第1页：材料力学的学科定位
- Mechanics of materials is a branch of applied mechanicsthat deals with the behavior of solid bodies subjected tovarious types of loading.This field of study is known by severalnames, including “strength of materials” and “mechanics ofdeformablebodies.
- ”这一领域的研究通过这么几个名字为大家所知，包括“材料强度”和“力学变形体系。
第2页：工程材料的力学模型
- In Mechanics of Engineering Materials the membershave shapes that either exist in actual structures or are beingconsidered for their suitability as parts of proposedengineering structures.
- 在工程材料中的力学模型，或者是在实际结构中有的或正在考虑其是否适合作为部分拟议工程结构。
第3页：材料力学的工程应用
- Asyoucanseealreadyfromthevarietyofmaterials,forces,andshapesmentioned,MechanicsofEngineeringMaterialsisofinteresttoallfieldsofengineering.TheengineerusedtheprinciplesofMechanicsofMaterialstodetermineifthematerialpropertiesandthedimensionsofamemberareadequatetoensurethatitcancarryitsloadssafelyandwithoutexcessivedistortion.
- 正如你已经从各种不同的材料，应力，和提到模型中看到的，工程材料力学涉及各个工程领域。
工程师运用材料力学的原理，确定材料特性和构件尺寸以确保它能够安全负载而不过度变形。
第4页：安全荷载与变形
- In general, then, we are interested in both the safe loadthat a member can carry and the associated deformation.Engineering design would be a dimple process if thedesigner could take into consideration the loads and themechanical properties of the materials, manipulate anequation, and arrive at suitable dimensions. Design isseldom that simple.
- 一般情况下，我们感兴趣的不仅是安全负荷，还有其与荷载有关的变形。
如果设计师考虑荷载和材料的力学性能，求解方程，得出合适的尺寸，那么工程设计将是一个波纹似的进程，设计很少这么简单。
第5页：理论分析与实验验证
- Theoretical analyses and experimental results haveequally important roles in the study of mechanics of materials.On many occasions, we will make logical derivations to obtainformulasand equations for predicting mechanical behavior,but we must recognize that these formulas cannot be used ina realistic way unless certain properties of the material areknown. These properties are available to us only after suitableexperiments have been carried out in the laboratory.
- 理论分析和实验结果对研究材料力学有同样重要的作用。
在许多情况下，我们将得出符合逻辑的推导公式和方程，并对力学行为进行预测，但我们必须认识到，这些公式在现实方法中不能被使用，除非材料的某些性能是已知的。这些属性只有在实验室进行实验以后才能够被获得。
第6页：设计循环与试验复算
- Also,becausemanypracticalproblemsofgreatimportanceinengineeringcannotbehandledefficientlybytheoreticalmeans,experimentalmeasurementsbecomeanecessity.此外，在很重要的工程当中，很多实际问题按理论的方法不能有效解决，因而，实验测量成为必要.Frequently,itdoesnotandthenanewtrialmemberisselectedandtheanalysisrepeated.Thisdesigncyclecontinuesuntilasatisfactorysolutionisobtained.
- 通常情况下，要是不满足，就会选一个新的试验构件并重新进行分析。
这样的设计周期将持续到获得一个令人满意的解决办法。
第7页：应力应变与挠度分析
- In general, the objectives of our analysis will be thedetermination of the stressed,strains, anddeflectionsproducedby the loads.If these quantities can be found for allvalues of load up to thefailureload, then we will have acomplete picture of the mechanical behavior of the body.
- 一般情况下，我们分析的目标是确定应力、应变和荷载引起的挠度。
如果可以找到导致破坏荷载的所有荷载值，那么我们将有一个体系的力学行为的完整构图。
第8页：结构的承载能力
- Animportantconsiderationinengineeringdesignisthecapacityoftheobjectbeingdesignedtosupportortransmitloads.Objectsthatmustsustainloadsincludebuildingstructures,machines,aircraft,vehicles,ships,andaseeminglyendlesslistofotherman-madethings.
