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Historical Development / Historical overview:
Past:
After making and controlling fire and inventing the wheel, spinning of continuous yarns is
probably the most important development of mankind, enabling him to survive outside the
tropical climate zones and spread across the surface of the Earth. Flexible fabrics made of locally
grown and spun fibres as cotton; flax and jute were a big step forward compared to animal skins.
More and more natural resources were used, soon resulting in the first composites; straw
reinforced walls, and bows (Figure M1.1.1 (a)) and chariots made of glued layers of wood, bone
and horn. More durable materials as wood and metal soon replaced these antique composites.
Present:
Originating from early agricultural societies and being almost forgotten after centuries, a true
revival started of using lightweight composite structures for many technical solutions during the
second half of the 20th century. After being solely used for their electromagnetic properties
(insulators and radar-domes), using composites to improve the structural performance of
spacecraft and military aircraft became popular in the last two decades of the previous century.
First at any costs, with development of improved materials with increasing costs, nowadays cost
reduction during manufacturing and operation are the main technology drivers. Latest
development is the use of composites to protect man against fire and impact (Figure M1.1.1 (b))
and a tendency to a more environmental friendly design, leading to the reintroduction of natural
fibres in the composite technology, see Figure M1.1.1 ©. Increasingly nowadays, the success of
composites in applications, by volume and by numbers, can be ranked by accessibility and
reproducibility of the applied manufacturing techniques. Some examples of use of natural fibers
Future:
In future, composites will be manufactured even more according to an integrated design process
resulting in the optimum construction according to parameters such as shape, mass, strength,
stiffness, durability, costs, etc. Newly developed design tools must be able to instantaneously
show customers the influence of a design change on each one of these parameters.
Concept of Composite:
Fibers or particles embedded in matrix of another material are the best example of modern-day
composite materials, which are mostly structural.
Laminates are composite material where different layers of materials give them the specific
character of a composite material having a specific function to perform. Fabrics have no matrix
to fall back on, but in them, fibers of different compositions combine to give them a specific
character. Reinforcing materials generally withstand maximum load and serve the desirable
properties.
Further, though composite types are often distinguishable from one another, no clear
determination can be really made. To facilitate definition, the accent is often shifted to the levels
at which differentiation take place viz., microscopic or macroscopic.
In matrix-based structural composites, the matrix serves two paramount purposes viz., binding
the reinforcement phases in place and deforming to distribute the stresses among the
constituent reinforcement materials under an applied force.
The demands on matrices are many. They may need to temperature variations, be conductors or
resistors of electricity, have moisture sensitivity etc. This may offer weight advantages, ease of
handling and other merits which may also become applicable depending on the purpose for
which matrices are chosen.
Solids that accommodate stress to incorporate other constituents provide strong bonds for the
reinforcing phase are potential matrix materials. A few inorganic materials, polymers and
metals have found applications as matrix materials in the designing of structural composites,
with commendable success. These materials remain elastic till failure occurs and show decreased
failure strain, when loaded in tension and compression.
Composites cannot be made from constituents with divergent linear expansion characteristics.
The interface is the area of contact between the reinforcement and the matrix materials. In some
cases, the region is a distinct added phase. Whenever there is interphase, there has to be two
interphases between each side of the interphase and its adjoint constituent. Some composites
provide interphases when surfaces dissimilar constituents interact with each other. Choice of
fabrication method depends on matrix properties and the effect of matrix on properties of
reinforcements. One of the prime considerations in the selection and fabrication of composites is
that the constituents should be chemically inert non-reactive. Figure M1.1.1 (f) helps to classify
matrices.
M1.2.1 Classification of Composites
Composite materials are commonly classified at following two distinct levels:
• The first level of classification is usually made with respect to the matrix constituent. The
major composite classes include Organic Matrix Composites (OMCs), Metal Matrix
Composites (MMCs) and Ceramic Matrix Composites (CMCs). The term organic matrix
composite is generally assumed to include two classes of composites, namely Polymer
Matrix Composites (PMCs) and carbon matrix composites commonly referred to as carboncarbon
composites.
• The second level of classification refers to the reinforcement form - fibre reinforced
composites, laminar composites and particulate composites. Fibre Reinforced composites
(FRP) can be further divided into those containing discontinuous or continuous fibres.
