机械设计制造及其自动化专业英语翻译第三单元

MechanicalpropertiesofMaterials材料的机械特性Thematerialspropertiescanbeclassifiedintothreemajorheading：（i）Physical，（ii）Chemical，（iii）Mechanical。材料的特性可以分为三个重要的部分：1是物理特性，2是化学特性，3是机械特性。PhysicalProperties物理特性Densityorspecificgravity,moisturecontent,etc.,canbeclassifiedunderthiscategory.密度或者比重，水分含量等，都被分为这一类的范畴ChemicalProperties化学特性Manychemicalpropertiescomeunderthiscategory.Theseincludeacidityoralkalinity,许多化学特性都归入到这个范畴。这些特性包括酸性或碱性reactivityandcorrosion.Themostimportantoftheseiscorrosionwhich反应性和腐蚀性。canbeexplainedinlayman’stermsastheresistanceofthematerialtodecaywhileincontinuoususeinaparticularatmosphere这些中最重要的是耐腐蚀性，它可以被解释为在外行人看来作为材料在特定大气中长期使用时抵抗腐蚀的能力。MechanicalProperties(机械特性)Mechanicalpropertiesincludethestrengthpropertiesliketensile,compression,shear,torsion,impact,fatigueandcreep.机械特性包括拉伸，压缩，剪切，扭转，冲击，疲劳和蠕变等强度特性。Thetensilestrengthofamaterialisobtainedbydividingthemaximumload,whichthespecimenbearsbytheareaofcross-sectionofthespecimen.材料的抗拉强度由试样承载的最大载荷除以试样的横截面积获得。ThisisacurveplottedbetweenthestressalongtheY-axis(ordinate)andthestrainalongtheX-axis(abscissa)inatensiletest.这是一个在拉伸试验中沿着Y轴（纵坐标）的应力和沿着X轴（横坐标）的应变的关系曲线。Amaterialtendstochangeorchangesitsdimensionswhenitisloaded,dependinguponthemagnitudeoftheload.当材料加载时尺寸的变化，取决于载荷大小的变化。Whentheloadisremoveditcanbeseenthatthedeformationdisappears.当卸载时可以看见变形消失。Formanymaterialsthisoccursuptoacertainvalueofthestresscalledtheelasticlimitσe.对于许多材料来说这种发生的应力极限值称为弹性极限。Thisisdepictedbythestraightlinerelationshipandasmalldeviationthereafter,inthestress-straincurve（Fig.3.1）.在应力-应变曲线中，直线关系和随后小的偏移描述了这个（加载和卸载的过程）。Withintheelasticrange,thelimitingvalueofthestressuptowhichthestressandstrainareproportional,iscalledthelimitofproportionalityσp.在弹性范围内，应力与应变的比例的极限值被称作比例极限。Inthisregion,themetalobeysHooke’slaw,whichstatesthatthestressisproportionaltostrainintheelasticrangeofloading(thematerialcompletelyregainsitsoriginaldimensionsaftertheloadisremoved).在这个区域中，金属服从胡克定律，阐述了在加载的弹性范围内应和应变成比例关系（材料卸载后能够恢复到原来的尺寸）。Intheactualplottingofthecurve,theproportionalitylimitisobtainedataslightlylowervalueoftheloadthantheelasticlimit.在曲线绘制中，载荷的比例极限值比弹性极限值稍低，Thismaybeattributedtothetime-lagintheregainingoftheoriginaldimensionsofthematerial.这可能由于材料恢复原尺寸时存在时间延迟。这种现象经常出现在有色金属中。Whileironandnickelexhibitclearrangesofelasticity,copper,zinctin,etc,arefoundtobeimperfectlyelasticevenatrelativelylowvaluesofstresses.当铁和镍存在明显的弹性范围，铜，锌，锡等，即使在相对低的应力下也表现为不完全弹性。