- 在工程设计中，一个很重要的问题是考虑物体支撑或传递荷载的能力。
负载物体包括建筑结构，机器，飞机，车辆，船舶，以及似乎永无休止的其他人为的东西。
第9页：强度与实际应力准则
- If structural failure is to be avoided, the loads that a structureactually can support must be greater than the loads it will berequired to sustain when in service.The ability of a structureto resist loads is called strength, hence the precedingcriterioncan be restated as follows: The actual strength of astructure must exceed the required strength.
- 如果结构的失效避免，结构的承载能力实际上可以比正常使用时更大。
结构抗负荷的能力称为强度，因此上述标准有如下的表述：结构的实际应力必须超过所要求的应力。
第10页：安全系数的取值
- Therationoftheactualstrengthtotherequiredstrengthiscalledthefactorofsafety:实际应力与所需应力的比值称为安全系数n：安全系数:Of course, the factor of safety must be greater than 1.0 if failureis to be avoided. Depending upon the circumstances, factors ofsafety from slightly above 1.0 are used.
- 当然，如果失效避免，安全系数必须大于1.0。
基于这种情况，使用时安全系数从略高于1.0考虑。
第11页：失效的多种含义
- The incorporation of factors of safety into design is not asimple matter, because both strength and failure have manydifferent meanings.Failure can mean thefractureor completecollapseof a structure, or it can mean that the deformationshave exceeded some limiting value so that the structure is nolonger able to perform its intender functions.
- 把安全系数纳入设计不是一件简单的事，因为强度和失效有很多不同的含义。
失效可能意味着破裂或结构的彻底坍塌，也可能意味着一些变形已超过限定值以至于这样的结构已不再能够履行既定的功能。
第12页：确定安全系数的因素
- The determination of a factor of safety must also take into account suchmatters as the following:the probability of accidental overloading of thestructure; the types of loadsstatic,dynamic,or repeated) and howaccurately they are known; the possibility of fatigue failure; inaccuracies inconstruction; quality of workmanship; variations in properties of materials;deterioration due to corrosion or other environmental effects; accuracy of themethods of analysis; whether failure is gradual (ample warning ) or sudden(no warning); consequences of failure (minor damage or majorcatastrophe);and other such considerations.
- 结构超载的可能性;负载的类型（静态，动态，或重复），以及如何准确地知道他们；疲劳破坏的可能性；施工时的不确性；工艺质量；材料性能的变化;确定一个安全系数也必须考虑到如下这些事项：由于腐蚀或其他环境影响引起的恶化；分析方法的正确性；失败是否是渐进的（足够的预兆）或突然的（没有预兆）；失效的后果（轻微损坏或重大灾难）；以及其他此类因素。
第13页：安全系数的工程判断
- If the factor of safety too low, the likelihood of failure will behigh and hence the structure will be unacceptable; if the factoris too large, the structure will be wasteful of materials andperhaps unsuitable for its function (for instance, it may be tooheavy).Because of these complexities, good engineeringjudgment is required when establishing factors of safety. Theyare usually determined by groups of experienced engineerswho write thecodesand specifications used by otherdesigners.
- 如果安全系数太低，失效的可能性将很高，因此，这样的结构将是不能接受的；如果系数过大，结构将浪费材料，而且可能不适合它的功能（例如，它可能过重）。
由于这些复杂性，建立安全系数时需要良好的工程判断，它们通常是由经验丰富的编写规范的工程师决定的，而这些规范是设计师在设计过程中使用的。
第14页：弹性范围内的设计方法
- In actual practice, there are several ways in which factors ofsafety are defined and implemented. For many structures, it isimportant that the material remain within thelinear elasticrangein order to avoid permanent deformations when the loads areremoved.Hence, a common method of design is to use a factorof safety with respect to yielding of the structure.The structurebegins to yield when the yield stress, we obtain anallowablestress, of working stress, that must not be exceeded anywherein the structure.
