19Y015 SELECTION OF MATERIALS

FACTORS OF MATERIAL SELECTION : Classes of engineering materials - Evolution of engineering materials- Definition of
materials properties- Design strengths and weakness of various materials and their processes, Displaying material
properties using materials selection charts- Forces for change in materials selection and design, Materials and the
environment. (9)

ROLE OF DESIGN : Design process - types of design, design requirements, Technical Factors - Function, Material attributes.
Shape and Manufacturing processes – Formulation of functional requirements, Constraints, Objectives and Free variable,
Materials processing and their influence on design, process attributes, Non-Technical Factors – Local conditions, Cost,
Availability, Reparability, Recyclability. Reliability, Environmental impact, Legal issues. (9)

MATERIAL SELECTION : Materials selection strategy and methods: Screening and Ranking- weighted ranking, performance
indices- materials selection charts, deriving property limits and material indices, structural indices, Multiple constraints
and multiple objectives, Role of local parameters, Post script on materials selection. (9)

PROCESS SELECTION : Process classification, Systematic process selection, process selection diagrams, process cost,
energy consumption for production, material and shape link with process, availability and environmental consideration,
Screening, Ranking – Process cost and Supporting information. (9)

MATERIAL SELECTION FOR INDUSTRIAL COMPONENTS : Introduction, materials for tie rods, columns, beams, oars,
flywheels, springs, safe pressure vessels, heat exchangers, disk brake caliber, connecting rods, automobile body, nuclear
reactors, boat hulls, etc (9)
 Selection of Materials
Systematic process of selecting one or more
equivalent, high performing, cost effective, eco friendly
materials for an application or for a service requirement
Applications : Parts, Components, Products, Devises, Structures
 The application
• Has a function or functions to do
• Has a shape and size
• Has to be manufactured
• Has to survive without failure (long lost)
• Has to be cost effective
Need
 Human’s thirst for sophistication, modernization and
innovations
 Changing international scenario
 Market competitiveness
 Attempts to delight the customers
 Miniaturization , Elegance, Customization
Driving Forces
Benefits - Good quality products at cheaper cost
- Better service life / reliability
- Minimizing or preventing accidents
- Profit maximization and market dominance
Factors affecting Materials Selection

 Service requirements / conditions

 Dimensional aspects
 Materials properties
 Processing requirements
 Assembly Requirements
 Service Life and Reliability
 Environmental impact & Recycling
 Various Cost involved
 Availability
 Safety & Regulatory compliances
 Maintenance, Service and Replacement
 Human ergonomics & Usability
Market need / New Idea / Innovation

Design Process

Functions and dimensions

Plenty of materials

Plenty of processes

Final choice

Product to market
Central Problem of Material Selection
Function
Transmit Power
Carry Load
Store Energy
Contain Pressure

Shape
2D or 3D
Solid or Hollow
Section Thickness
Tolerances
Surface Finish
Materials
Evolution of Engineering Materials
 COMPOSITES
1. CFRP (Carbon Fiber Reinforced Polymer)

Overview

• CFRP is a composite of long, fine carbon fibers embedded in a

polymer matrix (usually epoxy resin, or polyester).

• CFRP has low density, and high Young's modulus and strength.

• CFRP must be processed directly to shape by laying up partially-

cured layers of material, and then hot pressing - this is
expensive.

• Carbon fibers are also expensive to produce, and it is only 35

years since the process to manufacture them was invented.
Design strengths: Design weaknesses:

• High stiffness-to-weight ratio • Moderately high cost

• High strength-to-weight ratio • Cannot be recycled
• Difficult to shape
• Difficult to join

Typical Products

• Sports goods (tennis racquets, golf clubs, fishing rods)

• Performance racing bicycles
• Formula I car bodies
• Military aircraft skins
Process Notes

Forming
• All composite techniques are suitable - e.g. hand lay up, spray lay up.

Machining
• Requires special tools to drill and cut.
• Not used with other machining processes.

Joining
• Usually, made to shape.
• Fasteners rarely used because of risk of cracking at holes.
• Adhesives most common method of joining..

Environmental Issues

• Very difficult
2. GFRP (Glass Fiber Reinforced Polymer)
Overview

• GFRP is a composite of long, fine glass fibres embedded in a

polymer matrix (usually epoxy resin, or polyester).

• GFRP has low density, and fairly high Young's modulus and
strength.

• GFRP must be processed directly to shape by laying up partially-

cured layers of material, and then usually requires hot pressing -
this is expensive.

Design strengths: Design weaknesses:

• High stiffness-to-weight ratio • Cannot be recycled

• High strength-to-weight ratio • Difficult to shape
• Difficult to join
Typical Products

• Sports goods (tennis racquets, golf clubs, fishing rods)

• Boat hulls (yachts, canoes)
• Bathtubs

Process Notes

Forming
• All composite techniques are suitable - e.g. hand lay up, spray lay up.

Machining
• Requires special tools to drill and cut.
• Not used with other machining processes.

