1. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.2 Learning summary By the end of section 2.2 you will have learnt: the different classes of materials; the different bonding and structure observed in different material classes; some basic relationships between structure and properties for different classes of materials; the composition, properties and uses of some common engineering materials.

2. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: elastic modulus Elastic modulus, modulus of elasticity or stiffness, is defined as an object’s resistance to elastic deformation. Units are the pascal (Pa); most materials have elastic moduli ~109 Pa (GPa). The Young’s modulus can be determined from the slope of the straight line portion of stress–strain curves produced by tensile testing.

3. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: elastic modulus The origin of the elastic modulus of a material lies predominantly in the stiffness of the bonding between the atoms that comprise the material. Stiff bonding gives a high modulus. As little can be done to change the stiffness of the bonds, the elastic modulus varies very little with alloying, heat treatment and mechanical working. The elastic modulus enables the extension, compression, deflection and energy stored in structures under load to be determined (as long as the load produces reversible elastic deformation).

4. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: yield and tensile strength The yield stress is the stress above which non- reversible (or plastic) deformation occurs. The tensile strength, or ultimate tensile stress, is the maximum tensile strength a material can withstand before failure. Units are the pascal (Pa) most materials have yield and tensile strengths ~106 Pa (MPa). The yield and tensile strength can be determined from stress–strain curves, obtained from tensile tests, by taking the stress at which the stress-strain curve deviates from linearity and the maximum value of the stress on the curve respectively.

5. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: yield and tensile strength The origin of plastic deformation in metals is the shearing of planes of atoms past each other, rather than the breaking of bonds between neighbouring atoms. Ceramics do not show plastic deformation. In tension the fracture of ceramics occurs due to the presence of cracks at stresses below the tensile stress. In polymers, plastic deformation occurs as the chains slide past each other. As the strength of the bonding between chains increases, so does the yield strength.

6. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: yield and tensile strength The yield strength defines an upper limit to the tensile or compressive stress that can be applied to a component without irreversible plastic deformation.

7. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: ductility Ductility is a measure of the plastic strain at failure and is unit-less. The toughness is the amount of energy a material absorbs during facture and has units of J m -3. The ductility can be determined from a stress– strain curve produced by tensile testing and is roughly the strain corresponding to the breaking stress. The toughness of a material can be determined from a stress–strain curve by finding the area underneath the curve.

8. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: ductility The origin of the ductility of a material comes from its ability to undergo plastic deformation. The toughness of a material is in part determined by the degree to which a material can undergo plastic deformation before it fails but also the stress required to produce plastic deformation. The ductility of a material is not often considered in the design of a structure or component. The toughness is important in determining the energy that is absorbed during fracture either in an attempt to resist fracture or to absorb as much energy as possible during impact.

9. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: hardness The hardness of a material is an expression of its resistance to indentation and plastic deformation. Hardness has units equivalent to MPa. Hardness is normally measured using an indentation method. For a given load: the smaller the indent, the higher the hardness.

10. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: hardness The hardness of a material is dictated by its resistance to plastic deformation. Those materials that have high yield strengths will have high hardness. The hardness of metals, ceramics, polymers and coatings can be measured using this method. Hardness is a good indicator of a material’s ability to resist yield and deformation and can be used as a means of quality control.

11. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: density Density (ρ) is a measure of the mass of a material per unit volume. The SI unit of density is kilograms per cubic metre (kg m -3 ). The simplest way to determine the density of an object of regular shape is to measure its mass and divide this by its volume. The density of a material originates from the weight and packing of the atoms or molecules that comprise the material. Materials with closely packed structures will have high densities; those with open structures, low densities.

12. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: density The density of the material used in a component will affect its mass. The use of low-density materials is important to save weight and energy in structures that move.

13. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: thermal expansion Thermal expansion is the tendency of matter to increase in volume when heated. The coefficient of thermal expansion, multiplied by the change in temperature, gives the thermal strain. The coefficient of linear thermal expansion has units of K -1. The linear thermal expansion coefficient for a material can be measured by heating a material in a controlled manner and measuring the length dilation of the sample as a function of temperature.

14. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: thermal expansion When a material is heated, the energy in the bonds between atoms increases, as does the bond length. As a result, solids expand in response to heating and contract on cooling. Thermal expansion-induced strains can cause the distortion of components and structures. In most cases thermal strains should be minimized.

15. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: cost The cost of a material is defined as the price to the consumer to purchase a unit quantity, usually in terms of the mass. Described in this way it has units (in the UK) of £ kg -1. Values for materials prices can be determined directly from suppliers or from the commodities markets such as the London Metal Exchange. The cost of a material depends on the extraction, processing and transportation costs; it is sensitive to the cost of energy relating to processing and transportation and the abundance of the raw material and of scrap or recycled material.

16. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 – key points: cost Cost is an important aspect to consider as the choice of material is often constrained by the cost of the product and the best material for a particular application may be too expensive or in too limited a supply to be used.

17. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.3 Learning summary By the end of this section you will have learnt: the definitions for a number of important material properties and the testing methods which enable us to measure these properties; the origins of the properties for different material types; typical values for these properties for different materials; the relevance of these different properties to engineering applications and how to use property data tp solve design problems.

18. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.4 Learning summary By the end of this section you will have learnt: the material selection process for an engineering design requires a mathematical analysis of the problem, in combination with the use of relevant material property data; the generic method for this material selection process; that the method used is ideal for narrowing the field of candidate materials and that the final choice may require consideration of other factors such as cost and ease of manufacture.

19. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: casting By the end of this section you will have learnt: casting is the formation of shaped components involving filling a shaped mould with a liquid and solidifying it; casting is normally only employed for metallic components; many casting methods are available; casting may be a slow manual process or a fast automated process; the main phenomena which control casting are fluid flow, heat transfer and solidification behaviour of the liquid; certain rules need to be followed to reduce the possibility of casting defects.

20. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: moulding By the end of this section you will have learnt: moulding is the formation of shaped components involving pressing of a viscous solid into a shaped mould or through a shaped orifice; moulding is normally used for components being formed from glasses and polymers; polymers and glasses are normally (although not in the case of moulding of thermosetting polymers) heated to make them into mouldable viscous solids; when cooled they return to being solids which can be extracted from the moulds and handled; many moulding methods are available; moulding is employed to produce products as diverse as polymer sheet and optical fibre.

21. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: deformation By the end of this section you will have learnt: metals are commonly deformed to form shaped components; a wide range of specific methods are available; deformation can be conducted hot or cold; deformation produces an increase in dislocation density in the metals; the final properties of the material will depend critically upon the temperature at which the metal is when it is deformed; if the material being deformed is ‘hot’ (>0.55 Tm) the dislocations will be removed by in-process annealing, resulting in a fine recrystallised structure; if the material being deformed is ‘cold’ (<0.35 Tm) the dislocations will not be removed and the increased dislocation density will result in a stronger but less ductile material; deformation of metals to any shape may result in residual stresses in the components.

22. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: machining and cutting By the end of this section you will have learnt: as a material shaping process, machining and cutting involves material removal from a larger body to form a component; machining is commonly employed for metals, less for polymers (which can be easily moulded to shape) and ceramics (which are very hard to machine); it involves motion between tool and the workpiece; cutting is commonly used for metals and polymers.

23. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: heat treatment By the end of this section you will have learnt: some properties of metals can be substantially changed by heat treatment; the changes depend upon the alloy and the heat treatment; an understanding of the alloy phase diagram is required to understand many heat treatment schedules; the strength and toughness of many steels can be widely varied; structures such as ferrite-pearlite and tempered martensite can be formed by heat treatment; alloys (such as Al – 4wt% Cu) can be strengthened by controlling the formation of small precipitates in the metal via a controlled heat treatment schedule.

24. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: joining By the end of this section you will have learnt: materials and components are commonly joined to make complex structures; adhesive bonding is commonly used for bonding a wide variety of materials and can be used to join dissimilar materials; mechanical fasteners are commonly used for joining a wide variety of materials and can be used to join dissimilar materials; soldering and brazing are used to join metals – soldering is ideally used in the electronics industry; welding of metals involves joining with a molten metal; the components to be joined also melt close to the weld zone; welding can produce very strong joints but can also affect detrimentally the microstructure and properties of the components being welded; welding often results in residual stresses; (continued...)

25. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.5 Learning summary: joining By the end of this section you will have learnt: welding may anneal the components being joined close to the weld zone (in the HAZ); this could reduce the strength of a cold-worked metal considerably; in steels, brittle martensite can sometimes form in the HAZ; pre- heating before welding and PWHT can be used to avoid the problems associated with this.

26. Unit 2 An Introduction to Mechanical Engineering: Part One Materials and Processing 2.6 Learning summary: failure By the end of this section you will have learnt: a number of ways in which materials can fail in service and the relevance of these failure methods to engineering situations; the relevant equations that govern the failure process along with typical data for engineering materials; how to use these equations to design against material failure and to select suitable materials to avoid failure.