The earliest man-made composite materials were straw and mud
combined to form bricks for building construction. Ancient brick-
making was documented by Egyptian tomb paintings.
Wattle and daub is one of the oldest man-made composite
materials, at over 6000 years old. Concrete is also a composite
material, and is used more than any other man-made material in the
Woody plants, both true wood from trees and such plants as palms
and bamboo, yield natural composites that were used
prehistorically by mankind and are still used widely in construction
Plywood 3400 BC by the Ancient Mesopotamians; gluing wood at
different angles gives better properties than natural wood.
Cartonnage layers of linen or papyrus soaked in plaster dates to the
First Intermediate Period of Egypt c. 2181–2055 BC and was used
for death masks.
Papier-mâché, a composite of paper and glue, has been used for
hundreds of years.
The first artificial fiber reinforced plastic was Bakelite which dates to
1907, although natural polymers such as shellac predate it
WHAT ARE THEY?
A composite material can be defined as a macroscopic combination of
two or more distinct materials, having a recognizable interface between
them. Materials which differ by their nature or origin are combined in
order to take benefit from their different properties. Composite materials
are widely presented in the nature (e.g. wood, bones...) and also designed
and produced by humans (e.g. concrete, reinforced plastics, plywood
...). For technical applications, the definition can be restricted to include
only those materials that contain a reinforcement (such as fibers or
particles) supported by a binder (matrix) material.
According to their performance and properties, composite materials may
also be classified as:
General purpose composites
Composites used for aircraft structures are mainly classified as advanced
ADVANCED COMPOSITES IN AIRCRAFT INDUSTRY
Applications of composites on aircraft include:
Flight control surfaces
Landing gear doors
Leading and trailing edge panels on the wing and stabilizer
Floor beams and floor boards
Vertical and horizontal stabilizer primary structure on large aircraft
Primary wing and fuselage structure on new generation large
Turbine engine fan blades
These Advanced Composites combine high strength or high modulus
continuous fibers (as reinforcement) and a high performance matrix
The most common fiber types are:
The most common matrix types are resins like:
For some applications, Thermoplastics are also used.
CRITERIA FOR COMPOSITE MATERIALS CHOICE
Composites are used because of their:
Non sensitivity to corrosion
They also allow:
A reduction in the member of structure parts
The building of complex curved shapes
A good surface finish
But also some disadvantages, compared with metal, have to be taken into
Need for lightning strike protection
Need for a lightning protection
Need for an antistatic protection (not for carbon)
MATRICES USED IN ADVANCED COMPOSITES
The purpose of the composite matrix is to bind the fibers together by
virtue of its cohesive and adhesive characteristics to:
Transfer the loads to and between fibers.
Provide the required protection to the reinforcement,
Maintain the shape and geometry of the component.
Depending on the application (level of strength or rigidity, overall service
temperature, chemical environment compatibility with fibers, fire or
toxicity requirements and also: cost, processability.) the matrix will be
either organic, metallic or ceramic.
Organic matrices, commonly called resins, combined with long fiber
reinforcements, constitute the majority of applications in advanced
Resin types used are divided in two families:
Their molecular structure is completely different and therefore their
properties and behavior are different. The selection will depend on the
application and the environment.
These materials are hard at ambient temperature and become malleable
when heated. During the heating process there is no chemical reaction
and therefore the transformation is reversible. The manufacturing process
for molding requires heating, above the melt temperature, and cooling
down. The heating and cooling can be repeated several times without
damaging the material. There is no ageing process of the raw material
before transformation and therefore the raw material does not require a
particular storage, except a dust free and dry environment.
Several types of thermoplastics are available in the industry.