- 在实际工程中，有几种确定和实施安全系数的方法。
对于许多机构，重要的是，卸除载荷时这些材料仍在弹性线性范围内，从而避免永久变形。因此，通常的设计方法是使用与结构结构达到屈服应力时开始产生屈服，我屈服对应的安全系数。们会取得一个允许的工作应力，这个应力不能超过结构任何地方所能承受的压应力.
第15页：允许应力与屈服应力
- Thus,Orin which we have introduced the notations and for theallowable and yield stresses, respectively. In building design, atypical factor of safety n with respect to yielding is 1.67; thus, amild steelhaving a yield stress of 36 ksi has an allowablestress in tension of 21.6 ksi.
- 因而：或者上式，我们引出了和，分别表示允许应力和屈服应力。
在建筑设计中，一个典型的安全系数n对应的屈服应力为1.67；因此，低碳钢屈服应力为36 ksi对应有一个允许拉应力21.6 ksi。
第16页：基于极限应力的设计法
Anothermethodofdesignistoestablishtheallowablestressbyapplyingafactorofsafetywithrespecttotheultimatestress(极限应力)insteadoftheyieldstress.Thismethodissuitableforbrittlematerials,suchasconcrete,anditalsoisusedforwood.Theallowablestressisobtainedfromtheequation
另一种设计方法是通过应用与极限应力而不是屈服应力对应的安全系数。这种方法适用于脆性材料，如混凝土，也可用于木材。允许应力是从方程中获得:
第17页：荷载安全系数的应用
The last method we will describe involves the applicationof factors of safety to loads rather than to stresses. We willuse the term ultimate loads to mean the loads that producefailure or collapse of the structure.The loads that thestructure must support in service are called service loads orworking loads. The factor of safety is the ratio of the formerto the latter
我们描述的后一种方法是关于荷载安全系数的应用而不是应力安全系数的应用。我们将使用的极限荷载会意味着结构的断裂或倒塌。结构在使用期间需要支撑的载荷就是使用荷载或工作荷载。安全系数就是前者和后者的比值：
第18页：强度设计与极限荷载设计
Thenthestructureisdesignedsothatitcanjustsustaintheultimateloadsatfailure.Thismethodofdesignisknownasstrengthdesign,orultimate-loaddesign,andthefactorofsafetyiscalledtheloadfactorbecauseitisamultiplieroftheserviceloads:Ultimate load = (service load) (load factor)
极限荷载=（工作荷载）*（荷载系数）然后，所设计的结构正好能够保证在破坏时能够承担极限荷载。这种设计方法被称作强度设计或极限荷载设计，安全系数被称为荷载系数，因为它是工作荷载的乘数：
第19页：钢筋混凝土的荷载系数
- Typical load factors used in the design of reinforcedconcrete structures are 1.4 for dead load, which is the weightof the structure itself, and 1.7 for live loads, which are loadsapplied to the structure, The strength-design method is usedregularly for reinforced concrete structures and occasionallyfor steel structures.
- 典型的荷载系数在设计钢筋混凝土中使用，恒载为1.4，即是结构本身的重量，应用于结构中的活载为1.7，这种强度设计方法经常用于钢筋混凝土结构中，有时也用于钢结构中。
第20页：安全限度的定义
In aircraft design, it is customary to speak of the margin ofsafety rather than the factor of safety. Themargin[′mɑ:dʒin]n.
边缘部分，页面的空白，栏外of safety is defined as the factor ofsafety minus one:Margin of safety = n-1 (4A-6)在飞机设计中，习惯讲的是安全限度而不是安全系数。安全限度被定义为安全系数减去1：安全限度=安全系数n-1 (4A-6)
第21页：Section B 材料应力应变关系
- Section B Stress-strain Relationship of Materials（边坡上的基础）Every material undergoes deformation under the action ofexternal forces or loads. Deformation, for example, change indimensionor in shape of the body or bothsimultaneousl.While undergoing deformation, the particles of the material exerta resisting force.
