LEARNING OUTCOME 1 TUTORIAL 3

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UNIT 21: MATERIALS ENGINEERING

Unit code: F/601/1626

QCF level: 4

Credit value:

15

LEARNING OUTCOME 1

TUTORIAL 3

On successful completion of this unit a learner will:

1 Be able to determine the properties and selection criteria of materials from tests and data sources

Criteria for material selection: definitions of material properties and character appropriate to the learner’s programme of study e.g. mechanical, physical, chemical, process character and costs for range of materials (metals, ceramics, polymers, and composites)

Categorise materials: an appreciation of the properties of metals: ceramics, polymers and composites; recognition of micro structural characteristics of the more commonly used engineering materials

Materials testing: tests to determine the properties of commonly used engineering materials e.g. metals, ceramics, polymers and composites (such as electrical conductivity/resistivity, magnetic susceptibility, mechanical strength, hardness, toughness, fatigue and creep resistance, corrosion and reactivity, wear resistance, optical and thermal properties, formability); appropriate statistical methods and the processing of test data

Data sources: published data e.g. British Standards, ISO, product data sheets, IT sources, standard published data sources, manufacturers’ literature, job-specific information such as specifications, test data and engineering drawings; assessment of data reliability

Pre-Requisite Knowledge

Anyone studying this module needs to have studied the basic structures of materials probably by having completed the materials module at national level. Some of this material is repeated here and may be skipped if you have already completed it.

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CONTENTS

1. INTRODUCTION

2. INTRODUCTION TO MATERIALS CLASSIFICATION AND TERMINOLOGY 3. METALS Ferrous Non Ferrous 4. POLYMERS Thermoplastic Thermosetting Elastomer 5. CERAMICS 6. COMPOSITES 7. LAMINATES 8. SMART MATERIALS 1. INTRODUCTION

In engineering and technology the knowledge of materials has been at the forefront of science and has enabled us to produce amazing advances in all fields from medicines to electronics. We need to know about the mechanical properties (such as strength, durability, ductility and so on), the thermal properties (such as specific heat, melting point and conductivity), electrical properties (such as resistivity), magnetic properties, optical properties and many others.

This module is intended for students studying materials for mechanical and manufacturing, in particular metals, plastics and ceramics. The more you understand the molecular structure of atoms, the more you will understand the nature of the material that can be made from them. The goal of this module is to enable you to select the best materials to use for a given purpose so that it performs the desired task and can be made as economically as possible.

All materials are made up of atoms and combinations of atoms called molecules. The structure determines the engineering properties of the material. You should have studied the basic structure of materials in tutorial 1. There is a wealth of information on the internet and much of it appears contradictory. This is because explanations are often simplified to avoid going into too much detail. Hopefully the information here is sufficient to give you a good start on understanding engineering materials.

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3 2. INTRODUCTION TO MATERIAL CLASSIFICATION AND TERMINOLOGY

Engineering materials are classified in various ways depending on the properties of the materials you wish to highlight. The chart below shows the way they are classified in this tutorial and during the course of the tutorial you will learn what is special about them.

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4 3. METALS

FERROUS MATERIALS

Ferrous metals are alloys or compounds in which most of the atoms are iron. Iron ore is quite abundant and relatively cheap and can be made into a variety of iron based materials with many uses in structural and mechanical engineering. Iron is produced by melting the ore and other materials in a blast furnace and then refining it. In the early stages it contains many impurities including carbon which has a dramatic affect on its properties. Pure iron is very difficult to produce and it is rarely used on its own. Iron is one of the few substances that are magnetic.

CAST IRON

In the early stages of refining the iron contains a lot of carbon and this makes it very fluid in the molten state so it is cast into ingots and then processed. Historically, cast iron was one of the first materials to be used for large scale structures. The carbon forms as graphite flakes and this makes the material very brittle but it is good for casting complex shapes. It does not rust easily so it is used to make decorative outdoor structures such as garden furniture. Cast iron breaks very easily but when used in compression it is strong so it was widely used for making columns, pillars and arch bridges. Victorian shopping arcades had delicate cast arches and reached

its grandest level in the construction of the Crystal Palace. The very first cast iron bridge at Iron Bridge, Shropshire is a place well worth visiting or looking up on the internet. Graphite makes a good solid lubricant and so the slides on machine tools are often machined from cast iron. Many internal combustion engines have cast iron cylinder blocks and cylinder heads. The picture shows one illustrating the complexity of the casting and the need for machining.