1. Semicrystalline Thermoplastics
Semicrystalline thermoplastics possess properties of inherent flame
resistance, superior toughness, good mechanical properties at
elevated temperatures and after impact, and low moisture
absorption. They are used in secondary and primary aircraft
structures. Combined with reinforcing fibers, they are available in
injection molding compounds, compression-moldable random
sheets, unidirectional tapes, prepregs fabricated from tow
(towpreg), and woven prepregs. Fibers impregnated in
semicrystalline thermoplastics include carbon, nickel-coated
carbon, aramid, glass, quartz, and others.
2. Amorphous Thermoplastics
Amorphous thermoplastics are available in several physical forms,
including films, filaments, and powders. Combined with
reinforcing fibers, they are also available in injection molding
compounds, compressive moldable random sheets, unidirectional
tapes, woven prepregs, etc. The fibers used are primarily carbon,
aramid, and glass. The specific advantages of amorphous
thermoplastics depend upon the polymer. Typically, the resins are
noted for their processing ease and speed, high temperature
capability, good mechanical properties, excellent toughness and
impact strength, and chemical stability. The stability results in
unlimited shelf life, eliminating the cold storage requirements of
3. Polyether Ether Ketone (PEEK)
Polyether ether ketone, better known as PEEK, is a high
temperature thermoplastic. This aromatic ketone material offers
outstanding thermal and combustion characteristics and resistance
to a wide range of solvents and proprietary fluids. PEEK can also
be reinforced with glass and carbon.
These resin systems are in a liquid or a paste state at room temperature
before transformation. They require to be cured for polymerisation. The
polymerisation is a non-reversible chemical reaction between a resin base
and a hardener. Each Thermosetting material has its own curing cycle,
which has been qualified to meet the minimum required properties.
It is important to note that Thermosetting materials have a limit shelf life
when not cured. In order to limit the ageing effect it is required to store
them at low temperature.
There are several types such that:
1. Polyester Resins
Polyester resins are relatively inexpensive, fast processing resins
used generally for low cost applications. Low smoke producing
polyester resins are used for interior parts of the aircraft. Fiber-
reinforced polyesters can be processed by many methods. Common
processing methods include matched metal molding, wet layup,
press (vacuum bag) molding, injection molding, filament winding,
pultrusion, and autoclaving.
2. Vinyl Ester Resin
The appearance, handling properties, and curing characteristics of
vinyl ester resins are the same as those of conventional polyester
resins. However, the corrosion resistance and mechanical
properties of vinyl ester composites are much improved over
standard polyester resin composites.
3. Phenolic Resin
Phenol-formaldehyde resins were first produced commercially in
the early 1900s for use in the commercial market.
Ureaformaldehyde and melamine-formaldehyde appeared in the
1920–1930s as a less expensive alternative for lower temperature
use. Phenolic resins are used for interior components because of
their low smoke and flammability characteristics.
Epoxies are polymerizable thermosetting resins and are available in
a variety of viscosities from liquid to solid. There are many
different types of epoxy, and the technician should use the
maintenance manual to select the correct type for a specific repair.
Epoxies are used widely in resins for prepreg materials and
structural adhesives. The advantages of epoxies are high strength
and modulus, low levels of volatiles, excellent adhesion, low
shrinkage, good chemical resistance, and ease of processing. Their
major disadvantages are brittleness and the reduction of properties
in the presence of moisture. The processing or curing of epoxies is
slower than polyester resins. Processing techniques include
autoclave molding, filament winding, press molding, vacuum bag
molding, resin transfer molding, and pultrusion. Curing
temperatures vary from room temperature to approximately 350 °
F (180 °C). The most common cure temperatures range between
250° and 350 °F (120–180 °C).
Polyimide resins excel in high-temperature environments where
their thermal resistance, oxidative stability, low coefficient of
thermal expansion, and solvent resistance benefit the design. Their
primary uses are circuit boards and hot engine and airframe
structures. A polyimide may be either a thermoset resin or a
thermoplastic. Polyimides require high cure temperatures, usually
in excess of 550 °F (290 °C). Consequently, normal epoxy
composite bagging materials are not usable, and steel tooling
becomes a necessity. Polyimide bagging and release films, such as
Kapton® are used. It is extremely important that Upilex® replace
the lower cost nylon bagging and polytetrafluoroethylene (PTFE)
release films common to epoxy composite processing. Fiberglass
fabrics must be used for bleeder and breather materials instead of
polyester mat materials due to the low melting point of polyester.
