Material Science : Chapter 7: Composites

Environmental Pollution and Degradation
June 28, 2017
Environmental Impact Assessment
June 30, 2017
  1. Introduction

          1.1 History

  1. What are composites?

          2.1 Constituent phases

          2.2 Constituent conditions

          2.3 Composite properties

  1. Classification of Composites

         3.1 Based on size and shape of reinforcement

             3.1.1 Particle-reinforced composites

                 a) Dispersion strengthened

                 b) Large particle reinforced

             3.1.2 Fiber-Reinforced Composites (FRC)

        Influence of fiber length on composite strength

        Influence of fiber orientation on composite strength

                     a) Continuous fiber composites

                     b) Discontinuous and aligned fiber composites

                     c) Discontinuous and randomly oriented fiber composites

             3.1.3 Structural Composites

       Laminar composites

       Sandwich structures

        3.2 Based on type of matrix material

           3.2.1 Metal matrix composites

           3.2.2 Ceramics matrix composites

           3.2.3 Polymer matrix composites

               a) Carbon Fiber-Reinforced Polymer(CFRP) composites

               b) Glass Fiber-Reinforced Polymer(GFRP) composites

               c) Aramid Fiber-Reinforced Polymer(AFRP) composites

     4. Other important composites

        4.1 Carbon-Carbon composites

           4.2 Hybrid composites

  1. FAQs


1. Introduction 

With the emergence of technologies, there is a need for materials with special properties. Because, conventional materials are unable to meet these special properties like high strength and low density materials for the applications such as aircraft parts.

Composites are the new class of materials. These are multiphase materials which can be artificially made to get desired properties.

1.1 History

  • The use of natural composite materials has been a part of man’s technology since the first ancient builder, who used straw to reinforce mud bricks.
  • The 12th century Mongols made the advanced weapons of their day with archery bows consisting of composites structures made by combining cattle tendons, horn, bamboo and silk which bonded with natural pine resin.
  • In the years between 1870 and 1890, development of first synthetic resin revolutionized the composites usage, which would be used to convert a liquid to a solid by polymerization.

Now, this versatile material system has become a part of everyday life.

2. What are composites?

  • Composites are two or more materials with markedly different physical or chemical properties – categorized as “matrix” or “reinforcement” – combined in a way that they act in concert, yet remain separate and distinct at some level because they don’t fully merge or dissolve into one another
  • So composites are generally combination of two distinct engineering materials.

2.1 Constituent phases in composites

Matrix Dispersed phase
  • Continuous and surrounds the other phase, which is a dispersed phase.
  • This is inside of the matrix
  • Generally it carry most of the loading
  • Reinforcing material to strengthen overall combination

2.2 Conditions for constituents

  • These constituent phases differ in chemical composition.
  • Essential condition is insolubility in each other.

2.3 Composite properties depends on

  • Relative amounts of constituent phases
  • Geometry of the dispersed phase which means
    • Shape of the particles and the particle size,
    • Distribution and
    • Orientation

3. Classification of composites

Millions of material combinations are possible because of the combination but again to simplify complexity on our brain, they are classified based on two criteria

Type of Matrix material Size and shape of dispersed phase
Metal matrix composites Particle reinforced composites
Polymer matrix composites Fiber reinforced composites
Ceramic matrix composites Structural composites



Composite class based on size and shape of dispersed phase further divided as below

Why to study different composite types?

  • Properties of composite materials are nothing but improved version of properties of constituent.
  • So if we can understand the properties of constituent phases then it’s easy to understand the resulting properties of the composite.

3.1 Composites based on size and shape of reinforced

These dispersed or the reinforced phase inside matrix material is any size like particle, fiber as below.

