1、机械设计中应首先考虑材料的力学性能。通俗地讲力学性能决定了在多大和怎样形式的载荷条件下而不致于改变零件几何形状和尺寸的能力。Ch.1 Mechanical Properties 材料的力学性能材料的力学性能Usually,materials mechanical properties are the first consideration in structural objects design,such as architectures,vehicles,machinery,utensils,tools,to name just a few,which always withstand lo
2、ading/forces in service as prerequisite.The mechanical properties of a material are those properties that involve a reaction to an applied load.mechanical properties of a material are not constants and often change as a function of temperature,rate of loading,and other conditions.材料在外加载荷（外力或能量）作用下或载
3、荷与环境因素（温度、介质和加载速率）联合作用下表现出来的行为。The mechanical properties of metals determine the range of usefulness of a material and establish the service life(without failures like deformation or fracture)that can be expected.主要是指材料在力的作用下抵抗变形和开裂的性能。主要是指材料在力的作用下抵抗变形和开裂的性能。1.Definitions of Mechanical properties/力学
4、性能力学性能The most common properties considered are strength,ductility,hardness,impact resistance,and fracture toughness.tensile test:Ch.1.1 Steady-state Mechanical Properties Under Uniaxial TensionUniaxial Mechanical Response静载单向静拉伸静载单向静拉伸Stress-Strain CurveNote:Engineering stress and strain应力应力应变曲线应变曲
5、线Usually,four stages/segmentsFour stages/segments/四阶段：四阶段：1Seg.I（oab）Elastic deformation(弹性变形阶段)pa:Pp pb:Elastic Limit Pe the greatest stress that can be applied to a material without causing permanent deformation（不产生永久变形的最大抗力）poa段：L P Linear 直线阶段pab段：极微量塑性变形（0.001-0.005%)2Seg.II（bcd）Yielding(屈服变形)p
6、c:Yield Limit:屈服点 Ps 拉伸机上，低碳钢缓慢加载单向静拉伸曲线拉伸机上，低碳钢缓慢加载单向静拉伸曲线 Yielding屈服现象：屈服现象：金属材料开始产生明显塑性变形的标志。Yielding(or the yield strength or yield point)of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically.第一章第一章 绪论绪论3Seg.III（dB）Uniform pl
7、astic deformation(均匀塑性变形阶段)pB:Ultimate Tensile Strength Pb 材料所能承受的最大载荷4Seg.IV(BK)non-uniform/localized plastic deformation,concentration of plastic deformation 局部集中塑性变形 拉伸机上，低碳钢缓慢加载单向拉伸机上，低碳钢缓慢加载单向静拉伸曲线静拉伸曲线 Necking/Striction颈缩 Varying but characteristic types of curves shown by different materials
8、2.Stiffness and Elasticity/刚度和弹性刚度和弹性 2.1 Stiffness/刚度刚度u E=/Youngs modulus(杨氏弹性模量)GPa,MPauYoungs modulus,also known as the tensile modulus,is a measure of the stiffness of an elastic material and is a quantity used to characterize the materials resistant ability to elastic deformation!u 本质是：反映了材料内部
9、原子种类及其结合力的大小，组织不敏感的力系指标。材料在受力时，抵抗弹性变形的能力Stiffness is the rigidity of an object the extent to which it resists elastic deformation in response to an applied force.u比例极限：p=Pp/Fo 应力应变保持线性关系的极限应力值u弹性极限：e=Pe/Fo 不产永久变形的最大抗力。u工程上，p、e视为同一值，通常也可用0.01 Elasticity/弹性弹性材料不产生塑性变形的情况下，所能承受的最大应力 uElasticity is a pr
10、operty of materials which return to their original shape after the stress that caused their deformation is no longer applied.Elastic limits:the greatest stress that can be applied to a material without causing permanent deformation Ductility/塑性塑性材料在载荷作用下断开破坏前而能产生的塑性变形量的能力。材料在载荷作用下断开破坏前而能产生的塑性变形量的能力。
11、Ductility is a solid materials ability to deform under tensile stress before fracture Lk:试样拉断后最终标距长度延伸率与试样尺寸有关，d5,d10(Lo=5do,10do)1.Percent/specific elongation%EL/延伸率延伸率 是指试样拉断后的标距伸长量是指试样拉断后的标距伸长量L L k k与原始标距与原始标距L L 0 0之比。之比。10%属塑性材料属塑性材料Ductility/塑性塑性The percent elongation%EL reported in a tensile
12、 test is defined as the maximum elongation of the gage length divided by the original gage length.%ELu =F/Fo=(Fo-Fk)/Fo x 100%u 越大，塑性愈好u 5%,脆性材料试样拉断处横截面积试样拉断处横截面积k k的收缩量与原始横截面积的收缩量与原始横截面积F F0 0之比。之比。F0-Fk%RA=100%F02.Percentage reduction in area%RA/断面收缩率断面收缩率Ductility/塑性塑性The percentage decrease%RA i
13、n the cross-sectional area constriction/reduction of a tensile test piece caused by wasting or necking of the specimen.材料所能承受的极限应力材料所能承受的极限应力.strength/材料的强度材料的强度Strength of materials,also called mechanics of materials,is a subject which deals with the behavior of objects withstanding stresses and st