WROUGHT IRON

Wrought iron was another traditional material from the early times. It is produced by repeatedly heating strips, stretching it and folding it. This disperses the carbon and produces a material with properties similar to pure iron. Being difficult to make it is expensive and mainly finds use in wrought iron gates and similar structures because it can be bent and shaped into decorative shapes. CARBON STEELS

Steel is an alloy of iron and other elements that gives it the required properties. One of the most important elements is carbon. Pure iron is almost unknown as carbon always gets into it during the manufacturing stage when the ore is melted with coke. Steels with carbon fall between the extremes of pure iron and cast iron and are classified as follows.

NAME CARBON CONTENT % TYPICAL APPLICATION

Dead mild 0.1 – 0.15 pressed steel body panels

Mild steel 0.15 – 0.3 steel rods and bars

Medium carbon steel 0.5 – 0.7 forgings

High carbon steels 0.7 – 1.4 springs, drills, chisels

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5 STRUCTURE OF CARBON STEEL

All metals form crystals when they cool down and change from liquid into a solid. In carbon steels, the material that forms the crystals is complex. Iron will chemically combine with carbon to form IRON CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very hard and brittle. The

more cementite the steel contains, the harder and more brittle it becomes. When it forms in steel, it forms a structure of 13% cementite and 87% iron (ferrite) as shown. This structure is called PEARLITE. Mild steel contains crystals of iron (ferrite) and pearlite as shown.

As the % carbon is increased, more pearlite is formed and at 0.9% carbon, the entire structure is pearlite. If the carbon is increased further, more cementite is formed and the structure becomes pearlite with cementite as shown.

Carbon steel can have a wide range of mechanical properties (e.g. strength, hardness, toughness, and ductility) and these properties can be changed by heat treatment. Heat treatment changes the structure of the carbon and steel and this is a large area of study.

SELF ASSESSMENT EXERCISE No. 1

1. State 3 advantages and one disadvantage of making something from cast iron. Name some items that are made from cast iron.

2. If a component could perform equally well whether it is made from mild steel or titanium, for what simple reason would steel be chosen?

3. What % carbon does steel contain when its structure is entirely pearlite? 4. What is the main mechanical property of cementite?

5. Conduct further research and then sketch and describe the crystal structure of cementite. Here are two useful websites.

http://www.msm.cam.ac.uk/phase-trans/2003/Lattices/cementite.html)

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6 ALLOY STEELS

Besides carbon, iron is alloyed with other elements to produce desirable properties. Generally they fall into two groups: low alloy steels and high alloy steels depending on the % of alloying elements. This is done in order to improve the mechanical properties. Commonly alloy elements include manganese (the most-common one), nickel, chromium, molybdenum, vanadium, silicon and boron. Less common elements include aluminium, cobalt, copper, cerium, niobium, titanium, tungsten, tin and zirconium. Some of these find uses in exotic and highly-demanding applications, such as in the turbine blades of jet engines, in spacecraft and in nuclear reactors. Because of the ferromagnetic properties of iron, some steel alloys find important applications where magnetism is important including electric motors and transformers. This is a vast area of study and cannot possibly be covered here.

LIST OF ALLOYING MATERIALS IN STEEL

Element Percentage Primary function

Aluminium 0.95–1.30 Alloying element in nitriding steels

Bismuth - Improves machinability (makes it easier to cut on machine tools) Boron 0.001–0.003 A powerful hardenability agent

Chromium 0.5–2 Increases hardenability

4–18 Used in stainless steel and increases corrosion resistance Copper 0.1–0.4 This can help improve corrosion resistance

Lead - Often with sulphur makes the steel machinable at high speeds (free cutting)

Manganese

0.25–0.40 Combines with sulphur with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.

>1 Increases hardenability by lowering transformation points and causing transformations to be sluggish

Molybdenum 0.2–5

Stable carbides inhibit grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine tools and also the turbine blades of jet engines. Also used in rocket motors.

Nickel 2–5 Toughens the steel

12–20 Increases corrosion resistance Silicon

0.2–0.7 Increases strength

2.0 Spring steels

Higher

percentages Improves magnetic properties

Titanium - Fixes carbon in inert particles; reduces martensitic hardness in chromium steels

Tungsten - Also increases the melting point.