3.1.1 Particle-reinforced composites

  • These materials availability is more and cheap and thus widely used composites

They are again two kinds of composites based upon reinforcement or strengthening mechanism

  1.     a) Dispersion-strengthened
  2.     b) Particulate-reinforced or large particle composites.
  • The term “large” is used to indicate that particle–matrix interactions cannot be treated on the atomic or molecular level

Load interaction

  • These reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle.
  • In essence, the matrix transfers some of the applied stress to the particles, which bear a fraction of the load.
  • The degree of reinforcement or improvement of mechanical behavior depends on strong bonding at the matrix–particle interface.

a) Dispersion-strengthened

  • In this, particles are comparatively smaller, and are of 0.01-0.1μm in size.
  • Strengthening occurs at atomic/molecular level i.e. mechanism of strengthening is similar to that of a precipitation hardening in metals
    • This involves interactions between the particles and dislocations within the matrix

Load interactions

  • Matrix bears the major portion of an applied load, while dispersoids hinder/impede the motion of dislocations.
  • Thus, plastic deformation is restricted such that yield and tensile strengths, as well as hardness, improve.

How is this different from precipitation[FAQ1] hardening?

  • In the dispersed strengthening, particles are chosen to be unreactive, because of this, the strength is retained at elevated temperature for elongated times
  • But where as in case of precipitation-hardened alloy, upon temperature rise, dissolution of precipitation phase occurs, which decreases the strength


  • Thoria (ThO2) dispersed Ni-alloys (TD Ni-alloys) with high-temperature strength
  • SAP (sintered aluminium powder) – where aluminium matrix is dispersed with extremely small flakes of alumina (Al2O3).

b) Particulate-reinforced composites

  • Contain comparatively coarse particles in large amounts.
  • Designed to produce unusual combinations of properties rather than to improve the strength.
  • Concrete
    • This is most familiar large-particle composite, which is composed of cement (the matrix), and sand and gravel (the particulates for reinforcement)
    • The two most familiar concrete
      • Portland
        • Aggregate is gravel and sand
        • Mainly used in construction activities
      • Asphaltic cements
        • Composed of 5% asphalt/bitumen cement and 95% aggregates (stone, sand, and gravel)
        • Used primarily as a paving material
  • Particulate composites are used with all three material types –
  Examples Advantages Application
Metals Aluminium alloy castings containing dispersed SiC particles Resistance to wear and high cycle fatigue resistance Automotive parts like pistons and brakes.
Polymers Carbon black[FAQ2]  added to vulcanized rubber Enhances toughness and abrasion resistance of the rubber Automobile tyres
  • Cermets[FAQ3] in metallic matrix

tungsten carbide (WC) or titanium carbide (TiC) embedded cobalt or nickel used to make cutting tools

Increases toughness of the materials Cutting tools for hardened steels.

3.1.2 Fiber-Reinforced Composites(FRC)

  • These are Strong, stiff but brittle fibers are Incorporated into a softer and more ductile matrix.
  • This would provide improved strength and other mechanical properties and strength-to-weight ratio.

Load interaction

  • The matrix material acts as a medium to transfer the load to the fibers, which carry most of the applied load.
  • i.e fibers carry most of the load
  • The matrix also provides protection to fibers from external loads and atmosphere.
  • Apart from the properties of the fibers,the mechanical properties of FRCs depends on the degree of which an applied load is transmitted to the fibers by the matrix phase.
  • Other factors that affects mechanical properties of FRCs are
    • Length of fibers
    • Their orientation
    • Volume fraction in addition to direction of external load application Influence of fiber length on composite strength

  • Below figure shows small fiber that is not throughout matrix where  matrix deforms around the fiber such that there is virtually no stress transference and little reinforcement by the fiber.


