第5章材料的力学性能课件

11材料科学基础材料科学基础Chapter 5 材料的力学性能1材料科学基础Chapter 5 材料的力学性能22Objectives of Chapter 5oIntroduce the basic concepts associated with mechanical properties of materials.oEvaluate factors that affect the mechanical properties of materials.oReview some of the basic testing procedures that engineers use to evaluate many of these properties.2Objectives of Chapter 5Introd33Chapter Outline o5.1 材料承受静载荷时的力学性能o5.2 材料承受冲击载荷时的力学性能o5.3 材料的疲劳o5.4 材料的断裂韧性o5.5 材料的磨损性能o5.6 材料的蠕变性能3Chapter Outline 5.1 材料承受静载荷时的44Technological SignificanceFigure The materials used in sports equipment must be lightweight,stiff,tough,and impact resistant.Figure Aircraft,such as the one shown here,makes use of aluminum alloys and carbon-fiber-reinforced composites.4Technological SignificanceFig55Terminology for Mechanical Propertiesq Stress-Force or load per unit area of cross-section over which the force or load is acting.q Strain-Elongation change in dimension per unit length.q Youngs modulus-The slope of the linear part of the stress-strain curve in the elastic region,same as modulus of elasticity.q Shear modulus(G)-The slope of the linear part of the shear stress-shear strain curve.q Viscosity()-Measure of resistance to flow,defined as the ratio of shear stress to shear strain rate(units Poise or Pa-s).q Thixotropic behavior-Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time.5Terminology for Mechanical Pr66Figure (a)Tensile,compressive,shear and bending stresses.(b)Illustration showing how Youngs modulus is defined for elastic material.(c)For nonlinear materials,we use the slope of a tangent as a variable quantity that replaces the Youngs modulus constant6Figure (a)Tensile,compres77Section 5.1 材料承受静载荷时的力学性能材料承受静载荷时的力学性能5.1.1 材料的拉伸曲线材料的拉伸曲线oLoad-The force applied to a material during testing.oStrain gage or Extensometer-A device used for measuring change in length and hence strain.oGlass temperature(Tg)-A temperature below which an otherwise ductile material behaves as if it is brittle.oEngineering stress-The applied load,or force,divided by the original cross-sectional area of the material.oEngineering strain-The amount that a material deforms per unit length in a tensile test.7Section 5.1 材料承受静载荷时的力学性能5.1885.1.1 材料的拉伸曲线材料的拉伸曲线o单向静拉伸试验是广泛应用的材料性能检测方法。o负荷一伸长曲线-材料的拉伸曲线。o整个拉伸过程中的变形可分为弹性变形弹性变形、屈服变形屈服变形、均匀塑性变均匀塑性变形形及不均匀塑性变形不均匀塑性变形四个阶段。