TRANSCRIPTS


strain are reduced, and the component tries to return to its original shape. In the case of a rubber band or the beach ball, there is no problem because of the elasticity of rubber. Elasticity is the ability of a material to return to its original shape. Most metals are elastic up to a point. Steel, surprisingly, is very elastic, whereas concrete is definitely not. It’s knowing where that point is that is important.


If the load goes beyond the elasticity of the material, the component will never return to its original shape. The graph in slide 1 shows what happens. The solid line indicates that, as the load increases, the component deforms in a linear fashion, i.e., in a straight line, and will return along the same line when the load is removed. However, if the strain goes beyond the elastic limit, the deformation increases significantly, so that the return line is permanently offset – that’s the dotted line.


One aspect of elasticity is the rigidity of the material. A flexible material can flex – that’s where the word comes from, of course – but what does flex mean? It means that it can expand and contract – over a long period – without damage. However, if the material is more rigid, fatigue can set in and the result will be similar to the dotted line of the graph, or even total component failure.


Some materials can only withstand a very small amount of stress. This is called plasticity – that is, a plastic component under stress will never return to its original shape. Have you ever tried to break a plastic card by bending it repeatedly? A good example of plasticity is a ductile material such as copper, which is so soft it can be drawn out into a wire – and does not return to its original shape. So we have two key words – stress and strain, and two important qualities of materials – elasticity and plasticity.


Unit 3, Lesson 3.2, Exercise E ≤1.14


Part 4 So, to summarize, stress is the effect of a force on the component, and strain is the deformation resulting from it. Elasticity means a material can deform and still return to its original shape, whereas plasticity means the opposite. The effectiveness of a material used in a machine depends on how it reacts to stress.


OK, that’s it for today. Next time we’ll look at the factors which produce stress in a mechanical component. Don’t forget to do a bit of research on that before you come. Thanks. See you soon.


Unit 3, Lesson 3.2, Exercise F ≤1.15


1 When opposite forces are equal, a component is said to be in equilibrium.


2 Concrete is more elastic than steel. 3 Fatigue is more likely in flexible materials.


4 If the load goes beyond the elasticity of the material, the component will not return to its original shape.


5 Once materials are deformed, they can never return to their original shape.


6 Rigid materials always fracture when they deform.


Unit 3, Lesson 3.3, Exercise A ≤1.16 1 com'ponent 2 alu'minium 3 ela'sticity 4 'flexible 5 re'act 6 'copper 7 'rubber 8 'alloy 9 fa'tigue


10 'fracture 11 'rigid 12 'ductile 13 equi'librium 14 o'riginal 15 defor'mation 16 'cantilever 17 'plastic 18 e'lastic


Unit 3, Lesson 3.4, Exercise A ≤1.17


Part 4 So, to summarize, stress is the effect of a force on the component, and strain is the deformation resulting from it. Elasticity means a material can deform and still return to its original shape, whereas plasticity means the opposite. The effectiveness of a material used in a machine depends on how it reacts to stress.


OK, that’s it for today. Next time we’ll look at the factors which produce stress in a mechanical


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