6. Polybenzimidazoles (PBI)
Polybenzimidazole resin is extremely high temperature resistant
and is used for high temperature materials. These resins are
available as adhesive and fiber.
7. Bismaleimides (BMI)
Bismaleimide resins have a higher temperature capability and
higher toughness than epoxy resins, and they provide excellent
performance at ambient and elevated temperatures. The processing
of bismaleimide resins is similar to that for epoxy resins. BMIs are
used for aero engines and high temperature components. BMIs are
suitable for standard autoclave processing, injection molding, resin
transfer molding, and sheet molded compound (SMC) among
Thermosetting resins use a chemical reaction to cure. There are three
curing stages, which are called A, B, and C.
o A stage: The components of the resin (base material and hardener)
have been mixed but the chemical reaction has not started. The
resin is in the A stage during a wet layup procedure.
o B stage: The components of the resin have been mixed and the
chemical reaction has started. The material has thickened and is
tacky. The resins of prepreg materials are in the B stage. To
prevent further curing the resin is placed in a freezer at 0 °F. In
the frozen state, the resin of the prepreg material stays in the B
stage. The curing starts when the material is removed from the
freezer and warmed again.
o C stage: The resin is fully cured. Some resins cure at room
temperature and others need an elevated temperature cure cycle to
REINFORCEMENT IN ADVANCED COMPOSITES
In advanced composite design, different types of reinforcement materials
are used. They are used in combination with a selected matrix, to build up
Mainly two types of fiber are used:
short fibers (non-continuous reinforcement)
long fibers (continuous reinforcement)
The fibers are embedded in the matrix to meet the design requirements
(loads, environment, weight ...). When a design requires load transfer
and weight saving, long fibers are used.
The reinforcement is made of tows which are an arrangement of
thousands of filaments of a diameter between 4 and 15 micro meters.
Three kinds of reinforcements are used advanced composites on
To select the appropriate fiber many parameters should be noted such
Mechanical properties (strength and modulus) in direction of the
filaments in tensile and compression.
SEMI FINISHED PRODUCTS
In the aircraft industry, the most commonly used semi-finished products
A tow is an untwisted bundle of continuous filaments made of carbon,
glass or aramid. A tow designated as 3K has 3000 filaments. A yarn is an
assembly of twisted filaments to form a continuous length that is suitable
for use in weaving or interweaving into textile material. This product is
used in filament winding process. The dry tow is dipped in liquid resin
and applied on a mandrel.
This product is an arrangement of parallel tows in a single direction. The
mechanical properties are provided only in the direction of the fiber. They
are available on the market in prepreg form only.
This product is made of woven tows or yarns in perpendicular directions
to form such fabric patterns as plain or hardness satin. The mechanical
properties are provided in the two perpendicular directions.
Fabrics are used in a laying up process to carry loads in appropriate
directions. They are available on the market in dry sheet form, for wet
layup process (hand impregnation) or in prepreg form.
Main used woven fabrics:
Fabrics can be woven in various different types of weave patterns.
8 Harness satin weave.
5 Harness satin weave.
It is protected with release foils to prevent moisture absorption, dust
contamination and also to prevent solvent evaporation to maintain a
minimum tack level for material application.
Because resin base and hardener are already mixed together, prepregs
have a limited shelf life and shop life. Therefore they are stored and
shipped in a cold condition to limit the ageing effect.
Two distinct types of prepreg are available on the market where the
reinforcement is coated with a hot melt or a solvent system, to produce a
specific final product with calibrated resin content.
