Composites Notes FE Sem 1 Applied Chemistry | Mumbai University

Composites combine two or more different materials to obtain properties that none of them has alone, such as the high strength and low weight of carbon fibre reinforced plastics. This module defines composites and lists their characteristic properties. It explains the two constituents, the matrix phase and the dispersed (reinforcing) phase, and what each does. Finally it classifies composites by matrix and by reinforcement, knowledge needed to choose the right composite for aerospace, automobile, civil and sports applications.

Summary

A composite material is a multiphase material made by combining two or more chemically and physically different materials on a macroscopic scale, with a distinct interface between them, such that the combination has properties superior to, or not obtainable from, the individual constituents. The constituents keep their own identity and do not dissolve or merge into each other. Natural examples are wood (cellulose fibres in a lignin matrix) and bone (collagen with hydroxyapatite); engineered examples are reinforced concrete, glass fibre reinforced plastic (GFRP) and carbon fibre reinforced plastic (CFRP).

Characteristics of composites: high strength-to-weight and stiffness-to-weight ratios (low density with high strength); good corrosion and chemical resistance; good fatigue resistance; properties that can be tailored by choosing the constituents, their proportions and the fibre orientation; directional (anisotropic) properties where fibres are aligned; good dimensional stability with low thermal expansion; good thermal and electrical insulation for polymer composites; good wear and impact resistance for suitable combinations; and design flexibility allowing complex shapes and part consolidation. Limitations include high cost of fibres and processing, difficulty in repair and recycling, and possible delamination.

The matrix phase is the continuous phase that surrounds and binds the dispersed phase. It may be a polymer (epoxy, polyester, PEEK), a metal (Al, Mg, Ti) or a ceramic (SiC, alumina). Functions of the matrix: it binds the reinforcement together and holds it in position; transfers and distributes the applied load to the reinforcement; protects the reinforcement from chemical attack, moisture and mechanical abrasion; separates the fibres and stops the propagation of cracks from one fibre to another; gives the composite its shape and surface finish; and largely decides the maximum service temperature and the transverse and shear properties.

The dispersed phase (reinforcement) is the discontinuous phase embedded in the matrix, in the form of fibres, particles, flakes or whiskers. Common fibres are glass, carbon, aramid (Kevlar) and boron; particles include SiC, Al2O3, WC and gravel or sand in concrete. Functions of the dispersed phase: it carries most of the load and gives strength and stiffness; improves hardness, wear and abrasion resistance; improves creep and fatigue resistance; modifies thermal and electrical properties such as thermal expansion and conductivity; and, as fillers, may reduce cost. The interface between matrix and dispersed phase must give good bonding so that load is transferred efficiently.

Composites are classified on the basis of the matrix into polymer matrix composites (PMC: GFRP, CFRP, Kevlar-epoxy, used in boats, aircraft panels, sports goods), metal matrix composites (MMC: SiC or Al2O3 particles or fibres in Al, Mg or Ti, used in engine pistons, brake discs and aerospace parts) and ceramic matrix composites (CMC: SiC fibres in SiC, carbon-carbon composites, used in turbine parts, rocket nozzles and brake discs at very high temperatures).

On the basis of the reinforcement, composites are classified into (i) particle-reinforced composites, which include large-particle composites in which particles share the load (concrete: gravel and sand in cement; cermets: WC or TiC in Co or Ni for cutting tools) and dispersion-strengthened composites in which very fine particles (0.01 to 0.1 micrometre) hinder dislocation motion (Al2O3 in Al, ThO2 in Ni); (ii) fibre-reinforced composites, with continuous aligned fibres, discontinuous aligned fibres or discontinuous randomly oriented fibres (GFRP, CFRP, aramid composites), which give the best strength-to-weight ratio; and (iii) structural composites, which include laminar composites made by stacking and bonding sheets with different orientations (plywood, laminated sheets) and sandwich panels with strong thin face sheets bonded to a light core such as honeycomb or foam (aircraft floors, wings, building panels).

Mind map

How a fibre-reinforced composite carries a load

Important definitions

Key points to remember

• Constituents of a composite keep their identity; an alloy is not a composite.
• Matrix = continuous phase; dispersed phase = reinforcement.
• Matrix transfers load; reinforcement carries load.
• Matrix decides service temperature: polymer lowest, metal medium, ceramic highest.
• Fibre-reinforced composites give the best strength-to-weight ratio.
• Concrete is a large-particle composite; reinforced concrete adds steel rods as reinforcement.
• Cermets (WC in Co) are used for cutting tools.
• Plywood is a laminar composite; honeycomb panels are sandwich composites.
• Composites can be tailored but are costly and hard to repair or recycle.
• Good interfacial bonding is essential for load transfer.