Vanadium 0.15 Stable carbides; increases strength while retaining ductility; promotes fine grain structure. Increases the toughness at high temperatures

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7 SELF ASSESSMENT EXERCISE No. 2

Conduct some research on the internet to answer the following. In each case describe the properties of the steel that make them suitable for purpose and manufacture.

1. The kind of steel used to make modern railway track.

2. The kind of steel is used to make car panels and the properties of the steel that makes it suitable. 3. The kind of alloying elements used in the manufacture of high quality hack saw blades.

4. The kind of steel that can be made into cheap wire for garden fences. 5. The kind of steel used to make crank shafts in internal combustion engines. 6. The ferrous material commonly used to make engine blocks.

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8 STAINLESS STEEL

One important alloy steel group is Stainless Steel. The name covers a wide range of steel types and grades for corrosion or oxidation resistant applications. 'Stainless' is a term coined early in the development of these steels for cutlery applications. It was adopted as a generic name for these steels and now covers a wide range of steel types and grades for corrosion or oxidation resistant applications. Stainless steels are iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness. These include metals such as:

 Nickel  Molybdenum  Titanium  Copper 

Non-metal additions are also made, the main ones being:  Carbon

 Nitrogen

Stainless steels of various kinds are used in thousands of applications such as :-

Domestic Applications: - cutlery, sinks, saucepans, washing machine drums, microwave oven liners and razor blades.

Construction:- cladding, handrails, door and window fittings, street furniture, structural sections, reinforcement bar, lighting columns, lintels and masonry supports.

Transport: - exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers and refuse vehicles.

Chemical/Pharmaceutical:-pressure vessels and process piping.

Oil and Gas: - platform accommodation, cable trays, and sub-sea pipelines. Medical: - Surgical instruments, surgical implants and MRI scanners.

Food and Drink: - Catering equipment, brewing, distilling and food processing. Water: - Water and sewage treatment, water tubing and hot water tanks.

General: - springs, fasteners (bolts, nuts and washers) and wire.

This a useful link to find out more about stainless steel. http://www.bssa.org.uk/index.php

SELF ASSESSMENT EXERCISE No.3

1. Why is stainless steel used for containers where cleanliness and a sterile environment is required?

2. An architect is trying to decide whether a balcony safety rail and supports should be made from cast iron, steel or stainless steel. Discuss the relative merits of these materials?

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9 NON FERROUS METALS

There are a large number of metals with various properties that make them important. Here is a brief list of some of them with some of their properties.

COPPER - red colour.

- a good conductor of heat and electricity and widely used for electrical components. - good corrosion resistance.

- malleable and ductile and easily drawn into wire and tube. - easily joined by soldering.

ALUMINIUM - white colour

- not as good as copper for conducting electricity but cheaper and often used instead of copper. - good corrosion resistance.

- can be made into light and strong aluminium alloy and is used for many structural components. - easily rolled into thin sheets and foil.

- often extruded into various sections for light structures. LEAD

- bluish grey colour.

- very heavy (Dense). Used for screening from radiation. - soft.

- good corrosion resistance.

- added to other metals to make them more machinable. - added to tin it makes solder.

TIN

- silvery white colour.

- good corrosion resistance and used to coat other metals. - widely alloyed with other metals e.g. to make bearings. ZINC

- bluish white colour. - good corrosion resistance.

- used to coat steel sheets and components such as nails (galvanised). - widely alloyed with other metals to make a good casting material. TITANIUM

- low-density element (approximately 60% of the density of iron) - can be highly strengthened by alloying and working.

- nonmagnetic

- good heat-transfer properties.

- coefficient of thermal expansion lower than that of steels and less than half that of aluminium. - high melting point (higher than steel).

- immune to attack by most mineral acids and chlorides - non-toxic and very compatible with human tissue. SILVER

- the best electrical conductor of all but too expensive for making wires and cables. - mainly used for jewellery.

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10 GOLD

- very resistant to oxidisation and used for coating electrical contacts in high quality switches. - mainly used for jewellery.

PLATINUM

- better than gold but more expensive - mainly used for jewellery.

ALLOYS OF NON FERROUS METALS

Some of the alloys formed by non ferrous metals are:

Brass – Brass is basically a substitutional alloy of zinc in copper. It has a range of properties depending on the exact structure including strength, machinability, ductility, wear-resistance, hardness, colour, antimicrobial, electrical and thermal conductivity, and corrosion-resistance.

Brass is used in instruments (musical and other), coins, fixtures such as doorknobs, bolts, etc. Brass is also used to decorate many household items such as clocks and mirrors.