o应力应变曲线(工程应力应变曲线)o真实应力应变曲线85.1.1 材料的拉伸曲线单向静拉伸试验是广泛应用的材料性99Figure A unidirectional force is applied to a specimen in the tensile test by means of the moveable crosshead.The cross-head movement can be performed using screws or a hydraulic mechanism9Figure 1010Figure Tensile stress-strain curves for different materials.Note that these are qualitative10Figure 1111Figure The stress-strain curve for an aluminum alloy11Figure The stress-strain 1212121313True Stress and True StrainoTrue stress The load divided by the actual cross-sectional area of the specimen at that load.oTrue strain The strain calculated using actual and not original dimensions,given by t ln(l/l0).Figure The relation between the true stress-true strain diagram and engineering stress-engineering strain diagram.The curves are identical to the yield point13True Stress and True StrainT14145.1.2 材料的变形及其性能指标材料的变形及其性能指标oElastic limit oTensile strength,NeckingoHookes lawoPoissons ratiooModulus of resilience(Er)oTensile toughnessoDuctility145.1.2 材料的变形及其性能指标Elastic lim15155.1.2 材料的变形及其性能指标材料的变形及其性能指标1.材料变形的实质(1)弹性变形的实质(2)塑性变形的实质(3)超塑性2.材料变形的性能指标比例极限p弹性极限e弹性模量E屈服极限s(0.2)抗拉强度b断裂强度Sk延伸率断面收缩率155.1.2 材料的变形及其性能指标材料变形的实质1616Figure (a)Determining the 0.2%offset yield strength in gray cast ion,and(b)upper and lower yield point behavior in a low-carbon steel16Figure (a)Determining the1717Figure Typical yield strength values for different engineered materials.(Source:Reprinted from Engineering Materials I,Second Edition,M.F.Ashby and D.R.H.Jones,1996,Fig.8-12,p.85.Copyright Butterworth-Heinemann17Figure Typical yield streng1818181919Figure Range of elastic moduli for different engineered materials.(Source:Reprinted from Engineering Materials I,Second Edition,M.F.Ashby and D.R.H.Jones,1996,Fig.3-5,p.35,Copyright 1996 Butterworth-Heinemann.19Figure Range of elastic mo20205.1.3 材料的断裂及其性能指标材料的断裂及其性能指标（一）断裂的类型及断口特征根据断裂前后材料宏观塑性变形的程度，分为脆性断裂与韧性断裂；根据晶体材料断裂时裂纹扩展的途径，分为穿晶断裂和沿晶(晶界)断裂；根据微观断裂机理，分为解理断裂和剪切断裂等。材料的断裂表面称为断口。用肉眼、放大镜或电子显微镜等手段对材料断口进行宏观及微观的观察分析，称为断口分析。1韧性断裂与脆性断裂韧性断裂韧性断裂是材料断裂前产生明显塑性变形的断裂过程。韧性断裂的断口往往呈暗灰色、纤维状。塑性较好的金属材料和高分子材料，室温下的静拉伸断裂具有典型的韧性断裂特征。脆性断裂脆性断裂是材料断裂前不产生明显的塑性变形。脆性断裂的断口，一般与正应力垂直，宏观上比较齐平光亮，常呈放射状或结晶状。淬火钢、灰铸铁、陶瓷、玻璃等脆性材料的断口常具有上述特征。实际上，金属的脆性断裂与韧性断裂并无明显的界限，脆性断裂前也会产生微量塑性变形。因此，规定光滑拉伸试样的断面收缩率小于5为脆性断裂；大于5为韧性断裂。205.1.3 材料的断裂及其性能指标（一）断裂的类型及断口21215.1.3 材料的断裂及其性能指标材料的断裂及其性能指标2穿晶断裂与沿晶断裂根据材料(包括金属、陶瓷及结晶高分子)发生断裂时裂纹扩展的路径，分为穿晶断裂穿晶断裂和沿晶沿晶(晶界晶界)断裂断裂两种。穿晶断裂可以是韧性断裂，也可以是脆性断裂；而沿晶断裂则多为脆性断裂，断口呈结晶状；沿晶断裂是晶界结合力较弱的一种表现。例如共价键陶瓷晶界较弱，断裂方式主要是晶界断裂。离子键晶体的断裂往往以穿晶解理为主。