14、rains.Basically,a maximum stress(limit)a materials withstands to the extent of plastic deformation or fracture,when subjected to an applied load.Yield strengths/屈服强度屈服强度物理意义：物理意义：s代表材料开始产生明显塑性变形的抗力,是材料设计和选材的主要依据之一。The yield strength is defined as the stress at which a predetermined amount of permane
15、nt/plastic deformation,say 0.2%,gets started!Yield strength is an important indictor for the most engineering design,which is influenced by many factors such as raw material quality,chemical composition Point at which material exceeds the elastic limit and will not return to its original shape or le
16、ngth if the stress is removed.Yield strength is the amount of stress at which plastic deformation becomes noticeable and significant.Yield strength is a very important value for use in engineering structural design.If we are designing a component that must support a force during use,we must be sure
17、that the component does not plastically deform.We must therefore select a material that has high yield strength,or we must make the component large enough so that the applied force produces a stress that is below the yield strength.Yield point criteria/屈服标准屈服标准In Engineering,(1)Proportional limit 比例
18、极限 应力-应变曲线上符合线 性关系的最高应力，国际上常采用p表示，超过p时即认为材料开始屈服。(2)Elastic limit 弹性极限 以不出现残留的永久变形为标准，材料能够完全弹性恢复的最高应力。国际上通常以el表示。应力超过el时即认为材料开始屈服。(3)Yield point/onset of plastic deformation to a degree 屈服强度 以规定发生明显的残留变形为标准，如通常以0.2%残留变形的应力作为屈服强度，符号为0.2或ys。It is difficult to tell which of the two specimens is closer t
19、o the yield point or has even reached it,in particular for those without visible yield point phenomenon.Yield strength s&offset yield strength 0.02/屈服强度屈服强度和条件屈服强度和条件屈服强度 Ps s=(M pa)F0试样屈服时的载荷(N)试样原始横截面积(mm2)Yield strength s屈服强度屈服强度 （中高碳钢、无屈服点，以产生一定的微量塑性变形的抗力的极限应力值来表示。）脆性材料：b=s 灰口铸铁 P0.2 0.2=(M pa)S
20、0试样产生试样产生0.2%残余塑性变残余塑性变 形时的载荷形时的载荷(N)试样原始横截面积(mm2)Offset yield strength 0.2条件屈服强度条件屈服强度 The 0.2%yield strength or the 0.2%offset yield strengthHigh strength steel and aluminum alloys do not exhibit a yield point,so this offset yield point is calculated at 0.2%offset from the original cross-sectional
21、 area of the sample.Factors influencing yield strength Intrinsic/内在因素：Bonding states,Microstructure,Crystalline structure,Atoms type(结合键、组织、结构、原子本性)结合键的影响是根本性的。温度、应变速率温度、应变速率、应力状态应力状态。Extrinsic/外在因素：Temperature,Strain rate,Stress statesEffect of heat-treating process on the YS difference Effect of c
22、omposition on the YS difference Effect of microstructure on the YS difference Effect of grain size on YS yield strength There are several ways in which crystalline and amorphous materials can be engineered to increase their yield strength,Strengthening&mechanismsBy altering dislocation density,impur
23、ity levels,grain size(in crystalline materials),to prevent dislocations movements!While many material properties depend only on the composition of the bulk material,yield strength is extremely sensitive to the materials processing as well for this reason.The mechanism underlying is Work Hardening So
24、lid Solution Strengthening Particle/Precipitate Strengthening Grain boundary strengthening Thus,the yield strength of the material can be fine tuned!typically by introducing defects such as impurities dislocations in the material.Strain hardening/形变硬化形变硬化During yielding stage,the material deforms wi
25、thout an increase in applied load,but usually after a degree of yielding,the material undergoes changes in its atomic and crystalline structure,resulting in increased resistance of material to further deformation,leading the material into the strain hardening stage when increased stress is required
