Engg Materials.pdf

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Materials

Materials 1 Introduction to materials ................................................................................................................ 3 2 Properties of materials...................................................................................................................... 4 (a) Physical properties ........................................................................................................... 4 (b) Chemical properties ......................................................................................................... 5 (c) Mechanical properties ...................................................................................................... 8 (i) (ii) (iii) (iv) (v) (vi)

Tensile strength .................................................................................................................................... 8 Compressive strength ........................................................................................................................... 8 Ductility ............................................................................................................................................... 8 Toughness ............................................................................................................................................ 9 Hardness............................................................................................................................................... 9 Stiffness.............................................................................................................................................. 10

(d) Machining properties ..................................................................................................... 10 (i) (ii) (iii) (iv)

Casting properties .............................................................................................................................. 10 Forging properties .............................................................................................................................. 10 Welding properties ............................................................................................................................. 11 Cutting properties............................................................................................................................... 11

3 Material testing ....................................................................................................................................11 (a) Hardness testing ..............................................................................................................11 (b) Tensile testing ................................................................................................................ 12 (c) Stiffness testing .............................................................................................................. 14 (d) Toughness testing........................................................................................................... 14

4 Metal ........................................................................................................................................................... 14 (a) Ferrous metal ................................................................................................................. 15 (i) (ii) (iii) (iv) (v) (vi) (vii)

Pig iron ............................................................................................................................................... 15 Cast iron ............................................................................................................................................. 16 Wrought iron ...................................................................................................................................... 16 Steel ................................................................................................................................................... 16 Carbon steel ....................................................................................................................................... 17 Alloy steel .......................................................................................................................................... 17 Stainless steel ..................................................................................................................................... 18

(b) Non-ferrous metal .......................................................................................................... 18

5 Timber ....................................................................................................................................................... 20 (a) Softwood ........................................................................................................................ 21 (b) Hardwood ...................................................................................................................... 22 (c) Manufactured board ....................................................................................................... 23 (i) Plywood ............................................................................................................................................. 23 (ii) Fibreboard .......................................................................................................................................... 24

6 Plastics ....................................................................................................................................................... 25 (a) Thermoplastics ............................................................................................................... 25 (b) Thermosetting plastics ................................................................................................... 27

7 Concrete ................................................................................................................................................... 28 8 Compound materials....................................................................................................................... 28 1

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(a) Layer-formed compound materials ................................................................................ 28 (b) Fibre-formed compound materials ................................................................................ 29 (c) Particle-formed compound materials ............................................................................. 29

9 Degradation and protection of materials.......................................................................... 30 (a) Effects of the environment on materials ........................................................................ 30 (b) Protection of materials ................................................................................................... 31

10 Reinforcement of materials ..................................................................................................... 32 (a) Metal ............................................................................................................................. 32 (i) Hardening........................................................................................................................................... 32 (ii) Tempering .......................................................................................................................................... 32 (iii) Annealing ........................................................................................................................................... 33

(b) Concrete ......................................................................................................................... 33

Exercise ................................................................................................................................. 34

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Materials 1 Introduction to materials In modern society, people use a lot of tools, machinery and buildings, for example, stationery, wardrobes, cars and bridges, etc. All are made of different kinds of materials (Fig. 1). Each kind of material has its own unique characteristics and uses. For example, stationery items are usually made of plastics; wardrobes are usually made of wood; cars are usually made of metals; while bridges are usually made of concrete. In recent years, compound materials (such as glass fibre) are gradually being used in making light but strong tools and products, e.g. canoes.

(a) Stationery

(b) Wardrobe

(c) Car

(d) Bridge

Fig. 1 There are various materials used by people. Examples of common materials used in engineering are plastics, wood, metals, concrete and compound materials. In addition to the above, other commonly used materials include stone, ceramics, cloth, glass, paper, leather and rubber. Materials can be classified into two main categories: natural and artificial. Natural materials are those that can be found in nature. They only need simple processing before they can be used, like wood and metals. Artificial materials are materials that need more complicated processes to produce, like plastics, concrete and compound materials. Fig. 2 shows how materials are categorised.

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Fig. 2 Categories of commonly used materials

2 Properties of materials One must recognise the unique properties of materials before using them, in order to take full advantage of their benefits. The properties of materials can be classified into physical, chemical, mechanical and machining properties.

(a) Physical properties The properties that materials have due to the substance they are made of are called physical properties. Such physical properties will not change under any external forces. For example, the melting point of ice is 0℃, no matter what kind of heat energy is used to melt it. Common physical properties of materials include density, melting point, boiling point, specific heat capacity, latent heat of fusion, latent heat of vaporisation, coefficient of linear expansion, thermal conductivity and electrical conductivity. Table 1 lists the definitions of some physical properties and their examples.

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Physical property

Definition

Example

Density

Mass per unit volume

The density of gold is 6980 kg m-3

Melting point

The temperature at which a substance changes from a solid state to a liquid state

The melting point of copper is 1083℃

Boiling point

The temperature at which a substance changes from a liquid state to a vapour state

The boiling point of water is 100℃

Specific heat capacity

The amount of heat energy absorbed by a unit mass of substance when its temperature is increased by 1℃

The specific heat capacity of iron is 480 J kg –1℃-1

Latent heat of fusion

The amount of heat energy absorbed by a unit mass of substance at its melting point

The latent heat of fusion of soft ice is 3.4×105 J kg-1

Latent heat of vaporisation

The amount of heat energy absorbed by a unit mass of substance at its boiling point

The latent heat of vaporisation of aluminium is 10.1×106 J kg-1

Coefficient of linear expansion

The increase in length of a unit of a substance when its temperature is increased by 1℃

The linear expansion coefficient of silver is 20×10-6℃-1

Thermal conductivity

The ability of a substance to conduct heat energy

Silver is the metal with the best thermal conductivity

Electrical conductivity

The ability of a substance to conduct electricity

Silver is the metal with the best electrical conductivity

Table 1 Physical properties of materials

(b) Chemical properties Materials on earth can be divided into over one hundred types of elements, such as oxygen, hydrogen, mercury, gold, silver and iron. The smallest particle in the composition of an element is called an atom. This is also the basic particle that takes part in chemical reactions. An atom is composed of a number of fundamental particles, including protons, neutrons and electrons (Fig. 3). A proton carries positive charge, an electron carries negative charges, and a neutron does not carry any charge. Each atom of an element has an equal number of protons. This number is known as the atomic number of that element.