Bronze – Is an alloy of copper with up to 10% that may contain phosphor, silicon, manganese, aluminium, lead, iron and other elements. It can be quite hard and brittle. The tin gives the material resistance to wear and it is often better than brass in resisting corrosion.

The various types of bronze have different levels of wearability, machinability, corrosion-resistance and ductility for deep drawing. Bronze parts are typically used for bearings, clips, electrical connectors and springs.

Aluminium Bronze – This is a copper-aluminium alloy that may contain iron, nickel, and/or silicon for greater strength. It is used for tools and, because it will not spark when struck, for parts to be used around flammable materials. Aluminium bronze is frequently used for aircraft and automobile engine parts.

Manganese Bronze – This is often used for ship propellers because it is strong and resists saltwater corrosion.

Aluminium – This can be made strong by adding other elements and on a weight for weight basis is stronger than steel. These alloys are often classed as wrought or cast. Wrought alloys can be rolled into plates. Aluminium alloys are extensively used in the production of automotive engine parts, transport, packaging, electrical application, medicine, and construction of homes and furniture. High grade alloys are widely used in the aeronautical industry because of the lightness and strength. They are widely used to make tubes for structures requiring the same properties.

One of the best known alloys is Duralumin containing up of 90% aluminium, 4% copper, 0.5%-1% magnesium, and less than 1% manganese.

Titanium – Pure titanium is suited to many uses including use in surgery to support bones and teeth. The alloys have up to 6% aluminium and 4% vanadium by weight. This mixture forms a solid solution which varies with temperature and so allows it to be strengthened by heat treatment.

The combination of high strength, stiffness, toughness, lightness, and resistance to corrosion over a wide range of temperatures makes it highly suited for aerospace structures. The excellent corrosion resistance and biocompatibility makes it useful in chemical and petrochemical applications and salt water applications.

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11 SELF ASSESSMENT EXERCISE No. 4

1. Copper is a good conductor of electricity and heat. What mechanical properties make it suitable for forming into wires and tubes?

2. The leading edges of supersonic aeroplanes are often made from titanium alloy. What are the properties that make it suitable for this?

3. Titanium alloy is better than steel for most applications. Why is it not more widely used in engineering?

4. What are the mechanical properties of aluminium that makes it suitable for the manufacture of drink cans and foil wrapping?

5. Decorative frames for pictures and fire places are often made from brass or brass plated steel. What is the property of brass that makes it so suitable?

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12 4. POLYMERS

The basic microstructure of polymers is covered in tutorial 1. Here are some basic terms and definitions.

MONOMER

A monomer is a single molecule which when joined to others of the same kind in a chain is called a Polymer.

CO-POLYMERS

These are long chain molecules made up of different monomer joined together in a regular pattern. An example is polyvinyl chloride that has alternate molecules of vinyl chloride and vinyl acetate. THE MICROSTRUCTURE OF POLYMERS

When metals solidify we see a crystalline structure form as the small molecules move easily into regular shapes with ionic bonds holding them together. Polymers have long chain molecules entangled with each other and this makes it difficult for them to move and form a crystalline pattern. The solidification process of polymers may produce some regions in crystalline form and these are called crystallites. The rest of the material is amorphous. The crystalline region can be as much as 90% in some polymers.

MELTING

Metals and other crystalline materials melt at a fixed temperature but amorphous materials tend to soften and become more like a viscous liquid. If a polymer has a large crystalline content, the change from crystalline to amorphous structures when it melts is accompanied by an increase in the volume. The temperature at which this occurs is denoted Tm. Generally, the melting point increases in

temperature with the degree of crystallinity. GLASS TRANSITION TEMPERATURE

Polymers are generally soft at normal temperatures but they can become hard and brittle when cooled. The temperature at which it changes from soft and flexible to hard and glassy, is called the glass transition temperature denoted Tg. Some polymers are hard and rigid at normal temperatures

and these have many uses. THERMOPLASTIC

Heating the polymer vibrates the molecules and if they are not cross linked, the distance between the molecules will increase and the Van der Waal forces will be reduced. This will make the polymer soften so these types may be remoulded by heating.

THERMOSETTING

The raw material is in the form of soft dough like substance often in sheet form. They set into a hard state by initiating a chemical reaction either by heating them or mixing them with a catalyst. This process called curing makes the molecules become cross-linked forming a more rigid structure. Reheating will not soften the polymer.