26、to further-deform the material.Strain hardening is extremely important for metals from the materials engineering point of view,particularly in terms of practical processing and fabricating of products,otherwise large extent of plastic deformation processing is impossible!Mechanism of strain hardenin
27、gWork hardening,also known as strain hardening or cold working,is the strengthening of a metal by plastic deformation.This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material Work hardening(strain hardening)manifests as the in
28、crease in stress that is required to cause in increase in strain as a material is plastically deformed,as ascertained by the true stress-strain curves!Most non-brittle metals with a reasonably high melting point as well as several polymers can be strengthened in this fashion Alloys not amenable to h
29、eat treatment,including low-carbon steel,are often work-hardened.Empirical relations There are two common mathematical descriptions of the work hardening phenomenon,the latter is similar but includes the yield stress.The constant K is structure dependent and is influenced by processing while n is a
30、material property normally lying in the range 0.20.5.The strain hardening index can be described by Hollomons equation:Ludwiks equation:where is the stress,K is the strength index,p is the plastic strain and n is the strain hardening exponent.Tensile strength&Necking/抗拉强度与颈缩条件抗拉强度与颈缩条件 Ultimate tens
31、ile strength(UTS),often shortened to tensile strength(TS)or ultimate strength,is the maximum stress that a material can withstand while being plastic-deformed./材料被拉断前所承受的最大应力值（材料抵抗外力而不致断裂的极限应力值）。Prerequisite for necking/颈缩条件颈缩条件At the point the maximum of stress reaches,thus,dF=0 dF=d(SA)=AdS+SdA=0
32、(面积A与真实应力S)then -dA/A=dS/S volume constant theory/体积不变定理 dL/L=-dA/A=d Thus,dS/d=S,equally,n=en=eb b This is the prerequisite for necking/这就是出现颈缩的这就是出现颈缩的判据判据 Necking manifests the end of uniform plastic deformation or the starting point of localized plastic deformation,i.e.,the capacity/maximum stre
33、ss of materials to withstand loading stably 标志材料均匀塑性变形的结束、局部塑性变形的开始和材料稳定承载能力达到极限。For those material shows Hollomon relation characteristic,its necking occurs when its strain hardening exponent equates the true stress it withstands.满足Hollomon关系的材料，其强化指数之值等于该材料颈缩开始时之真实应变值。1.True fracture strength/真实断裂
34、强度真实断裂强度Actually,inasmuch as there exist always micro cracks in metals that the materials would fracture at the stress point far below its Sk.In such case,fracture toughness KIC is practically ascertained!由于实际金属材料往往存在裂纹缺陷，其断裂时所承受的应力值要比Sk低得多！需要用断裂韧性KIC表示.Fracture strength/断裂强度Fracture/断裂断裂：Solid stat
35、e materials decompose into separate parts under loading/forces,indicating the completely mechanical failure of the materials.固体材料在力固体材料在力的作用下分成若干部分的现象。意味作材料的彻底失效。的作用下分成若干部分的现象。意味作材料的彻底失效。The true stress when the materials fracture by tension testSk:k=Fk/Ak practically non-useful!Idealistic crystalli
36、ne materials TFS/理想晶体脆性（解理）断裂的理论强度。210)(aEmsWhere,E is the Youngs Modulus,s surface tension of materials and a0 inter-atomic distance between neighboring atoms.、a0一定，一定，m与与s有关，实际解理面的有关，实际解理面的s越低，越低，m小而易解理小而易解理。In most cases,metals true fracture stress is far lower than TFS(usually by 2 to 3 orders i
37、n magnitude!),and brittle materials like ceramic and glass show even much lower!实际金属材料的断裂应力仅为理论m的1/101/1000，而陶瓷、玻璃等脆性材料则更低。2.Theoretical fracture strength/理论断裂强度理论断裂强度(无缺陷理无缺陷理想材料下的理论值预测想材料下的理论值预测)3.Theoretical fracture strength for a cracked crystal/含裂纹材料理论断裂强度（格里匪斯裂纹理论）含裂纹材料理论断裂强度（格里匪斯裂纹理论）21)(aEc
38、s无限大平板，无限大平板，2a长度裂纹扩展失稳的临界应力值：长度裂纹扩展失稳的临界应力值：21)(0aamc裂纹在其两端引起的应力集中，将外加裂纹在其两端引起的应力集中，将外加应力放大倍。应力放大倍。局部区域达到理论断裂强度，而断裂。局部区域达到理论断裂强度，而断裂。21)(0aacmA length scale for optimized fracture strength in mineral platelet.(a)A schematic diagram of mineral platelet with a surface crack.(b)Comparison of the fract