Solved examples

Example 1. Identify the matrix, the dispersed phase and the class of composite for: (a) concrete, (b) GFRP boat hull, (c) WC-Co cutting tool, (d) plywood.

(a) Concrete: matrix = cement paste; dispersed phase = sand and gravel particles; class = large-particle reinforced, ceramic-type matrix. (b) GFRP: matrix = polyester or epoxy resin; dispersed = glass fibres; class = polymer matrix, fibre-reinforced. (c) WC-Co: matrix = cobalt metal; dispersed = tungsten carbide particles; class = metal matrix, large-particle (cermet). (d) Plywood: thin wood veneers bonded with adhesive, grain of alternate layers at right angles; class = structural, laminar composite.

Example 2. An aircraft floor panel must be very light and stiff in bending. Which type of composite would you choose and why?

A sandwich panel: two thin, strong face sheets (CFRP or aluminium) bonded to a light honeycomb (aramid paper or aluminium) core. The faces carry the bending tension and compression, the core separates them, increasing stiffness greatly with very little added weight, and also resists shear. Hence high stiffness-to-weight ratio, as required.

Example 3. A brake disc for a high-performance vehicle operates at very high temperatures. Suggest a suitable composite class and justify.

A ceramic matrix composite such as carbon-carbon or carbon fibre reinforced SiC. Polymer matrices degrade at a few hundred C and metals soften, but ceramic and carbon matrices retain strength at very high temperatures; the fibres add toughness that monolithic ceramics lack, so the disc does not fail by brittle fracture.

Example 4. Classify Al-SiC and Ni-ThO2 composites and explain how their strengthening differs.

Both are metal matrix, particle-reinforced composites. Al-SiC (particles of a few micrometres) is a large-particle composite in which the hard SiC particles share the load and restrain matrix deformation, improving stiffness and wear resistance. Ni-ThO2 (TD nickel, particles 0.01-0.1 micrometre) is dispersion-strengthened: the fine particles block dislocation movement, so the matrix itself becomes stronger and retains strength at high temperature.

Example 5. In a GFRP rod, explain what happens when a single glass fibre breaks under load.

The broken fibre stops carrying load at the break. The matrix transfers that load through shear at the interface to the neighbouring intact fibres and to the broken fibre a short distance away from the break. The ductile matrix also blunts the crack tip and keeps the crack from jumping to adjacent fibres. Therefore the rod does not fail suddenly, showing the crack-arresting function of the matrix.

Important questions and answers

Q1. Define composite material. Give two natural and two synthetic examples. (3 marks)

A composite is a multiphase material formed by combining two or more chemically and physically distinct materials with a distinct interface, giving properties superior to those of the individual constituents. Natural: wood (cellulose fibres in lignin), bone (collagen and hydroxyapatite). Synthetic: GFRP, CFRP, reinforced concrete, cermets.

Q2. State the characteristics of composite materials. (4 marks)

High strength-to-weight and stiffness-to-weight ratios; low density; good corrosion and chemical resistance; good fatigue and creep resistance; tailorable properties by choosing constituents and fibre orientation; anisotropic behaviour in aligned-fibre composites; low thermal expansion and good dimensional stability; good thermal and electrical insulation (polymer composites); design flexibility for complex shapes. Limitations: high cost, difficult repair and recycling.

Q3. Explain the constituents of composites: matrix and dispersed phase, with their functions. (6 marks)

Matrix: continuous phase (polymer, metal or ceramic). Functions: binds the dispersed phase and holds it in place; transfers and distributes load to the reinforcement; protects reinforcement from moisture, chemicals and abrasion; separates fibres and prevents crack propagation; gives shape and surface finish; determines service temperature and transverse properties. Dispersed phase: discontinuous reinforcing phase as fibres (glass, carbon, aramid), particles (SiC, Al2O3, WC) or flakes. Functions: bears most of the load; increases strength and stiffness; improves hardness, wear, creep and fatigue resistance; modifies thermal expansion and conductivity; fillers reduce cost. Good interfacial bonding is needed for load transfer.