• Fibre Reinforced Composites are composed of fibres embedded in matrix material. Such a
composite is considered to be a discontinuous fibre or short fibre composite if its properties
vary with fibre length. On the other hand, when the length of the fibre is such that any further
increase in length does not further increase, the elastic modulus of the composite, the
composite is considered to be continuous fibre reinforced. Fibres are small in diameter and
when pushed axially, they bend easily although they have very good tensile properties. These
fibres must be supported to keep individual fibres from bending and buckling.
• Laminar Composites are composed of layers of materials held together by matrix.
Sandwich structures fall under this category.
• Particulate Composites are composed of particles distributed or embedded in a matrix body.
The particles may be flakes or in powder form. Concrete and wood particle boards are
examples of this category.
M1.2.2 Organic Matrix Composites
M1.2.2.1 Polymer Matrix Composites (PMC)/Carbon Matrix Composites or CarbonCarbon
Composites
Polymers make ideal materials as they can be processed easily, possess lightweight, and
desirable mechanical properties. It follows, therefore, that high temperature resins are
extensively used in aeronautical applications.
Two main kinds of polymers are thermosets and thermoplastics. Thermosets have qualities
such as a well-bonded three-dimensional molecular structure after curing. They decompose
instead of melting on hardening. Merely changing the basic composition of the resin is enough to
alter the conditions suitably for curing and determine its other characteristics. They can be
retained in a partially cured condition too over prolonged periods of time, rendering Thermosets
very flexible. Thus, they are most suited as matrix bases for advanced conditions fiber reinforced
composites. Thermosets find wide ranging applications in the chopped fiber composites form
particularly when a premixed or moulding compound with fibers of specific quality and aspect
ratio happens to be starting material as in epoxy, polymer and phenolic polyamide resins.
Thermoplastics have one- or two-dimensional molecular structure and they tend to at an elevated
temperature and show exaggerated melting point. Another advantage is that the process of
softening at elevated temperatures can reversed to regain its properties during cooling,
facilitating applications of conventional compress techniques to mould the compounds.
Resins reinforced with thermoplastics now comprised an emerging group of composites. The
theme of most experiments in this area to improve the base properties of the resins and extract
the greatest functional advantages from them in new avenues, including attempts to replace
metals in die-casting processes. In crystalline thermoplastics, the reinforcement affects the
morphology to a considerable extent, prompting the reinforcement to empower nucleation.
Whenever crystalline or amorphous, these resins possess the facility to alter their creep over an
extensive range of temperature. But this range includes the point at which the usage of resins is
constrained, and the reinforcement in such systems can increase the failure load as well as creep
resistance. Figure M1.2.1 shows kinds of thermoplastics.
A small quantum of shrinkage and the tendency of the shape to retain its original form are also to
be accounted for. But reinforcements can change this condition too. The advantage of
thermoplastics systems over thermosets are that there are no chemical reactions involved, which
often result in the release of gases or heat. Manufacturing is limited by the time required for
heating, shaping and cooling the structures.
Thermoplastics resins are sold as moulding compounds. Fiber reinforcement is apt for these
resins. Since the fibers are randomly dispersed, the reinforcement will be almost isotropic.
However, when subjected to moulding processes, they can be aligned directionally.
There are a few options to increase heat resistance in thermoplastics. Addition of fillers raises the
heat resistance. But all thermoplastic composites tend loose their strength at elevated
temperatures. However, their redeeming qualities like rigidity, toughness and ability to
repudiate creep, place thermoplastics in the important composite materials bracket. They are
used in automotive control panels, electronic products encasement etc.
Newer developments augur the broadening of the scope of applications of thermoplastics. Huge
sheets of reinforced thermoplastics are now available and they only require sampling and heating
to be moulded into the required shapes. This has facilitated easy fabrication of bulky
components, doing away with the more cumbersome moulding compounds.
Thermosets are the most popular of the fiber composite matrices without which, research and
development in structural engineering field could get truncated. Aerospace components,
automobile parts, defense systems etc., use a great deal of this type of fiber composites. Epoxy
matrix materials are used in printed circuit boards and similar areas. Figure M1.2.2 shows some
kinds of thermosets.
Direct condensation polymerization followed by rearrangement reactions to form heterocyclic
entities is the method generally used to produce thermoset resins. Water, a product of the
reaction, in both methods, hinders production of void-free composites. These voids have a
negative effect on properties of the composites in terms of strength and dielectric properties.
