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1、MechanicalBehaviorofMaterialsKnowtheconceptsofmechanicalpropertiesofmaterials.Understandthefactorsaffectingthemechanicalproperties.Beawareofthebasictestingproceduresthatengineersusetoevaluatemanyoftheseproperties.ObjectiveOutlineMechanicalPropertiesofMaterialsStress-StrainDiagram&PropertiesBendTesto
2、fMaterialsHardnessTestofMaterialsImpactTestingofMaterialsFractureMechanicsofMaterialsFatigueofMaterialsandApplicationCreepofMaterials,StressRupture,andStressCorrosionEvaluationofCreep&UseofCreepDataMechanicalBehaviorofMaterialsBehaviorandManufacturingPropertiesofMaterials 2003 Brooks/Cole Publishing
3、 / Thomson LearningRepresentativeStrengthsofVariousCategoriesofMaterialsMaterials Design and Selection1.Densityismassperunitvolumeofamaterial,usuallyexpressedinunitsofg/cm3orlb/in.32.Strength-to-weightratioisthestrengthofamaterialdividedbyitsdensity;materialswithahighstrength-to-weightratioarestrong
4、butlightweight.l lMost Most common common test test for for determining determining such such mechanical mechanical properties properties as as strength, strength, ductility, toughness, elastic modulus, and strain hardening. ductility, toughness, elastic modulus, and strain hardening. l lThe The tes
5、t test specimen specimen made made according according to to standard standard specifications. specifications. Most Most specimens specimens are are solid and round, some are flat-sheet. solid and round, some are flat-sheet. lIn this test a metal sample is pulled to failure at a constant rate. lThe
6、load displacement relationship is plotted on a moving chart graph paper, with the signals coming from a load cell fixed at the top of the testing machine, and an extensometer (strain gauge) attached to the sample.lThe load displacement data obtained from the chart paper can be converted to engineeri
7、ng stress/strain data, and a plot of engineering stress vs. engineering strain can be constructed.TensionTestMechanicalBehaviorofMaterialsTensionTestingMachineTensileSpecimensMechanicalBehaviorofMaterialsEngineeringStressStrainDiagramForAHigh-StrengthAluminumAlloy.(c)2003 Brooks/Cole, a division of
8、Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Aunidirectionalforceisappliedtoaspecimenin the tensile test by means of the moveablecrosshead. The cross-head movement can beperformed using screws or a hydraulicmechanismMechanicalBehaviorofMaterialsMechanical property
9、 data obtained from the tensile test are of engineering importance for structural design. These are:1.modulus of elasticity2.yield strength at 0.2 percent offset3.ultimate tensile strength4.percent elongation at fracture5.percent reduction in area at fracture- Stress () = Force or load per unit area
10、 of cross-section.- Strain () = Elongation change in dimension per unit length- Youngs modulus (E)= The slope of the linear part of the stress- strain curve in the elastic region (stress) = E x (strain)or E = (stress)/(strain) psi or paMechanicalBehaviorofMaterialslSlopeofstressstrainplot(whichispro
11、portionaltotheelasticmodulus)dependsonbondstrengthofmetalAdapted from Fig. 6.7, Callister 7e. MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Comparisonoftheelasticbehaviorofsteelandaluminum.Foragivenstr
12、ess,aluminumdeformselasticallythreetimesasmuchasdoessteelMechanicalBehaviorofMaterialsMechanicalBehaviorofMaterialsIn In industry, industry, components components are are formed formed into into various various shapes shapes by by applying applying external external forces forces to to the the workp
13、iece workpiece using using specific specific tools tools and and dies. dies. A A typical typical operation operation is is rolling rolling of of a a flat flat sheet to be processed into a car body. sheet to be processed into a car body. Because Because deformation deformation in in these these proce
14、sses processes is is carried carried out out by by mechanical mechanical means, means, an an understanding understanding of of the the behavior behavior of of materials materials in in response response to to externally externally applied applied forces forces is is important. important. Forming For
15、ming operations operations may may be be carried carried out out at at room room temperature temperature or or at at higher higher temperatures and at a low or a high rate of deformation.temperatures and at a low or a high rate of deformation.The The behavior behavior of of a a manufactured manufact
16、ured part part during during its its expected expected service service life life is is an an important important consideration. consideration. For For example example the the wing wing of of an an aircraft aircraft is is subjected subjected to to static static as as well well as as dynamic dynamic f