图图56 穿晶断裂与沿穿晶断裂与沿晶断裂示意图晶断裂示意图215.1.3 材料的断裂及其性能指标2穿晶断裂与沿晶断裂2222Figure Localized deformation of a ductile material during a tensile test produces a necked region.The micrograph shows necked region in a fractured sample22Figure Localized deformatio23235.1.3 材料的断裂及其性能指标材料的断裂及其性能指标Microstructural Features of Fracture in Metallic MaterialsoTransgranular-Meaning across the grains(e.g.,a transgranular fracture would be fracture in which cracks would go through the grains).oMicrovoids-Development of small holes in a material.oIntergranular-In between grains or along the grain boundaries.oChevron pattern-A common fracture feature produced by separate crack fronts propagating at different levels in the material.235.1.3 材料的断裂及其性能指标Microstruct2424Figure When a ductile material is pulled in a tensile test,necking begins and voids form starting near the center of the bar by nucleation at grain boundaries or inclusions.As deformation continues a 45 shear lip may form,producing a final cup and cone fracture24Figure 2525Figure Dimples form during ductile fracture.Equiaxed dimples form in the center,where microvoids grow.Elongated dimples,pointing toward the origin of failure,form on the shear lip25Figure Dimples form during 2626Figure Scanning electron micrographs of an annealed 1018 steel exhibiting ductile fracture in a tensile test.(a)Equiaxed dimples at the flat center of the cup and cone,and(b)elongated dimples at the shear lip(x 1250)26Figure Scanning electron m2727Figure Scanning electron micrograph of a brittle fracture surface of a quenched 1010 steel(x 5000).27Figure Scanning electron m2828Figure The Chevron pattern in a 0.5-in.-diameter quenched 4340 steel.The steel failed in a brittle manner by an impact blow28Figure The Chevron pattern2929Figure The Chevron pattern forms as the crack propagates from the origin at different levels.The pattern points back to the origin29Figure The Chevron pattern30305.1.4 材料的弯曲及其性能指标材料的弯曲及其性能指标oBend test-Application of a force to the center of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load.oFlexural strength or modulus of rupture-The stress required to fracture a specimen in a bend test.oFlexural modulus-The modulus of elasticity calculated from the results of a bend test,giving the slope of the stress-deflection curve.305.1.4 材料的弯曲及其性能指标Bend test 31315.1.4 材料的弯曲及其性能指标材料的弯曲及其性能指标1.弯曲试验测定的力学性能指标o弯曲试验在万能试验机上进行，其试样分圆柱和方形两种。加载方式有三点弯曲加载和四点弯曲加载两种。通过记录载荷F(或弯矩)与试样最大挠度f之间的关系曲线弯曲图，来确定材料在弯曲载荷下的力学性能。o对于脆性材料，可根据弯曲图计算抗弯强度抗弯强度式中：Mb 为试样断裂时的弯矩。W为试样抗弯截面系数，对于直径为d0 的圆柱试样，对于宽度为b，高度为h 的矩形试样，。材料的塑性用最大弯曲挠度fmax 表示，fmax 值可由百分表或挠度计直接读出。