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Fig. 3 The structure of an atom If elements are arranged in ascending order of their atomic numbers, we shall discover that elements with similar chemical properties appear in a repetitive manner. This phenomenon can be represented in the periodic table (Fig. 4). The English letters in the boxes represent the chemical symbols of the elements, and the numbers represent the atomic number of the elements. In the periodic table, elements in the same column are called a group. They all have similar chemical properties within a group.

Fig. 4 Periodic Table

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For example, the group on the left-hand side of the periodic table is alkali metals. These include metals like lithium (Li), Sodium (Na) and Potassium (K). They all have similar chemical properties. The group on the right hand side of the periodic table is called inert gases. These include Neon (Ne), Argon (Ar) and Krypton (Kr). They are all very inactive. Hence, inert gases are usually used instead of air in light bulbs, to prevent the oxidation of the tungsten filament at high temperatures.

Fig. 5 Inside a light bulb

Fig. 6 Titanium is usually used in space aeronautics

Elements can be roughly classified into metals and non-metals. Most of the elements in the periodic table are metals. They are located on the left-hand side of the table, for examples, Potassium (K) and Magnesium (Mg). In contrast, non-metals are located on the right hand side of the periodic table, for examples, Phosphorus (P) ad Sulphur (S). Transition metals are located in the centre of the periodic table. They usually have some special properties, for example, they are hard and durable, good conductors of electricity; and they have an attractive appearance. Therefore, they are often used as engineering materials. Iron (Fe), Copper (Cu) and Titatium (Ti) are examples of transition metals (Fig. 6). To prevent the periodic table from being too wide, the elements known as lanthanides and actinides are located separately at the bottom. Elements can be combined to form tens of thousands of compounds with different properties. For example, water (H2O) is a compound of Hydrogen (H) and Oxygen (O) (Fig. 7). The process of forming a new compound from an old compound is called a chemical reaction. The smallest particle of a compound is called a molecule. Molecules of compounds are composed of atoms. The properties of atoms do not change during a chemical reaction. The atoms are only rearranged. For example, Oxygen (O2) and Ozone (O3) molecules are both composed of Oxygen (O) atoms (Fig. 8). However, the number of Oxygen atoms and their arrangement differ in the two types of molecules, hence they have different properties.

Fig. 7 Water molecule

Fig. 8 Oxygen (O2) and Ozone (O3) molecules

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The chemical properties of a substance include its various chemical reactions such as the effects of oxidation on the substance, its ability to resist acidification and alkalization. For example, one of the chemical properties of aluminium is that after its oxidation, a harder aluminium oxide is formed. However, when it is in contact with strong acid, it is easily corroded, and releases oxygen. Hence, aluminium plates and acid may be used together to produce the oxygen that is needed in oxygen balloon. On the other hand, gold is resistant to both oxidation, and corrosion by acids and alkali. Since it may be stored for a long time without damage, it is suitable for making coins or souvenirs. Furthermore, metals are easily decomposed by acids, but glass and plastics are resistant to strong acids.

(c) Mechanical properties The mechanical properties of a material refer to the characteristics shown by the material in a solid state when a force is exerted on it. Common physical properties include tensile strength, compressive strength, ductility, malleability, toughness, hardness and stiffness.

(i) Tensile strength Tensile strength is the ability of a material to resist deformation under a tensile force (Fig. 9a). For example, materials used for making steel cables in cranes must have good tensile strength.

(ii) Compressive strength Compressive strength is the ability of a material to resist deformation under a compressive force (Fig. 9b). For example, the outer shell of cars is usually made of steel, because it must be able to resist the compressive force produced on impact during an accident.

Fig. 9 (a) Tensile strength

(b) Compressive strength

(iii) Ductility Ductility is the ability of a material to maintain its strength during the process of shaping by being stretched out, without fracturing. For example, copper and tin are materials with good ductility. They can be drawn out into copper and tin wire.

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Fig. 10 (a) Tin is the material with good ductility

(b) Tin wire

Besides, some of the materials have malleability feature. Malleability of a material is the ability to maintain its strength during the process of shaping by being hammered and compressed, without fracturing. For example, gold and aluminium are the materials with good malleability, which can be compressed to very thin gold sheets and aluminium sheets.

(iv) Toughness Toughness is the ability of a material to undergo hammering or twisting without fracturing. For example, steel is a very tough metal that is able to withstand strong impact.

(v) Hardness Hardness is the ability of a material to resist cutting, penetrating or grinding (Fig. 11). For example, diamond is the hardest substance, and it may be used to cut other materials. Usually, the greater the hardness of a material, the greater is its brittleness, i.e. the more easily it will fracture.

(a)

(b)

Fig. 11 Using the same pricker for testing the hardness of different materials.

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(vi) Stiffness Stiffness is the ability of a material to resist bending. For example, a diving board is easily bent for diving, so it is less stiff. On the other hand, a balance beam has to be stronger to resist bending, so it is stiffer (Fig. 12).

Fig. 12 (a) A diving board with low stiffness

(b) A balance beam with high stiffness

(d) Machining properties The machining properties of a material refer to the characteristics of the material when it is being cut, shaped or joined, for example, casting, forging, welding and cutting.

(i) Casting properties Casting refers to the process of using heat to melt a material, and then pouring it into a mould and allowing it to cool to produce a solid component. Casting properties refer to the degree to which a metal is suitable for casting to produce good quality casts. Metal is a major casting material that can be used to produce tools of irregular shapes (Fig. 13a).

Fig. 13 (a) Irregular shapes of casts

(b) Casing of the car

(ii) Forging properties Forging refers to the process of heating a material to soften it, and then shaping it by compressing it in a mould. Forging properties refer to the degree to which a metal is able to undergo the exertion of an external force to produce a good quality forged product. Metal is a major forging material. For example, it can be used to make the soft steel casing of cars (Fig. 13b).  10 

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(iii) Welding properties Welding refers to the process of heating or compressing materials in order to melt or join them together. Welding properties refer to the degree to which the materials can be welded together with a tight joint during the process of welding. For example, two steel plates can be tightly joined together using an electric welder (Fig. 14a).

Fig. 14 (a) Welded products

(b) An example of cutting: metal chess pieces

(iv) Cutting properties Cutting is the processing of a material from its original shape (for example, a cylinder) to produce a final shape with desired measurements. Cutting properties refer to the properties of a material to give good cutting effects. Materials such as wood and metal often undergo the process of cutting to produce various types of tools (Fig. 14b).

