M. Ibbotson - Professional english in use engineering (794233), страница 18
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This is different to a scalar qu antity, which has amagnitude only.Note: See Appendi x Von page 106 for more on types of load.-Stress and strainThe extract below is fro m an engineering textbook.In a test, a thick cable is used to pick up a heavyobject. The cable stretches slightly, but lifts theweight.
A second test is done using a thinnercable- one with only half the cross-sectional areaof the thick cable. This time, the cable stretches,then breaks.Why did the thinner cable fail? Not due to ahigher load, as the weight was the same. Thefailure was due to stress. Stress is force per unitof area, and is measured in newtons per squaremetre, or Pascals (1 N/m 2 = 1 Pa). The thinnercable was therefore stressed twice as much as thethick cable, as the same load was concentratedinto a cross-sectional area that was 50% smaller.Why did the thick cable stretch but not break?When objects are stressed, they deform -that is,they change size (if only slightly). In the tests, thecable extended- it increased in length.
Extensioncan be measured as a change in an object's lengthcompared with its original length before stresswas applied. This measurement is called strain.According to a law called Young's Modulus ofElasticity, stress is proportional to strain . In otherwords, a percentage increase in stress will causethe same percentage increase in strain . However,th is is only true up to a point called the limit ofproportionality. If a materia l is overstressedbeyond this limit - it will start to become strainedby a higher proport ion . Stress and strain willtherefore become disproportional.Small lengths of material, calledspecimens, can be stressed in amaterials testing machine to measurestrain and test their strength.Note: See Unit 18 fo r more on elasticity and th e limit of pro portion ality.66Professional English in Use Engineering30.1Replace the underlined words and expressions with alternative words and expressions fromA opposite and Appendix Von page 106.If you look at the objects around you, it's difficult to find something that couldn't besmashed with a hammer.
But if you laid a hammer down carefully on any of thoseobjects, the (1) force which it (2) put on them wouldn't be sufficient to cause even theslightest damage. This comparison illustrates the difference between:• a (3) moving force, which combines mass and movement to apply (4) a shock• a (5) still force, which consists only of an object's (6) own mass.Between the two situations, the (7) size of the load (8) placed on the surface isdramatically different.The above comparison illustrates another difference in the way surfaces are (9) pressured.When a hammer is laid horizontally on a surface, its weight is spread over a relativelylarge area.
It therefore applies a (1 0) spread out force. By contrast, when a hammer hitssomething, only the edge of the hammer head comes into contact with the surface. Theforce is therefore (11) focused in a small area, applying a (12) localized pressure.30.2Complete the technical checklist (1-7) based on the questions (a-g), using words from A andB opposite and Appendix Von page 106. The first one has been done for you.a Which components need to carry load?b What types of load will be carried by each part? Which loads will remain constant, andwhich will differ depending on use and circumstances?c What amount of load will be exerted, in newtons?d In what directions will the loads act?e For the materials used, how concentrated can maximum loads be without putting thecomponent under too much pressure?f How much deformation can be expected?g If something breaks, will the assembly collapse dangerously, or in a controlled, relativelysafe way?1 Determine which components are ..........
!!?.?.1.0.:.~~9.!.!!:l.g........... .2 Analyze the types of load that will ................................................ on each part. Assess................................................ loads and ................................................ loads.3 Calculate the ................................................ of loads as ................................................ quantities.4 Evaluate loads as ................................................
quantities.5 Determine the maximum level of ................................................ that can be carried by materialswithout causing them to be ................................................ .6 Calculate percentages of ................................................ .7 Assess the consequences if a component ................................................ , determining thepotential dangers of the ................................................ .Ove,r ..f-o tjou~Think about a machine or structure you're familiar with. Give examples of types of loadwhich act on specific components or members. Say which components are stressed the most.and explain why.Professional English in Use Engineering67IIJ Force, deformation and failure-Types of force and deformationNon-technicalwordTechnicalterm (noun)Adjective used withthe nouns stress,load and forceInitial deformation ofcomponent or memberstretchingtensiontensile stressIt will extend (lengthen).squashingcompressioncompressive stressIt will compress (shorten).bendingbendingbending stressIt will bend- we can also say itwill deflect or flex.
