Anna University Long Questions and Answers

Unit - III

Chapter 4

Composites

Anna University Long Questions & Answers

Part - B

 

1. What are composites? Explain the constituents of composites.

A composite material may be defined as, “a material system consisting a mixture of two (or) more micro-constituents, which are mutually insoluble, differing in form (or) composition and forming distinct phases”.

Composites consist of two important constituents.

(i) Matrix phase (or) Matrix resin

(ii) Dispersed phase (or) Reinforcement.

1. Matrix Phase (or) Matrix resin

Matrix phase is the continuous body constituent, which encloses the composite. Matrix phase may be metals, ceramics (or) polymers (liquid resins), composites using these matrix are known as

(i) Metal Matrix Composites (MMC)

(ii) Ceramic Matrix Composites (CMC)

(iii) Polymer Matrix Composites (PMC) respectively.

 

2. Dispersed Phase (or) Reinforcement

Dispersed phase is the structural constituent, which determines the internal structure of composite.

Examples : Fibres, Particulates, Flakes, Whiskers.

1. Fibres

Fibres are long and thin filament of any macromolecular substance such as polymer, metal (or) ceramic having high length to diameter ratio at least 100:1.

2. Particulates

Particulates are small pieces of hard solid materials. They may be metallic (or) non-metallic. Generally distribution of particles in a given matrix is random, so the resulting composites are usually isotropic. Due to the inherent hardness, the particles in a composite cause constraints on the plastic deformation on the matrix.

3. Flakes

Flakes are very thin solid like materials.

Example: Mica flakes

4. Whiskers

Whiskers are thin strong fibre like material made by growing a crystal.

Example: Graphite, silicon carbide, silicon nitride

 

2. What are the advantages of polymer composites? Explain using FRP as an example. (CBE. A.U. Jan 2009)

1. Polymer composite has very high strength to weight ratio than the metal.

2. The weight of FRP is 75% lower than the weight of steel, so it improves performance, reduces energy need, simplifies handling.

3. Using polymer composite devices both capacitance and leakage current can be reduced.

4. FRP offers better corrosion resistance to a wide range of chemicals resulting in longer service life, less maintenance and repair down time.

5. FRP possesses excellent durability and fire resisting properties.

6. It is ideal for outdoor installation, because it is impervious to the effect of sunlight, heat, cold and other weather conditions.

7. Specifying the orientation of reinforcing fibres, can increase the strength and resistance to deformation of the polymer.

8. GRP (glass reinforced polymer) are strongest and most resistive to deforming forces, when the polymer fibres are parallel to the force being exerted and are weakest when the fibres are perpendicular.

9. CRP (carbon fibre reinforced polymer) in place of sheet aluminium in traditional rudded are

(i) 25% reduction in weight.

(ii) 95% reduction in components by combining parts and forming simpler moulded parts.

 

3. Write a note on FRP. (Chen. A.U. Jan 2009, June 2009, TNV A.U. Jan 2009, Coim A.U. Jan 2010)

(Or)

Write notes on polymer matrix composites. (Coim. A.U. Jan 2010)

The fibre reinforced polymer composites constitute more than 90% (by weight) of the total composites used in various industries. The advantages of using polymer in structural composites is the ease of fabrication of complex structural shapes. It can be easily manufactured.

Preparation

: . Fibre-reinforced plastics are produced by suitably :bonding a fibre material with a resin matrix and curing them under pressure and heat. The main reinforcing agents used in FRP composites are, glass, graphite, alumina, carbon, boron, etc. The reinforcement material can be in different forms such as short fibres, continuous filaments (or) woven fabrics.

The resin matrix commonly used in FRP are polyesters, epoxy, phenolic, silicone, and polyamide polymer resins.