此外，从弯曲挠度曲线上还可测算弯曲弹性模量Eb。315.1.4 材料的弯曲及其性能指标1.弯曲试验测定的力32325.1.4 材料的弯曲及其性能指标材料的弯曲及其性能指标2.弯曲试验的特点及应用o弯曲加载时受拉一侧的应力状态基本上与静拉伸时相同，且不存在拉伸试验时试样装卡偏斜对实验结果造成的影响。对于难以加工成拉伸试样的硬脆材料，可用弯曲试验测定断裂强度，并能显示出它们的塑性差别。o弯曲试验时，试样截面上的应力分布是表面上应力最大，故可灵敏地反映材料的表面缺陷。因此，常用来比较和评定材料表面处理层的质量，例如检验渗碳层的质量和性能。o不能使塑性材料断裂，虽可测定规定非比例应力和弯曲应力，但实际上很少应用。o主要用于测定灰铸铁、硬质合金、陶瓷等材料的抗弯强度。灰铸铁弯曲试样一般采用圆柱毛坯试样，实验加载速度不大于0.1mm/s。硬质合金由于硬度高，难以加工成拉伸试样，故常用弯曲试验评价其性能和质量。陶瓷材料脆性大，测定抗拉强度困难，不能得到精确的结果，主要以抗弯强度作为评价陶瓷材料性能的指标。325.1.4 材料的弯曲及其性能指标2.弯曲试验的特点3333Figure The stress-strain behavior of brittle materials compared with that of more ductile materials33Figure The stress-strain b3434Figure (a)The bend test often used for measuring the strength of brittle materials,and(b)the deflection obtained by bending34Figure (a)The bend test o3535Figure Stress-deflection curve for Mg0 obtained from a bend test35Figure 3636Figure (a)Three point and(b)four-point bend test setup36Figure (a)Three point an37373738385.1.5 材料的硬度材料的硬度Hardness of MaterialsoHardness test-Measures the resistance of a material to penetration by a sharp object.oMacrohardness-Overall bulk hardness of materials measured using loads 2 N.oMicrohardness Hardness of materials typically measured using loads less than 2 N using such test as Knoop(HK).oNano-hardness-Hardness of materials measured at 110 nm length scale using extremely small(100 N)forces.385.1.5 材料的硬度Hardness of Mater39395.1.5 材料的硬度材料的硬度Hardness of Materials硬度硬度是衡量材料软硬程度的一种力学性能，其物理意义是材料表面上不大体积内抵抗变形或破裂的能力。硬度试验方法有十几种，按加载方式不同，可分为压人法和刻划法两大类。布氏硬度布氏硬度、洛氏硬度、维氏硬度和显微硬度洛氏硬度、维氏硬度和显微硬度属于压人法。刻划法包括莫氏硬度和挫刀法等。395.1.5 材料的硬度Hardness of Mater40405.1.5 材料的硬度材料的硬度Hardness of Materials1.布氏硬度o布氏硬度是1900年由瑞典工程师J.B.Brinell提出。测量方法是，在负荷F的作用下，将直径为D的淬火钢球压人试样表面，保持一定时问后卸除载荷，以试样压痕的表面积A去除负荷F所得的商，作为硬度的计算指标，用符号HB表示。压痕直径越大，布氏硬度值HB越小；o布氏硬度值为450650的材料，用硬质合金压头，用“HBW”表示；对于布氏硬度值低于450的材料，使用淬火钢球压头，用“HBS”表示。o布氏硬度值的表示方法，一般记为“数字+硬度符号(HBS或HBW)+数字/数字/数字”的形式，符号前面的数字为硬度值，符号后面的数字依次表示钢球直径、载荷大小及载荷保持时间等试验条件。o布氏硬度试验的优点是压痕面积大，试验数据稳定，重复性高。其硬度值能反映材料在较大区域内各组成相的平均性能，最适合测定灰铸铁、轴承合金等材料的硬度。o操作较为麻烦，对不同的材料需要更换压头直径和载荷F，压痕直径需要测量。因压痕直径较大，一般不宜在成品件上直接进行检验。405.1.5 材料的硬度Hardness of Mater41415.1.5 材料的硬度材料的硬度Hardness of Materials1.洛氏硬度o洛氏硬度是1919年由美国人S.P.Rockwell和H.M.Rockwell提出，以压痕深度作为计量硬度的依据。o洛氏硬度试验时，采用的压头为120的金钢石圆锥或直径为1.588mm、3.175的钢球。载荷先后分两次施加。o金属越硬压痕深度越小，金属越软压痕深度越深。用常数k减去压痕深度h，所得差值作为洛氏硬度的指标HR。o硬度值可由表盘上直接读出。显然，材料越软则压痕 越深；o洛氏硬度试验的优点是操作简便；压痕面积较小，可检测成品、小件和薄件；测量范围大，从很软的有色金属到极硬的硬质合金；测量迅速，可直接从表盘上读出硬度值。其缺点是压痕较小，代表性差；所测硬度值的重复性差、分散度大；不适于检测灰铸铁、滑动轴承合金及偏析严重的材料。用不同标尺测得的硬度值既不能直接进行比较，又不能彼此互换。