39、ure strength of a cracked mineral platelet calculated from the Griffith criterion with the strength of a perfect crystal.Critical stress at the sharp point of crack of 2a length in infinite slab/plateNoteInfinite slab model For most metals,micro cracks within the bulk might present in various types
40、and sizes,so that the materials would fracture at the stress point quite different(usually far lower than)with theoretical fracture strength above-mentioned.As a matter of fact,In such case materials ability to resist crack-propagating ,namely,fracture toughness KIC is ascertained practically as the
41、 most important mechanical property in terms of fracture failure,will be discussed later!Energy storage&conversion in materials deformation/材料变形过程的能量转换材料变形过程的能量转换(应力与应变的综合应力与应变的综合)For the sake of simplicity,we didnt consider so far some important factors like time/loading rate which might be of crit
42、ical importance in practice!In such case,energy becomes more scientific and technical consideration as the combined contributions from both stress and strain(usually influenced largely by time factor,say loading rate!)Resilience/回弹性回弹性/顺应力顺应力 Resilience is the capacity of a materials to absorb energ
43、y when it is deformed elastically and then,upon unloading,to have this energy recovered.Modulus of resilience,Ur/弹性比功弹性比功 The energy that can be absorbed per unit volume without creating a permanent distortion材料在发生永久变形前吸收弹性变形能的能力，所以又称作材料的弹性应变能密度。它在数值上等于弹性应力应变曲线下的面积 。弹性比功是一个韧度指标，它表征材料的储能能力和释能能力弹性比功是一
44、个韧度指标，它表征材料的储能能力和释能能力 W e=0 e d =12ee =e2/2 E How to improve Ur of a spring materials?AnelasticityAnelasticity/非理想弹性非理想弹性&Internal Damping/&Internal Damping/内耗内耗 Anelasticity/滞弹性滞弹性 The property of a solid in which deformation depends on the time rate of change of stress as well as on the stress its
45、elf.The property of a solid in which deformation depends on the time rate of change of stress as well as on the stress itself Or Relating to the property of a substance in which there is no definite relation between stress and strainViscoelasticity/粘弹性粘弹性Viscoelasticity is the property of materials
46、that exhibit both viscous and elastic characteristics when undergoing deformation.Basically,time dependent effects indicate that the stress-strain behavior of a material will change with time.The classic material model for time dependent effects is viscoelasticity.As the name implies,viscoelasticity
47、 incorporates aspects of both fluid behavior(viscous)and solid behavior(elastic).pseudoelasticity/伪弹性伪弹性 Pseudoelasticity,sometimes called superelasticity,is an elastic(reversible)response to an applied stress,caused by a phase transformation.Example:shape memory alloys-形状记忆合金形状记忆合金Schematic of a st
48、ress-strain-temperature curve showing the shape memory effect.Bauschinger effect/包申格效应包申格效应The Bauschinger effect refers to a property of materials where the materials stress/strain characteristics change as a result of the microscopic stress distribution of the material.For example,an increase in t
49、ensile yield strength at the expense of compressive yield strength.anelasticity/internal damping/滞弹性滞弹性内耗内耗damping is an effect that reduces the amplitude of oscillations in an oscillatory system.Toughness,static toughness/静力韧度静力韧度韧度:材料断裂前吸收塑性变形功和断裂功的能力。材料的韧性用韧度度量(习惯上，韧度和韧性二词混用，不作区别)。Toughness is th
50、e ability of a material to absorb energy and plastically deform without fracturing.Modulus of Toughness/静力韧性From the stress strain diagram,the area under the complete curve gives the measure of modules of toughness.It is the materials.Ability to absorb energy upto fracture.It is clear that the tough