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13 ELASTOMERS

These are virtually the same as thermosetting but they have a very high degree of elasticity and although they stretch easily compared to metals, they spring back into shape. The tangled molecules tend to straighten when pulled but spring back when released.

GENERAL PROPERTIES

Polymers are often referred to as plastics because they often have a very large plastic range. This is not always the case and polymers exhibit a wide range of mechanical properties (strength, toughness and hardness etc.) In general polymers are very resistant to attack from chemical reagents. They have a low density compared to other materials and so for example, a plastic bottle is much lighter than a glass bottle of equivalent strength. They can be coloured or transparent and give a pleasing finished appearance to many household items. These properties make them suitable for a wide range of manufactured items such as:

 Plastic tubes/pipes

 Bottles

 Car shells/interior linings

 Cases for electronic goods

 Springs/shock absorbers

 Tool handles/cases

 Toys

 Electric wire insulation

 Seals used in hydraulics and pneumatics.

 Packaging.

 Linings to vessels.

Useful web sites

www.Vakoseals.com www.Matweb.com

http://www.efunda.com/materials/polymers/history/history.cfm?list_order=time

List typical products that are made from thermoplastics and name the type of plastic. List typical products that are made from thermosets.

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14 5. CERAMICS

The word “ceramic” is traced to the Greek term Keramos, meaning pottery or potter. Ceramics may be crystalline (e.g. diamond) or amorphous (e.g. glass). They may be broken into small particles and bonded into a matrix (e.g. a grinding wheel). Here is a list of some of the modern ceramics materials.

Alumina Zirconia Silicon Carbide

Silicon Nitride Boron Carbide Beryllia

Steatite Sterite

Whilst Bricks, Pottery, Glass and so on are widely used for every day objects, modern ceramics for engineering components have been produced for the following purposes.

 HIGH MELTING POINT – e.g. furnace linings.

 HIGH HEAT ABSORPTION (specific heat capacity) e.g. space shuttle tiles and storage heaters.

 HARDNESS – e.g. cutting tools such as Tungsten Carbide tips and grinding wheels.

 LOW CREEP AND THERMAL EXPANSION – e.g. turbine blades where any elongation would wreck the engine.

 POROSITY – e.g. used to make very tight filters whose absolute filtration rating is too tight to allow the passage of bacteria and pathogens like cryptosporidium. Such filters are used in survival kits for filtering urine and making it drinkable.

 ELECTRONIC PROPERTIES – e.g. used in semi conductors and microelectronics as parts of components, substrate, or package.

 ELECTRIC PROPERTIES – e.g. used for insulators on high power transmission lines.

 HIGH RESISTANCE TO DEGRADATION – e.g. corrosion and chemical attack There are a wide range of products. Here is a partial list.

 Bottles and glasses.

 Sanitary ware (WCs, Sinks and so on like in the picture).

 Tableware.

 Electrical insulators.

 Cooker hobs.

 Parts for heaters.

 Hair rollers and curlers.

 Parts for electronics.

 Fire bricks and tiles.

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15 Here are some more pictures of just a few of the many things made from ceramic.

Halogen Lamp Holder GLASS

The main constituent of most commercial glass is sand in other words - SILICA. This is mixed with other substances to produce the required properties.

A typical composition of glass is : 70% - 74% SiO2 (silica)

12% - 16% Na2O (sodium oxide)

5% - 11% CaO (calcium oxide) 1% - 3% MgO (magnesium oxide) 1% - 3% Al2O3 (aluminium oxide)

SELF ASSESSMENT EXERCISE No. 6 Visit http://www.dynacer.com and then :

Name and describe three components made from ceramics for electronic purposes. Name and describe three components made from ceramics for their thermal properties. Name and describe three components made from ceramics for their biological properties. Name and describe three components made from ceramics for their refractory properties.

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16 6. COMPOSITES

A composite material is a combination of two or more materials to obtain the best properties of both. There are broadly two classifications, PARTICLE and FIBRE.

PARTICLE COMPOSITES

This is a material in which particles of one material is fixed in a matrix of another. Some of the simplest examples are materials used for the construction industry. Here are some examples.

Tarmac –gravel held in a matrix of tar, ideal for roads.

Mortar and Concrete – sand, gravel and stone bonded into a matrix of cement that sets and forms a light material strong in compression. Since it can be moulded or laid down wet, it is an ideal building material.

Particle composites are used in engineering to make a range of hard cutting tools. The main product is called cermet.