3 Material testing We must thoroughly understand the properties of a material so as to use it in the appropriate way. Material testing refers to the process of using objective measures to determine the properties of a material. Material testing includes tests on hardness, tensile, stiffness, and toughness. The following is an introduction to some simple methods of material testing that can be carried out in school.

(a) Hardness testing The hardness of a material can be tested using a steel pricker of a certain weight. Place the steel pricker at an assigned height above the material to be tested. Allow it free-fall in a straight line, towards the surface of the material, as shown in Fig. 15a. The steel pricker will leave a hole on the material sample. Measure the diameter of the hole. The smaller the diameter means that the harder it is for the steel pricker to move into the material, and hence the harder the material is (Fig. 15b).

Fig. 15

(a)

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(b) Tensile testing

Fig. 16

(a)

(b)

The tensile strength of a material may be tested using the tensile testing machine shown in Fig. 16b. Shape the material (for example aluminium) in the form shown in Fig. 16a. Measure the cross-sectional area and original length of the material sample before clamping it in the tensile testing machine. Turn the handle of the tensile testing machine to stretch the material sample. It is possible to determine the extent of elongation of the sample by reading the value in the elongation meter on the machine. A separate dial on the machine displays the tensile force. Record the tensile force and the degree of elongation of the sample. The tensile properties of a material may be illustrated using a stress-strain graph. force per unit area acting on a material sample. It can be expressed as:

Stress 

Stress is the

Force Cross - sectional area

Strain is the ratio of the length of elongation to its original length. It can be expressed as: Strain 

Elongation Original length

Fig. 17a illustrates what happens when a material is being stretched. Fig. 17b shows the stress-strain graph for mild steel. Point A on the chart is called the elastic limit. This is the maximum level of stress beyond which it will yield to enduring deformation. If the load is removed before the specimen has reached its elastic limit, it will restore to its original length. This stage is called its elastic stage. Beyond its elastic stage stretching results in enduring deformation. This stage is called its plastic stage. If the load is removed within the plastic stage, the increase in length of the mild steel will be reduced, but it will not return to its original length. This means that the internal structure of the mild steel has already been altered.

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Fig. 17 (a) Stretching of a material sample

(b) The stress-strain graph for mild steel

Point B in Fig. 17b is the yield stress. At this point, the material sample will elongate even without an increase in load. Mild steel has an elastic yield point, but not all materials have such a property. Moreover, point C represents the maximum stress that the material can withstand, and is called the tensile strength. At this point, the cross-sectional area at the centre of the material sample will greatly decrease, forming a section that is narrower (Fig. 18a). Beyond point C, the load required to increase the degree of elongation decreases. At point D, the material sample will break apart at the place where its cross-sectional area is the narrowest (Fig. 18b).

Fig. 18 (a) The decrease in cross-sectional area of a material sample

(b) The material sample breaks apart

Different materials have different tensile properties. Ductile materials will undergo extreme plastic deformation prior to breaking apart, but brittle materials will not undergo any obvious plastic deformation (Fig. 19).

(a) Brittle materials

(b) Ductile materials

Fig. 19 Chart showing stress-strain relationships

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(c) Stiffness testing The stiffness of a material can be tested by using simple apparatus like that shown in Fig. 20. Firstly, make samples of identical size and shape from different materials. Clamp a material sample in place, leaving a specific length of the sample protruding from the opening of the clamp. Then hang a load of a specific weight from the sample. Measure and record the distance to which the section of the material sample deflects from the horizontal level. The smaller the deflection, the greater the stiffness of the material. The stiffness of different materials can be compared by repeating this test with samples of different materials.

Fig. 20 Stiffness testing

(d) Toughness testing The toughness of a material can be tested using an impact experiment (Fig. 21a). First make a sample of material (Fig. 21b)(for example, mild steel) into a specific size and shape. Saw a narrow groove in the sample, and then clamp it onto the testing machine. Raise the plumb to a specific height, and allow it free-fall to strike the material sample. Record the weight of the plumb that causes the material sample to break apart. The greater the weight of the plumb, the greater the toughness of the material is.

Fig. 21 (a) Impact experiment

(b) Different material samples

4 Metal Metal is a material which is extracted from an ore, for examples, gold, silver, copper, iron, aluminium, lead, tin, zinc, nickel, chromium and potassium (Fig. 22a). Most metals appear as shiny solids at room temperature (25℃), and are good conductors of heat and electricity. There are basically two types of metals: ferrous metal and non-ferrous metal.  14 

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Fig. 22 (a) Metal

(b) Bronze sculpture

Many commonly used metals are in fact alloys. An alloy is a mixture of two or more types of metal or a mixture of a metal and other materials. Most alloys contain a great quantity of one type of metal, and a small quantity of other metals. The metal that makes up most of the alloy is called the main composition of that alloy. For example, bronze is an alloy of copper, zinc and tin. It is harder than copper, but it is easier to cast. Hence it is used for casting large souvenirs and sculptures (Fig. 22b).

(a) Ferrous metal Ferrous metal is mainly formed from iron atoms. These metals include iron and steel. They are the most frequently used metals in modern society. Pig iron is produced by placing iron ore, coke and limestone together in a blast furnace for heating, and by removing the impurities from the iron ore (Fig. 23a).

(i) Pig iron Pig iron contains many impurities, for example, silicon, carbon, phosphorus, sulphur, and manganese. It contains 3.5% carbon. Hence it is rather hard and brittle. Its compressive strength is great, but its malleability and toughness is poor. It is the main composition of steel and other ferrous metals. It can be used to make wheels and iron pipes. Other ferrous metals such as cast iron and wrought iron can be obtained from pig iron after refining by various methods.

Fig. 23 (a) Iron ore

(b) Iron products

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(ii) Cast iron Cast iron contains 3% carbon, and approximately 1% silicon, phosphorus, sulphur, and manganese. Due to its high carbon content, cast iron is extremely hard. It has a very low tensile strength and very brittle. Cast iron can generally be divided into white cast iron and grey cast iron. White cast iron is formed by cooling molten iron at a rapid rate. It fractures to produce a silvery white surface on the broken surface. It is very hard. Therefore it is suitable for making surfaces that resist abrasion, such as brake drums, clutch plates and sliding parts of machinery. Grey cast iron is formed by cooling molten iron at a slow rate. It fractures to produce a dark grey surface on the broken surface. It is used in the manufacture of lathe bodies and slideways in machine tools (Fig. 24) since it is a softer and tougher metal.