  • So, some critical fiber length is necessary for effective strengthening and stiffening of the composite material.
  • This critical length (lc) is dependent on
    • The fiber diameter d
    • It’s ultimate (or tensile) strength(σf)
    • Fiber–matrix bond strength (or the shear yield strength of the matrix, whichever is smaller) τc according to

  • As fiber length l increases, the fiber reinforcement becomes more effective
  • Based on the length of fiber reinforcement, these can be classified as
    • Continuous when fiber length l>>lc
    • Discontinuous when fiber length is less than critical length
  • So from the above analogy, continuous reinforcement gives highest strength and stiffness Influence of fiber orientation and concentration

  • Reinforcing fibers can be introduced into matrix in any orientation.
  • If the fiber is short and randomly oriented then
    • Can be easily introduced into matrix
    • Exhibit relatively isotropic(same in all directions) behaviour in the composite
  • If the fiber is long and continuous unidirectional arrangement then
    • Exhibit anisotropic properties

Below are some of the orientation examples

a) Continuous fiber composites

  • The reinforcement fibers are continuous from one end of matrix to the other end.

  • Maximum strength and reinforcement are achieved along the alignment (longitudinal) direction
  • In the transverse direction, fiber reinforcement is virtually nonexistent: fracture usually occurs at relatively low tensile stresses
  • In the other directions the reinforcement is between above two extremes

b) Discontinuous and aligned fiber composites

  • Although reinforcement efficiency is lower, these are commercially gaining an important place.  
  • Eg: Chopped glass fibers, carbon and aramid discontinuous fibers in many applications

c) Discontinuous and randomly oriented fiber composites

  • Reinforcement efficiency of these fiber composites is difficult to calculate, and is usually characterized by a parameter known as fiber efficiency parameter, K.

3.1.3 Structural composites

  • Properties of these composites depend on the properties of the constituents as well as on geometrical design of various structural elements.
  • Two classes of these composites widely used are
    • Laminar composites  
    • Sandwich structures. Laminar composites

  • These are composed of two-dimensional sheets/layers that have a preferred strength direction.
  • These layers are stacked and cemented together according to the requirement.
  • General materials used in their fabrication are: metal sheets, cotton, paper, woven glass fibers embedded in plastic matrix, etc.
  • Eg: thin coatings, thicker protective coatings, claddings, bimetallics, laminates.

Applications: Designed to increase corrosion resistance while retaining low cost, high strength or light weight. Sandwich structures

  • As in the figure, these consist of thin layers of a facing material joined to a light weight filler(core) material.
  • Neither the filler material nor the facing material is strong or rigid, but the composite possesses both properties. Example: corrugated cardboard.
  • The faces
    • Bear most of the in-plane loading and also any transverse bending stresses.
    • General face materials are Al-alloys, fiber-reinforced plastics, titanium, steel and plywood.
  • The core serves two functions –
    • Separates the faces and resists deformations perpendicular to the face plane
    • Provide shear rigidity along planes that are perpendicular to the faces.
    • General core materials are: foamed polymers, synthetic rubbers, inorganic cements, balsa wood.

Applications: Roofs, floors, walls of buildings, and in aircraft for wings, fuselage and tailplane skins.

3.2 Composites type based on matrix material

3.2.1 Metal matrix composites

  • The matrix is ductile metal
  • These are having higher operating temperatures, non flammability, and greater resistance to degradation by organic fluids.
  • Metal-matrix composites are employed in driveshafts (that have higher rotational speeds and reduced vibrational noise levels), extruded stabilizer bars, and forged suspension and transmission components.

3.2.2 Ceramic matrix composites

  • Ceramic materials are inherently resilient to oxidation and deterioration at elevated temperatures
  • Ideal candidates for use in
    • high-temperature and severe-stress applications
    • specifically for components in automobile and aircraft gas turbine engines.
  • The fracture toughness of ceramics have been improved significantly by the development of a new generation of ceramic-matrix composites (CMCs)— particulates, fibers, or whiskers of one ceramic material that have been embedded into a matrix of another ceramic

3.2.3 Polymer matrix composites

Polymer-matrix composites (PMCs) consist of a polymer resin as the matrix, with fibers as the reinforcement medium