415.1.5 材料的硬度Hardness of Mater42425.1.5 材料的硬度材料的硬度Hardness of Materials标尺标尺测量范围测量范围初载荷初载荷/N(Kgf)主载荷主载荷N(Kgf)压头类型压头类型K/mmk/mmHRA608598.1（10）490.3（50）金刚石圆锥体金刚石圆锥体0.20.002HRB2510098.1（10）882.6（90）钢球钢球0.260.002HRC206798.1（10）1373（140）金刚石圆锥体金刚石圆锥体0.20.002表表52 洛氏硬度试验条件及应用洛氏硬度试验条件及应用425.1.5 材料的硬度Hardness of Mater4343Figure Indentors for the Brinell and Rockwell hardness tests43Figure Indentors for the B4444444545Section 5.2 材料承受冲击载荷时的力学性能材料承受冲击载荷时的力学性能oImpact test-Measures the ability of a material to absorb the sudden application of a load without breaking.oImpact energy-The energy required to fracture a standard specimen when the load is applied suddenly.oImpact toughness-Energy absorbed by a material,usually notched,during fracture,under the conditions of impact test.oFracture toughness-The resistance of a material to failure in the presence of a flaw.45Section 5.2 材料承受冲击载荷时的力学性能I46465.2.1 冲击弯曲试验冲击弯曲试验o缺口试样一次冲击弯曲试验在摆锤式冲击试验机上进行，将试样水平放置于试验机支座上，缺口位于冲击相背方向。冲击时将质量为G 的摆锤举至高度h0的位置，使其获得位能Gh0。释放摆锤冲断试样后，摆锤的剩余能量为Ghf，则摆锤冲断试样失去的位能为Gh0-Ghf。此即为试样变形和断裂所吸收的功，称为冲击功，以Wk(Ak)表示，单位为J。o国家标准规定，冲击弯曲试验用试样分为夏比U型缺口试样和夏比V型缺口试样，所测得的冲击功分别记为Whu和Whv。测量陶瓷、铸铁或工具钢等脆性材料的冲击功时，常采用101055(mm)的无缺口冲击试样。465.2.1 冲击弯曲试验缺口试样一次冲击弯曲试验在摆锤式47475.2.2 多次冲击试验多次冲击试验o实践表明，承受冲击载荷的机件多数是经过多次冲击后断裂的，其破坏是各次冲击损伤积累的结果，根本不同于一次冲击破坏的过程。o多次冲击试验后可绘制出冲击功W一冲断次数N曲线o随冲击功W的减少，冲断次数N增加。图图515 多次冲击曲线多次冲击曲线475.2.2 多次冲击试验实践表明，承受冲击载荷的机件多48485.2.3 冲击韧性及其意义冲击韧性及其意义冲击韧性冲击韧性K式中，Wk为冲断试样所消耗的冲击功，MJ；A0为试样缺口处横截面积，m2oK值越大，表示材料的冲击韧性越好。冲击韧性表示材料抵抗冲击破坏的能力。同一条件下，同一材料的两种试样，其U型缺口试样的K值显著大于V型缺口试样，所以它们的K值不能互相比较。o材料的K值随温度的降低而减小。在某一温度范围内，K值急剧降低，这种现象称为冷脆。这个温度范围称为冷脆转变温度范围。oK值对材料的缺陷，如淬火过热造成的晶粒粗大、回火脆性、时效不充分、夹杂物形态、纤维方向等非常敏感，故常用于检验冶炼、热加工、热处理工艺的质量。也常用于检验材料的冷脆性、以确定材料的冷脆转变温度。485.2.3 冲击韧性及其意义冲击韧性K4949Figure The impact test:(a)The Charpy and Izod tests,and(b)dimensions of typical specimens49Figure The impact test:(a5050oDuctile to brittle transition temperature(DBTT)-The temperature below which a material behaves in a brittle manner in an impact test.oNotch sensitivity-Measures the effect of a notch,scratch,or other imperfection on a materials properties,such as toughness or fatigue life.50Ductile to brittle transitio5151Figure Results from a series of Izod impact tests for a super-tough nylon thermoplastic polymer51Figure 5252Figure The Charpy V-notch properties for a BCC carbon steel and a FCC stainless steel.The FCC crystal structure typically leads top higher absorbed energies and no transition temperature52Figure 5353Figure The area contained within the