Cermets – This is a material in which ceramic powder is bonded in a metallic matrix to get the best properties from both such as hardness, high working temperatures and strength. The ceramic materials are often oxides, borides and carbides. The metals are nickel, molybdenum and cobalt. The volume of a typical cermet is about 20% to bond the ceramic particles.

Cermets are used widely in electronics to make resistors and capacitors for high temperature use. Cermets are also used to make dies and cutting tips for tools used in machining and sawing of hard materials. Particles of very hard ceramic materials are embedded in a metal. They have good resistance to oxidation and keep their hardness at high temperatures. Typically the cermet contains titanium carbide and titanium nitride. For example, tungsten carbide embedded in cobalt make very hard cutting tools and dies. They can be compacted into the required shape and then heated to sinter them. This means the cobalt is hot enough to re-crystallise and form a matrix around the tungsten. The pictures show examples of cutting tools with cermet tips.

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17 FIBRE COMPOSITES

You will find a good illustrated tutorial on composites at this link.

Examples are:

 Reinforced concrete.

 Glass reinforced plastics (GRP)

 Carbon fibres.

 Aramid fibres.

 Natural composites such as wood.

Concrete is very brittle and weak in tension so it is normally only used for support structures (columns and solid floors). By adding steel rods, the structure becomes stronger in tension and withstands some bending. Hence bridges, unsupported floors and other structures where some bending occurs can be made to take the tension. The resulting structure is lighter than steel on its own. Brittle materials fail by cracks spreading through them with little resistance. Adding fibres prevents the crack opening and spreading.

Glass and carbon fibres when made new are very strong and flexible and if they are imbedded in a matrix of plastic (thermosetting) they retain their high tensile strength. The result is a very strong flexible and light structure. Many things are made from these materials such as boat hulls, tennis rackets, fishing poles and racing cycle frames like the one shown.

Many GRP products are made from sheets of chopped fibres laying in random directions. This is formed into the shape required often in a mould and pasted with epoxy resin. All the air must be forced out of the fibres and resin forced in either with rollers, brushes or with a vacuum process. A gel coat is often used to form an outer layer with a smooth coloured finish. You can see the process on this video link

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18 7. LAMINATES

An important type of composite material is those made up from laminated layers of either the same or different materials glued to each other in layers to obtain an overall structure with the combined properties of each layer. For example one layer may make the material water proof as in laminate flooring. Other examples are snow boards and skis that need to be strong and flexible. Many aircraft parts are made from sheets of laminated material.

PLYWOOD

Grainy materials like wood have strength in one direction only so if they are layered with the grain at 90o to each other, equal strength is obtained in both.

TYRES

An ideal tyre must have strength, good grip, not wear and not puncture. For this reason a tyre consists of laminated layers of Rayon, Nylon and Steel in a rubber matrix with cross plies to produce strength in all directions.

Tyre Laminate Flooring Plywood

It is very important that laminated structures do not come apart between any layers (de – lamination) so appropriate bonding materials must be used. This can occur due to stress or environmental conditions such as chemical spills.

1. Explain how a material that is strong in one direction and weak in another may be formed to be strong in both directions.

2. Find out what materials are used to manufacture snow boards and how they are joined together. Explain the desired properties that are produced as a result of this process.

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19 8. SMART MATERIALS

This topic is not specifically mentioned in the syllabus but is added to complete the picture.

The structures and types of smart materials were described in outcome 1. The syllabus item is “The effects of post-production use”. This phrase does not make a lot of sense to the author and has been construed as meaning “post-production uses”. Here we will look at some uses for these materials. PIEZOELECTRIC

These are materials that change shape when a voltage is applied to it so if an alternating voltage is applied it can be made to emit sound. It works in reverse and produces a voltage when stressed in various ways and so is used to make microphones and instruments such as pressure transducers. The material is mainly crystalline ceramics and the most common one is

called QUARTZ which is Silicon Dioxide (SiO2). Silicon dioxide is the most

abundant mineral in the Earth's crust. Sand is mainly composed of this and used to make glass. It occurs in many crystalline forms and is known as a polymorph, which means many forms. Quartz is mostly a trigonal crystal structure of SiO2 shown in the picture. There are other materials mineral,

biological and man made that exhibit piezoelectric properties.

Electronic oscillators/timers - Quartz can be cut into precise crystals and used in electronic oscillators to regulate the frequency. The crystal resonates at the frequency defined by its dimensions and so it can be used to regulate or filter electronic oscillations. A good post production use is in quartz in watches.