Fig. 24 (a) Lathe bodies

(b) Slideways of machine tools

(iii) Wrought iron Wrought iron is mainly composed of highly pure iron and iron silicide slag. It contains a small quantity of carbon and other impurities, giving it great resistance to corrosion, great ductility, hardness and toughness. It is also easily welded. Therefore it is commonly used in the manufacture of iron pipes, ships, railway tracks, anchors, gears and chains.

Fig. 25 Gears

(iv) Steel The strength of pure iron is relatively low. Once it is refined and has an appropriate quantity of carbon added to it, it can be made into extremely strong and tough steel. Steel is more suitable for making cutting tools, machinery and various structures. Steel is very important to modern society because its uses are wide-ranging. Steel can be classified as carbon steel or alloy steel.

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(v) Carbon steel Carbon steel contains a small quantity of carbon elements. Its surface is greyish black in colour. It can be classified into three types: low carbon steel, medium carbon steel and high carbon steel. The properties of the different types of carbon steel are listed in Table 2, along with examples of their uses. Some medium carbon steel and high carbon steel products are shown in Fig. 26.

Carbon steel

Carbon content

Properties

Examples of their uses

Less than 0.3%

Not too hard and stiff, easier to work and weld

Wire net, screws, structural steel

Medium carbon steel

0.3% to 0.7%

Very hard but rather brittle

Mechanical components (such as bearings, gears, screws), tools (such as screwdrivers)

High carbon steel

0.7 to 1.5%

Extremely hard and brittle

Measuring tools, cutting tools, stamping moulds, hammer heads

Low carbon steel (mild steel)

(tool steel)

Table 2 Properties of the different types of carbon steel and examples of their uses

Fig. 26 (a) Medium carbon steel products

(b) High carbon steel products

(vi) Alloy steel Alloy steel can be made by adding small quantities of other elements to carbon steel, for example, boron, chromium, cobalt, copper and nickel. The different elements added are able to improve the physical and mechanical properties of steel. There are many types of alloy steel, including high speed steel and stainless steel. The carbon content of high speed steel is approximately 0.75%. It also contains small quantities of chromium, vanadium and tungsten. It is harder than carbon steel, and can even retain its hardness while being cut at high speed. Therefore it is suitable for manufacturing various cutting tools such as turning tools and drill bits (Fig. 27a).

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Fig. 27 (a) High speed steel products

(b) Stainless steel products

(vii) Stainless steel Stainless steel contains chromium and a small quantity of nickel. It is very resistant to corrosion, and does not easily undergo oxidation or rusting. As it does not rust easily, its surface can usually remain smooth. It is shiny and silvery white in colour. Stainless steel is often used in the manufacturing of products with a high degree of resistance to rust, such as cutlery, kitchen utensils, sinks and moving blades of steam turbines (Fig. 27b).

(b) Non-ferrous metal Non-ferrous metals are metals or alloys that do not contain any iron. Many non-ferrous metals are materials commonly used in engineering. Pure metals include aluminium, copper, lead, tin, zinc, silver and gold. Some commonly used pure metals and their properties are listed in Table 3, along with examples of their uses. Pure metal

Properties

Examples of their uses

Gold

Making coins, jewellery Extremely good ductility and resistance to corrosion, extremely high density

Silver

Extremely good electrical conductivity, thermal conductivity, and resistance to corrosion

Making coins, jewellery, high-quality electric wires

Copper

Good ductility, electrical conductivity and thermal conductivity

Making electric wires, electric soldering irons, copper alloys

Aluminium

Good ductility, corrodes easily

Making aluminium alloys

Zinc

Good resistance to corrosion and good Making zinc-plated iron sheets, used anti-oxidation ability as electroplating materials

Tin

Soft in texture but tough with good malleability, good electrical conductivity, strong resistance to corrosion, does not easily oxidize, relatively low melting point

Making tin-plated iron sheets, used as welding and electroplating materials

Lead

Extremely high density, soft in texture, weak ductility, toxic, can effectively block out radiation rays

Making accumulators, welding materials, anti-radiation devices

Table 3 The properties and uses of pure metals  18 

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Fig. 28 Tin rod

Fig. 29 Lead sinkers

The properties of metals can be improved when different metals are mixed together to form alloys. For example, their strength, hardness and resistance to corrosion can be increased. Alloys include brass, bronze, and duralumin. Different types of alloys can be categorized according to their differences in composition. Some commonly used alloys and their properties are listed in Table 4, along with examples of their uses. Examples of the uses of alloys are shown in Fig. 30 and 3 Alloy

Properties

Examples of their uses

Brass

Making imitation gold jewellery, Mainly composed of copper and zinc, yellowish in colour. Highly resistant to ornaments, screws and rivets. corrosion, good ductility, and easily bent, rolled and welded.

Bronze

Making bronze statues and bronze Mainly composed of copper and tin. bells (bell metal), pumps used in Classified as bell metal or gun metal according to its tin content. Bell metal navigation (gun metal) is easy to process and cast, while gun metal is highly resistant to corrosion and has good toughness.

Duralumin (hard aluminium)

Mainly composed of aluminium, with small quantities of copper and manganese. Its hardness is similar to that of mild steel, but its density is lower. It has high ductility and is easy to process and shape.

Making fuselages of aircraft, space vehicles, spare parts of cars, pulleys

Table 4 The properties and uses of alloys

Fig. 30 (a) Aluminium window frames

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Fig. 31 (a) Brass products

(b) Bronze products

5 Timber The uses of timber are extremely wide-ranging. Men have been using timber since historical times, for example, to make household tools and buildings (Fig. 32).

Fig. 32 (a) Wooden products

(b) Wooden buildings

In general, timber can be divided into two types, solid wood and manufactured board. Solid wood is the wood that comes directly from trees after the removal of their branches, leaves and bark. Manufactured board is produced by gluing together many layers of planks of wood. This will be discussed in detail in Chapter 2.

