Module 3-2

Module 3Properties Characterization of Materials3.1 Properties and Evironmental Variables 3.2 Mechanical 3.2 Mechanical Properties 3.2.8 Ductility 3.2.9 Toughness3210 Processing Properties Properties3.3 Electrochemistry 3.4 Thermal Properties 3.5 Determining Materials Structure3.2.10 Processing Properties 3.2.11 Flexural or bending strength3.2.12 Fatigue (Endurance) strength 3.2.13 Creep 3.2.14 3.2.14 Torsional Torsional strength 3.2.15 Hardness3.2.8Ductility 延展性/可塑性3.2.8 Ductility 延展性,可塑性z Ductile Ductile materials materials 韧性材料: undergo undergo large plastic large plasticdeformation without fracture 破坏z Brittle materials : 脆性材料show an show an absence of ductility absence of ductility or little evidence of forthcoming fracture by yielding9The difference in the amount of plastic deformation shown by each curve prior to fractureDuctility Ductility --------------Stiffness?Stiffness?Sudden fracture9The difference between yield strength and tensile strength9The difference in the amount of plastic deformation shown by each curve prior to fractureQ3.4How do you distinguish a ductilematerial from a brittle material with the aid of a stress aid of a stress--strain diagram?Brittle material Ductile materialvery little differenceLittle plastic deformation wide differencelarge plastic deformation1)Percent elongation=length at fracture, l =original length Measurement of ductilityl 0(l F -l 0)% elongation=±100%where l F length at fracture, l 0original length2)Percent reduction in area where A 0 = original cross = original cross--sectional area,A F = cross = cross--sectional area at fractureA 0(A 0-A F )% RA in area reduction =±100%Notes for measuring of ductilityz Both these measures of ductility do not take into account the nonuniform on elongation and area reduction due to necking 颈缩z Necking phenomenon 颈缩现象9At the tensile strength, the sample begins to collapse inward (向内塌陷）Notes for measuring of ductilityzHigh ductility results in large % elongation9Sample can neck downto a pointzModerate ductilityexhibits some necking9Cup Cup--cone fracturezBrittle fractureexhibits no ductility9Flat fracturezDuctility values vary with materials–A specimen that has A specimen that has 5%5%or or less elongation less elongation isconsidered as a considered as a brittlebrittle material –Pure metals can undergo elongations of 40Pure metals can undergo elongations of 40--60% without fracture i l ll h d ili i f l hApplications of ductility–Typical Typical superalloys superalloys have have ductilities ductilities of less than 20%zDuctility Ductility vs vs toughness–Having a Having a good amount of ductility good amount of ductility makes manymaterials materials toughtough because the plastic flow permitted by the ductility prevents a concentration of stress at the tip of a crack.3.2.9Toughness 韧性3.2.9 Toughness 韧性z The ability or capacity of a material to absorb energy during plastic deformationModulus of Toughness （T ）韧性模量9The total area under the stress stress--strain curve up to the point of rupture9Represents the energy per unit volume of a material required to produce fracture under static conditionsz Impact (冲击):a a sudden application sudden application of a loadconfined to a localized 局部area of a material9Quick application of force can cause considerable damage to a material that Impact Impact & impact resistance & impact resistance (抗冲击性)cannot adequately redistribute the stresses caused by the impact.Examples:1.Metals: good toughness, good impact resistance2.Ceramics: poor toughness, poor impact resistance, poor fracture toughness3.An increase in ductility improves toughness thus the impact resistance Impact strength & Impact tests冲击强度和冲击实验z Impact strength measured by impact tests gives an indication of the relative toughnessz Dynamic testing (static testing, i.e.: tensile & compression)9performed to measure the response (i.e.: response (i.e.: impact energy impact energy) ) of a material to dynamic loadingPendulum Impact Tester摆锤冲击试验机A heavy pendulum (摆锤)swinging through its arc strikes and breaks the specimen9Record the initial and final elevations (高度)of thependulum, and their potential energy (势能) from a dial indicator connected to the pendulum arm9Calculate the difference in Measurement of impact energypotential energy9The above difference is the impact energy absorbed by the specimen during failure 9Unit:•foot foot--pounds (ft lb), •joules (J)Universal impact tester万能冲击试验机Charpy & Izod impact testingNotched Notched--bar impact tests （缺口样条冲击试验）Izod impact testing 悬臂梁式冲击试验Charpy impact testing 分简支梁式冲击试验Standard notched specimensThe notched specimens of the material