M. Ibbotson - Professional english in use engineering (794233), страница 12
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This is a measureof how easily cracks that have already formedcontinue to open up and increase in length.Another problem is creep - where componentsbecome permanently deformed (stretched, forexample), due to loads. Creep increases overtime. The problem is made worse by heat, so isa major issue in engines, where both loads andtemperatures are high.Basic thermal propertiesSome materials conduct (carry or transmit) heat better than others.
Therefore, thermalconductivity varies, depending on the material. Copper, for example, is an excellent thermalconductor. Polystyrene, on the other hand, is an excellent thermal insulator (and so a verypoor thermal conductor).As temperature increases, most materials expand (increase in size due to heating), and astemperature falls, they contract (decrease in size due to cooling) .
The extent to which expansionand contraction occur is measured by a material's coefficient of thermal expansion - that is, itschange in size for a given change in temperature. The coefficient for aluminium, for example,is 0.000023 . This means that for an increase in temperature of one degree Celsius, a one-metrelength of aluminium will increase in length by 0.000023 metres. This figure can also be referred toas the coefficient of linear expansion, since it describes change in length (a linear measurement).44Professional English in Use Engineering19.
1 Complete the design brief for part of a cutting machine using four of the words in the box.Look at A opposite to help you.abrasiondurabilityharddurableindentationso11scratch12uu15softes•s?uThe cutting wheel will be surrounded by transparent guards. These will allow the operator to see thecutting wheel at all times, and will shield the operator from flying metal fragments. The guards musttherefore be constructed from material with a high degree of (1) ................................................ hardness, toprotect it from impacts. As the guards will require regular cleaning, the action of wiping away metalfragments will result in (2) .................................................
The guards must, therefore, have sufficient(3) ................................................ hardness in order to retain their transparency and ensure adequate(4) ................................................ .19.2~--------------------------------------------------------------------~'~Match the descriptions (1-4) to the technical terms (a-d) . Look at B opposite to help you.the cause of fatigueabcd2 the consequence of fatigue3 a material property that helps to slow down cracking4 permanent changes in shape due to the action of loads over time19.3creepcyclic loadsmicro-crackingfracture toughnessComplete the extract from an electrical design handbook using words and expressions fromC opposite.When comparing copper and aluminium as materialsfor electrical wires, it is necessary to consider theirthermal properties.
For instance, in situations wherehigh temperatures are involved, it is important tounderstand how quickly wires (1) ................................ heatalong their length- for example, away from hot parts,such as motors, towards heat-sensitive electricalcomponents. In this regard, the (2) ................................ ofcopper is roughly 40% greater than that of aluminium,so copper is a much more effective (3) ................................
.In the example above, a designer might therefore preferaluminium wiring over copper wiring.Another issue is thermal movement- the extent to which the metals (4) ................................ whenheated, and (5) ................................ as they cool. In situations where temperature continually risesand falls, the resulting (6) ................................ and (7) ................................ can be problematic, as it cancause mechanical electrical connections to loosen over time.
In this regard, copper has a(8) ................................ approximately 40% lower than that of aluminium. Copper therefore has theadvantage in this respect, as it is less susceptible to movement.Ove,r 1-o ':10t.c~For a product you know about, say what the designer needed to consider with regard to:•abrasion•indentations•fatigue•creep•thermal issues.What materials were chosen as a result of these considerations?Professional English in Use Engineering45Btl Forming, working and heat-treating metal-Casting, sintering and extruding metalMetal can be formed into shapes using heat and pressure.
Casting involves heating metaluntil it becomes molten (liquid) and pouring it, or forcing it under pressure, into a mouldcalled a die. Instead of being cast, metal components can be formed by sintering. This isdone by using metal powder instead of molten metal. The powder is placed in a die andcompressed into a solid mass. It is then heated (though not melted) until it becomes sintered- that is, the powder particles join together structurally, due to the heat.Metal can also be shaped by extruding it into long lengths. Extrusion involves heating metaluntil it is molten, then forcing it at high pressure through a shaping tool- also called a dieto form bars or tubes, for example.
At the same time, the metal cools and becomes solid.-Working metalTraditionally, many metal tools were made by heating iron bars in a fire, called a forge, untilthey were red hot or (hotter still) white hot. The metal was then worked- in other words,shaped by hammering it. Working metal using compression (for example, hammering) is alsocalled forging. The same basic technique is still in use today, especially with steel.
However,large, automated machines are now used. Metal is often worked (or forged) when hot (hotforged), but may also be worked when it is cold (cold forged) .A common forging technique is drop forging, where a heavy hammer is dropped onto a piece ofmetal. A die fixed to the hammer compresses the metal into the required shape. Rollers can alsobe used to apply compression, with or without heat, to produce hot rolled or cold rolled metal.Forging also increases the hardness of metal.This is called work hardening. Metal becomeswork hardened because its structure is changedby compression.
The same result can be achievedwithout hammering or rolling - and thereforewithout changing the component's shape - byshot-peening. This involves firing small metal balls(metal shot) at the surface of components (whencold), at high speed. After components have beenshot-peened, their surface is significantly harder.-Dro p forged steelHeat treating metalThe properties of a metal can be changed by heat treating it- that is, heating and cooling themetal. The table below, from the technical information section of a steel supplier's website,summarizes the main types of heat treatment.46Type of heat treatmentDescription of processProperties of treated metalquenchingMetal is heated, then di pped in water oroil to cool it ra pidly.Quenched metal is harder, but tends tobe more brittle.annealingMetal is heated, then allowed to coolslowly.Annealed metal is generally softer andmore elastic.temperingMetal is heated and kept at a hightemperatu re for a period of time.Tempered metal possesses a balancebetween hardness and elasticity.precipitation hardening(also called age hardening)A process similar to tempering, but heatis mai ntained for longer.Precipitation-hardened metal is harderthan tempered metal.case hardening (alsocalled surface hardening )Metal is heated in specific types of gas(not in air), causing its surfa ce to absorbelements such as carbon.Only the outer surface of case-hardenedmetal becomes harder.Professional English in Use Engineering20.1Tick what is usually required in the metal forming processes (1-3) .
Look at A opposite tohelp you.casting2 sintering3 extrusion20.2molten metalmetal powderheatpressurea dieDDDDDDDDDDDDDDDDecide whether the sentences below are true or false, and correct the false sentences. Look atB opposite to help you .1 Metal must always be heated before it can be forged .2 When referring to metals, the terms working and forging mean the same.3 A common reason for forging metal is to increase its hardness.4 One way of forging metal is by heating it and then rolling it.5 Metal can only be rolled after it has been heated to a high temperature.6 When metal is drop forged, it is subjected to compression.7 Metal can only be work hardened by the process of hot forging.8 Shot-peening is a hot forging technique used to work harden metal.20.3Make correct sentences using one part from each column. Look at C opposite to help you .The first one has been done for you.If a metal is precipitation \it is held at a hightemperature for a time,making it harder, but morebrittle.2 When metal is annealed,it is heated within a gasto improve its hardness withoutreducing its elasticity too much.3 If metal is quenched, thismeansit can also be describedas age hardened, ---......._to harden only the metal nearthe surface.4 When a metal is tempered,its temperature is allowed ._because it is heated for a longto decrease graduallytime.5 If a metal case is hardened,its temperature is reducedrapidly,1hardened,in order to make it more elasticand less brittle.20.4 Replace the highlighted expressions in the report extract with alternative words andexpressions from A, B and C opposite.
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