Joining
• Usually, made to shape.
• Fasteners rarely used because of risk of cracking at holes.
• Adhesives most common method of joining..

Environmental Issues
• Very difficult
 POLYMERS
1. Polycarbonate
Overview
• Polycarbonate (PC) is a quite expensive thermoplastic, used for
its relatively high strength and toughness.

• Like all thermoplastics, polycarbonate is easy to shape and join.

Design strengths: Design weaknesses:

• Good strength (for a polymer) .
Quite expensive
• Low density
• Transparent, or easily coloured
• High toughness

Typical Products
• Crash and safety helmets
• Lightweight armour (e.g. riot shields)
• Street light covers
Process Notes
Polymer forming
• Most suitable - Injection moulding.
•Compression moulding is not used.

Machining
• Relatively soft, so readily machined.
• However, usually formed to near-net-shape so little cutting is done in
practice.
• Surface finish usually good enough after moulding, so no polishing
required.

Joining
• Adhesive bonding is most common.
• Fusion (e.g. arc) welding is not suitable, although some hot welding
(non-melt) processes are okay.

Environmental Issues
• Thermoplastics can be reheated and reshaped.
• No toxic fumes when burnt.
2. Polythene
Overview
• Comes in various forms, of which LDPE (low density) and HDPE
(high density) are the most common.

• Low density polythene is the only polymer which floats, high

density polythene does not.

• Polythene is the polymer used in the largest quantities

• Like all thermoplastics, polythene is easy to shape and join.

Design strengths:
• Very simple polymer structure, so easy to process.
• Transparent, or easily coloured
• Can be drawn to very large elongations, and very thin sheet

Design weaknesses:
• Quite expensive
Typical Products

• Dustbins
• Water and gas pipes
• Carrier bags
• Food packaging
• Sandwich boxes

Process Notes

Polymer forming

• Injection moulding is the most common.

• Often extruded for pipes
• Rotational moulding for large products like dustbins
• Blow moulding for bottles
• Vacuum forming for packaging.
• Compression moulding is not used.
Machining

• Relatively soft, so readily machined.

• However, usually formed to near-net-shape so little cutting is done in
practice.
• Surface finish usually good enough after moulding, so no polishing
required.

Joining

• Adhesive bonding is most common.

• Fusion (e.g. arc) welding is not suitable, although some hot welding
(non-melt) processes are okay - e.g. friction welding for pipes.

Environmental Issues

• Polymers are derived from hydrocarbons, and require energy to extract

and purify them.
• Thermoplastics can be reheated and reshaped.
• No toxic fumes when burnt.
 CERAMICS
1. Glasses
Overview
• Glasses are amorphous solids based on silicon oxide.

• Glass is soft and mouldable when hot, making shaping straightforward; when
cool and solid it is strong in compression, but brittle and weak in tension.

• Glass is transparent or can be easily coloured. Special glasses are made into
fibres for optical communications.

Design strengths: Design weaknesses:

•Transparent, or easily coloured •Low tensile strength
•High resistance to corrosion •Low toughness
•Easy to shape
Typical Products
• windows
• bottles
• ovenware
• optical fibres
Process Notes

Forming
• Blowed or rolled by a variety of process when hot (it softens).

Machining
• Extremely difficult to machine at room temperature as too brittle.
• Mechanical cutting usually by score-and-snap.
• Can be drilled, but special tools required.

Joining
• Best to avoid fasteners because of cracking at holes.
• Usually adhesive bonded.

Environmental Issues

• Can be recycled
2. Silicon carbide
Overview

• Silicon carbide is a covalent ceramic. It is mainly used for its very high
hardness (e.g. cutting tools), and for its electrical properties.

• Like all ceramics, silicon carbide is intrinsically hard and strong in

compression, but has low toughness and tensile strength.

• Due to its high melting point, silicon carbide can only be processed in
powder form.

Design strengths: Design weaknesses:

• Excellent corrosion resistant • Low tensile strength

• Low density • Low toughness
• Resistant to high temperatures • Difficult to shape
• High electrical resistance.
• High hardness
Typical Products
• electrical insulators (e.g. semiconductor substrate)
• cutting tools
• grinding wheels

Process Notes
Forming
• Sintering and HIPping are the dominant shaping processes.

Machining
• Requires special tools to drill and cut.
• Not used with other machining processes.

Joining
• Usually, made to shape.
• Can be brazed, but with some difficulty.
• Fasteners rarely used because of risk of cracking at holes.
• Adhesives not really suitable.

Environmental Issues
•Energy intensive process
 Metals & Alloys
1. Alloy steels
• Alloy steels are mostly fairly cheap, covering a range of carbon contents
(0.1-1.0%).

• The medium to high carbon content steels respond well to quenching and
tempering heat treatment to give very high strength and good
toughness for gears, drive shafts, pressure vessels, tools.

• Alloy steels containing other elements as well as carbon are classified

into low alloy and high alloy, depending on the amount of
additional alloying elements.

• Heat-treated high alloy steels give very high strengths, but are more
expensive.

