外文翻譯--應(yīng)力應(yīng)變.doc

五、外文資料翻譯StressandStrain1.IntroductiontoMechanicsofMaterialsMechanicsofmaterialsisabranchofappliedmechanicsthatdealswiththebehaviorofsolidbodiessubjectedtovarioustypesofloading.Itisafieldofstudythatisknownbyavarietyofnames,including“strengthofmaterials”and“mechanicsofdeformablebodies”.Thesolidbodiesconsideredinthisbookincludeaxially-loadedbars,shafts,beams,andcolumns,aswellasstructuresthatareassembliesofthesecomponents.Usuallytheobjectiveofouranalysiswillbethedeterminationofthestresses,strains,anddeformationsproducedbytheloads；ifthesequantitiescanbefoundforallvaluesofloaduptothefailureload,thenwewillhaveobtainedacompletepictureofthemechanicsbehaviorofthebody.Theoreticalanalysesandexperimentalresultshaveequallyimportantrolesinthestudyofmechanicsofmaterials.Onmanyoccasionwewillmakelogicalderivationstoobtainformulasandequationsforpredictingmechanicsbehavior,butatthesametimewemustrecognizethattheseformulascannotbeusedinarealisticwayunlesscertainpropertiesofthebeenmadeinthelaboratory.Also,manyproblemsofimportanceinengineeringcannotbehandledefficientlybytheoreticalmeans,andexperimentalmeasurementsbecomeapracticalnecessity.Thehistoricaldevelopmentofmechanicsofmaterialsisafascinatingblendofboththeoryandexperiment,withexperimentspointingthewaytousefulresultsinsomeinstancesandwiththeorydoingsoinothers.SuchfamousmenasLeonardodaVinci(1452-1519)andGalileoGalilei(1564-1642)madeexperimentstoadequatetodeterminethestrengthofwires,bars,andbeams,althoughtheydidnotdevelopanyadequatetheories(bytodaysstandards)toexplaintheirtestresults.Bycontrast,thefamousmathematicianLeonhardEuler(1707-1783)developedthemathematicaltheoryanyofcolumnsandcalculatedthecriticalloadofacolumnin1744,longbeforeanyexperimentalevidenceexistedtoshowthesignificanceofhisresults.Thus,Eulerstheoreticalresultsremainedunusedformanyyears,althoughtodaytheyformthebasisofcolumntheory.Theimportanceofcombiningtheoreticalderivationswithexperimentallydeterminedpropertiesofmaterialswillbeevidenttheoreticalderivationswithexperimentallydeterminedpropertiesofmaterialswillbeevidentasweproceedwithourstudyofthesubject.Inthissectionwewillbeginbydiscussingsomefundamentalconcepts,suchasstressandstrain,andthenwewillinvestigatebathebehavingofsimplestructuralelementssubjectedtotension,compression,andshear.2.StressTheconceptsofstressandstraincanbeillustratedinelementarywaybyconsideringtheextensionofaprismaticbarseeFig.1.4(a).Aprismaticbarisonethathascrosssectionthroughoutitslengthandastraightaxis.InthisillustrationthebarisassumedtobeloadedatitsendsbyaxisforcesPthatproduceauniformstretching,ortension,ofthebar.Bymakinganartificialcut(sectionmm)throughthebaratrightanglestoitsaxis,wecanisolatepartofthebarasafreebodyFig.1.4(b).Attheright-handendtheforcePisapplied,andattheotherendthereareforcesrepresentingtheactionoftheremovedportionofthebaruponthepartthatremain.Theseforceswillbecontinuouslydistributedoverthecrosssection,analogoustothecontinuousdistributionofhydrostaticpressureoverasubmergedsurface.Theintensityofforce,thatis,theperunitarea,iscalledthestressandiscommonlydenotedbytheGreekletter.AssumingthatthestresshasauniformdistributionoverthecrosssectionseeFig.1.4(b),wecanreadilyseethatitsresultantisequaltotheintensitytimesthecross-sectionalareaAofthebar.Furthermore,fromtheequilibriumofthebodyshowinFig.1.4(b),Fig.1.4PrismaticbarintensionwecanalsoseethatthisresultantmustbeequalinmagnitudeandoppositeindirectiontotheforceP.Hence,weobtain=P/A(1.3)astheequationfortheuniformstressinaprismaticbar.Thisequationshowsthatstresshasunitsofforcedividedbyarea-forexample,Newtonspersquaremillimeter(N/mm)orpoundsofpersquareinch(psi).WhenthebarisbeingstretchedbytheforcesP,asshowninthefigure,theresultingstressisatensilestress；iftheforcearereversedindirection,causingthebattobecompressed,theyarecalledcompressivestress.AnecessaryconditionforEq.(1.3)tobevalidisthatthestressmustbeuniformoverthecrosssectionofthebat.Thisconditionwillberealizediftheaxialforcepactsthroughthecentroidofthecrosssection,ascanbedemonstratedbystatics.WhentheloadPdosesnotactatthuscentroid,bendingofthebarwillresult,andamorecomplicatedanalysisisnecessary.Throughoutthisbook,however,itisassumedthatallaxialforcesareappliedatthecentroidofthecrosssectionunlessspecificallystatedtothecontrary.Also,unlessstatedotherwise,itisgenerallyassumedthattheweightoftheobjectitselfisneglected,aswasdonewhendiscussingthisbarinFig.1.4.3.StrainThetotalelongationofabarcarryingforcewillbedenotedbytheGreekletterseeFig.1.4(a),andtheelongationperunitlength,orstrain,isthendeterminedbytheequation=/L(1.4)WhereListhetotallengthofthebar.Nowthatthestrainisanondimensionalquantity.ItcanbeobtainedaccuratelyformEq.(1.4)aslongasthestrainisuniformthroughoutthelengthofthebar.Ifthebarisintension,thestrainisatensilestrain,representinganelongationorastretchingofthematerial；ifthebarisincompression,thestrainisacompressivestrain,whichmeansthatadjacentcrosssectionofthebarmoveclosertooneanother.(SelectedfromStephenP.TimoshenkoandJamesM.Gere,MechanicsofMaterials,VanNostrandReinholdCompanyLtd.,1978.)應(yīng)力應(yīng)變1、材料力學(xué)的介紹材料力學(xué)是應(yīng)用力學(xué)的分支，它是研究受到各種類型載荷作用的固體物。材料力學(xué)所用的方面就我們所知道的類型名稱包括：材料強(qiáng)度和可變形物體的力學(xué)。在本書中考慮的固體物有受軸向載荷的桿、軸、梁和柱以及用這些構(gòu)件所組成的結(jié)構(gòu)。通常我們分析物體由于載荷所引起的應(yīng)力集中、應(yīng)變和變形作為目的。如果這些是能夠獲得增長(zhǎng)直到超載的重要性。我們就能夠獲得這種物體的完整的機(jī)械行為圖。理論分析和實(shí)驗(yàn)結(jié)論是研究材料力學(xué)的相當(dāng)重要的角色。在許多場(chǎng)合，我們要做出邏輯推理獲得機(jī)械行為的公式和方程。但是同時(shí)我們必須認(rèn)識(shí)到這些公式除非已知這些材料的性質(zhì)，否則不能用于實(shí)際方法中，這些性質(zhì)只有通過一些合適的實(shí)驗(yàn)之后才能用。同樣的，許多重要的問題也不能用理論的方法有效的處理，只有通過實(shí)驗(yàn)測(cè)量才能實(shí)際應(yīng)用。材料力學(xué)的發(fā)展歷史是理論與實(shí)驗(yàn)極有趣的結(jié)合。在一些情況下是指明了得以有用結(jié)果的道路，在另一些情況下則是理論來做這些事。例如著名人物萊昂納多達(dá)芬奇（1452-1519）和伽利略加能（1564-1642）做實(shí)驗(yàn)以確定鐵絲、桿、梁的強(qiáng)度。盡管他們沒有得出足夠的理論（以今天的標(biāo)準(zhǔn)）來解釋他們的那些實(shí)驗(yàn)結(jié)果。相反的，著名的數(shù)學(xué)家利昴哈德尤勒（1707-1783）在1744年就提出了柱體的數(shù)學(xué)理論計(jì)算出其極限載荷，而過了很久才有實(shí)驗(yàn)證明其結(jié)果的重要性。雖然其理論結(jié)果并沒有留存多少年，但是在今天他仍是柱體理論的基本形式。隨著研究的不斷深入，把理論推導(dǎo)和在實(shí)驗(yàn)上已確定的材料性質(zhì)結(jié)合起來形容的重要性是很顯然的。然后，調(diào)查研究簡(jiǎn)單結(jié)構(gòu)元件承受拉力、壓力和剪切的性質(zhì)。2、應(yīng)力應(yīng)力和應(yīng)變的概念可以用圖解這種方