Beams usuallysag, deflecting downwards. Insome cases deflection or flexure isupward - the beam hogs.sctssonngshear orshearingshear stressIt will deform very little, failingsuddenly.twistingtorsion ortorquetorsional stressIt will twist.tensioncompressiontorsion or torqueBending comprises two opposite stresses:tension and compression. This is shown inthe diagram of a simply supported beam.As a result of the bending force, the lowerhalf of the beam is in tension and the upperhalf is in compression.
These oppositestresses reach their maximum at the upperand lower surfaces of the beam, andprogressively decrease to zero at the neutralaxis - an imaginary line along the centre ofthe beam which is free from stress.shearA simply supported beamTypes of failureThe ultimate failure of a component or structural member depends on the type of force:•in tension - it will fracture•in compression- if it is thick, it will crush (squash). If it is slender (long and thin), it willbuckle, bending out of shape•in bending - it will fracture on the side of the component which is in tension, or crush onthe side which is in compression- or fail due to a combination of both•in shear- it will shear (break due to shear force)•in torsion - it will fracture or shear.When vertical members can no longer resist a load they either crush or buckle.68Professional English in Use Engineering31.1Complete the word puzzle and find the word going downthe page.
Look at A and B opposite to help you.1 bend downwards2 a twisting force2~3~3 take a force without breaking4 increase in length, due to tension4~5~6~5 long and thin, likely to buckle rather than crush6 a scissoring force31.2~IIThe question below, which was posted on a forum on a construction website, contains amistake about a technical fact. Can you find the mistake? Look at A opposite to help you.n nPost 1:I was under the impression that concrete and steel bars were used together in reinforcedconcrete (RC) because concrete is good at resisting compression and poor at resistingtension , whereas steel is strong in tension. I also thought the steel always went at thebottom of an RC beam because that's the part that's in tension, whereas the top of thebeam is free from stress. But if that 's the case , when you see reinforcement being fixed inbig RC beams, why are there bars both at the bottom and at the top?31.3Now complete a structural engineer's answer to the question in 31.2 using the words in thebox.
Look at A and B opposite and 31.2 above to help you.bendingcompressJOncompresstvecrushingdeflectdeflectionfracturinghogneutralsagtensiletensionReplies to post 1:Let me start by clarifying something. When a beam is subjected to (1) ................................stress, the bottom part is generally in tension, as you rightly say. But the top part is not'free from stress' , as you suggest.
It's in (2) ................................ . Only the horizontal centrelineof the beam- a zone called the (3) ................................ axis- is not stressed. It's alsoimportant to be clear about the strengths of concrete and steel. You're right that concreteis poor at resisting (4) ................................ stress as it's prone to failure by (5) ................................suddenly.
it's also true that concrete is good at resisting {6) ................................ stress. Butsteel is much stronger than concrete, not just in (7) ................................ as you point out, butalso when it's compressed. So steel is often put in the tops of beams in cases wherethe beam is subjected to high levels of compression, meaning that the concrete requiresreinforcing to prevent it from (8) ................................ and failing.Another possible reason for a beam having steel in the top is to take tension. Why wouldyou get tension in the top of a beam? It's true that most beams want to (9) ..............................
..downwards because gravity causes them to (10) ................................ ,putting only the bottomof the beam in tension . But in some structures, there are beams or parts of beams thatwant to (11) ................................ - being forced into upward (12) ................................ .0V~r ..f-o tjOtc•Think about the d ifferent forces acting on a machine or structure you're familiar with .How would the different components or members deform or fail if they were not adequatelydesigned, or if they were overstressed?Professional English in Use Engineering69I!J Structural mechanics-Statically determinate structuresWhen a structural member- that is, part of a structure- is loaded in a certain direction,the load will cause a reaction.
This means that another force, equal to the load, will act inthe opposite direction. The reaction will counteract (resist) the load and stop the memberfrom moving. When the loads and reactions acting on a member are equal, we say it is inequilibrium. Structures that are designed not to move - to stay in equilibrium- are calledstatically determinate structures.Note: See Appendix Von page 106 for the names of specific structural members.-===--•Resultant forces and centre of gravityOften, a structural member will need to carrydifferent loads of different magnitudes, actingin different directions. To calculate the result ofthese different component forces, an engineer cancalculate a resultant force (or resultant). This is aforce with a magnitude and direction equal to allthe component forces together.component force 1resultantcomponentforce 2One of the main forces exerted on a structure isgravity.
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