Fig. 33 (a) Solid wood

(b) Manufactured board

Solid wood may be classified into softwood and hardwood according to the variety of tree it is felled from. Softwood is the wood from coniferous or needle-leafed trees, and hardwood is from the wood of deciduous or broad-leaved trees. The names of some commonly used woods and their classifications are listed in Fig. 34. Hardwood tends to be harder in texture, darker in colour, and the wood grain tends to be more colourful. Examples of softwood and hardwood are shown in Fig.e 35.  20 

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Fig. 34 Names of some commonly used woods and their classifications

Fig. 35 (a) Examples of softwood: redwood and yellow pine

(b) Examples of hardwood: maple planks and teak floorboards

(a) Softwood The types of softwood commonly used in Hong Kong include pine, fir and white poplar. Table 5 lists the properties of these woods, and some examples of their uses. Fig. 36 shows some of the uses of softwood. Softwood Properties Examples of their uses Pine (redwood, yellow pine, whitewood, Chinese pine)

Household furniture, tools, The wood grain is straight and prominent knots are materials for school projects. often seen. Its texture is relatively soft. Hence it splits easily, but the wood grain is attractive in appearance and the wood is easy to process.

Fir (yellow fir, yew, spruce)

The wood grain is straight and prominent. The wood is yellowish-white in colour, and somewhat oily.

Wooden ladders, wooden chests used for packaging, external structures of buildings.

White poplar

The wood grain is straight and fine. The wood is soft in texture, and it is tough and elastic. It easily absorbs water; but mold grows easily on it.

Drawing boards, matchsticks, plywood surfaces.

Table 5 Properties of some softwood and their uses.  21 

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Fig. 36 (a) Wooden ladder made from fir

(b) Drawing board made from poplar

(b) Hardwood The types of hardwood commonly used in Hong Kong include serayah, teak and maple. Table 6 lists the properties of some hardwoods and examples of their uses. Hardwood

Properties

Examples of their uses

Teak

The wood grain is attractive in appearance. The wood is hard in texture, and somewhat oily. It is highly damp-proof, and highly resistant to corrosion by acids.

High-quality household furniture, floorboards, surfaces of laboratory workbenches and tools on ships.

Maple

The wood grain is either straight or curly. High-quality household The wood is hard in texture, high in furniture, floorboards, doors. density, attractive in appearance and is extremely durable.

Camphorwood

The wood grain is coarse and straight. The wood is hard in texture and high in density, with small splinters. It is easy to process.

Surfaces of worktops in workshops and schools, facilities in country parks.

Beech

The wood is hard in texture, and water-resistant.

Sleepers for railway tracks, boats and ships, handles of hand tools.

Balsa

Light in colour. The wood grain is straight and coarse. The wood is light in weight, and soft in texture. It is easy to cut and process.

Making models.

Ash

The wood grain is straight and prominent. Exercise equipment, handles The wood is highs elastic, and its texture of hand tools. is tough. It is easy to process and bend.

Serayah (Cover term for various types of wood)

Frame construction, boards The wood grain is mixed. The wood is highly tough in texture, and may be either enclosing construction sites, soft or hard. It is easily affected by and inner layers of plywood. humidity, which causes it to expand, contract and deform.

Table 6 Properties of some hardwood and their uses  22 

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(a) Balsa

Fig. 37

(b) Wooden handles made of Ash

(c) Traditional Chinese furniture

(c) Manufactured board There are two types of manufactured board: plywood and fibreboard.

(i) Plywood Plywood is made by gluing and pressing together three or more sheets of wood, with the grain of each consecutive piece positioned at 90° to the preceding one. This method of layering the wood prevents deformation through warping. There must be an odd number of layers of wood making up the plywood. Some good-quality veneer or fireproof board may be glued onto the surface of the plywood, to change its appearance. Plywood may also be classified as multi-layered plywood (Fig. 38) or solid corestock-laminated board (Fig. 39) according to its composition.

Fig. 38 Multi-layered plywood

Fig. 39 Solid corestock-laminated board

The layers of wood in solid corestock-laminated board are mainly composed of strips. Solid corestock-laminated board is often used in construction or in the making of furniture. The strips of wood are usually made from pine, birch or lauan. Decorative wooden sheets are usually used for the face plate. Commonly found types of solid corestock-laminated board include laminboard and  23 

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blockboard.

Fig. 40 (a) Thin bending plywood

(b) Thick bending plywood

The layers of wood in multi-layered plywood are composed of many wooden sheets. This type of wood is suitable for making better-quality bending tools and products (Fig. 40). Plywood has a number of advantages:

(ii)

1.

It has a flat and broad area.

2.

It has a high resistance to bending.

3.

It is tough and durable.

4.

It does not easily expand, contract or deform due to humidity.

5.

It is easily stored, and does not need to be dehydrated.

6.

Thin plywood is very tough, and is easy to bend into shape.

Fibreboard

The making of fibreboard involves crushing and combining together wood, paper, bagasse and plant fibres. Glue is added to the mixture. It is thoroughly mixed before compressing it tightly into boards. Examples of fibreboard include hardboard and chipboard. The advantages of hardboard include the following: inexpensive, evenly structured and easily cut. As hardboard is not very strong, it is easily abraded. Therefore, when hardboard is used, other types of wood are often used for support. Hardboard is often used for making the backs of furniture or table tops (Fig. 41).

Fig. 41 Hardboard

Fig. 42 Wooden closet made of Chipboard

Chipboard is made from sawdust, woodshavings, and wood fragments mixed together with glue, which is then compressed by using hot compression machines. If suitable chemical compounds are added during the manufacturing process, its damp-proof ability, resistance against corrosion, and resistance to heat will be greatly improved. The inner layers of chipboard are usually made from larger material fragments to increase its strength. In contrast, the outer layers of  24 

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chipboard are usually made from smaller fragments for a more attractive appearance. The texture of chipboard is hard but relatively brittle. Therefore it is not suitable for joining by mortise and tenon. Chipboard is commonly used for making the spare parts of furniture that can be taken apart (Fig. 42).

6 Plastics All plastic materials are made of composites. Common properties of plastics include the following: low density, good recipient of colours and additives which are easily added to improve their properties, low thermal and electrical conductivity, good resistance to corrosion, easy to produce in large quantities and inexpensive. Therefore, many products and utensils for daily use in modern society are made of plastics (Fig. 43 and 44). There are many kinds of plastics but they can be largely classified into two categories: thermoplastics and thermosetting plastics.