These are further divided based on the type of fiber used as reinforcement as below

a) Carbon Fiber-Reinforced Polymer (CFRP) Composites

  • The stable form of crystalline carbon at ambient conditions is graphite
  • Carbon fibers are not totally crystalline, but are composed of both graphitic and noncrystalline regions
  • To obtain high strength, the layer planes of  the graphite have to aligned parallel to the axis of the fiber in the matrix


  • These are having highest specific strength of all reinforcing fiber materials even at elevated temperatures
  • Carbon fibers are not affected by the moisture or wide variety of solvents


  • Utilized extensively in sports and recreational equipment (fishing rods, golf clubs)
  • Filament-wound rocket motor cases, pressure vessels, and aircraft structural components
  • Both military and commercial, fixed wing and helicopters (e.g., as wing, body, stabilizer, and rudder components).

b) Glass Fiber-Reinforced Polymer (GFRP) Composites

  • The composition of the glass that is most commonly drawn into fibers (sometimes referred to as E-glass)
  • The surface characteristics of glass fibers are extremely important because even minute surface flaws can deleteriously affect the tensile properties


  • In spite of having high strengths, they are not very stiff and do not display the rigidity that is necessary for some applications


  • Automotive and marine bodies, plastic pipes, industrial floorings and containers

c) Aramid Fiber-Reinforced Polymer Composites

  • Aramid fibers are high-strength, high-modulus materials
  • They are especially desirable for their outstanding strength-to- weight ratios, which are superior to metals
  • Examples of aramid are Kevlar and Nomex


  • These fibers have longitudinal tensile strengths compared to other fibers but they are relatively weak in compression
  • Even though the aramids are thermoplastics, they are, nevertheless, resistant to combustion and stable to relatively high temperature


  • The aramid fibers are most often used in composites having polymer matrices; common matrix materials are the epoxies and polyesters

4. Other important composites

4.1 Carbon-Carbon composites

  • One of the most advanced and promising engineering material is the carbon fiber- reinforced carbon-matrix composite, often termed a carbon–carbon composite
  • as the name implies, both reinforcement and matrix are carbon
  • Properties like high-tensile moduli and tensile strengths that are retained to temperatures in excess of 2000 C


  • Rocket motors, as friction materials in aircraft and high-performance automobiles
  • For hot-pressing molds, in components for advanced turbine engines
  • As ablative shields for re-entry vehicles.

4.2 Hybrid composites

  • This is obtained by using two or more different kinds of fibers in a single matrix
  • The glass–carbon hybrid is one such example which is stronger and tougher, has a higher impact resistance
  • In case of failure, the carbon fibers are the first to fail, at which time the load is transferred to the glass fibers.


1. What is Precipitation hardening?

Ans: It is a heat treatment process that produces uniformly dispersed particles within a metal’s grain structure that hinder dislocation motion, thereby strengthening the metal.

The formation of these precipitates is done by using a solution treatment at high temperatures prior to a rapid cooling process

2. What is carbon black?

Ans: Carbon black consists of very small and essentially spherical particles of carbon, produced by the combustion of natural gas or oil in an atmosphere that has only a limited air supply.

3. What are cermets?

Ans: Cermets are composites in which ceramic materials and metals join together, typically to give something with the high temperature performance or wear resistance of a ceramic and the toughness, flexibility, or electrical conductivity of a metal


  1. William D. Callister, Jr.David G. Rethwisch: Materials Science and Engineering: An Introduction, Wiley publication, 2014
  2. NPTEL material science material by Satish Vasu Kailas (IISc)



  1. Alpesh says:

    Also include elstic behaviour of continuous and oriented fiber composite in longitudinal and transverse loading.

  2. sagar suroshi says:

    Sir critical length of fibre, the denominator should have 4 instead of 2. Am i wrong ??

  3. adil says:

    Are u planning to launch current affair annual magazine

  4. Harsh says:

    Nice Work Sir !

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