Basic introduction to composite materials

1. COMPOSITE MATERIALS
A composite material is a non uniform solid consisting of two or more different
materials that are mechanically or metallurgically bonded together. Each of the
various components retains its identity in the composite and maintains its
characteristic structure and properties. The composites possesses combinations of
properties
Properties depend on­
(1) the properties of the individual components;
(2) the relative amounts of the components;
(3) the size, shape, and distribution of the discontinuous components;
(4) the orientation of the various components;
(5) the degree of bonding between the components
(1)LAMINAR OR LAYERED COMPOSITES
Laminar composites have distinct layers of material bonded together in some
manner and include thin coatings, thicker protective surfaces, claddings,
bimetallics, laminates, sandwiches. Properties of laminar composites are always
anisotropic.
Plywood, Bimetallic strip are examples of a laminate material.
Aramid–aluminum­laminates (Arall) consist of thin sheets of aluminum bonded
with woven adhesive­impregnated aramid fibers. The combination offers light
weight coupled with high fracture, impact, and fatigue resistance.
(2)PARTICULATE COMPOSITES
Particulate composites consist of discrete particles of one material surrounded by
a matrix of another material. Properties of particulate composites are usually
isotropic.
Concrete is a classic example, consisting of sand and gravel particles surrounded
by hydrated cement.
DISPERSION­STRENGTHENED MATERIALS are particulate composites
where a small amount of hard, brittle, small­sized particles (typically, oxides or
carbides) are dispersed throughout a softer, more ductile metal matrix;dispersed
material is not soluble in the matrix, it does not redissolve, overage, or over
temper when the material is heated. Creep resistance, therefore, is improved
significantly.

2. Examples ­sintered aluminum powder (SAP), consists of an aluminum matrix
strengthened by up to 14% aluminum oxide, and thoria­dispersed (or TD) nickel,
a nickel alloy containing 1 to 2 wt% thoria (ThO2). Dispersion­strengthened
composites are generally produced by powder metallurgy techniques.
TRUE PARTICULATE COMPOSITES contain large amounts of coarse
particles. They are usually designed to produce some desired combination of
properties rather than increased strength.
Cemented carbides, for example, consist of hard ceramic particles, such as
tungsten carbide, tantalum carbide, or titanium carbide, embedded in a metal
matrix, which is usually cobalt; the hard, stiff carbide could withstand the high
temperatures and pressure of cutting, it is extremely brittle. Toughness is
imparted by combining the carbide particles with cobalt powder, pressing the
material into the desired shape, heating to melt the cobalt, and then resolidifying
the compacted material.
METAL–MATRIX COMPOSITES of the particulate type have been made by
introducing a variety of ceramic or glass particles into aluminum or magnesium
matrices. Particulate toughened ceramics using zirconia and alumina matrices
are being used as bearings,bushings, valve seats, die inserts, and cutting­tool
inserts
Example­ combination of granite particles in an epoxy matrix that is currently
being used in some machine tool bases. This unique material offers high strength
and a vibration­damping capacity that exceeds that of gray cast iron
(3)FIBER­REINFORCED COMPOSITES
In the fiber­reinforced composite geometry continuous or discontinuous thin fibers
of one material are embedded in a matrix of another objective is usually to
enhance strength, stiffness, fatigue resistance, or strength­to­weight ratio by
incorporating strong, stiff, but possibly brittle, fibers in a softer, more ductile
matrix. Matrix supports and transmits forces to the fibers, protects them from
environments and handling, and provides ductility and toughness, while the
fibers carry most of the load and impart enhanced stiffness.
Fiber of material tends to be stronger than the same material in bulk form The
orientation of the fibers within the composite is often key to properties and
performance

3. The properties of fiber­reinforced composites depend strongly on several
characteristics:
(1) properties of the fiber material;
(2) volume fraction of fibers;
(3) aspect ratio of the fibers, that is, the length­to diameter ratio;
(4) orientation of the fibers;
(5) degree of bonding between the fiber and the matrix; and
(6) properties of the matrix
Volume fraction of fibers generally cannot exceed 80% to allow for a continuous
matrix. Long, thin fibers (higher aspect ratio) provide greater strength, and a
strong bond is usually desired between the fiber and matrix
Wood and bamboo are two naturally occurring fiber composites, consisting of
cellulose fibers in a lignin matrix.
Glass fibers are still the most widely used reinforcement, primarily because of
their lower cost;
Boron–tungsten fibers (boron deposited on a tungsten core) offer an elastic
modulus of 55e6 psi with tensile strengths in excess of 400 ksi;
Silicon carbide filaments (SiC on tungsten) have an even higher modulus of
elasticity;
Kevlar is an organic aramid fiber with a tensile strength up to 650 ksi, elastic
modulus of 27e6psi ,a density approximately one­half that of aluminum, and
good toughness
Ceramic fibers, metal wires, and specially grown whiskers have also been used as
reinforcing fibers for high­strength, high­temperature applications. Metal fibers
can also be used to provide electrical conductivity or shielding from
electromagnetic interference to a lightweight polymeric matrix
Graphite’s negative thermal expansion coefficient can also be used to offset the
positive values of most matrix materials, leading to composites with low or zero
thermal expansion
(4) ADVANCED FIBER­REINFORCED COMPOSITES
Advanced composites are materials that have been developed for applications
requiring exceptional combinations of strength, stiffness, and light weight. Fiber
content generally exceeds 50% (by weight), and the modulus of elasticity is
typically greater than 16 E6 psi; Superior creep and fatigue resistance, low
thermal expansion, low friction and wear, vibration­damping characteristics, and
environmental stability are other properties.