Fig. 43

Fig. 44

(a) Thermoplastics Thermoplastics will soften or melt after heating, just like wax. However, the molten plastic will harden again after cooling, so it can be shaped by moulding. This process can be done repeatedly without altering its characteristics (Fig. 45).

Fig. 45 Properties of thermoplastics Some commonly used thermoplastics are listed in Table 7, along with their properties and main applications. In reality, there are numerous types of plastics, and each type of plastics can be classified into yet different subsidiaries. For example, polystyrene can be classified into High Impact Polystyrene (commonly called non-breakable plastic) and Expanded Polystyrene (commonly called EPS). Each has slightly different properties. Fig. 46 shows some examples of thermoplastics. Type of plastics

Common name

Abbreviations

Properties

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Polyethylene

Soft plastics

PE

Strong toughness; smooth; easy to process; non-toxic; highly resistant to corrosion; poor thermal and electrical conductivities

Plastic bags, bottles, buckets; Saran wrap; sheath of wire; insulating board

Polycarbonates

Bullet-resistan t plastics

PC

Strong toughness; transparent; thermal resistance; good electrical conductivity; highly resistant to corrosion

Helmet; bullet-resistant glass; goggles; casing of electrical appliances; electrical tools; baby's bottle

Polystyrene

Hard plastics

PS

Hard; brittle; tough; good insulation of electricity

Toys; stationery; tableware; outer casing of home appliances; furniture; essential goods; foam board

Polypropylene

Polypropylene

PP

Strong toughness; tasteless; non-toxic; high strength; can subject to repeated bending without breaking up

Wrapping bags; essential goods; parts of car; video tape box; stationery; toys

Polyvinyl chloride

PVC

PVC

Tough; high elasticity; water-resistant; resistant to scraping; highly resistant to corrosion; easy to paint

Artificial leather; water pipes; soles of shoes; water resistant cloths; floorboards; wire coating

Polymethyl methacrylate

Acrylic

PMMA

Hard; tough; easy to paint; good photo-conductivity; resistant to efflorescence; resistant to chemical corrosion

Artificial jewellery; transparent plastic sheets; decorations; sunglasses; advertising signs; stationery; lamps screen; lens of camera; surface of watches; sign boards

Polyamide

Nylon

PA

Tough; tolerant to collision; resistant to abrasion; after being made as threads, tensile strength raised

Bearing; gear; snowboard; zip; nylon socks; fishing lines; rope; nylon bags

Table 7 Some properties and examples of applications of thermoplastics

Fig. 46 (a) Polyethylene toys (thermoplastics)

(b) Polypropylene straps (thermoplastics)

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(b) Thermosetting plastics Thermosetting plastic changes into a state after solidification by chemical processing. This process can only happen cannot revert to its original state even re-heating, as shown in Fig. 47.

solid once. It after

Fig. 47 Properties of thermosetting plastics Table 8 shows the properties and examples of applications of some commonly used thermosetting plastics. Fig. 48 shows some examples of thermosetting plastics. Name

Common name Abbreviations

Properties

Main applications

Ureaformaldehyde Ureaformaldehyde, resin glue

UF

Semi-transparan t; dye can be added; resistant to fire; hardly occurs ageing

Tableware, adhesives, decorations

Polyester resin

UP

Hard; transparent

Desktop decorations, artistic knick knacks, models, fibre glass-reinforced plastics

PF

Tough and hard; heat and electricity resistant; resistant to fire; weakly resistant to UV light; easily processed

Casing of electrical appliances, gears, lamp holders, plugs, switches

Phenolic resin

Polyester

epoxy Phenolic moulding powder

Melamine formaldehyde

Melamine-formald ehyde resin

MF

Colorless; tasteless; weak water absorption; hard; resistant to scraping and corrosion, and difficult to decompose after heating

Tableware, bowls and dishes, decorations , spare parts and casing of electrical appliances

Epoxy resin

Epoxy resin

EP

Water resistant; efflorescence resistant; rapid hardening and tough; easy to glue with other materials; high resistance to corrosion

Adhesives, paints, construction materials, metalwork materials

Table 8 Some properties and applications of thermosetting plastics

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Fig. 48 (a) Bowls (Melamine formaldehyde) (b) Socket (Phenolic moulding powder)

7 Concrete The composition of concrete includes cement, sand, gravel and water. Cement, made from a mixture of limestone and clay, is used to glue together other substances. After the various components are mixed together, water and cement undergo a chemical reaction to become a liquid adhesive, and adhere together all the substances. Concrete becomes extremely hard after drying and solidifying, and can be used in different applications (Fig. 49a).

Fig. 49

(a) Concrete slabs

(b) Concrete buildings

Concrete has various properties. It is fireproof, bug-proof, easy to maintain, inexpensive and easy to cast. Moreover, it can be processed and painted, so it is usually used for making structures such as buildings, bridges, roads and dams. Modern cities usually make use of concrete for building skyscrapers, thus giving rise to the term “concrete jungles” (Fig. 49b).

8 Compound materials Compound materials are made from mixtures of various materials, and can be divided into layer-formed, fibre-formed and particle-formed compound materials.

(a) Layer-formed compound materials Layer-formed compound materials are types of compound material made by gluing together layers of plastics (Fig. 50a). Formica is a type of thin artificial plastic board. Its surface is hard and resistant to abrasion. It is also heat resistant and damp-proof, so it is often used for making tabletops or surfaces of kitchen furniture, wardrobes and household utensils (Fig. 50b). Formica is available in various colours and decorative patterns. Its thickness generally ranges from 0.17 mm to 2 mm.  28 

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Fig. 50 (a) Layer-formed compound materials

Materials

(b) Formica used as the surfaces of the Kitchen furniture (left) and bookshelf (right)

(b) Fibre-formed compound materials Fibre-formed compound materials are made from bundles or yarns of wire-like materials (such as glass fibre) woven into the form of a mat. Examples include crushed-ply mats, strand-ply bundled-yarn mats, scattered-yarn plainly-woven mats, and woven mats (Fig. 51a). The mat is then put onto a mould brushed with mould release wax. A fibre-formed compound material is produced after applying a coat of suitable adhesive (such as epoxy resin). For example, glass reinforced plastics (GRP) is a fibre-formed compound material usually known by its abbreviated name of glass fibre.

Fig. 51 (a) Woven mat

(b) Windsurfing board made from glass fibre

There are many types of glass fibre. Normally they are harder than other plastics. Some types of glass fibre are even harder than steel. Their elasticity is similar to that of steel but they have a lower density. Glass fibre can be used to make canoes, windsurfing boards (Fig. 51b), hulls of speedboats, and outer shells of cars.