Examples for Fiber
Fiberglass is often used for secondary structure on aircraft, such as
fairings, radomes, and wing tips. Fiberglass is also used for helicopter
rotor blades. There are several types of fiberglass used in the aviation
industry. Electrical glass, or E-glass, is identified as such for electrical
applications. It has high resistance to current flow. E-glass is made from
borosilicate glass. S-glass and S2-glass identify structural fiberglass that
have a higher strength than E-glass. S-glass is produced from magnesia-
alumina-silicate. Advantages of fiberglass are lower cost than other
composite materials, chemical or galvanic corrosion resistance, and
electrical properties (fiberglass does not conduct electricity). Fiberglass
has a white color and is available as a dry fiber fabric or prepreg material.
Kevlar is DuPont’s name for aramid fibers. Aramid fibers are light
weight, strong, and tough. Two types of Aramid fiber are used in the
aviation industry. Kevlar 49 has a high stiffness and Kevlar® 29 has a
low stiffness. An advantage of aramid fibers is their high resistance to
impact damage, so they are often used in areas prone to impact damage.
The main disadvantage of aramid fibers is their general weakness in
compression and hygroscopy. Service reports have indicated that some
parts made from Kevlar® absorb up to 8 percent of their weight in water.
Therefore, parts made from aramid fibers need to be protected from the
environment. Another disadvantage is that Kevlar® is difficult to drill
and cut. The fibers fuzz easily and special scissors are needed to cut the
material. Kevlar is often used for military ballistic and body armor
applications. It has a natural yellow color and is available as dry fabric
and prepreg material. Bundles of aramid fibers are not sized by the
number of fibers like carbon or fiberglass but by the weight.
One of the first distinctions to be made among fibers is the difference
between carbon and graphite fibers, although the terms are frequently
used interchangeably. Carbon and graphite fibers are based on graphene
(hexagonal) layer networks present in carbon. If the graphene layers, or
planes, are stacked with three dimensional order, the material is defined
as graphite. Usually extended time and temperature processing is required
to form this order, making graphite fibers more expensive. Bonding
between planes is weak. Disorder frequently occurs such that only two-
dimensional ordering within the layers is present. This material is defined
as carbon. Carbon fibers are very stiff and strong, 3 to 10 times stiffer
than glass fibers. Carbon fiber is used for structural aircraft applications,
such as floor beams, stabilizers, flight controls, and primary fuselage and
wing structure. Advantages include its high strength and corrosion
resistance. Disadvantages include lower conductivity than aluminum;
therefore, a lightning protection mesh or coating is necessary for aircraft
parts that are prone to lightning strikes. Another disadvantage of carbon
fiber is its high cost. Carbon fiber is gray or black in color and is
available as dry fabric and prepreg material. Carbon fibers have a high
potential for causing galvanic corrosion when used with metallic
fasteners and structures.
Boron fibers are very stiff and have a high tensile and compressive
strength. The fibers have a relatively large diameter and do not flex well;
therefore, they are available only as a prepreg tape product. An epoxy
matrix is often used with the boron fiber. Boron fibers are used to repair
cracked aluminum aircraft skins, because the thermal expansion of boron
is close to aluminum and there is no galvanic corrosion potential. The
boron fiber is difficult to use if the parent material surface has a
contoured shape. The boron fibers are very expensive and can be
hazardous for personnel. Boron fibers are used primarily in military
5. Ceramic Fibers
Ceramic fibers are used for high-temperature applications, such as turbine
blades in a gas turbine engine. The ceramic fibers can be used to
temperatures up to 2,200 °F.