Q4. Classify composites with suitable examples. (6 marks)

Based on matrix: PMC (GFRP, CFRP, Kevlar-epoxy), MMC (SiC in Al, Al2O3 in Mg), CMC (SiC-SiC, carbon-carbon). Based on reinforcement: (1) Particle-reinforced: large-particle (concrete, cermets like WC-Co) and dispersion-strengthened (Al2O3 in Al, ThO2 in Ni). (2) Fibre-reinforced: continuous aligned, discontinuous aligned, discontinuous random (GFRP, CFRP, aramid composites). (3) Structural: laminar (plywood, laminates) and sandwich panels (honeycomb core with face sheets). A tree diagram should be drawn.

Q5. Differentiate between matrix phase and dispersed phase. (4 marks)

Matrix: continuous phase; binds, protects and transfers load; in PMCs and MMCs usually softer and more ductile than the reinforcement (ceramic matrices are brittle); determines service temperature; examples epoxy, Al, SiC. Dispersed phase: discontinuous phase; carries the load; usually stronger, stiffer and harder; determines strength and stiffness; examples glass fibres, carbon fibres, SiC particles.

Q6. Write a note on fibre-reinforced composites. (4 marks)

Fibres of glass, carbon, aramid or boron are embedded in a matrix, usually polymer. Fibres may be continuous and aligned (highest strength along the fibre axis, anisotropic), discontinuous and aligned, or discontinuous and randomly oriented (nearly isotropic, lower strength). The matrix transfers load to the fibres. They give the highest strength-to-weight ratio of all composites and are used in aircraft parts, boat hulls, car bodies, sports goods, wind turbine blades and pressure vessels.

Q7. Write a note on structural composites. (4 marks)

Structural composites owe their properties to the geometry of their elements as well as to the materials. Laminar composites consist of layers (laminae) bonded together, each with a preferred high-strength direction placed at different angles, giving strength in several directions, e.g. plywood, CFRP laminates. Sandwich panels consist of two thin, stiff, strong face sheets (aluminium, CFRP, GFRP) bonded to a light, thick core (honeycomb, polymer foam, balsa wood); the core separates the faces and resists shear, giving very high bending stiffness at low weight; used in aircraft floors, wings, helicopter blades and building panels.

Q8. Write short notes on PMC, MMC and CMC. (5 marks)

PMC: polymer (epoxy, polyester) matrix with glass, carbon or aramid fibres; light, corrosion resistant, easy to fabricate; limited to low service temperatures; used in boats, aircraft panels, sports goods. MMC: metal (Al, Mg, Ti) matrix with SiC, Al2O3 or boron reinforcement; higher service temperature, good thermal conductivity and wear resistance; used in pistons, brake rotors, aerospace frames. CMC: ceramic matrix (SiC, Al2O3, carbon) with ceramic fibres; very high temperature resistance with better toughness than monolithic ceramics; used in turbine components, rocket nozzles, heat shields and high-performance brakes.

Q9. Why are composites preferred in aerospace applications? (3 marks)

They have high strength-to-weight and stiffness-to-weight ratios, so structures are lighter and fuel-efficient; excellent fatigue and corrosion resistance; properties can be tailored by fibre orientation; low thermal expansion gives dimensional stability; and complex shapes can be moulded with fewer joints.

Exam tips

• Always draw the classification tree for the 6-mark classification question.
• Give at least one example for every class and sub-class.
• List functions of matrix and dispersed phase as separate numbered points.
• Use the distinction table format for matrix vs dispersed phase questions.
• Do not call alloys composites; their constituents do not retain separate identity with a distinct interface on a macroscale.

FAQs

Is reinforced concrete a composite?

Yes. Concrete itself is a particle-reinforced composite, and adding steel bars gives a further reinforced composite in which steel carries tension.

Why is the matrix usually ductile?

A ductile matrix can deform to transfer load to fibres and blunt cracks, preventing sudden brittle failure.

Which composite has the highest strength-to-weight ratio?

Continuous fibre-reinforced polymer composites such as CFRP, along the fibre direction.

Why are composites hard to recycle?

The constituents, especially thermoset matrices and fibres, are intimately bonded and cannot easily be separated or remelted.

These notes follow the University of Mumbai first-year engineering syllabus (NEP 2020, Applied Chemistry) and were prepared with AI help and checked.