(c) Particle-formed compound materials Particle-formed compound materials are compound materials made from gluing together numerous types of particles. Concrete is an example. Fig. 52 shows the types of particles to be glued together to form concrete.

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Fig. 52 Types of particles found in concrete

9 Degradation and protection of materials (a) Effects of the environment on materials Materials will often be affected by the environment when they are in use, causing a decrease in their efficiency. This is known as the degradation of materials. For example, timber may contract and deform in a dry environment. Environmental factors include: temperature, humidity, sunlight, acidity and alkalinity, insects and pests. Under their influences, materials will suffer from different forms of degradation. Timber is more easily affected by the environment than other materials. In a dry environment (with very low humidity), timber deforms and the compressive strength decreases, its junctions fall apart or its structure will shift. In a damp environment (with very high humidity), timber will easily expand and become moldy, causing its texture to suffer from deterioration and its strength to decrease. Moreover, timber is easily harmed by white ants, birds or other organisms, that also destroy its texture and decrease its strength (Fig. 53a). High temperatures will also cause timber to wither or scorch, and even catch fire.

Fig. 53 (a) Timber that has been eaten by insects

(b) Oxidized steel

Metal is not affected by temperature, sunlight or insects, but it is easily oxidized in a humid environment (Fig. 53b). Since oxidized iron is softer, its strength may decrease and it may even fracture. Furthermore, metals are easily corroded by acidic substances, so acid rain caused by air pollution will accelerate the degradation of metals, destroying their surface or structure. Besides, fatigue occurs when steel is subject to excessive high speed and pressure resulting in crack and fracture. Plastics and compound materials are not affected by acidity, alkalinity, humidity or insects, but they will soften, melt or decompose under heat. Moreover, ultraviolet rays of sunlight will also cause plastics to soften or harden, and will accelerate their aging process.  30 

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Concrete is not affected by acidity, alkalinity, humidity, sunlight or insects. However, if it endures tensile force after expansion and contraction due to changes in temperature, concrete of especially low tensile strength will fragment. Since the environment will cause materials to degrade to different extents, environmental factors must be carefully considered and protective measures must be taken before materials are selected for use. On the other hand, aging of materials also causes the occurrence of the degradation of materials, while the environmental factors also influence the occurrence of aging of materials. Therefore, they are inter-correlated.

(b) Protection of materials Corrosive strength means the ability of materials to resist corrosion. Increasing the corrosive strength of materials will increase their durability, and thus prevent accidents or financial loss due to damages on the materials. Therefore, effective protective measures must be taken when materials are being selected and used. When using timber, its surface should be treated first, for example, by coating it with protective substances, such as shellac, varnish, lacquer, oil-based paint, wax and enamel paint (Fig. 54a). Another method is to cover the surface of the timber with protective materials, including veneer, wood grain plywood and formica.

Fig. 54 (a) Wooden product coated with lacquer

(b) Electroplated products

Protective procedures for metals include electroplating and the addition of protective substances, such as lacquer, brushing lacquer, enamel paint and enamel. Usually, harder and more abrasion-resistant metals, such as Chromium and Nickel are selected for electroplating. This does improve not only the corrosive strength of metals, but also their mechanical properties (Fig. 54b). Sometimes, protective substances such as plastics may also be used to cover the surface of metals. Plastics that are more resistant to ultraviolet rays or plastics containing appropriate additives, should be used to make plastic toys and facilities in playgrounds (Fig. 55a) when they are placed outdoors or in the sun for long periods of time.

Fig. 55 (a) Outdoors plastic recreational facilities

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To decrease the tensile force created by expansion and contraction due to temperature changes in concrete, juncture points should be designed appropriately or be filled with materials with good elasticity. For example, placing see-saw shaped metal joints at the junctures of a concrete bridge allows the surface of the bridges to expand freely at high temperature (Fig. 55b). Another method is to put one end of the bridge onto ball bearings, and to allow the bridge sufficient space for expansion during heating.

10 Reinforcement of materials When using various materials, it is often necessary to increase their efficiency. This is known as reinforcement. For example, the hardness of steel iron can be increased by heat treatment and the strength of concrete can be increased by changing its composition.

(a) Metal The physical properties of metals can be changed by heating or cooling. This is known as heat treatment. Heat treatment is done by heating the metal to an appropriate temperature, before passing it through a process of heat preservation and various methods of cooling, in order to change the molecular structure of the metal. For example, steel can be hardened and become abrasion-resistant after heat treatment, but it can also be softened after heat treatment to make it easier to process.

Fig. 56 Heat treatment process

(i) Hardening The process of applying heat treatment to materials to increase their hardness is called hardening. Carbon steel is hardened by heating it to threshold temperature or higher, and then putting it into cold water or oil for rapid cooling. The carbon steel will become harder and more brittle, because its molecular structure is changed. This is called the hardening of carbon steel. The threshold temperature of different types of carbon steel varies according to their content.

(ii) Tempering Carbon steel will become brittle and will break easily after the process of hardening, and may not be suitable for use. Therefore, tempering can be carried out to reduce its hardness and brittleness slightly, while increasing its ductility and toughness, making it more resistant to breaking. The process of tempering involves reheating the hardened carbon steel to a specific temperature below its threshold temperature, and then cooling it at an appropriate rate, in order to precisely control its physical properties.

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(iii) Annealing The internal molecular structure of steel will change after processing (for example, after hammering and rolling), causing the steel to harden and its ductility to decrease. The main purpose of annealing is to soften the steel, to make it easier to cut or process. Annealing involves heating the steel to a temperature slightly higher than its threshold temperature, preserving it at that temperature for a period of time, and then allowing it to cool slowly in the open air. This will remove its internal stress, causing the steel to soften and its ductility to increase.

