Lecture 2

Lecture 2 Engineering

Materials and Their Properties

Materials Selection in Mechanical Design, 4th Edition, 2010 Michael Ashby

Jayant Jain Assistant Professor,

Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016


Historically, we have gone through a long way, many choices of

materials are available today.

However the challenge is to select the best for a given application.

Understanding of materials properties and tailoring them for need

is one of the key aspect in the future of materials development

Recap

It is not necessarily a material that we seek, but a

certain profile of properties the one that best meets the

needs of the design


Materials are the food of design

Menu of Engineering Materials

The members of a material family

have certain features in

common: similar properties, similar processing routes, and, often, similar

applications


Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

Examples of Each Material Family

Our daily life uses most of them in some form or the other!!

Stiff high E

Hard

Abrasion resistant

Good high temperature strength

Good corrosion resistance

Brittle


Ceramics

Hard

Corrosion resistant

Electrically insulating

Transparent

Brittle low KIC

Glasses

Light low

Easily shaped

High strength per unit weight (/)

Lack stiffness low E (50X less than metals)

Properties highly sensitive to temperature


Polymers

Lack stiffness low E (500 5000X less than metals)

Able to retain initial shape after being stretched

Relatively strong and tough

Elastomers

Expensive

Difficult to shape and join

Properties dependent on combination of materials

Tough high KIC Stiff high E

Ductile

Wide range of strengths depending on composition and processing

Thermally and electrically conductive

Reactive low corrosion resistance


Metals

Hybrids

Material Properties and Their Units

Each material can be thought of as having a set of

attributes or properties

The combination that characterizes a given

material is its property profile

It is the favorable property profile that qualifies the

material to be as one of the potential choice


Figure 3.3


Why metals are heavy??

Why polymers are light??

Why ceramics are slightly lighter than

metals

r metals > r ceramics > r polymers

Density of solids

Density of engineering materials


Factors that affect the density of solid

1. Mass and size of the atoms that make them up

2. Efficiency with which they are packed to fill the space

Metals: atoms are heavy and close packed

Polymers: less close packed, C, H, O atoms are light

Ceramics: also contains light atoms O, N and C, atoms are

packed closely

Composites: densities are average of the materials of which

they are made

How do we measure the density of solids? Hydrostatic weighing method: Archimedes principle


Elastic moduli

Do we get this response for all materials?

It is resistance of material to elastic or springy deformation Stretching rubber band as opposed to steel piece

Hookes law


Non-linear elastic solids


How do we measure elastic modulus of materials?

Tensile test Need a precise measurement of elastic strain, stretching of test fixture: data needs to be compliance corrected, cannot be used for all materials 3 point bend test

Particularly good for brittle materials As you cannot load them in tension

Measure the velocity of sound wave in the material

Documents

Lecture 2