Transducers – Many transducers involve making electrical measurements based on some form of mechanical movement or stress. If the thing being measured can be made to stress the piezoelectric material, a charge will be produced that forms the basis of the electrical signal that can be electronically processed to display the thing being measured. Here are some examples.

Strain Gauge – The piezo-resistive material forms the basis of gauges fixed to structures so that any changes in dimensions produce a change in resistance that is electronically processed to indicate the strain. This can be applied to a variety of instruments where the strain is produced by some other affect.

Force Gauge - Force produces strain so the strain gauge is the basis of many weighing systems. The gauge can be incorporated into many systems such as Torque measurement. The picture shows a button load cell.

Pressure Transducer – The pressure deforms a surface with piezoelectric material on it and so the electric charge represents the pressure. A typical sensor is shown.

Accelerometers – Acceleration produces a force which produces strain and hence

strain gauges form the basis of instruments to measure acceleration. This forms the basis of many systems ranging from navigation to computer toys.

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20 Vibration monitor – This is fundamentally a microphone that converts vibrations into a proportional electric signal.

Audio Devices – Microphones pick up air vibrations so a piezoelectric material will produce an electric signal representing the sound. When an oscillating electric signal is applied to piezoelectric material, it vibrates so simple loudspeakers and tone generators are made from this material. The picture shows an audio alarm using this principle.

Igniters – When a piezoelectric material is struck the high fast rate of strain produces a large charge of electricity that is sufficient to produce a spark between two electrodes and this is used in devices for igniting gas flames such as that shown.

Actuators and Motors– Piezoelectric materials change dimensions under the control of an electric charge so they can produce small mechanical motion that can produce linear

movement or rotational movement. The movement is precise and is ideal for control devices. ELECTRO-RHEOSTATIC (ER) FLUIDS

These change their viscosity in the presence of an electric field. This may be used to change a liquid into a gel or almost solid structure. The basic structure of these fluids is fine solid particles (colloidal) mixed and suspended in a fluid. ER fluids can be as simple as milk chocolate or cornstarch and oil. In a strong electric field, the particles are polarised like little magnets and form

chains or columns parallel to the field. You can see a simulation at this link.

http://www.ssslab.com/ehtml/3_1.php

This may be used to change a liquid into a gel or almost solid structure. Here are some applications. Hydraulic Valves – The change in the fluid is very quick so valves can be made to open and close very quickly at a flick of a switch.

Clutches and Brakes – The plates of the clutch are locked together by applying a charge to the fluid separating them. If one set of plates is fixed, the system is a brake.

Shock Absorbers – Many shock absorbers consist of a piston sliding inside a cylinder full of fluid. The fluid is forced through holes to produce a damping force. Increasing the viscosity of the fluid increases the resistance to motion so the stiffness of the shock absorber can be controlled electrically if the fluid is ER.

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21 MAGNETO-RHEOSTATIC (MR) FLUIDS

These change their viscosity in the presence of a magnetic field. This may be used to change a liquid into a gel or almost solid structure. MR fluid is composed of carbonyl iron particles, 'soft' iron particles which are only 3-5 µm in diameter, hydrocarbon oil and other additives to produce the required fluid properties. Applying a magnetic field forces the particles to line up so that the liquid becomes solid. The stronger the field, the more viscous the fluid becomes. Removing the magnetic field unlocks the particles and turns the solid back to liquid. MR fluids are being used in exercise machines, washing machines, car shock absorbers, and artificial legs.

The applications are similar to those of ER fluids except that the change is produced by using an electromagnet instead of directly applying the electric charge to the fluid. Dampers and shock absorbers containing MR fluids are used in a variety of things like washing machines to damp out vibrations. Here are additional applications.

Prosthetics – Artificial limbs make use of damping devices (shock absorbers) and MR fuids are better for this than ER fluids.

Body Armour – It may become possible in the near future to make bullet proof clothes containing MR fluid.

Exercise machines - The stiffness or resistance in an exercise machine is sometimes electrically controlled with MR dampers.

SHAPE-MEMORY ALLOYS (SMA)

This is a metal alloy that "remembers" its original, cold-forged shape. When deformed beyond the yield point normal metals will stay deformed but SMA will return to the pre-deformed shape when it is heated. Imagine a car that can have its dents removed by simply heating the panel.

Alloys exhibiting this property are

 Copper/zinc/aluminium/nickel.