• Alloy carbon steels rust easily, and must be protected by painting or other
coatings.
Design strengths:
• High strength with good toughness
• High stiffness
• Mostly very cheap
• Quite easy to shape
• Easy to weld
• Easy to recycle

Design weaknesses:
• High density
• Poor electrical and thermal conductivity

Typical Products
• High integrity structures - oil rigs
• Bicycles
• Railway track
• Bearings, gears, shafts
• Cutting tools
• Pressure vessels
• Hand tools (spanners, hammers etc)
Process Notes

Metal forming
• Usually wrought, not cast.
• Powder metal forming is most commonly used with high alloy steels
• Rolling, extrusion and sheet forming are only used with low alloy (lower
strength) alloys.

Machining
• Gets more difficult for the stronger alloys (usually those with higher
alloy content).

Welding
• Suitable for use with most techniques.

Environmental Issues
• Steel production uses a lot of energy, but less than most metals.
• Steel is easily recycled.
2. Nickel alloys
Overview

• Nickel alloys are dense, stiff, strong alloys used primarily for their
strength and corrosion resistance at high temperatures.

• Magnetic alloys

Design strengths:
• High strength at high temperature
• High corrosion resistance
• High stiffness
• Easy to shape

Design weaknesses: Typical Products

• High density • Jet engines for aircraft
• Coins
• Tanks for chemicals
Process Notes

Metal forming

• Not used in sheet forming.

• Forging is important for coins and jet engine parts.
• Special die casting processes are used for jet engine blades.

Machining

•Usually readily machined.

Joining

•Arc welding only with inert gas or in vacuum.

Environmental Issues

• Nickel production uses quite a lot of energy, but the volume in use is
small.
SUMMARY
PROPERTIES
 Stiff, Strong, Tough, Light

Not Stiff

Not Strong

Not Tough

Not Light
 Young's Modulus and Specific Stiffness

Overview

 measures the resistance of a material to elastic (recoverable)

deformation under load.

 A stiff material has a high E and changes its shape only

slightly under elastic loads (e.g. diamond)

 A flexible material has a low E and changes its shape

considerably (e.g. rubbers).
 A stiff material requires high loads to elastically deform it -
not to be confused with a strong material, which requires high
loads to permanently deform (or break) it.

 The stiffness of a component means how much it deflects

under a given load.

 This depends on the Young's modulus of the material, but

also on how it is loaded (tension, or bending) and the shape and
size of the component.

 Specific stiffness is Young's modulus divided by density (but

should more properly be called "specific modulus“)
Design issues
• Stiffness is important in designing products which can only be
allowed to deflect by a certain amount (e.g. bridges, bicycles,
furniture).
• Stiffness is important in springs, which store elastics energy (e.g.
vaulting poles, bungee ropes).
• In transport applications (e.g. aircraft, racing bicycles) stiffness is
required at minimum weight.
• In these cases materials with a large specific stiffness are best.

Measurement
• Tensile testing is used to find many important material properties.
The compression test is similar but uses a stocky specimen to
prevent bending.
 Strength and Specific Strength
Overview

• The strength of a material is its resistance to failure by permanent

deformation (usually by yielding).

• A strong material requires high loads to permanently deform (or

break) it - not to be confused with a stiff material, which requires
high loads to elastically deform it.

• For metals, polymers, woods and composites, "strength“ refers to

loading in tension (as failure is by yielding).

• For brittle materials (ceramics), failure in tension is by fracture,

and the "tensile strength" is very variable.

• The "strength“ is then "compressive strength" (which requires a

much higher load).

• Specific strength is strength divided by density.

Design issues

• Many engineering components are designed to avoid failure by yield or

fracture (cranes, bikes, most parts of cars, pressure vessels).

• In structural applications, brittle materials are nearly always used in

compression (e.g. brick, stone and concrete for bridges and buildings).

• In transport applications (e.g, aero planes, racing bikes) high strength is

needed at low weight. In these cases materials with a large "specific
strength" are best.

Measurement

• Tensile testing

• Two measures of strength are defined - yield strength and ultimate tensile
 Where from?
• Data Books / Hand Books
• Standard / Reference Books
• Industrial Reports / Communications
• Research Articles / Patents
• Internet
 What form?
• Text and Tables

• Graphs and Bar charts

• Bubble charts / Material selection charts

Text and Tables
Bar Charts
Bubble Charts / Asbhy’s Charts
Applications of the chart

• Stiff lightweight materials are hard to find, for things like sports products and
bicycles

• Many applications require stiff materials, e.g. roof beams.

• Many applications require low density materials, e.g. packaging foams.

• Composites appear to offer a good compromise, but they are usually quite
expensive, and wood is still used for cheaper products (e.g. oars).

• Polymers don't seem like a good choice for stiff, lightweight products – but they can
be reinforced by incorporating stiffening ribs into the design (for instance, look
inside a plug).

• Ceramics are quite light and very stiff – but their poor tensile strength and
toughness means they are likely to fracture.