6. Lightning Protection Fibers
An aluminum airplane is quite conductive and is able to dissipate the high
currents resulting from a lightning strike. Carbon fibers are 1,000 times
more resistive than aluminum to current flow, and epoxy resin is
1,000,000 times more resistive (i.e., perpendicular to the skin). The
surface of an external composite component often consists of a ply or
layer of conductive material for lightning strike protection because
composite materials are less conductive than aluminum. Many different
types of conductive materials are used ranging from nickel-coated
graphite cloth to metal meshes to aluminized fiberglass to conductive
paints. The materials are available for wet layup and as prepreg. In
addition to a normal structural repair, the technician must also recreate
the electrical conductivity designed into the part. These types of repair
generally require a conductivity test to be performed with an ohmmeter to
verify minimum electrical resistance across the structure. When repairing
these types of structures, it is extremely important to use only the
approved materials from authorized vendors, including such items as
potting compounds, sealants, adhesives, and so forth.
COMPOSITE STRUCTURE TYPES
1. SANDWICH DESIGN
Basically, the sandwich design consists of two approximately parallel thin
skins with a thick core between. The sandwich design is mainly used to
provide a great strength in bending. Core materials can be wood,
Aramid/Phenolic or Glass/Phenolic honeycomb, metal honeycomb or
foam materials. On Airbus products mainly Aramid/Phenolic,
Glass/Phenolic, or metal honeycombs are used.
Two main processes are used to manufacture sandwich parts:
The two skins are laid-up and cured in a first step. Then the final
sandwich part is manufactured in a second step, using adhesive to bond
the skins to the core.
The skins are cured in a single operation together with the core.
1. MONOLITHIC DESIGN
Basically the monolithic design consists of a composite material made of
a continuous lamination of tapes or fabrics without any core material.
The construction of these elements can be made by:
One-Shot Bonding Technique:
Different elements like skins, stringers or stiffeners are cured in one shot.
To get self-stiffened panels the modular technique is used (for example
Different elements are cured and bonded in several steps.
DAMAGES AND DEFECTS IN COMPOSITE MATERIALS
A delamination is a separation between different plies. This can be caused
by an impact, or when there is a resin (bonding) failure for any other
A debonding is a separation between different materials or parts of a
component. This could be, for example, in a sandwich, a separation
between skin and core material or on a monolythic part the separation
between skin and stringer/stiffener. A debonding can be caused by an
A scratch is linear damage of any depth and length caused by contact
with a sharp object.
A gauge is wider and might be deeper than a scratch. It is usually caused
by contact with a sharp object which causes a continuous, sharp or
smooth channel, like a groove, in the material
Liquid ingress is mainly caused by damage (cracks) of the skin and/or a
failure in the surface protection of the sandwich components. Liquid
ingress (for example, water or skydrol) can cause further damage like
An abrasion is the wearing a way of surface material caused by contact
with other surfaces.
Erosion is the wearing away of the material surface during flight by wind,
Lightning strikes can cause burn marks on composite materials.
Depending on the intensity, delamination and disbondings are also
Chemicals like paint strippers might cause a degradation on composite
Visual damage mainly caused by impact. Impacts could occur during
flight (bird impact, hail) or during ground handling and maintenance (for
example, collision with ground equipment or dropped tools).
Impacts can cause a complete perforation of a monolithic or sandwich
component. On sandwich components, the component may only be partly
perforated. In this case only one skin is damaged and the other remains
Light impacts can cause dents or, on sandwich parts, a depression as
visual damage. Very often the visual damage is combined with a
delamination or a debonding.
MMC SYSTEMS-Metal Matrix Sys
A metal matrix composite system is generally designated simply by the
metal alloy designation of the matrix and the material type, volume
fraction and form of the ceramic reinforcement.
MMCs differ from other composite materials in several ways. Some of
these general distinctions are as follows:
1. The matrix phase of an MMC is either a pure or alloy metal as
opposed to a polymer or ceramic.
2. MMCs evidence higher ductility and toughness than ceramics or
CMCs, although they have lower ductility and toughness than their
respective unreinforced metal matrix alloys.
