graphite - in advance composites, carbon
fibers are the material of choice. Carbon is a very light element,
with a density of about 2.3 g/cc and its stiffness is considerable
higher than glass. Carbon fibers can have up to 3 times the
stiffness of steel and up to 15 times the strength of construction
steel. The graphitic structure is preferred over the diamond-like
crystalline forms for making carbon fiber because the graphitic
structure is made of densely packed hexagonal layers, stacked
in a lamellar style. This structure results in mechanical and
thermal properties are highly anisotropic and this gives component
designers the ability to control the strength and stiffness
of components by varying the orientation of the fiber.
Polymer – the strong covalent bonds
of polymers can lead to impressive properties when aligned along
the fiber axis of high molecular weight chains. Kevlar is an
aramid (aromatic polyamide) composed of oriented aromatic chains,
which makes them rigid rod-like polymers. Its stiffness can
be as high as 125 GPa and although very strong in tension, it
has very poor compression properties. Kevlar fibers are mostly
used to increase toughness in otherwise brittle matrices.
Ceramic – fibers made from materials
such as Alumina and SiC (Silicon carbide) are advantageous in
very high temperature applications, and also where environmental
attack is an issue. Ceramics have poor properties in tension
and shear, so most applications as reinforcement are in the
Metallic - some metallic fibers have high
strengths but since there density is very high they are of little
use in weight critical applications. Drawing very thin metallic
fibers (less than 100 micron) is also very expensive.
In dispersion strengthened composites, small particles on the
order of 10-5 mm to 2.5 x 10-4 mm in diameter are added to the
matrix material. These particles act to help the matrix resist
deformation. This makes the material harder and stronger. Consider
a metal matrix composite with a fine distribution of very hard
and small secondary particles. The matrix material is carrying
most of the load and deformation is accomplished by slip and dislocation
movement. The secondary particles impede slip and dislocation
and, thereby, strengthen the material. The mechanism is that same
as precipitation hardening but effect is not quite as strong.
However, particles like oxides do not react with the matrix or
go into solution at high temperatures so the strengthening action
is retained at elevated temperatures.
The particles in these composite are larger than in dispersion
strengthened composites. The particle diameter is typically on
the order of a few microns. In this case, the particles carry
a major portion of the load. The particles are used to increase
the modulus and decrease the ductility of the matrix. An example
of particle reinforced composites is an automobile tire which
has carbon black particles in a matrix of polyisobutylene elastomeric
polymer. Particle reinforced composites are much easier and less
costly than making fiber reinforced composites. With polymeric
matrices, the particles are simply added to the polymer melt in
an extruder or injection molder during polymer processing. Similarly,
reinforcing particles are added to a molten metal before it is
- The interface is a bounding surface or zone where a discontinuity
occurs, whether physical, mechanical, chemical etc.
- The matrix material must "wet" the fiber. Coupling
agents are frequently used to improve wettability. Well "wetted"
fibers increase the interface surface area.
- To obtain desirable properties in a composite, the applied
load should be effectively transferred from the matrix to the
fibers via the interface. This means that the interface must
be large and exhibit strong adhesion between fibers and matrix.
Failure at the interface (called debonding) may or may not be
desirable. This will be explained later in fracture propagation
- Bonding with the matrix can be either weak van der Walls forces
or strong covalent bonds.
- The internal surface area of the interface can go as high
as 3000 cm2/cm3.
- Interfacial strength is measured by simple tests that induce
adhesive failure between the fibers and the matrix. The most
common is the Three-point bend test or ILSS (interlaminar shear
We will consider the results of incorporating fibers in a matrix.
The matrix, besides holding the fibers together, has the important
function of transferring the applied load to the fibers. It is
of great importance to be able to predict the properties of a
composite, given the component properties and their geometric
Isotropy and Anisotropy
- Fiber reinforced composite materials typically exhibit anisotropy.
That is, some properties vary depending upon which geometric
axis or plane they are measured along.
- For a composite to be isotropic in a specific property, such
as CTE or Young’s modulus, all reinforcing elements, whether
fibers or particles, have to be randomly oriented. This is not
easily achieved for discontinuous fibers, since most processing
methods tend to impart a certain orientation to the fibers.
- Continuous fibers in the form of sheets are usually used to
deliberately make the composite anisotropic in a particular
direction that is known to be the principally loaded axis or