材料性能-Ch1_Uniaxial-Mechanical-Properties讲解课件

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 l

2、oading/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）

3、作用下或载荷与环境因素（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。1. Definitions of Mechanical properties /力学性能The most com

4、mon 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Stress-Strain CurveNote: Engineering stress and strain应力应变曲线Usually, four stages/se

5、gmentsFour stages/segments/四阶段：1Seg.I（oab）Elastic deformation(弹性变形阶段)a: Pp b: 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）oa段：L P Linear 直线阶段ab段：极微量塑性变形（0.001-0.005%)2Seg. II（bcd）Yielding(屈服变形)c: Yield Limit: 屈服点 Ps 拉伸机上，低

6、碳钢缓慢加载单向静拉伸曲线 Yielding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 绪论3Seg. III（dB） Uniform plastic deformation (均匀塑性变形阶段)B: Ultimate Tensile Str

7、ength 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静拉伸曲线 Necking/Striction颈缩 Varying but characteristic types of curves shown by different materials 2. Stiffness and Elasticity/刚度和弹性 2.1 Stiffness /刚度 E=/ Youngs modul

8、us (杨氏弹性模量) GPa, MPaYoungs 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! 本质是：反映了材料内部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 a

9、n object the extent to which it resists elastic deformation in response to an applied force. 比例极限：p=Pp/Fo 应力应变保持线性关系的极限应力值弹性极限：e=Pe/Fo 不产永久变形的最大抗力。工程上，p、e视为同一值，通常也可用0.01 Elasticity/弹性材料不产生塑性变形的情况下，所能承受的最大应力 Elasticity is a property of materials which return to their original shape after the stress t

10、hat 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。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

11、试样尺寸有关， d5 , d10 (Lo=5do, 10do)1. Percent/specific elongation %EL /延伸率 是指试样拉断后的标距伸长量L k与原始标距L 0之比。 10% 属塑性材料Ductility/塑性The percent elongation %EL reported in a tensile test is defined as the maximum elongation of the gage length divided by the original gage length. %EL =F/Fo=(Fo-Fk)/Fo x 100% 越大，塑性

12、愈好 5%, 脆性材料试样拉断处横截面积k的收缩量与原始横截面积F0之比。 F0 - Fk %RA = = 100% F02. Percentage reduction in area %RA/断面收缩率Ductility/塑性The percentage decrease %RA in 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. strength/材料的强度Strength

13、of materials, also called mechanics of materials, is a subject which deals with the behavior of objects withstanding stresses and strains. 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

14、服强度物理意义：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 permanent/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 ma

15、terial quality, chemical composition Point at which material exceeds the elastic limit and will not return to its original shape or length 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 import

16、ant value for use in engineering structural design. If we are designing a component that must support a force during use, we must be sure 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 th

17、at the applied force produces a stress that is below the yield strength.Yield point criteria /屈服标准In Engineering, (1)Proportional limit 比例极限应力-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 d

18、eformation 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 to 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/屈服强度和条件屈服强度

19、 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 （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 ) S0试样产生0.2%残余塑性变 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 The 0.2% yield strength or the 0.2% offset yield strengthHigh strength steel and aluminum al

20、loys do not exhibit a yield point, so this offset yield point is calculated at 0.2% offset from the original cross-sectional 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 、

21、应力状态。 Extrinsic /外在因素： Temperature, Strain rate, Stress statesEffect of heat-treating process on the YS difference Effect of composition 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 amorph

22、ous materials can be engineered to increase their yield strength, Strengthening & mechanismsBy altering dislocation density, impurity levels, grain size (in crystalline materials), to prevent dislocations movements! While many material properties depend only on the composition of the bulk material,

23、yield strength is extremely sensitive to the materials processing as well for this reason.The mechanism underlying is Work Hardening Solid Solution Strengthening Particle/Precipitate Strengthening Grain boundary strengthening Thus, the yield strength of the material can be fine tuned! typically by i

24、ntroducing defects such as impurities dislocations in the material.Strain hardening/形变硬化During yielding stage, the material deforms without an increase in applied load, but usually after a degree of yielding, the material undergoes changes in its atomic and crystalline structure, resulting in increa

25、sed resistance of material to further deformation, leading the material into the strain hardening stage when increased stress is required to further-deform the material. Strain hardening is extremely important for metals from the materials engineering point of view, particularly in terms of practica

26、l processing and fabricating of products, otherwise large extent of plastic deformation processing is impossible! Mechanism of strain hardeningWork hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of

27、dislocation movements and dislocation generation within the crystal structure of the material Work hardening (strain hardening) manifests as the increase 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!M

28、ost non-brittle metals with a reasonably high melting point as well as several polymers can be strengthened in this fashion Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Empirical relations There are two common mathematical descriptions of the work harde

29、ning 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 material property normally lying in the range 0.20.5. The strain hardening index can be described by Hollomons equation :Ludwiks equation:where is

30、 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 Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being p

31、lastic-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At the point the maximum of stress reaches, thus, dF=0 dF=d(SA)=AdS+SdA=0 (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=ebThis is the prerequisite for necking/这就是出现颈缩的

32、判据 Necking manifests the end of uniform plastic deformation or the starting point of localized plastic deformation, i.e., the capacity/maximum stress 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

33、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 cas

34、e, 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：Solid state materials decompose into separate parts under loading/forces, indicating the completely mechanical failure of the materials. 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