Polyesters phenolic and Epoxies are the two important classes of thermoset resins.
Epoxy resins are widely used in filament-wound composites and are suitable for moulding
prepress. They are reasonably stable to chemical attacks and are excellent adherents having
slow shrinkage during curing and no emission of volatile gases. These advantages, however,
make the use of epoxies rather expensive. Also, they cannot be expected beyond a temperature of
140ºC. Their use in high technology areas where service temperatures are higher, as a result, is
ruled out.
Polyester resins on the other hand are quite easily accessible, cheap and find use in a wide range
of fields. Liquid polyesters are stored at room temperature for months, sometimes for years and
the mere addition of a catalyst can cure the matrix material within a short time. They are used in
automobile and structural applications.
The cured polyester is usually rigid or flexible as the case may be and transparent. Polyesters
withstand the variations of environment and stable against chemicals. Depending on the
formulation of the resin or service requirement of application, they can be used up to about 75ºC
or higher. Other advantages of polyesters include easy compatibility with few glass fibers and
can be used with verify of reinforced plastic accoutrey.
Aromatic Polyamides are the most sought after candidates as the matrices of advanced fiber
composites for structural applications demanding long duration exposure for continuous service
at around 200-250ºC .
M1.2.2.2 Metal Matrix Composites (MMC)
Metal matrix composites, at present though generating a wide interest in research fraternity, are
not as widely in use as their plastic counterparts. High strength, fracture toughness and
stiffness are offered by metal matrices than those offered by their polymer counterparts. They
can withstand elevated temperature in corrosive environment than polymer composites. Most
metals and alloys could be used as matrices and they require reinforcement materials which need
to be stable over a range of temperature and non-reactive too. However the guiding aspect for the
choice depends essentially on the matrix material. Light metals form the matrix for temperature
application and the reinforcements in addition to the aforementioned reasons are characterized by
high moduli.
Most metals and alloys make good matrices. However, practically, the choices for low
temperature applications are not many. Only light metals are responsive, with their low density
proving an advantage. Titanium, Aluminium and magnesium are the popular matrix metals
currently in vogue, which are particularly useful for aircraft applications. If metallic matrix
materials have to offer high strength, they require high modulus reinforcements. The strength-toweight
ratios of resulting composites can be higher than most alloys.
The melting point, physical and mechanical properties of the composite at various temperatures
determine the service temperature of composites. Most metals, ceramics and compounds can be
used with matrices of low melting point alloys. The choice of reinforcements becomes more
stunted with increase in the melting temperature of matrix materials.
M1.2.2.3 Ceramic Matrix Materials (CMM)
Ceramics can be described as solid materials which exhibit very strong ionic bonding in general
and in few cases covalent bonding. High melting points, good corrosion resistance, stability at
elevated temperatures and high compressive strength, render ceramic-based matrix materials a
favourite for applications requiring a structural material that doesn’t give way at temperatures
above 1500ºC. Naturally, ceramic matrices are the obvious choice for high temperature
applications.
High modulus of elasticity and low tensile strain, which most ceramics posses, have combined
to cause the failure of attempts to add reinforcements to obtain strength improvement. This is
because at the stress levels at which ceramics rupture, there is insufficient elongation of the
matrix which keeps composite from transferring an effective quantum of load to the
reinforcement and the composite may fail unless the percentage of fiber volume is high enough.
A material is reinforcement to utilize the higher tensile strength of the fiber, to produce an
increase in load bearing capacity of the matrix. Addition of high-strength fiber to a weaker
ceramic has not always been successful and often the resultant composite has proved to be
weaker.
The use of reinforcement with high modulus of elasticity may take care of the problem to some
extent and presents pre-stressing of the fiber in the ceramic matrix is being increasingly resorted
to as an option.
When ceramics have a higher thermal expansion coefficient than reinforcement materials, the
resultant composite is unlikely to have a superior level of strength. In that case, the composite
will develop strength within ceramic at the time of cooling resulting in microcracks extending
from fiber to fiber within the matrix. Microcracking can result in a composite with tensile
strength lower than that of the matrix.
