Introduction to Composite Materials

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Questions and Answers

What property is essential for a material to function effectively as an electrical insulator?

High resistivity (correct)

High dielectric constant

Low tensile strength

High thermal conductivity

Which of the following materials is commonly used as a dielectric in capacitors?

Mica (correct)

Silver

Iron

Copper

At what range of breakdown electric fields do most insulators typically operate?

1 to 5 kV/mm

20 to 30 kV/mm

5 to 15 kV/mm (correct)

10 to 20 kV/mm

Which characteristic is NOT typical of dielectrics?

They allow free charges to move Signup and view all the answers

Why is it critical to understand how a material stores electrical charge when selecting insulating materials?

It determines the maximum voltage the material can withstand. Signup and view all the answers

Which material is NOT mentioned as an effective insulator?

Silicon Signup and view all the answers

What is the typical resistivity range for materials used as insulators?

<blockquote> 10^11 W-m </blockquote> Signup and view all the answers

Which property would NOT be desirable in a dielectric material for capacitors?

High ability to conduct electricity Signup and view all the answers

Which parameter primarily affects the electrical conductivity of a material?

Charge carrier mobility and number Signup and view all the answers

What happens to the resistivity of metals and alloys as temperature increases?

It rises linearly with temperature. Signup and view all the answers

What is the form of the equation used to calculate resistivity with temperature changes?

$ ho = ho_{R_T}(1 + eta_R riangle T)$ Signup and view all the answers

Which of the following statements is true regarding charge carriers in metallic bonds?

Valence electrons can move easily within the metallic structure. Signup and view all the answers

When considering impurities in a metal, what effect do they typically have on resistivity?

They increase resistivity by scattering charge carriers. Signup and view all the answers

What is the drift velocity $v$ in terms of charge carrier mobility $ ho$ and electric field $E$?

$v = ho E$ Signup and view all the answers

What type of movement do electrons exhibit when deflected by atoms in a conductor?

Irregular path movement Signup and view all the answers

In which scenario would mobility of charge carriers be decreased?

In the presence of atomic-level defects Signup and view all the answers

What is the approximate maximum tensile strength of ceramics?

700 MPa Signup and view all the answers

How does the compressive strength of ceramics compare to its tensile strength?

It is 10 times higher than tensile strength Signup and view all the answers

What is the typical range of stiffness for ceramics?

400-600 GPa Signup and view all the answers

What property of ceramics is characterized by brittle fracture?

Toughness Signup and view all the answers

What is the maximum temperature ceramics can withstand before melting?

5000°C Signup and view all the answers

What defines creep strength in materials?

The maximum stress that causes deformation in a specified time Signup and view all the answers

What risk does creep pose to structural materials?

It reduces the structural integrity Signup and view all the answers

How can the fracture strength of ceramics be improved?

By creating compression on the surface Signup and view all the answers

What material is primarily used in cemented carbides for wear resistance applications?

Tungsten carbides Signup and view all the answers

Which of the following acids can ceramics generally resist?

Nitric acid Signup and view all the answers

What characteristic defines a soft magnet?

Magnetism depends on an external magnetic field Signup and view all the answers

Why should ceramics not be used in solutions with a pH greater than 10 at 100°C?

They are highly reactive with alkaline solutions Signup and view all the answers

What is a typical application for silicon carbide?

Resistance heating elements Signup and view all the answers

Which material is known for high-temperature applications in ceramics?

Alumina Signup and view all the answers

Which of the following statements about ferrites is true?

Ferrites can be either soft or hard magnets Signup and view all the answers

What type of carbon materials are considered ceramics despite not being made of inorganic compounds?

Diamond and graphite Signup and view all the answers

What is the primary role of the matrix in composite materials?

To protect the fibres from environmental damage Signup and view all the answers

Which of the following is NOT a type of polymer matrix in composite materials?

Silicon carbide Signup and view all the answers

How does reinforcement change the characteristics of composite materials?

Provides greater mechanical properties Signup and view all the answers

What constitutes the critical fibre length in a composite material?

The minimum length for effective stress transfer Signup and view all the answers

Which type of composite is typically reinforced with natural fibres?

Polymer matrix composites Signup and view all the answers

Which of the following types of fibres is commonly used in polymer composites?

Glass fibres Signup and view all the answers

What transformation involves the transfer of load from matrix to fibre in composites?