true stress-true strain curve is related to the tensile toughness.Although material B has a lower yield strength,it absorbs a greater energy than material A.The energies from these curves may not be the same as those obtained from impact test data53Figure 5454Section 5.3 材料的疲劳材料的疲劳oFatigue is the lowering of strength or failure of a material due to repetitive stress which may be above or below the yield strength.oCreep-A time dependent,permanent deformation at high temperatures,occurring at constant load or constant stress.oBeach or clamshell marks-Patterns often seen on a component subjected to fatigue.oRotating cantilever beam test-An older test for fatigue testing.oS-N curve(also known as the Whler curve)-A graph showing stress as a function of number of cycles in fatigue.54Section 5.3 材料的疲劳Fatigue is5555Section 5.3 材料的疲劳材料的疲劳o许多机件承受的是大小及方向不断变化的交变载荷，例如轴、齿轮、弹簧等。在交变载荷作用下，材料经常在远低于其屈服强度的载荷下发生断裂，这种现象称为“疲劳”。o疲劳断裂时，材料没有明显的塑性变形，断裂是突然发生的，常常造成严重的事故。o按应力状态，分为弯曲疲劳弯曲疲劳、扭转疲劳扭转疲劳、拉压疲劳拉压疲劳、接触疲劳接触疲劳和复复合疲劳合疲劳；o按应力高低和断裂寿命，分为高周疲劳高周疲劳和低周疲劳低周疲劳。55Section 5.3 材料的疲劳许多机件承受的是大小56565.3.1 疲劳曲线疲劳曲线o以max为纵坐标，以疲劳断裂周次N为横坐标绘制的曲线，称为疲劳曲线。简写为-N曲线。实验表明，金属材料所受的最大交变应力max越大，则断裂前所能承受的应力循环次数N越少。当应力循环中的最大应力max降低到某一数值，材料可以经受无限次应力循环而不发生疲劳断裂，-N曲线上出现了趋于水平部分。o不同材料的疲劳曲线形状不同，大致可分为两类。一类有水平线，如一般结构钢和球墨铸铁的疲劳曲线，据此，可标定出无限寿命的疲劳强度；另一类无水平线，如有色合金、不锈钢和高强钢的疲劳曲线。565.3.1 疲劳曲线以max为纵坐标，以疲劳断裂周次5757Figure The stress-number of cycles to failure(S-N)curves for a tool steel and an aluminum alloy57Figure The stress-number o58585.3.2 疲劳极限疲劳极限o当应力低于某一值时，材料经无限循环周次也不发生断裂，此值称为疲劳极限疲劳极限或疲劳强度疲劳强度。疲劳极限是保证机件疲劳寿命的重要性能指标，是评定材料、制订工艺和疲劳设计的依据。光滑试样的对称疲劳极限用-1表示，单位MPa。对于无水平线的疲劳曲线，只能根据材料的使用要求，确定有限寿命下的疲劳极限。例如，钢材的循环基数为107，有色金属和某些超高强度钢的循环基数为108。超过这个基数就认为该材料不再发生疲劳破坏。o常见的对称循环载荷有对称弯曲、对称扭转、对称拉压等，对应的疲劳极限分别记为-1、-1及-1p，其中-1是最常用的。一般情况下-1-1p-1o对于中、低强度钢，-1=0.5b。但抗拉强度较高时，这种线性关系要改变，因为强度较高时，材料的塑性和断裂韧性降低，裂纹易于形成和扩展。585.3.2 疲劳极限当应力低于某一值时，材料经无限循环59595.3.3 疲劳断口疲劳断口o一般来说，典型疲劳断口由3个特征区组成，即疲劳裂纹产生区疲劳裂纹产生区、疲疲劳裂纹扩展区劳裂纹扩展区和最后断裂区最后断裂区。o疲劳裂纹萌生的地方，多出现在机件表面，常和缺口、裂纹、刀痕、蚀坑等缺陷相连。若材料内部存在严重冶金缺陷(夹杂、缩孔、偏析、白点等)，也会因局部材料强度降低而在机件内部引发出疲劳源。o疲劳裂纹产生后，在交变应力作用下，继续扩展长大，这个区域称为疲劳裂纹扩展区。其宏观特征是：断口较光滑并分布有贝纹线(或海滩花样)，有时还有裂纹扩展台阶。贝纹线是一簇以疲劳源为圆心的平行弧线，凹侧指向疲劳源，凸侧指向裂纹扩展方向。近疲劳源区贝纹线较细密，表明裂纹扩展较慢；远离疲劳源区贝纹线较稀疏、粗糙，表明此段裂纹扩展较快。595.3.3 疲劳断口一般来说，典型疲劳断口由3个特征区60605.3.3疲劳断口疲劳断口o最后断裂区，随着疲劳裂纹的扩展，零件的有效截面不断减小，剩余断面上的应力不断增加。当应力超过材料的断裂强度时，发生断裂，形成最后断裂区。该区的断口比疲劳区粗糙，宏观特征如同静载，随材料性质而变。脆性材料断口呈结晶状；韧性材料断口为纤维状，暗灰色，在心部平面应变区呈放射状或人字纹状，边缘平面应力区则有剪切唇区存在。o疲劳裂纹扩展区与最后断裂区所占面积的相对比例，随应力大小和材料的断裂韧性而变化。所受应力小而无大的应力集中时，则疲劳裂纹扩展区大；反之，则小。