建筑专业英语21511.docx

建筑专业英语21511.docx

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建筑专业英语21511

UNITONE

TextIntroductiontoMechanicsofMaterials

1.Mechanicsofmaterialsisabranchofappliedmechanicsthatdealswiththebehaviorofsolidbodiessubjectedtovarioustypesofloading。

Itisafieldofstudythatisknownbyavariety

ofnames,including“strengthofmaterials”and“mechanicsofdeformablebodies。

”The

solidbodiesconsideredinthisbookincludeaxially-loadedbars,shafts,beams,andcolumns,

aswellasstructuresthatareassembliesofthesecomponents。

Usuallytheobjectiveofouranalysiswillbethedeterminationofthestresses,strains,anddeformationsproducedbytheloads:

ifthesequantitiescanbefoundforallvaluesofloaduptothefailureload,thenwewillhaveobtainedacompletepictureofthemechanicalbehaviorofthebody.

2Theoreticalanalysesandexperimentalresultshaveequallyimportantrolesinthestudy

ofmechanicsofmaterials。

Onmanyoccasionswewillmakelogicalderivationstoobtainformulasandequationsforpredictingmechanicalbehavior,butatthesametimewemustrecognizethattheseformulascannotbeusedinarealisticwayunlesscertainpropertiesofthematerialareknown.Thesepropertiesareavailabletousonlyaftersuitableexperimentshavebeenmadeinthelaboratory。

Also,manyproblemsofimportanceinengineeringcannotbehandledefficientlybytheoreticalmeans,andexperimentalmeasurementsbecomeapracticalnecessity.Thehistoricaldevelopmentofmechanicsofmaterialsisafascinatingblendofboththeoryandexperiment,withexperi1nentspointingthewaytousefulresultsinsomeinstancesandwiththeorydoingsoinothers.SuchfamousmenasLeonardodaVinci（1452—1519）and

Galileo（1564—1642）madeexperimentstodeterminethestrengthofwires,bars,andbeams,

althoughtheydidnotdevelopanyadequatetheories（bytoday’sstandards）toexplaintheir

testresults.Bycontrast,thefamousmathematicianLeonhardEuler（1707—1783）developedthemathematicaltheoryofcolumnsandcalculatedthecriticalloadofacolumnin1744,longbeforeanyexperimentalevidenceexistedtoshowthesignificanceofhisresults.Thus,Euler’stheoreticalresultsremainedunusedformanyyears,althoughtodaytheyformthebasisofcolumntheory.

3Theimportanceofcombiningtheoreticalderivationswithexperimentallydetermined

propertiesofmaterialswillbeevidentasweproceedwithourstudyofthesubject。

Inthisarticlewewillbeginbydiscussingsomefundamentalconcepts,suchasstressandstrain,andthenwewillinvestigatethebehaviorofsimplestructuralelementssubjectedtotension,compression,andshear.

4Theconceptsofstressandstraincanbeillustratedinanelementarywaybyconsideringtheextensionofaprismaticbar（seeFig1-1a）.Aprismaticbarisonethathasconstantcrosssectionthroughoutitslengthandastraightaxis。

Inthisillustrationthebarisassumedtobe1oadedatitsendsbyaxialforcesPthatproduceauniformstretching,ortension,ofthebar,Bymakinganartificialcutthroughthebaratrightangletoitsaxis,wecanisolatepartofthebarasafreebody,Attheright-handendthetensileforcePisapplied,andattheotherthereareforcesrepresentingtheactionoftheremovedportionofthebaruponthepartthatremains.Theseforcesw⒒lbecontinuouslydistributedoverthecrosssection,analogoustothecontinuousdistributionofhydrostaticpressureoverasubmergedsurface。

Theintensityofforce,thatis,theforceperunitarea,iscalledthestressandiscommonlydenotedbytheGreekletterσ。

Assumingthatthestresshasauniformdistributionoverthecrosssection（seeFig.1-1b）,wecanreadilyseethatitsresultantisequaltotheintensityσtimesthecross-sectionalarea'Aofthebar。

Furthermore,fromtheequilibriumofthebodyshowninFig.1-1b,wecanalsoseethatthisresultantmustbeequalinmagnitudeandoppositeindirectiontotheforceP,Hence,weobtainastheequationfortheuniformstressinaprismaticbar,Thisequationshowsthatstresshasunitsofforcedividedbyarea——forexample,poundspersquareinch（psi）orkipspersquareinch（ksi）.WhenthebarisbeingstretchedbytheforceP,asshowninthefigure,theresultingstressisatensilestress;iftheforcesarereversedindirection,causingthebartobecompressed,theyarecalledcompressivestresses。

5AnecessaryconditionforEq（1-1）tobevalidisthatthestressσmustbeuniformoverthecrosssectionofthebar,ThisconditionwillberealizediftheaxialforcePactsthroughthe

centroidofthecrosssection,ascanbedemonstratedbystatics.WhentheloadPdoesnotactatthecentroid,bendingofthebarwillresult,andamorecomplicatedanalysisisnecessary。

Throughoutthisbook,however,itisassumedthatallaxialforcesareappliedatthecentroidofthecrosssectionunlessspecificallystatedtothecontrary,Also,unlessstatedotherwise,itisgenerallyassumedthattheweightoftheobjectitselfisneglected,aswasdonewhendiscussingthebarinFig。