under test have:keyhole cutV -notch cutUsed to determine theresistance of the material to crack propagationEffects of temperature on the impactresistance of metalsmuch greater 转变温度区little absorption ofenergyabsorption of energyCritical temperatures 1.Cleavage 开裂破裂9Cracks propagate most rapidly along specificcrystallographic planes2.Intergranular path 晶粒间破裂Brittle fractures9Crack grows along the grain boundaries particularly when the grain boundaries are weakened by inclusions 掺杂or segregation 晶间偏析3.Chevron pattern 回纹状破裂9Crack fronts propagate at different levels in the materials from a common originz abrupt or considerable rangeTransition temperature range转变温度区What are the factors influencing TTR ？9Raise TTRQ3.69Coarse grain size9Strain hardening (应变硬化)9The addition of certain elements9Decrease TTR (enhance notch toughness at rather low temps.)9fine grain size 9Heat treatment9The addition of certain elementsTransition temperature range转变温度区T or F questions ：All materials have a distinct Q3.7transition temperature.•Metals with an fcc crystal structure do not have such a chracteristic•Metals with an bcc crystal structure do4.2.10Processing Properties g p加工性能Terms:Melleability Workablity Formability Processing PropertiesWeldability Machinability4.2.11Flexural or bending strength g g挠曲强度4.2.11 Flexural or Bending strength(挠曲强度)Beam 梁: a structural member that bends or flexes when subjected to forces perpendicular to its longitudinal axisNormal stressesNormal stressesSimple supported beam 简支梁A beam supported at its ends with a pin and a rollerNeutral axis (NA)中性轴Simple supported beam 简支梁Normal stressesNormal stressesLoaded beam showing deformation横向载荷compressivecompressiv eThese bending stresses are normal to any cross These bending stresses are normal to any cross--sectional area through the beam, parallel to the NANeutral axis (NA)中性轴No normal stresses are assumedto exist along its lengthtensiletensilezAssumptions:9Homogeneous(均相的)and and isotropic(isotropic(各向同性的)beam 9max. normal stresses near the top & bottom z Flexure formula 弯曲公式:Flexural or bending stressesMc Where, σ:bending stressesM:M:bending moment bending moment (弯曲距)c: c: maximum distance maximum distance from the beam’s neutral axis to the outer surface of the beamI:rectangular rectangular moment of inertia moment of inertia of the cross of the cross--sectional area of the beam(转动惯量)Iσ=1.Load simple Load simple--supported beam2.Record max.bending stress and Flexure test (挠曲试验)deformation for increments of load P 3.Plot data to obtain a stress stress--strain diagram9Flexural strength 挠曲强度: max. bending stress at failure•For ceramics: bend strength or modulus of rupture (MOR)9Flexural yield strength: for those materials that do notcrack 断裂, the max. bending or flexural stress4.2.12Fatigue (Endurance) strength g g疲劳强度Cyclic loading (周期性承受载荷)9A transverse load 横向载荷is applied andremoved in some cyclic fashion¾Type:•Stress repetitious 重复应力•Stress reversal 交变应力Stress repetitious 重复应力Downward Downward--Condition of zero stress acting load Stressed Condition Condition of zero stressStress reversal 交变应力Sc:S il Downward Downward--acting loadcompressionCompressive stressSt: Tensile stressUpward Upward--acting loadtensionFatigue (Endurance) failure 疲劳破坏•Fatigue(疲劳)9Cyclic load under load under long long timeFatigue (Endurance) failure9Failures have occurred when the stressesdeveloped were developed were well below well below the the ultimate ultimate stress and frequently below the yield strength.9>50%of the of the failures failures occurring under service conditionszDetermine the strength ofmaterials under fatigue loadingzThe expense involved in running fatigue tests is high–Test the material until it failsFatigue testing machine 疲劳测试机under varying conditions of load and ambient temperatureszS (stress)S (stress)--N (cycles) diagrams 应力-周数曲线9semilog or log or log--logFatigue limit or The The maximum maximum stress below which amaterial can Fig. 3Fig. 3--15 s 15 s--N diagram for a typical fiberglass composite materialendurance limitpresumably endure aninfinite number of stress cycles疲劳极限Fig. 3Fig. 3--16 s 16 s--N diagram for a typicallow low--carbon steelFatigue limit orendurance limit 9Ferrous alloys: About 1/2 the tensile strengthFig. 