因此，可以根据疲劳断口上两个区所占的比例，估计所受应力高低及应力集中程度的大小。o对疲劳断口的分析是研究疲劳过程、分析疲劳失效原因的一种重要方法。605.3.3疲劳断口最后断裂区，随着疲劳裂纹的扩展，零件的6161Figure Fatigue fracture surface.(a)At low magnifications,the beach mark pattern indicates fatigue as the fracture mechanism.The arrows show the direction of growth of the crack front,whose origin is at the bottom of the photograph.(Image(a)is from C.C.Cottell,Fatigue Failures with Special Reference to Fracture Characteristics,Failure Analysis:The British Engine Technical Reports,American Society for Metals,1981,p.318.)(b)At very high magnifications,closely spaced striations formed during fatigue are observed (x 1000)61Figure Fatigue fracture s6262Figure Schematic representation of a fatigue fracture surface in a steel shaft,showing the initiation region,the propagation of fatigue crack(with beam markings),and catastrophic rupture when the crack length exceeds a critical value at the applied stress62Figure 6363Results of the Fatigue TestoEndurance limit-An older concept that defined a stress below which a material will not fail in a fatigue test.oFatigue life-The number of cycles permitted at a particular stress before a material fails by fatigue.oFatigue strength-The stress required to cause failure by fatigue in a given number of cycles,such as 500 million cycles.oNotch sensitivity-Measures the effect of a notch,scratch,or other imperfection on a materials properties,such as toughness or fatigue life.oShot peening-A process in which metal spheres are shot at a component.63Results of the Fatigue TestE6464Application of Fatigue TestingFigure Examples of stress cycles.(a)Equal stress in tension and compression,(b)greater tensile stress than compressive stress,and(c)all of the stress is tensile64Application of Fatigue Testi6565Figure Crack growth rate versus stress-intensity factor range for a high-strength steel.For this steel,C=1.62 1012 and n=3.2 for the units shown65Figure Crack growth rate 6666Section 5.4 材料的断裂韧性材料的断裂韧性oFracture mechanics-The study of a materials ability to withstand stress in the presence of a flaw.oFracture toughness-The resistance of a material to failure in the presence of a flaw.o工程设计中，一般根据材料的屈服强度 确定许用应力。机件在许用应力下工作，不会发生塑性变形，更不会发生断裂，应该是安全的。然而，有些机件会在很低应力的状态下，发生脆性断裂。这是因为，一般讨论材料的力学性能时，假定材料的内部是完整的、连续的。而实际材料中不可避免地存在着各种缺陷。例如，夹杂物、气孔等冶金缺陷和在使用、加工过程中产生的机械缺陷。这些缺陷破坏了材料的连续性，成为材料中的裂纹。实验分析表明，低应力脆性断裂是由材料中裂纹的扩展引起的。66Section 5.4 材料的断裂韧性Fracture67675.4.1 断裂韧性的概念断裂韧性的概念o断裂力学运用连续介质力学的弹塑性理论，考虑了材料的不连续性，研究材料中裂纹扩展的规律，确定材料抵抗断裂的力学性能指标断裂韧性断裂韧性。