- 任何材料在外力或荷载的作用下都要变形。
例如，变形有尺寸的改变或形体的变化或二者同时进行。经历变形的同时，材料各部分会产生抵抗力。
第22页：材料的变形
- Adeflectionofafewthousandthsofaninchmightmakeaboringmachineuseless,whereastheboomonadraglinemightdeflectseveralincheswithoutimpairingitsusefulness.Itisoftennecessarytorelatetheloadsonastructure,oronamemberinastructure,tothedeflectiontheloadswillproduce.
- 变形几千分之一英寸就有可能使钻床毫无用处，而挖土机的悬臂可能偏移几英寸也不会损害其效用。
关心结构上或构件上的荷载以及荷载产生的挠度是必要的。
第23页：变形容许与挠度控制
- Typical foundation types in Hong Kong are spreadfootings,caissons(hand-dug and machine bored) andpiles (percussion, bored,precastandcast-in-place).Spread footings and hand-dug caissons are the mostcommon because heavy machinery is not required forconstruction, the operation of which is difficult on steephillsides.
- 在香港，典型的基础形式包括扩展基础、沉箱（人工挖掘及机械钻挖）以及桩基础（冲击、钻孔、预制和现浇）。
因为重型机械很难在陡坡上进行施工操作，所以扩展基础和人工挖掘沉箱是最常见的基础形式。
第24页：应力应变图的由来
- Such information can be obtained by plotting diagramsshowing loads and deflections for each member and type ofloading in a structure,but suchdiagramswill vary with thedimensions of the members, and it would be necessary to drawnew diagrams each time the dimensions were varied.A moreuseful diagram is one showing the relation between the stressand strain. Such diagrams are called stress-strain diagrams.
- 这些信息的获得是通过绘制表格来表示构件的荷载和变形以及在结构上的荷载类型。
但是这种图表随着构件尺寸的变化而变化，所以每次尺寸变化的时候绘制新表格是有必要的。显示应力和应变关系的图更有用，这种图表被称为应力应变图。
第25页：应力应变数据的测定
- Data for stress-strain diagrams are usually obtained byapplying an axial load to a test specimen and measuring theload and deformation simultaneously.A testing machine isused to strain the specimen and to measure the loadrequired to produce the strain. The stress is obtained bydividing the load by the initialcross-sectionalarea of thespecimen.
- 通常应力应变图表的数据是根据轴向载荷作用到试件上，测量荷载和同时发生的变形来获得的。
测试机是用来测量产生应变的试样的荷载。应力是通过试样的荷载除以初始横截面积得出的。
第26页：名义应力与实际应力
It is the stress most commonly used, however, in designingstructures. The stress obtained by dividing the load by theactual area is frequently called the true stress and is useful inexplaining the fundamental behavior of materials. The stressbeing defined as
在结构设计中，最常用到的是应力。通过荷载除以实际面积而得到的应力通常叫做实际应力，这种应力对解释材料的基本行为很有用处。应力被定义为：
第27页：应变的定义
Strainisameasureofthedeformationproducedbytheapplicationofexternalforces.Strainismeasuredastheratioofthechangeinlength,totheoriginallength.Itisthelinearchangeinlengthperunitlength.Itisusuallydenotedbyε.Thestrainproducedisdefinedasfollows
应变是衡量应用外部力而产生变形的。应变是用长度的变化除以原始长度的比率来衡量的。每单位长度的变化是线性变化。它就是通常所指的ε。应变的产生的定义如下：
第28页：真实应变与工程应变
- Truestrain,liketruestress,iscomputedonthebasisoftheactuallengthofthetestspecimenduringthetestandisusedprimarilytostudythefundamentalpropertiesofmaterials.Strainsareusuallyrelativelysmallinmaterialsusedinengineeringstructures,oftenlessthan0.1%,andtheiraccuratedeterminationrequirespecialmeasuringequipment.