35、彻底失效。The true stress when the materials fracture by tension testSk: k=Fk/Ak practically non-useful! Idealistic crystalline materials TFS/理想晶体脆性（解理）断裂的理论强度。Where, E is the Youngs Modulus, s surface tension of materials and a0 inter-atomic distance between neighboring atoms. 、a0一定，m与s有关，实际解理面的s越低， m小而

36、易解理。In most cases, metals true fracture stress is far lower than TFS (usually by 2 to 3 orders in 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(无缺陷理想材料下的理论值预测)3.Theoretical fracture

37、strength for a cracked crystal/含裂纹材料理论断裂强度（格里匪斯裂纹理论）无限大平板，2a长度裂纹扩展失稳的临界应力值：裂纹在其两端引起的应力集中，将外加应力放大倍。局部区域达到理论断裂强度，而断裂。A length scale for optimized fracture strength in mineral platelet. (a) A schematic diagram of mineral platelet with a surface crack. (b) Comparison of the fracture strength of a cracke

38、d 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 and sizes, so that the

39、 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 most important

40、 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(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 critical importance in practice!In

41、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/顺应力 Resilience is the capacity of a materials to absorb energy when it is deformed elastically

42、and then, upon unloading, to have this energy recovered. Modulus of resilience, Ur /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 How to improve Ur of a spring material

43、s?Anelasticity /非理想弹性&Internal Damping /内耗 Anelasticity/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 itself.The property of a solid in which deformation depends on the time rate of change of stress as well as on the stress its

44、elf Or Relating to the property of a substance in which there is no definite relation between stress and strainViscoelasticity/粘弹性Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Basically, time dependent effects indicate

45、 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 incorporates aspects of both fluid behavior (viscous) and solid behavior (elastic). pseudoelasticity/伪弹性 Pseudoelastici

46、ty, 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Schematic of a stress-strain-temperature curve showing the shape memory effect. Bauschinger effect/包申格效应The Bauschinger effect refers t

47、o 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 tensile yield strength at the expense of compressive yield strength. anelasticity/internal damping /滞弹性内耗damping is an ef

48、fect that reduces the amplitude of oscillations in an oscillatory system.Toughness, static toughness/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 the ability of a material to absorb energy and plastically deform without fracturing. Modulus of Toughness/静力韧性From the stre

49、ss 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 toughness of a material is related to its ductility as well as to its ultimate strength and that the capacity of a structu

50、re to withstand an impact Load depends upon the toughness of the material used. 静力韧性应力应变曲线下的面积减去弹性变形功所得的差 Where, D形变强化模数(tg ) 静力韧度依赖于材料的强度和塑性 Impact/Notch toughness/冲击韧性 材料在冲击载荷作用下抵抗破坏的能力。A k = m g H m g h (J) AK a k = (J/cm) S0The impact toughness (AKA Impact strength) of a material can be determin

51、ed with a Charpy or Izod test.These tests are named after their inventors and were developed in the early 1900s before fracture mechanics theory was available.The impact toughness of a metal is determined by measuring the energy absorbed in the fracture of the specimen. Impact toughness/properties a

52、re not directly used in fracture mechanics calculations as discussed later, but the economical impact tests continue to be used as a quality control method to assess notch sensitivity and for comparing the relative toughness of engineering materials. Notch-Toughness Definitely, Notch toughness is th

53、e ability that a material possesses to absorb energy in the presence of a flaw. As mentioned previously, in the presence of a flaw, such as a notch or crack, a material will likely exhibit a lower level of toughness. When a flaw is present in a material, loading induces a triaxial tension stress sta

54、te adjacent to the flaw. The material develops plastic strains as the yield stress is exceeded in the region near the crack tip. However, the amount of plastic deformation is restricted by the surrounding material, which remains elastic. When a material is prevented from deforming plastically, it fa

55、ils in a brittle manner. Notch-toughness is measured with a variety of specimens such as the Charpy V-notch impact specimen or the dynamic tear test specimen. With these specimens and by varying the loading speed and the temperature, it is possible to generate curves such as those shown in the graph

56、. Typically only static and impact testing is conducted but it should be recognized that many components in service see intermediate loading rates in the range of the dashed red line. Toughness is greatly affected by temperature, a Charpy or Izod test is often repeated numerous times with each speci

57、men tested at a different temperature. The transition temperature Tk is the boundary between brittle and ductile behavior and this temperature is often an extremely important consideration in the selection of a material. An impact toughness versus temperature graph for a steelHardness/硬度抵抗外物压入的能力，称为

58、硬度综合性能指标。 Resistance to permanently indenting the surface. Large hardness means: -resistance to plastic deformation or cracking in compression. -better wear properties.28Adapted from Fig. 6.18, Callister 6e. (Fig. 6.18 is adapted from G.F. Kinney, Engineering Propertiesand Applications of Plastics,

59、p. 202, John Wiley and Sons, 1957.)Hardness is a measure of the materials resistanceto localized plastic deformation (e.g. dent or scratch)Brinell-hardness/布氏硬度HB 450HBS; 650HBW;The oldest of the hardness test methods in common use on engineering materials today is the Brinell hardness test(Dr. J. A

60、. Brinell, Sweden in 1900 ). The Brinell test is frequently used to determine the hardness metal forgings and castings that have a large grain structures. The Brinell test provides a measurement over a fairly large area that is less affected by the course grain structure of these materials than are