断裂韧性反映了材料抵抗裂纹失稳扩张的能力。o张开型裂纹(通常称之为I型裂纹)，其大小可以用应力强度因子应力强度因子 来描述。o对一个存在裂纹的试样施加拉伸载荷时，其Y值是一定的。随应力 逐渐增大，或者裂纹长度2a逐渐扩展，裂纹尖端的K1增大到某一数值时，可使裂纹前沿某一区域的内应力大到足以使裂纹产生失稳扩展，即发生脆断。这个强度因子的临界值，称为材料的断裂韧性，用K1c 表示。它反映了有裂纹存在时，材料抵抗脆性断裂的能力。当 K1 K1c时裂纹失稳扩展，发生脆断；当 K1 K1c时，裂纹不扩展或扩展很慢，不发生快速脆断；当 K1=K1c时，裂纹处于临界状态。675.4.1 断裂韧性的概念断裂力学运用连续介质力学的弹68685.4.1 断裂韧性的概念断裂韧性的概念oK1是描述裂纹尖端应力场大小的力学参量，它与裂纹类型、物体的形状、大小以及外加应力等参数有关，与材料无关；而断裂韧性K1c 是评定材料阻止裂纹失稳扩展能力的力学性能指标，它与裂纹本身的大小、形状无关，也和外加应力无关，是材料本身的特性，只和材料的成分、热处理及加工工艺等有关。o应力强度因子和断裂韧性的提出，在工程上有重要的意义。例如，知道材料的断裂韧性，再测出构件中的最大裂纹长度，就可以计算出裂纹失稳扩展的临界载荷，即构件所能承受的最大载荷。或者己知材料的断裂韧性，根据构件实际所受的外加应力，确定构件中允许存在的最大裂纹长度。所以，断裂韧性为安全设计提供了一个重要的力学性能指标，尤其在疲劳、冲击、高低温强度、应力腐蚀、辐照损伤等强度领域得到了广泛的应用。同时也为发展新材料、新工艺及合理选材指出了方向。685.4.1 断裂韧性的概念K1是描述裂纹尖端应力场大小69695.4.2 影响材料断裂韧性的因素影响材料断裂韧性的因素o化学成分对于金属材料，提高韧性的元素，均提高材料的断裂韧性。加入细化晶粒的元素，可使金属的断裂韧性提高；强烈固溶强化的合金元素使断裂韧性降低；形成脆性化合物的元素降低断裂韧性。o晶粒尺寸晶粒越细，材料的强度和韧性同时提高，另外，细化晶粒有助于减轻杂质在晶界上的偏析，减少沿晶断裂，从而提高材料的断裂韧性。o杂质及第二相钢中的夹杂物以及第二相，如硫化物、氧化物、碳化物等，都是脆性相，这些相的存在，降低钢的断裂韧性；脆性相以细小球状存在时，对断裂韧性的有害作用减小。o温度和加载速度大多数材料，温度降低，断裂韧性降低。对于存在韧脆转变温度的材料，在韧性温度区，材料发生韧性断裂，有较高的断裂韧性；而在韧脆转变温度以下，材料主要是解理性脆性断裂，断裂韧性较低。增加形变速度，与降低温度有类似的效果，使断裂韧性下降。695.4.2 影响材料断裂韧性的因素化学成分7070Figure Schematic drawing of fracture toughness specimens with(a)edge and(b)internal flaws70Figure Schematic drawing o7171Figure The fracture toughness Kc of a 3000,000psi yield strength steel decreases with increasing thickness,eventually leveling off at the plane strain fracture toughness Klc71Figure The fracture toughn7272727373Figure A scanning electron micrograph showing crack propagation in a PZT ceramic.(Courtesy of Wang and Raj N.Singh,Ferroelectrics,207,555575(1998).)Figure Secondary cracks developed during hardness testing can be used to assess the fracture toughness of brittle materials73Figure A scanning electron7474Figure Fracture toughness versus strength of different engineered materials.(Source:Adapted from Mechanical Behavior of Materials,by T.H.Courtney,2000,p.434,Fig.9-18.Copyright 2000 The McGraw-Hill Companies.Adapted with permission.)74Figure 7575The Importance of Fracture MechanicsoSelection of a MaterialoDesign of a ComponentoDesign of a Manufacturing or Testing MethodoGriffith flaw-A crack or flaw in a material that concentrates and magnifies the applied stress.75The Importance of Fracture M7676Section 5.5 材料的磨损性能材料的磨损性能5.5.1 磨损过程和磨损的分类磨损过程和磨损的分类1.磨损过程跑合阶段（磨合阶段），图中OA阶段，在此阶段内，摩擦表面逐渐被磨平，实际接触面积不断增大，同时，接触表面因塑性变形产生形变强化以及表面形成牢固的氧化膜，使得磨损速率不断减小。稳定磨损阶段，图中AB阶段，这是磨损速率稳定的阶段，线段的斜率就是磨损速率。大多数机器零件都在此阶段内服役，磨损实验就是根据工件在该段经历的时间、磨损速