(b) Concrete Concrete has many advantages, but its efficiency will only be at its peak if its composition is appropriately proportioned and treated. The ultimate properties of concrete mainly depend on the properties of the cement, the quantity and types of filling (sand and gravel) in the concrete, its water content and the chemical reactions that take place. The ratio of water to cement is particularly important. Too much water will decrease the strength of the concrete, but too little water will prevent the concrete from appropriately sticking together and hardening. Cement is the most expensive constituent of concrete. Hence, 70-80% of concrete is composed of filling material (sand and gravel) to reduce costs. This affects the strength of the concrete. Concrete used for different purposes will make use of different quantities and types of filling materials. When concrete is being made, additives are sometimes added to it to change its properties. Such additives include colourings, accelerators and moderators. Colourings can change the colour of concrete, but they do not affect its strength. Accelerators can accelerate the chemical reactions that take place during the solidification process, causing the concrete to solidify at a faster rate, and increasing its initial strength so that it becomes more resistant to cold weather. In contrast, moderators slow down the chemical reactions that take place during solidification, causing the concrete to solidify at a slower rate, and increasing its strength in the long-term so that it becomes more resistant to hot weather. Concrete can resist huge pressure, but it has very weak tensile strength and thus cracks easily. If concrete is used to build a bridge, cracks will easily appear at the bottom of the bridge where it has to endure more tensile force, and it will break apart as a result (Fig. 57a). If a steel reinforcing bar is added to the concrete before it is moulded, its structure will be strengthened and its tensile strength will be increased. This type of concrete is called reinforced concrete (Fig. 57b).

Fig. 57 (a) Concrete cracks under tensile force

(b) Reinforced concrete

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Exercise 1.

Explain the following definitions: (a) Density

(b) Melting point

(c) Boiling point

(d) Specific heat capacity

(e) Specific latent heat of fusion

(f) Specific latent heat of vaporization

(g) Coefficient of linear expansion 2.

(a) State the differences in chemical reactions of aluminium and gold when they react with strong acids. (b) Explain why gold is commonly used in coin making due to its chemical properties.

3.

(a) What is the meaning of “mechanical properties” of a material? (b) Explain the following machining properties of materials below: (i) Tensile strength

4.

(ii) Bending strength

(iii) Shearing strength

(a) What is the “machining properties” of a material? (b) Give four machining properties of materials.

5.

(a) Explain how to do a “hardness test” by using a steel pricker. (b) Compare the hardness of the following materials: (i) Wood

6.

(ii) Steel

(iii) Diamond

(iv) Aluminium

(a) Draw the stress-strain graph of a soft steel while being stretched, and state the following positions on the graph: (i) Elastic limit

(ii) Yield stress

(iii) Tensile strength

(b) Draw the stress-strain graphs of the following materials: (i) Brittle material

(b) Ductile material

 34 

(iv) Break point

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7.

The above stress-strain graph shows the tensile properties of two different metals A and B. (a) Compare the properties of ductility of A and B. (b) Choose one of the above metals for manufacturing a thin pipe. Explain your answer. 8.

One of the elements in product design is the choose of materials. The following materials are available: cast iron, high carbon steel, pig iron and mild steel. In the following table, choose appropriate material for the corresponding product, and explain your choice. Product

Material chosen

Reasons

Casing of car Wok Bearing Center punch 9.

The following materials are available: wrought iron, cast iron, stainless steel and high speed steel. In the following table, choose appropriate material for the corresponding product, and explain your choice. Product

Material chosen

Reasons

Knife and fork Twist bit Clutch Anchor

10. The bench of the MTR train can be made by plastics or stainless steel.

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(a) Give a merit and a drawback of the above materials for manufacturing the bench. (b) Give two consequences of the impact of using plastics on the environment. (You may refer to the Environmental Protection Department and Green Power webpages for further information.) 11. The following types of wood are available: Maple, White poplar, Balsa and Beech. Fill in the following table suitable types of wood for manufacturing the products listed and their characteristics. Sleepers for railway tracks

Product

Model

Furniture

Matchsticks

Best choice of wood Hardwood or softwood Characteristics 1 Characteristics 2

12. A new wooden bookshelf is to be placed in each classroom in the school to accommodate the heavy books of the students. (a) Which of the following types of wood would you suggest for the making of the new bookshelf? Explain. (i) Fir

(ii) Serayah

(iii) Ash

(iv) Teak

(b) Give one example of the practical use of each of the above wood types. 13. The table below lists some information of the plastic materials. Plastic materials

M1

M2

M3

M4

Plasticity

Poor

Fair

Poor

Good

Conductivity

Low

High

Medium

Low

Strength of anti-acids

Poor

Best

Good

Best

Efficiency of combustion

Low

Low

Maximum working temperature (℃)

80

120

Incombustible Incombustible 200

100

For the materials stated in the above table, a long pipe is to be designed for discharging industrial acidic wastes, which can bear temperature as high as 60℃.  36 

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(a) Upon choosing the suitable material, which two characteristics have the most priority of consideration? (b) Which plastic material should be chosen? (c) Is aluminium the substitute of plastics in this case?

Explain briefly.

14. Fill in the following table the most suitable plastic material with their characteristics for the manufacturing of the products listed in the table. Products

Water pipe

Gear

Tableware

Most suitable plastic material Thermoplastic or thermosetting Characteristics 1 Characteristics 2

15. (a) The following products can be manufactured by aluminium or copper. Fill in the following table the suitable materials chosen for manufacturing the products below with explanation. Product

Material chosen

Reason

Parts of aircraft Conductor in electric cable Cooling pipe of engine Casing of toothpaste

(b) State the main compositions of bronze and brass respectively. What are the differences in the metallic properties between these two metals and pure copper? (c) Give two examples of the practical uses of bronze and brass respectively. 16. What is the usual sales mode of metallic materials in the market of industrial materials? Students are advised to form groups of 4 to 6 and find the information of the sales of the following metallic materials from the market or the internet. Use a “” in the following table to show the shape/class of that material on sales in the market.

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Shape/class

(a)

Octagonal Bar

(b)

Circular bar

(c)

Chunk

(d)

Sheet

(e)

Channel

(f)

Rectangular bar

Bronze

Red Zinc-plated Tin-plated Aluminum Steel copper iron iron

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Shape/class

(g)

Hexagonal bar

(h)

Flake

(i)

Square bar

(j)

Rectangular through hole

(k)

Circular through bar

Materials

Bronze

Red Zinc-plated Tin-plated Aluminum Steel copper iron iron

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Shape/class

(l)

Corner block

(m)

I-bar

(n)

T-bar

Bronze

Red Zinc-plated Tin-plated Aluminum Steel copper iron iron

17. Are there any other shapes or classes of the metallic materials available in the market? and draw those materials found in the market in the table below. Material

Shape/Class

a.

b.

c.

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Write