4. ADVANCED ORGANIC OR RESIN­MATRIX COMPOSITES­use high­strength,
high­modulus fibers of graphite, aramid (Kevlar), or boron; Properties can be put
in desired locations or orientations at about one­half the weight of aluminum (or
one­sixth that of steel); Thermal expansion can be designed to be low or even
negative; have a maximum service temperature of about 315ºC
METAL­MATRIX COMPOSITES (MMCs)­ductile matrix material can be
aluminum, copper, magnesium, titanium, nickel, superalloy, or even intermetallic
compound, and the reinforcing fibers may be graphite, boron carbide, alumina, or
silicon carbide; used for operating temperatures up to 1250ºC; where the
conditions require high strength, high stiffness, good electrical and/or thermal
conductivity, exceptional wear resistance, and good ductility and toughness. They
are nonflammable, do not absorb water or gases, and are corrosion resistant to
fuels and solvents
CARBON–CARBON COMPOSITES­graphite fibers in a graphite or carbon
matrix; offer the possibility of a heat­resistant material that could operate at
temperatures above 2000ºC; strength is 20 times that of conventional graphite,
density is 30% lighter (1.38g/cm3), low coefficient of thermal expansion; it
actually gets stronger when heated; properties include good toughness, good
thermal and electrical conductivity, and resistance to corrosion and abrasion.
For temperatures over 540ºC, composite requires some form of coating to protect it
from oxidizing.
Current applications include the nose cone and leading edge of the space shuttle,
aircraft and racing car disc brakes, automotive clutches, aerospace turbines and
jet engine components, rocket nozzles, and surgical implants
CERAMIC­MATRIX COMPOSITES (CMCs)­offer light weight and stiffness,
good dimensional environmental stability; matrix provides high temperature
resistance. Glass matrices can operate at temperatures as high as 1500ºC;
crystalline ceramics, usually based on alumina, silicon carbide, silicon nitride,
boron nitride, titanium diboride, or zirconia reinforcements include carbon fiber,
glass fiber, fibers of the various matrix materials, and ceramic whiskers.
Composites with discontinuous fibers tend to be used primarily for wear
applications, such as cutting tools, forming dies, and automotive parts such as
valve guides.
Continuous­fiber ceramic composites are used for applications involving the
combination of high temperatures and high stresses, and have been shown to fail
in a non catastrophic manner.
Application examples include gas­turbine components, high­pressure heat
exchangers, and high­temperature filters. Unfortunately, the cost of ceramic–
ceramic composites ranges from high to extremely high.

5. HYBRID COMPOSITES­involve two or more different types of fibers in a
common matrix
Types of hybrid composites include­
(1) interply (alternating layers of fibers),
(2) intraply (mixed strands in the same layer),
(3) interply–intraply,
(4) selected placement(where the more costly material is used only where needed),
(5) interply knitting (where plies of one fiber are stitched together with fibers of
another type
DESIGN, FABRICATION, LIMITS AND APPLICATIONS OF
COMPOSITES
Design of composite materials involves­
1) selection of the component materials;
2) determination of the relative amounts of each component;
3) determination of size, shape, distribution, and orientation of the
components;
4) selection of an appropriate fabrication method
Fibrous composites can be manufactured into useful shapes through compression
molding, filament winding, pultrusion (where bundles of coated fibers are drawn
through a heated die), cloth lamination, and autoclave curing (where pressure
and elevated temperature are applied simultaneously)
The greatest limitations of composites are their relative brittleness and the high
cost of both materials and fabrication. Manufacturing with composites can still
be quite labor intensive, and there is a persistent lack of trained designers,
established design guidelines and data, information about fabrication costs, and
reliable methods of quality control and inspection; difficult to predict the
interfacial bond strength, the strength of the composite and its response to
impacts, and the probable modes of failure. Defects can involve delaminations,
voids, missing layers, contamination, fiber breakage, and (hard­to­detect)
improperly cured resin. Many composites with polymeric matrices are sensitive to
moisture, acids, chlorides, organic solvents, oils, and ultraviolet radiation, and
they tend to cure forever, causing continually changing properties.
In addition, most composites have limited ability to be repaired if damaged,
preventive maintenance procedures are not well established, and recycling is often
extremely difficult.