 Copper-aluminium-nickel.

 Nickel/Titanium.

You should have come across the terms Austenite and Martensite in tutorial 2 and learned that the terms also apply to other alloys. Alloys that have these two phases form the basis of SMA. It is found that Austenitic structures can be transformed into Martensitic structures by stress and strain as well as by heat. The crystalline changes produce distortion in the crystal lattice and dimensional changes in the bulk material. Heating can reverse this and this forms the basis of memory metals.

An important property of these alloys is that they are hard and springy above the transition temperature (the Austenitic form). Below the transition temperature they are soft and easy to bend (the Martensitic form).

NiTi alloys are more expensive. The transition from the martensitic phase to the austenitic phase is only dependent on temperature and stress, not time, as with carbon steel. The change does not involve the diffusion of atoms and it is reversible. This gives it the memory properties. While martensite can be formed from austenite by rapidly cooling carbon steel this process is not reversible, so steel does not have shape-memory properties.

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22 One of the most popular memory metals is called Nitinol. It is mainly produced as wire and thin sheets before being turned into products.

You can download a very good tutorial on this material on this web site.

http://www.ccmr.cornell.edu/education/modules/documents/Nitinol.pdf

There is a lot of information about Nitinol manufacture and properties at this link.

http://www.memry.com/nitinol-iq/nitinol-fundamentals

You can see a Video demonstrating Nitinol wire at this link.

http://www.youtube.com/watch?v=Y7jjqXh7bB4

An important property of these alloys is that they are hard and springy above the transition temperature when in the Austenitic form. Below the transition temperature when in the Martensitic form it is soft and easy to bend.

Nitinol is super elastic, biocompatible and resistant to fatigue. Here are some applications:- Glasses – The alloy has a low transition temperature is makes it ideal for

the frames for optical glasses which are almost indestructible except at cold temperatures. The picture demonstrates the super elastic properties at normal temperatures.

Stents - are spring like devices inserted into arteries and veins to stop them narrowing. Nitinol is ideal as it stretches easily and is biocompatible.

Teeth Brace - In dentistry the tooth brace has an archwire that is bent into the shape of the patient’s teeth. When the wire warms up in the mouth it tries to change shape and pushes on the teeth to gently force them into a new shape. The transition temperature for this use must be less than the body temperature (typically 27oC).

Thermostats – When used in devices like electric kettles, the change in shape can be made to switch off the power at say 98oC and this is an example of a thermostat. This implies that SMA can be made to have a range of transition temperatures. There are many applications for these devices such as anti-scalding valves can be used in taps (water faucets) and shower heads.

Aircraft - Variable Geometry e.g. to reduce engine noise.

Pipe Couplings - oil line pipes for industrial applications, water pipes and similar types of piping for consumer/commercial applications.

Muscle wire – This is nitinol wire that actually shortens in length when electrically powered. They can lift thousands of times their own weight. The direct linear motion is ideal for robotic use and small motors or solenoid activated devices. You can see a small robot in action with this wire at this link.

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23 COLOUR CHANGING MATERIALS

Thermochromic – Some substances change colour with temperature and this property is called Thermochromism. This has many uses such as indicating if a drink is too hot. There are two basic types, liquid crystals and leuco dyes. Leuco dyes allow wider range of colours to be used, but their response temperatures are more difficult to set with accuracy.

Liquid crystals – are the name given to the material used in liquid crystal displays (LCD) but the optical properties are controlled by an electric field. These are widely used in display screens for many electronic devices.

Photochromic – Some materials change colour with light intensity. Glasses that darken in bright sunlight are an example. Usually, they are colourless in a dark place, and when sunlight or ultraviolet radiation is applied the molecular structure of the material changes and it exhibits colour. When the relevant light source is removed the colour disappears. This is used with T shirts to make logos appear. The picture shows colour changing threads. These materials may be purchased from http://www.mindsetsonline.co.uk

Electroluminescent – These are materials that emit light of various colours when electricity passes through them. The material may be organic or inorganic. A typical EL material is a thin film of zinc sulphide with manganese and gives out a yellow-orange light.

SELF ASSESSMENT EXERCISE No. 8 1. List some uses for piezoelectric.

2. List some uses for ER.

3. List some uses for MR fluids. 4. List some uses for SMA.

5. Define the meaning of thermochromatic, photochromic and electroluminescent.

6. The SMA Nitinol stands for nickel, and tin and the “nol” part stands for something also. Find out what this is.