Actuallytheelasticlimitisdistinguishablefromtheproportionalitylimitmoreclearlydependinguponthesensitivityofthemeasuringinstrument.实际上弹性极限和比例极限可以区分的更清晰，依靠测试仪器的精确性。Whentheloadisincreasedbeyondtheelasticlimit,plasticdeformationstarts.当在超过了弹性极限载荷时会产生塑性变形。Simultaneouslythespecimengetswork-hardened.同时试件会发生加工硬化。Apointisreachedwhenthedeformationstartstooccurmorerapidlythantheincreasingload.到达某点后变形的速度要比载荷增加的速度快一些。ThispointiscalledtheyieldpointQ.这一点叫做屈服极限点（Q）。themetalwhichwasresistingtheloadtillthen,startstodeformsomewhatrapidly,i.e.,yield.一当到达屈服极限后开始一直在抵抗载荷的金属开始迅速地发生形变也就是屈服。Theyieldstressiscalledyieldlimit.屈服应力叫做屈服极限。TheelongationofthespecimencontinuesfromQtoSandthentoT.试件的延伸率从Q到S再到T持续地延长。Thestress-strainrelationinthisplasticflowperiodisindicatedbytheportionQRSTofthecurve.在这个塑性流动期间的应力-应变关系表示为曲线上的QRST段。AtTthespecimenbreaks,andthisloadiscalledthebreakingload.在T点试件断裂，此时的载荷称为断裂载荷。ThevalueofthemaximumloadSdividedbytheoriginalcross-sectionalareaofthespecimenisreferredtoastheultimatetensilestrengthofthemetalorsimplythetensilestrength.最大载荷值S除以试件的横截面积为金属的最大拉伸强度或简单地称为拉伸强度。Logicallyspeaking,oncetheelasticlimitisexceeded,themetalshouldstarttoyield,andfinallybreak,withoutanyincreaseinthevalueofstress.逻辑上来说，一旦超过弹性极限，金属应该开始屈服直至最后断裂，在应力值上应该没有增加。Butthecurverecordsanincreasedstressevenaftertheelasticlimitisexceeded.但是实际的曲线却记录了在超过弹性极限后的增加了的应力。Tworeasonscanbegivenforthisbehavior:1.thestrainhardeningofthematerial;2.thediminishingcross-sectionalareaofthespecimen,sufferedonaccountoftheplasticdeformation.这种现象的发生可能有两种原因。1.材料的应变强化。2.由于塑性变形引起的试件横截面积的缩小。Themoreplasticdeformationthemetalundergoes,theharderitbecomes,duetowork-hardening.由于加工硬化,金属在发生塑性变形时会变得越来越硬。Themorethemetalgetselongatedthemoreitsdiameter(andhence,cross-sectionalarea)isdecreased.金属拉伸越长它的直径越小。ThiscontinuesuntilthepointSisreached.这种现象一直持续到曲线上的S点。AfterS,therateatwhichthereductioninareatakesplace,exceedstherateatwhichthestressincreases.超过S点后,面积减少的速度超过了应力增加的速度。Strainbecomessohighthatthereductioninareabeginstoproducealocalizedeffectatsomepoint.应变变得很大,面积的减少在某些点产生了局部效应。Thisiscallednecking.这种效应称为颈缩。Reductionincross-sectionalareatakesplaceveryrapidly;sorapidlythattheloadvalueactullydrops.ThisisindicatedbyST.FailureoccursatthispointT.横截面积减小的速度非常快以至于实际上载荷降低。Thenpercentageelongationandreductioninareaindicatetheductilityorplasticityofthematerial:伸长率和面积减少率表示了材料的延展性:δ=LLL00100%Ψ=AAA00100%WhereLandL0aretheoriginalandthefinallengthofthespecimen;A0andAaretheoriginalandthefinalcross-sectionalarea.其中，L0和L为试样的原始和最终的长度；A0和A为最初的和最后的横截面积。