- 真正的应变，和实际应力一样，在实验当中试样的实际长度的基础上计算出来的，而且主要用来研究材料的基本性能。
用于工程结构中的材料，应变相对较小，通常低于0.1％，它的准确得出需要特殊的测量设备。
第29页：名义与实际应力应变差异
- Thedifferencebetweennominalstressandstrain,computedfrominitialdimensionsofthespecimen,andtruestressandstrainisnegligibleforstressesusuallyencounteredinengineeringstructures,butsometimesthedifferencebecomesimportantwithlargerstressesandstrains.
- 名义应力和应变之间的差异，是从试样初始的尺寸中算出的。
而通常在遇到的工程结构中，实际的应力和应变是可以忽略的，但有时候在较大的应力应变中这种差异就变得很重要。
第30页：应力应变图的初始直线段
- The initial portion of the stress-strain diagram for most materials used inengineering structures is a straight line. The stress-strain diagrams for somematerials, such asgraycast ironand concrete, show a slight curve even atvery small stresses, but it is common practice to draw a straight line toaverage the data for the first part of the diagram and neglect thecurvature.Thomas Young, 1807, suggested what amounts to using the ratio of stress tostrain to measure the stiffness of a material. This ratio is given the symbol Eand termedYoung’s modulusor the modulus of elasticity and is the slope ofthe straight-line portion of the stress-strain diagram.
- 在工程结构中使用的大部分材料最初的应力应变图是直线型的。
一些材料的应力应变图，例如灰口铸铁和混凝土，即使应力非常小也会呈现轻微曲线，但通常的做法是利用图表第一部分的平均数据绘制直线，且忽略弯曲。1807年，托马斯杨，建议使用应力应变比率测量材料的刚度。符号E表示了这个比率，称为杨氏模量或弹性模量，是应力应变图的初始直线部分的斜率。
第31页：杨氏模量
Thus, Young’s modulus is written asYoung’s modulus E is generally assumed to be the samein tension or compression and for most engineeringmaterials has a high numerical value.Typically, E=200×109N/m2 for steel, so that it will be observed from Eq. thatstrains are normally very small. The actual value of Young’smodulus for any materials is normally determined by carryingout a standard test on a specimen of the material.因此，杨氏模量被写为：一般认为，对于大多工程材料，在具有相同拉力或压力下，杨氏模量E具有很高的数值。典型地如，对于E= 200×109N/m2
的钢，从上面等式中，可观察到的应变通常很小。对于任何材料的实际杨氏模量，通常是由材料样本的标准测试来确定的。
第32页：比例极限与弹性极限
- The maximum stress for which stress and strain areproportionalis called the proportional limit.The action is said tobe elastic limit (or proportional limit for practical purposes), it isfound that a portion of the deformation remains after the load isremoved. The deformation remaining after an applied load isremoved is calledplastic deformation.Plastic deformationindependent of the time duration of the applied load is known asslip.Creepis plastic deformation that continues to increaseunder a constant stress.
- 应力最大时，应力和应变的比例被称为比例极限。
该作用被认为是弹性极限（或实际应用的比例极限），我们发现荷载卸掉后仍有一部分变形保留，这种荷载被移除后保持的变形叫做塑不取决于加载的持续时间的塑性变形被称为滑移。徐性变形。变就是应力持续时不断增加的塑性变形。
第33页：徐变与温度影响
- Inmanyinstancescreepcontinuesuntilfractureoccurs;however,inotherinstancestherateofcreepdecreasesandapproacheszeroasalimit.Somematerialsaremuchmoresusceptibletocreepthanareothers,butmostmaterialsusedinengineeringexhibitcreepatelevatedtemperatures.Thetotalstrainisthusmadeupofelasticstrain,possiblycombinedwithplasticstrainthatresultsfromslip,creep,orboth.Whentheloadisremoved,theelasticportionofthestrainisrecovered,buttheplasticpart(slipandcreep)remainsaspermanentset.