Interface interaction Signup and view all the answers

Which of the following is true regarding particulate reinforcements?

They can be hollow microspheres or nanoparticles Signup and view all the answers

What defines a 'Green composite'?

Composites reinforced with natural fibres Signup and view all the answers

Which of the following is NOT a role of the matrix in composites?

Providing an electrical conductive path Signup and view all the answers

Which of the following accurately describes continuous fibres in composites?

Enhance the mechanical properties significantly Signup and view all the answers

Which component in a composite is characterized as being isotropic before reinforcement?

Matrix Signup and view all the answers

What type of natural fibres can be categorized as bast fibres?

Hemp Signup and view all the answers

Which of the following correctly lists the phases in a composite material?

Matrix, reinforcement, interface Signup and view all the answers

Study Notes

Composite Materials Introduction

Composite materials are a combination of two or more distinct materials, each material contributing its properties to create a new material with a combined effect.
Some examples of composite materials are wood, metal matrix composite, polymer composite, ceramic matrix composite, and carbon/carbon composite.

Composite Materials Explained

A composite material is composed of two or more distinct materials that retain their identities within the mixture, generating characteristics unlike those of the individual constituents.
There are three phases in composite materials: a reinforcement phase, a matrix phase, and an interface or interphase.

Stress-Strain Relationships

In the stress-strain relationships of composite materials, the reinforcement is stiff but brittle, while the matrix, most commonly a polymer, is soft but ductile.

Types of Composites

Polymer matrix composites (PMCs) are the most common type of composite.
Metal matrix composites (MMCs) are used in high-temperature applications.
Ceramic matrix composites (CMCs) are used where high strength and stiffness are required at high temperatures.
Natural fibre composites, also known as “Green composites,” are reinforced with natural fibres, primarily plant-derived materials. An example of this is wood fibre composites (WPC).

Matrix Types

Polymeric Matrix: Thermoset polymers (resins) provide high strength and stiffness, while thermoplastic polymers offer flexibility and ease of processing.
Metallic Matrix: Aluminium and its alloys, titanium alloys, copper-nickel-based superalloys, and stainless steel offer high strength, high temperature resistance, and good conductivity.
Ceramic Matrix: Aluminium oxide (Al2O3), silicon carbide (SiC), and silicon nitride (Si3N4) offer high stiffness, high-temperature resistance, and low thermal expansion.

Roles of the Matrix

The matrix holds the reinforcement in the desired orientation, protects the fibres from damage, and transfers loads into and between the fibres.

Reinforcement Phase

Reinforcements can be particulate, short fibres, long fibres, or continuous fibres.
Reinforcements provide strength and stiffness, influencing the formability and machinability of the resulting structure.
They change the nature of the composite material, making it anisotropic rather than isotropic.

Types of Reinforcements

Particulate reinforcements include micro-balloons, nano particles, and short fibres.
Discontinuous fibre reinforcements include chopped strand and chipped fibres for injection moulding, 100 μm long.
Continuous fibre and fabric reinforcements include weaving, non-crimp fabrics, knitting, braiding, mats and non-woven, combination fabrics, and preforms.

Common Fibres for PMCs

Glass fibres (SiO2 E-glass fibre) are commonly used due to their low cost and high strength-to-weight ratio.
Carbon fibres (Graphite fibres) offer high strength and stiffness but are more expensive than glass fibres.
Aramid fibre (Kevlar) is known for its high tensile strength and outstanding impact resistance.
Natural fibres are used in “Green composites” for their renewable and biodegradable characteristics.

Types of Natural Fibres

Bast fibres come from the stems of plants, including flax, hemp, jute, and kenaf.
Leaf fibres consist of sisal, banana, palm, etc., and are extracted from leaves.
Seed fibres are derived from seeds, such as cotton, coconut (coir), and kapok.
Wood fibres, mostly used in Wood Plastic Composites (WPC), are extracted from wood.

Composite Materials Applications

Composites are used in various industries, including aerospace, automotive, construction, and marine.
Examples include aircraft components, boat hulls, and building materials.

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Description

Explore the world of composite materials, their characteristics, and the various types that exist today. This quiz covers the fundamental concepts of composite materials, including stress-strain relationships and the phases involved. Test your knowledge on polymer matrix composites and their applications.