Characteristics (or) Properties of FRP

1. It possess superior properties like higher yield strength, fracture strength and fatigue life.

2. Since fibre prevents slip and crack propagation, the mechanical properties of FRP gets increased.

3. It possess high corrosion resistance and heat resistance property.

 

4. What are composites? Give the preparation and uses of glass fibre reinforced composites and carbon fibre reinforced composites. (Chen. A.U. Jan 2010)

Composites:

A composite material may be defined as, “a material system consisting a mixture of two (or) more micro-constituents, which are mutually insoluble, differing in form (or) composition and forming distinct phases”.

Composites consist of two important constituents.

(i) Matrix phase (or) Matrix resin

(ii) Dispersed phase (or) Reinforcement.

1. Matrix Phase (or) Matrix resin

Matrix phase is the continuous body constituent, which encloses the composite. Matrix phase may be metals, ceramics (or) polymers (liquid resins), composites using these matrix are known as

(i) Metal Matrix Composites (MMC)

(ii) Ceramic Matrix Composites (CMC)

(iii) Polymer Matrix Composites (PMC) respectively.

 (i) Glass fibre reinforced composites

Preparation

Glass fibre reinforced composites are prepared by suitably bonding (mixing) epoxy resin with a glass fibre.

Process

First the raw materials are thoroughly mixed and the batch is fed into the hooper. The batch, from the hooper, is forced into the chamber (furnace), maintained at about 1540°C, where the glass melts and converted into molten glass. The viscosity of the molten glass is maintained in between 600 and 1000 p.

The molten glass, from the chamber, moves to refiner part where dust and foreign substances are removed and the temperature gets reduced by 30°C. Finally the refined molten glass moves to forehead (criticle part of glass fibre making Winding part), which draw the molten glass into glass fibre. Glass fibre is cooled at out let by water spray and GFRP is obtained.

Properties

(i) Lower densities, dielectric constants.

(ii) Higher tensile strengths and impact resistance.

(iii) Excellent corrosion resistance, chemical resistance, Non-flammability.

Uses :

Automobile parts, storage tanks, plastic pipes, flourings, transportation industries, etc.

2. Carbon fibre reinforced composites

Preparation

Carbon fibre reinforced composites are prepared by suitably mixing polyester resin with a carbon fibre as per the above method.

Properties

(i) Lighter density.

(ii) Excellent resistance to corrosion.

(iii) Retention of desired properties even at elevated temperatures.

Uses

Structural components of aircraft and helicopters, recreational equipments, sports materials, antenna disc, solar panal etc.

 

5. Write notes on the followings (i) Ceramic matrix composites (ii) Hybrid composites

(i) Ceramic matrix composites

Properties

1. CMCs are used at temperatures above 1500°C, because of their good corrosion resistance, stability at elevated temperatures and high compressive strength.

2. Ceramics provide necessary strength at high temperature with good oxidation resistance.

3. The matrixes used are glass, ceramics, carbides, nitrides, oxides and borides, the reinforcements are Al₂O₃, B, C, SiC and SiO₂

Uses

They are used in re-entry thermal shields in space vehicles and tiles, pump seal, round rings, brake linings etc.

(ii). Hybrid composites

Hybrid composites are new class of materials composed of a suitable polymer matrix reinforced with two different fibres (or) fillers.

Properties

Hybrid composites possess very good properties compared to their single fibre composites.

1. They possess strong, tough and higher impact resistance.

2. When hybrid composites are stressed in tension, failure does not occur suddenly.

3. They possess balanced strength and stiffness.

4. They also have balanced thermal distortion stability, reduced weight, improved fatigue resistance.

5. They exhibit superior mechanical and tribological properties than other composites.

Uses

1. It is used in light-weight transport (land, water (or) oil) structural components.

2. It is also used in light weight orthopedic components and sporting goods.

3. It is used to make furniture like chair, table and bath tubs

4. It is used in railway coach interiors.

5. It is also used in making daily used applications like plates and spoons.

6. Automobile industry utilizes hybrid composites in many of the interior and exterior applications.