M1.2.3 Classification Based on Reinforcements
M1.2.3: Introduction to Reinforcements
Reinforcements for the composites can be fibers, fabrics particles or whiskers. Fibers are
essentially characterized by one very long axis with other two axes either often circular or near
circular. Particles have no preferred orientation and so does their shape. Whiskers have a
preferred shape but are small both in diameter and length as compared to fibers. Figure M1.2.3
shows types of reinforcements in composites.
Reinforcing constituents in composites, as the word indicates, provide the strength that makes
the composite what it is. But they also serve certain additional purposes of heat resistance or
conduction, resistance to corrosion and provide rigidity. Reinforcement can be made to perform
all or one of these functions as per the requirements.
A reinforcement that embellishes the matrix strength must be stronger and stiffer than the matrix
and capable of changing failure mechanism to the advantage of the composite. This means that
the ductility should be minimal or even nil the composite must behave as brittle as possible.
M1.2.3.1 Fiber Reinforced Composites/Fibre Reinforced Polymer (FRP) Composites
Fibers are the important class of reinforcements, as they satisfy the desired conditions and
transfer strength to the matrix constituent influencing and enhancing their properties as desired.
Glass fibers are the earliest known fibers used to reinforce materials. Ceramic and metal fibers
were subsequently found out and put to extensive use, to render composites stiffer more resistant
to heat.
Fibers fall short of ideal performance due to several factors. The performance of a fiber
composite is judged by its length, shape, orientation, and composition of the fibers and the
mechanical properties of the matrix.
The orientation of the fiber in the matrix is an indication of the strength of the composite and the
strength is greatest along the longitudinal directional of fiber. This doesn’t mean the longitudinal
fibers can take the same quantum of load irrespective of the direction in which it is applied.
Optimum performance from longitudinal fibers can be obtained if the load is applied along its
direction. The slightest shift in the angle of loading may drastically reduce the strength of the
composite.
Unidirectional loading is found in few structures and hence it is prudent to give a mix of
orientations for fibers in composites particularly where the load is expected to be the heaviest.
Monolayer tapes consisting of continuous or discontinuous fibers can be oriented unidirectional
stacked into plies containing layers of filaments also oriented in the same direction. More
complicated orientations are possible too and nowadays, computers are used to make projections
of such variations to suit specific needs. In short, in planar composites, strength can be changed
from unidirectional fiber oriented composites that result in composites with nearly isotropic
properties.
Properties of angle-plied composites which are not quasi-isotropic may vary with the number
of plies and their orientations. Composite variables in such composites are assumed to have a
constant ratio and the matrices are considered relatively weaker than the fibers. The strength of
the fiber in any one of the three axes would, therefore be one-third the unidirectional fiber
composite, assuming that the volume percentage is equal in all three axes.
However, orientation of short fibers by different methods is also possible like random
orientations by sprinkling on to given plane or addition of matrix in liquid or solid state before or
after the fiber deposition. Even three-dimensional orientations can achieve in this way.
There are several methods of random fiber orientations, which in a two-dimensional one, yield
composites with one-third the strength of a unidirectional fiber-stressed composite, in the
direction of fibers. In a 3-dimension, it would result in a composite with a comparable ratio,
about less than one-fifth.
In very strong matrices, moduli and strengths have not been observed. Application of the
strength of the composites with such matrices and several orientations is also possible. The
longitudinal strength can be calculated on the basis of the assumption that fibers have been
reduced to their effective strength on approximation value in composites with strong matrices
and non-longitudinally orientated fibers.
It goes without saying that fiber composites may be constructed with either continuous or short
fibers. Experience has shown that continuous fibers (or filaments) exhibit better orientation,
although it does not reflect in their performance. Fibers have a high aspect ratio, i.e., their
lengths being several times greater than their effective diameters. This is the reason why
filaments are manufactured using continuous process. This finished filaments.
Mass production of filaments is well known and they match with several matrices in different
ways like winding, twisting, weaving and knitting, which exhibit the characteristics of a fabric.
Since they have low densities and high strengths, the fiber lengths in filaments or other fibers
yield considerable influence on the mechanical properties as well as the response of composites
to processing and procedures. Shorter fibers with proper orientation composites that use glass,
ceramic or multi-purpose fibers can be endowed with considerably higher strength than those
that use continuous fibers. Short fibers are also known to their theoretical strength. The
continuous fiber constituent of a composite is often joined by the filament winding process in
which the matrix impregnated fiber wrapped around a mandrel shaped like the part over which
the composite is to be placed, and equitable load distribution and favorable orientation of the
fiber is possible in the finished product. However, winding is mostly confined to fabrication of
bodies of revolution and the occasional irregular, flat surface.