1-1。

6.ThetotalelongationofabarcarryinganaxialforcewillbedenotedbytheGreekletterε（seeFig.1-1a）,andtheelongationperunitlength,orstrain,isthendeterΠ1inedbytheequationwhereListhetotallengthofthebar。

NotethatthestrainCisanondimensionalquantity,ItcanbeobtainedaccuratelyfromEq。

（1~2）asIongasthestrainisuniformthroughoutthelengthofthebar。

Ifthebarisintension,thestrainisatensilestrain,representinganelongationorstretchingofthematerial;ifthebarisincompression,thestrainisacompressivestrain,whichmeansthatadjacentcrosssectionsofthebarmoveclosertooneanother。

NewWordsandExpressions

（be）subjectedto承受,经受deformable可变形的axially轴向地

shaft轴,杆状物derivation推导realistic现实的,实际的

fascinate迷住,强烈吸引blend混合,融合prismatic等截面的

tensile拉力的,拉伸的sectional截面的,部分的hydrostatic静力学的

analogous类似的analogousto类似于submerged浸在水中的

uniform均匀的denote指示,表示equilibrium平衡

resu1tant合力magnitude大小,尺寸equation方程

kip千磅tensile拉力的compressive压力的,压缩的

centroid矩心,形心specifically具体地,特定地elongation伸长,拉长

nondimensional无量纲的adjacent相邻的

UNITTWO

TextTheTensileTest

[1]Therelationshipbetweenstressandstraininaparticularmaterialisdeterminedbymeansofatensiletest.Aspecimenofthematerial,usuallyintheformofaroundbar,isplacedinatesting1nachineandsubjectedtotension。

'Γheforceonthebarandtheelongationo￡thebararemeasuredastheloadisincreased.Thestressinthebarisfoundbydividingtheforcebythecross-sectionalarea,andthestrainisfoundbydividingtheelongationbythelengthalongwhichtheelongationoccurs。

Inthis1nanneraCompletestress-straindiagran1canbeobtainedforthematerial。

[2]Thetypicalshapeofthestress-straindiagramforstructuralsteelisshowninFig.2-1（a）,wheretheaxialstrainsareplottedonthehorizontalaxisandthecorrespondingstressesaregivenbytheordinatestotheCurveOABCDE.FromOtoAthestressandstrainaredirectlyproportionaltooneanotherandthediagramislinear.Beyondpoint'⒋the1inearrelationshipbetweenstressandtrainnolongerexists;hencethestressatz⒋iscalledtheproportionallimit.Forlow-carbon（structural）steels,thislimitisusuallybetween30,000psi,and36,000psi,butforhigh-strengthsteelsitmaybemuchgreater。

Withanincreaseinloading,thestrainincreasesmorerapidlythanthestress,untilatpointBaconsiderableelongationbeginstooccurwithnoappreciableincreaseinthetensileforce。

Thisphenomenonisknownasyieldingofthematerial,andthestressatpointBiscal1edtheyieldpointoryieldstress。

IntheregionBCthematerialissaidtohavebecomeplastic,andthebarmayactuallyelongateplasticallybyanamountwhichisto10or15timestheelongationwhichoccursuptotheproportionallimit。

AtpointCthematerialbeginstostrainhardenandtoofferadditionalresistancetoincreaseinload。

Thus,withfurtherelongationthestressincreases,anditreachesitsmaximumvalue,orultimatestress,atpointD。

Beyondthispointfurtherstretchingofthebarisaccompaniedbyareductionintheload,andfractureofthespecimenfinallyoccursatpointEonthediagram。

[3]Duringelongationofthebaralateralcontractionoccurs,resultinginadecreaseinthecross-sectionalareaofthebar。

Thisphenomenonhasnoeffectonthestress-straindiagramup

toaboutpointC,butbeyondthatpointthedecreaseinareawillhaveanoticeableeffectupon

thecalculatedvalueofstress。

Apronouncedneckingofthebaroccurs（seeFig。

2-2）,andifthe

actualcross-sectionalareaatthenarrowpartoftheneckisusedincalculatingσ,itwillbefoundthatthetruestress-strainCurvefollowstheashedlineCE。

Whereasthetotalloadthebarcancarrydoesindeeddiminishaftertheultimatestressisreached（⒒neDE）,thisreductionisduetothedecreaseinareaandnottoalossinstrengthofthematerialitself。

Thematerialactuallywithstandsanincreaseinstressuptothepointoffailure。

Formostpracticalpurposes,however,theconventionalstress-straincurveo⒕BCDE,basedupontheoriginalcross-sectionalareaofthespecimen,providessatisfactoryinformationfordesignpurposes。

[4]ThediagraminFig。

2-1（a）hasbeendrawntoshowthegeneralcharacteristicsofthestress-straincurveforsteel,butitsproportionsarenotrealisticbecause,asalreadymentioned,

thestrainwhichoccursfromBtoCmaybe15timesasgreatasthestrainoccurringfromOto

A.Also,thestrainsfromCtoEareevengreaterthanthosefromBtoC.AdiagramdrawninproperproportionsisshowninFig。

2-1（b）。

InthisfigurethestrainsfromCtoAareso

SmallincomparisontothestrainsfromAtoEthattheycannotbeseen,andthel