- 在许多情况下，徐变会一直持续到断裂发生，但是，在其他情况下，徐变率降低而且一直接近到0作为极限。
有些材料比其他材料更容易发生徐变，但是工程中，大部分材料在温度升高时表现出徐变。这样，应变包含了弹性应变、滑移导致的塑性应变，徐变或者两者皆有。当荷载被移走时，应变的弹性部分恢复，但塑性应变部分（滑移和徐变）却会永久保留。
第34页：屈服点与屈服强度
- A precise value for the proportional limit is difficult toobtain, particularly when the transition of the stress-straindiagram from a straight line to a curve is gradual.For thisreason, other measures of stress that can be used as apractical elastic limit are required. The yield pointand theyield strength for a specified offset are frequently used forthis purpose.
- 比例极限的精确值是很难获得的，尤其是在应力应变图由直线逐渐变为曲线的过渡时候。
出于这个原因，其它一些在实用弹性极限中使用的应力测量方法是必须的。通常出于这一目的，使用屈服点和屈服强度来作为补偿。
第35页：屈服点与屈服强度定义
- The yield point is the stress at which there is an appreciableincrease in strain with no increase in stress, with the limitationthat, if straining is continued, the stress will again increase.The yield strength is defined as the stress that will induce aspecified permanent set, usually 0.05 to 0.3 percent, which isequivalent to a strain of 0.0005 to 0.003. The yield strength isparticularly useful for materials with on yield point.
- 屈服点是在应变有可观增加，应力没有增加的应力极限，在这个极限下，如果应变继续增加，应力就也会增加。
屈服强度的定义是应力会产生特定的永久变形，通常是0.05~0.3%，这相当于0.0005至0.003的应变。在屈服点，材料的屈服强度尤其有用。
第36页：失效与屈服的关联
- We might take the obvious definition that a material has failed when ithas broken into two or more parts. However, it has already been pointedout that in most applications a member would be unserviceable due toexcessive distortion long before it actually ruptured .Consequently, we willrelate failure to yielding and consider that a material has failed when it willno longer return to its original shape upon release of the loads.In a simpletensile test we would then say that aductilematerial has failed when thematerial begins to yield. Then for uniaxial stress, failure occurs when thestress reaches the yield stress, in either tension or compression.
- 对一种材料破坏为两部分或更多部分时，我们会采取明确的定义。
但是，已经指出，大多数应用的材料中，一个构件在破裂之前，由于长期过度变形将不能使用。因此，我们将失效和屈服联系起来，认为当这种材料在卸载时不能在一个简单的拉伸试验中，我们会说，当恢复原来的形状时，材料已经破坏。材料开始屈服时，韧性材料就已经失效了。对于单轴应力来说，不管拉力或压力，当应力达到屈服应力时，破坏就发生了。
第37页：极限强度的定义
- The maximum stress, based on the original area,developed in a material before rupture is called theultimate strength of the material, and the term maybe modified as the ultimate tensile, compressive, orshearing strength of the material. Ductile materialsundergo considerable plastic tensile or shearingdeformation before rupture.
- 材料在原始面积基础上破裂之前发展的最大应力，称为材料的极限强度，这个术语被修正为材料的极限抗拉强度、抗压强度、抗剪强度。
在破裂之前，韧性材料经历相当的塑性拉伸或剪切变形。
第38页：韧性材料的颈缩现象
- When the ultimate strength of a ductile material is reached,the cross-sectional area of the test specimen starts todecrease orneck down, and the resultant load that can becarried by the specimen decreases.Thus, the stress basedon the original area decreases beyond the ultimate strength ofthe material, although the true stress continues to increaseuntil rupture.
- 当达到韧性材料的极限强度时，测试试样的横截面积开始减小或颈缩，由试样承担的荷载会减小。
这样，超过了材料的极限强度时在原始面积基础上的应力就减小，虽然真正的应力会继续增加直至破裂。