3. The role of the reinforcement in MMCs is to increase strength and
modulus as is the case with PMCs. Reinforcement in CMCs is
generally to provide improved damage tolerance.
4. MMCs have a temperature capability generally higher than
polymers and PMCs but less than ceramics and CMCs.
5. Low to moderately reinforced MMCs are formable by processes
normally associated with unreinforced metals.
The choice of a matrix alloy for an MMC is dictated by several
considerations. Of particular importance is whether the composite is to be
continuously or discontinuously reinforced. The use of continuous fibers
as reinforcements may result in transfer of most of the load to the
reinforcing filaments and hence composite strength will be governed
primarily by the fiber strength. The primary roles of the matrix alloy, then
are to provide efficient transfer of load to the fibers and to blunt cracks in
the event that fiber failure occurs and so the matrix alloy for a
continuously reinforced MMC may be chosen more for toughness than
for strength. On this basis, lower strength, more ductile, and tougher
matrix alloys may be utilized in continuously reinforced MMCs. For
discontinuously reinforced MMCs, the matrix may govern composite
strength. Then, the choice of matrix will be influenced by consideration
of the required composite strength and higher strength matrix alloys may
Additional considerations in the choice of the matrix include potential
reinforcement/matrix reactions, either during processing or in service,
that might result in degraded composite performance; thermal stresses
due to thermal expansion mismatch between the reinforcements and the
matrix; and the influence of matrix fatigue behavior on the cyclic
response of the composite. Indeed, the behavior of MMCs under cyclic
loading conditions is an area requiring special consideration. In MMCs
intended for use at elevated temperatures, an additional consideration is
the difference in melting temperatures between the matrix and the
reinforcements. A large melting temperature difference may result in
matrix creep while the reinforcements remain elastic, even at
temperatures approaching the matrix melting point. However, creep in
both the matrix and reinforcement must be considered when there is a
small melting point difference in the composite.
Many different metals have been employed in MMCs and the choice of
matrix material provides the basis for further classification of these
composites. Alloy systems such as aluminum, copper, iron (steels),
magnesium, nickel, and titanium.
Reinforcements can be divided into two major groups: (a) particulates or
whiskers; and (b) fibers. Fiber reinforcements can be further divided into
continuous and discontinuous. Fibers enhance strength in the direction of
Most often reinforcement materials for MMCs are ceramics (oxides,
carbides, nitrides, etc.) which are characterized by their high strength and
stiffness both at ambient and elevated temperatures. Examples of
common MMC reinforcements are SiC, Al2O3, TiB2, B4C, and graphite.
Metallic reinforcements are used less frequently.
In many MMCs, it is necessary to apply a thin coating on the
reinforcements prior to their incorporation into the metal matrix.
In general, coatings on the fibers offer the following advantages:
1. Protection of fiber from reaction and diffusion with the matrix by
serving as a diffusion barrier
2. Prevention of direct fiber-fiber contact
3. Promotion of wetting and bonding between the fiber and the matrix
4. Relief of thermal stresses or strain concentrations between the fiber
and the matrix
5. Protection of fiber during handling
In some instances particulates are coated to enhance composite
processing by enhancing wetting and reducing interfacial reactions.
Choice of the primary manufacturing process for the fabrication of any
MMC is dictated by many factors, the most important of which are:
1. Preservation of reinforcement strength
2. Minimization of reinforcement damage
3. Promotion of wetting and bonding between the matrix and
4. Flexibility that allows proper backing, spacing and orientation of
the reinforcements within the matrix
These primary industrial manufacturing processes can be classified into
liquid phase and solid state processes. Liquid phase processing is
characterized by intimate interfacial contact and hence strong bonding,
but can lead to the formation of a brittle interfacial layer. Solid state
processes include powder blending followed by consolidation, diffusion
bonding and vapor deposition. Liquid phase processes include squeeze
casting and squeeze infiltration, spray deposition, slurry casting
(compocasting), and reactive processing (insitu composites).