Short-length fibers incorporated by the open- or close-mould process are found to be less
efficient, although the input costs are considerably lower than filament winding.
Most fibers in use currently are solids which are easy to produce and handle, having a circular
cross-section, although a few non-conventional shaped and hollow fibers show signs of
capabilities that can improve the mechanical qualities of the composites.
Given the fact that the vast difference in length and effective diameter of the fiber are assets to a
fiber composite, it follows that greater strength in the fiber can be achieved by smaller diameters
due to minimization or total elimination of surface of surface defects.
After flat-thin filaments came into vogue, fibers rectangular cross sections have provided new
options for applications in high strength structures. Owing to their shapes, these fibers provide
perfect packing, while hollow fibers show better structural efficiency in composites that are
desired for their stiffness and compressive strengths. In hollow fibers, the transverse
compressive strength is lower than that of a solid fiber composite whenever the hollow portion
is more than half the total fiber diameter. However, they are not easy to handle and fabricate.
M1.2.3.2 Laminar Composites
Laminar composites are found in as many combinations as the number of materials. They can
be described as materials comprising of layers of materials bonded together. These may be of
several layers of two or more metal materials occurring alternately or in a determined order more
than once, and in as many numbers as required for a specific purpose.
Clad and sandwich laminates have many areas as it ought to be, although they are known to
follow the rule of mixtures from the modulus and strength point of view. Other intrinsic values
pertaining to metal-matrix, metal-reinforced composites are also fairly well known.
Powder metallurgical processes like roll bonding, hot pressing, diffusion bonding, brazing and
so on can be employed for the fabrication of different alloys of sheet, foil, powder or sprayed
materials. It is not possible to achieve high strength materials unlike the fiber version. But sheets
and foils can be made isotropic in two dimensions more easily than fibers. Foils and sheets are
also made to exhibit high percentages of which they are put. For instance, a strong sheet may use
over 92% in laminar structure, while it is difficult to make fibers of such compositions. Fiber
laminates cannot over 75% strong fibers.
The main functional types of metal-metal laminates that do not posses high strength or stiffness
are single layered ones that endow the composites with special properties, apart from being costeffective.
They are usually made by pre-coating or cladding methods.
Pre-coated metals are formed by forming by forming a layer on a substrate, in the form of a thin
continuous film. This is achieved by hot dipping and occasionally by chemical plating and
electroplating. Clad metals are found to be suitable for more intensive environments where
denser faces are required.
There are many combinations of sheet and foil which function as adhesives at low temperatures.
Such materials, plastics or metals, may be clubbed together with a third constituent. Pre-painted
or pre-finished metal whose primary advantage is elimination of final finishing by the user is the
best known metal-organic laminate. Several combinations of metal-plastic, vinyl-metal
laminates, organic films and metals, account for upto 95% of metal-plastic laminates known.
They are made by adhesive bonding processes.
M1.2.3.3 Particulate Reinforced Composites (PRC)
Microstructures of metal and ceramics composites, which show particles of one phase strewn in
the other, are known as particle reinforced composites. Square, triangular and round shapes of
reinforcement are known, but the dimensions of all their sides are observed to be more or less
equal. The size and volume concentration of the dispersoid distinguishes it from dispersion
hardened materials.
The dispersed size in particulate composites is of the order of a few microns and volume
concentration is greater than 28%. The difference between particulate composite and dispersion
strengthened ones is, thus, oblivious. The mechanism used to strengthen each of them is also
different. The dispersed in the dispersion-strengthen materials reinforces the matrix alloy by
arresting motion of dislocations and needs large forces to fracture the restriction created by
dispersion.
In particulate composites, the particles strengthen the system by the hydrostatic coercion of
fillers in matrices and by their hardness relative to the matrix.
Three-dimensional reinforcement in composites offers isotropic properties, because of the three
systematical orthogonal planes. Since it is not homogeneous, the material properties acquire
sensitivity to the constituent properties, as well as the interfacial properties and geometric
shapes of the array. The composite’s strength usually depends on the diameter of the particles,
the inter-particle spacing, and the volume fraction of the reinforcement. The matrix properties
influence the behaviour of particulate composite too.