3Fig. 3--17 s 17 s--N diagram for nonferrous metal and some plasticsNo No fatigue limit or fatigue limit or d li itz Both the stress & the & the number of cycles number of cycles are needed in describing the life of the materialendurance limit Fatigue strength 疲劳强度l¾The The maximum stress maximum stress that can be sustained for a specified number of number of cycles cycles without fracture without fracture ¾Any value on the ordinate of the s Any value on the ordinate of the s--N diagram ¾The corresponding N value must be reportedExamples:1)low-carbon steel 940kpsi9N:105cycles 2)Bolt910-20% ultimate strength9N:107cyclesQ3.7What are the major difference between the fatigue limit and fatigue strength?z Fatigue limit: the maximum stress that can be sustained for an can be sustained for an infinite number infinite number of stress stress cycles;cycles;z Fatigue strength: the maximum stress that can be sustained for a that can be sustained for a specified specified number of of cycles.cycles.z The quotient 商of of endurance limit endurance limit to to tensile tensilestrengthz Varies between 0.25Varies between 0.25--0.45, depending on the Fatigue (Endurance) ratio 耐久比materialz For many ferrous alloys, the endurance limit is about one about one--half the tensile strength of the metal.zwhen is fatigue ratio needed–Avoid the high cost in running fatigue tests –Attempt to duplicate service conditions in a lab3.2.13CCreep 蠕变Definition for CreepzA A slow process slow process of of plastic deformation plastic deformation thattakes place when a material is subjected to a a constant constant condition of condition of loading loading (stress) below its normal yield strength.zA material plastically deforms or flows very slowly under load as a function of time.Characteristics of CreepzStatic stresses Static stresses: constant load for : constant load for long time long time9The elapsed time increase, plastic deformation (creepstrain) strain) increase increase, sometimes leading to creep rupture , sometimes leading to creep rupture(蠕变断裂)Creep zCreep occurs occurs at at any temperature any temperature, but is , but is usually usually associated with associated with high temperature high temperature–higher temp., high creep rates9Cold flow (冷变形): low ): low--temperature creep of polymeric materialsTensile creep testszDevelop data at a constant level (stress) and at a constant temperature.z Creep curve (蠕变曲线):9The amount of The amount of creep strain creep strain is plottedagainst against time timeCreep curve & creep rate 蠕变曲线&蠕变速率zDetermined at anypoint by the slope of the curve i n , εzExpressed in percent creep strain per hour zTypical rate 10-4%per hourper hour C r e e p s t r a TimeCreep curve 蠕变曲线, εInitial stage(Primary or transient creep)decreasing creep ratedecreasing creep rate.C r e e p s t r a i n TimeCreep curve 蠕变曲线, εMiddle stage (Secondary creep)Steady Steady--state creep rated ε/dt ~ constant, minimum rateC r e e p s t r a i n TimeCreep curve 蠕变曲线t r a i n , εC r e e p s TimeFinal stage (Tertiary creep Tertiary creep))accelerated creep rate and failure!Creep of polycrystalline materialsA : Creep resistance is dependent on t t d th t t fQ3.8What are the factors influencing creep resistance of polycrystalline materials ？temperature and the structure of materials9At high temperatures coarse coarse--grained > fine grained > fine--grained 9At low temperatures coarse coarse--grained < fine grained < fine--grainedCreep forming 预模压加热蠕变成形z A A workpiece workpiece is is loaded loaded against a die or tool fora a specified time specified timez The load is normally applied using pressure or vacuum at temp. around 160C.zThe process results in The process results in predetermined shapes predetermined shapes that required little or no machining.Factors should be taken into consideration1)The amount of creep resulting from such operations2)Springback 回弹the elastic recovery3.2.14 T i l St th Torsional Strength 抗扭强度Torsion 扭转z the process of twisting9A force acts to turn one end around the longitudinal axis of the rod while the other end is axis of the rod while the other end is fixed fixed or twisted in in opposite direction opposite direction zTorque (Torque (T T ) [tC:k] n., : : P P ±r (Pr)q 扭矩,转矩P rrP PzThe external The external torquetorque produces both produces both stress stress and a and a deformation deformation of the rod The angle of twist 扭转角γθ9Stress: Stress: torsional torsional stress stress, shear stress , shear stress 9Deformation: Deformation: shear shear deformation¾measured by angle γor by angle θ¾the the angle of twist angle of twist::θTT -torque (clockwise)torque (clockwise)r -radius of rod/shaftTorsion formulaTr τmax =z Used to calculate the Used to calculate the torsional stress (torsional stress (τ)produced by the applied torqueWhere,T: T: the applied torque; the applied torque;r:the radius of the cross the radius of the cross--sectional area; J:the the centroidal centroidal polarmoment of inertia 矩心极化转动惯量JττmaxrzMaximum Maximum torsional