MMC joining is not yet a mature technology and many important details
are still being developed. Therefore, the applicability of a specific MMC
joining method depends on the types of MMC materials being joined.
They are classified to:
1. Solid State Processes
Inertia Friction Welding
Friction Stir Welding
2. Fusion Processes
Laser Beam Welding
Electron Beam Welding
Gas Metal Arc Welding
Gas Tungsten Arc Welding
Resistance Spot Welding
Capacitor Discharge Welding
3. Other Processes
Transient Liquid Phase
Rapid Infrared Joining
After decades of high-tech developments of artificial fibers like aramid,
carbon and glass it is remarkable that natural fibers have gained a
renewed interest, especially as a glass fiber substitute in automotive
industries. New environmental regulations and societal concern have
triggered the search for new products and processes that are compatible to
the environment. The incorporation of bio-resources in to composite
materials can reduce further dependency of petroleum reserves. The
major limitations of present biopolymers are their high cost. Again
renewable resource based bio-plastics are currently being developed and
need to be researched more to overcome the performance limitations.
Bio-composites can supplement and eventually replace petroleum based
composite materials in several applications thus offering new agricultural,
environmental, manufacturing and consumer benefits. The main
advantage of using renewable materials is that the global CO2 balance is
kept at a stable level.
Classification of Bio-composites
Classification of Bio-fibers
Among all the matrix polymers; polypropylene (PP) has attained much
commercial success in bio-composites for automotive applications.
Although unsaturated polyester resin can be used in bio-composite
applications commercially, the non-recyclable nature of this thermoset
resin over thermoplastic recyclable PP is hindering its growing
SMART COMPOSITE MATERIALS
Smart material systems often consist of mixtures of several different
passive and active materials. Mixing the constituent materials in the right
way makes it possible to make new smart composites with properties
beyond those of the individual constituents.
Intrinsically smart structural composites are multifunctional structural
materials which can perform functions such as sensing strain, stress,
damage or temperature; thermoelectric energy generation; EMI shielding;
electric current rectification; and vibration reduction. These capabilities
are rendered by the use of materials science concepts to enhance
functionality without compromising structural properties. They are not
achieved by the embedding of devices in the structure. Intrinsically smart
structural composites have been attained in cement-matrix composites
containing short electrically conducting fibers and in polymer-matrix
composites with continuous carbon fibers. Cement-matrix composites are
important for infrastructure, while polymer-matrix composites are useful
for lightweight structures.
Smart structures have the ability to sense certain stimuli and respond in
an appropriate fashion, somewhat like a human being. Sensing is the most
fundamental aspect of a smart structure. A structural composite which is
itself a sensor is said to be self-sensing. It is multifunctional.
There are: cement-matrix and polymer-matrix composites
Cement-matrix composites include concrete (containing coarse and fine
aggregates), mortar (containing fine but no coarse aggregate), and cement
paste (containing no aggregate, neither coarse nor fine). Other fillers,
called admixtures, can be added to the mix to improve the properties of
the composite. Admixtures are discontinuous, so that they can be
included in the mix. They can be particles, such as silica fume (a fine
particulate) or latex (a polymer in the form of a dispersion). Short fibers,
such as polymer, steel glass or carbon fibers, and liquids, such as
methylcellulose aqueous solution, water reducing agents or defoamers,
can be used.
Polymer-matrix composites for structural applications typically contain
continuous carbon, polymer, or glass fibers, as continuous fibers tend to
be more effective than short fibers as reinforcement. Polymer-matrix
composites with continuous carbon fibers are used for aerospace,
automobile and civil structures. (In contrast, continuous fibers are too
expensive for reinforcing concrete.) The fact that carbon fibers are
electrically conducting, whereas polymer and glass fibers are not, means
that carbon fiber composites are predominant among polymer-matrix
composites that are intrinsically smart.