torsional stress occurs 9when r is a maximum9at the outer surfaces of the circular rodTorsional yield strengthττmax τ=torsionalstressτmax =maximumtorsional stressrTorsion Torsion--test machine 扭曲试验机zA torsion A torsion--test machine measures the test machine measures the torque (T)torque (T)扭矩applied to a specimen of material along with the corresponding with the corresponding angle of twist (angle of twist (θ)扭转角9The point on thetorque torque--twist diagram where the curve begins Torque Torque--twist diagram 扭矩-扭转角曲线Torsional Yield Strength 抗扭屈服强度g to depart from a straight line9Is roughly corresponds to the yield strength in shear9A measure of the ability of material to withstand a i il dTorque Torque--twist diagram 扭矩-扭转角曲线Ultimate Ultimate torsionaltorsional strength 极限抗扭强度(Modulus of Rupture 断裂模量)twisting load 9Is roughly equivalent to the ultimate shear strength9Is approximatelyTorque Torque--twist diagram 扭矩-扭转角曲线Torsional modulus of elasticity 抗扭弹性模量(modulus of rigidity 刚性模量)equal to the shear modulus or the modulus ofelasticity in shear (G)3.2.15 Hardness 硬度DefinitionzA measure of a material’s resistance to penetration 抗穿刺(local plastic deformation) or deformation) or scratching scratching 抗划伤z Hard materials have good resistance to d b i wear and abrasion 耐磨性9Cutting tools and drillingMeasurement of hardness1)the degree of penetration ，压痕法2)the resistance of scratching ，划痕法3)the rebound of a small weight bounced ff h f f h l 回弹法off the surface of the material ，回弹法Hardness testingz Simple to performz Test apparatus indents (在…上压凹痕) the materialzSize of indentation is related to a hardness scale(硬度标度)9Smaller indents (凹痕)Higher hardness numbers Harder materialszMany types of indenters 压头9Spherical steel balls 钢球压头9Diamond cones 金刚石锥压头9Diamond pyramids 金刚石棱锥Brinell hardness (HB)布氏硬度,压球硬度z Test sample must have a smooth flat surface zindenters 压头–1010--mm mm steel steel (HB 459)(HB 459)–tungsten carbide 碳化钨sphere sphere (HB600)(HB600)z 500500--3000kg load in 500 kg increments z The The indentation indentation (压痕) ) size size is measured using a microscope containing an ocular scale (目镜标尺)zValue of HB (Value of HB (Brinell Brinell hardness number hardness number) is a ) is a function of load and indent size HB ＝2F/πD(D D(D--(D 2-d 2)1/2)F -compressive force, D compressive force, D--sphere dia., d sphere dia., d--indent dia.Vickers hardness(HV) 维氏硬度zSimilar to Brinell hardnessz Employ a Employ a diamond pyramid diamond pyramid indentor (金刚石棱锥压头)z The Vicker method is commonly used–Suitable for all materials –HV is proportional tohardness (HV400 & HV200)–Indentation size: very small –Time Time--consuming measurementRockwell hardness (HR) 洛氏硬度zRockwell hardness testers (洛氏硬度试验)9direct direct--reading instrumentzA variety of A variety of indentors indentors: :91/8 and 1/16 inch ball,diamond cone 金刚石锥压头zA variety of loads:z Scales (硬度标): HRB, HRB, HRC HRC, etc., etc.z Difference in Difference in penetration penetration depth gives hardnessnumberReport Rockwell hardness readings along with the scale letter (to get inform. on the type of indentor & the size of load)Various standard loads for the Rockwell hardness testerMohs hardness 莫式硬度z use scratch resistancez Comparison test9Compares a material’s hardness to some10 known minerals arranged in order ofhardnessz Mohs scale莫式硬度等级9Classes hardness of all materials between1 and 109Based on the ability of a hard material toscratch a softer materialMohs scale 莫式硬度等级1=talc [tAlk]滑石2=gypsum[5dVipsEm]石膏3=calcite [5kAlsait]方解石4=fluorite [5flu(:)Erait]萤石5tit[5t it5=apatite [ApEtait]磷灰石6=orthoclase [5C:WEukleis]正长石7=quartz [kwC:ts]石英8=topaz [5tEupAz]黄玉9=corundum [kE5rQndEm]刚玉10=diamond金刚石HarderOther hardness testsz File test 锉磨试验9Resistance to scratching9Filing an edge of materialsz Scleroscope(HScHSc) test) test回跳硬度试验9Measures the rebound of a small weightbounced off the surface of the materialOther hardness testsz Microhardness tester微硬度计9very thin or very brittle or very small materials9Laboratory research in materials1th diff i h d th i t1.the difference in hardness over the minutearea of a single grain of metal2.the difference in hardness between themiddle of a crystal and the grain boundaryarea•Measurement: small indentation require microscopic viewing with appropriatescalesCorrelation between hardness andtensile strengthz A given ductile metal9Tensile strength can be estimated only from the hardness reading within an error of less than10%9Steel9Tensile strength (psi) is about 500 times the HBz Table 4Table 4--1, a comparison of the approximate hardness of a variety of materials using 11 different hardness scales。