17、orces. forces. If If excessive, excessive, dynamic dynamic forces forces can can lead lead to to cracks cracks and and can can cause cause failure of the component.failure of the component.MechanicalBehaviorofMaterialsEngineeringstress-strain.Elasticrangeinstress-strain.MechanicalBehaviorofMaterials
18、Engineeringstress-straincurve,showingvariousfeaturesEngineeringstress-straincurve,showingvariousfeaturesYieldstress(Y),Ultimatetensilestrength(UTS),andFracture.Yieldstress(Y),Ultimatetensilestrength(UTS),andFracture. 1.ElasticandPlastic,2.UniformelongationandNecking.1.ElasticandPlastic,2.Uniformelon
19、gationandNecking.MechanicalBehaviorofMaterialsAlloying a metal with other metals or nonmetals and heat treatment can greatly affect the tensile strength and ductility of metals. During the tensile test, after necking of the sample occurs, the engineering stress decreases as the strain increases, lea
20、ding to a maximum engineering stress in the engineering stress-strain curve. Thus, once necking begins during the tensile test, the true stress is higher than the engineering stress. Engineering stress = P/A0 and Engineering strain =(l-l0)/l0 True stress T = F/Ai = (1+ ) and True strain T =ln (li/l0
21、) = ln (1+ )MechanicalBehaviorofMaterialsMechanicalBehaviorofMaterialsEngineeringstress-straincurvesforsomemetalsandalloysChapter 4, mechanical properties of metalsMechanicalBehaviorofMaterialsChapter 4, mechanical properties of metalsComparisonbetweenengineeringandtuestress-straincurveMechanicalBeh
22、aviorofMaterialsYield strength is a very important value in engineering structural design since it is the strength at which a metal or alloy begins to showsignificant plastic deformation. Since there is no definite point on the stress-strain curve where elastic strain ends and plastic strain begins,
23、 the yield strength is chosen to be that at which a finite amount of plastic strain has occurred. For American structural design, the yield strength is chosen at 0.2% plastic strain. The ultimate tensile strength (UTS) is the maximum strength reached in the engineering stress-strain curve. If the sp
24、ecimen develops a localized reduction in cross-sectional area (necking), the engineering stress will decrease with further strain until fracture.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Determinin
25、gthe0.2%offsetyieldstrengthingraycastion,and(b)upperandloweryieldpointbehaviorinalow-carbonsteelMechanicalBehaviorofMaterialsResilience,UrlAbilityofamaterialtostoreenergyEnergystoredbestinelasticregionIf we assume a linear stress-strain curve this simplifies toAdapted from Fig. 6.15, Callister 7e.yy
26、r21UMechanicalBehaviorofMaterialsTheThe areaarea underunder thethe elasticelastic regionregion isis thethe elasticelastic strainstrain energyenergy (in.lb./in.(in.lb./in.3 3),), a ameasuremeasureofofthetheamountamountofofelasticelasticenergyenergythatthatcancanbebestoredstoredinineacheachcubiccubici
27、nchinchofofthespecimen.thespecimen.ForForspringspringsteel,steel,MMR R =385385in.lb./in.in.lb./in.3 3 oror13551355./lb./lb. ForForrubber,rubber,MMR R =16801680385385in.lb./in.in.lb./in.3 3 oror48,00048,000./lb./lb. RubberRubbercancanstorestoremuchmuchmoremoreenergyenergyperperunitunitvolumevolumeorw
28、eightthancansteel.orweightthancansteel.MechanicalBehaviorofMaterialsElasticStrainRecoveryAdapted from Fig. 6.17, Callister 7e.1. Initial2. Small load3. UnloadFd dMechanicalBehaviorofMaterialsThe more ductile a metal is, the more the decrease in the stress on the stress-strain curve beyond the maximu
29、m stress. For high strength aluminum alloy, there is only a small decrease in stress beyond the maximum stress because this material has relatively low ductility.The ultimate tensile strength is not used much in engineering design for ductile alloys since too much plastic deformation takes place bef
30、ore it is reached. However, the ultimate tensile strength can give some indication of the presence of defects. If the metal contains porosity or inclusions, these defects may cause the ultimate tensile strength of the metal to be lower than normal.MechanicalBehaviorofMaterialsDuctility of metals is
31、most commonly expressed as percent elongation and percent reduction in area. The percent elongation and percent reduction in area at fracture is of engineering importance not only as a measure of ductility but also as an index of the quality of the metal. Percent elongation is the amount of elongati
32、on that a tensile specimen under goes during testing provides a value for the ductility of a metal. Percent reduction in area is usually obtained from a tensile test using a specimen 0.50 in (12.7 mm) in diameter. x 100LLLEL%oof-=100xAAARA%ofo-=MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a div
33、ision of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Localizeddeformationofaductilematerialduringatensiletestproducesaneckedregion.ThemicrographshowsneckedregioninafracturedsampleMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, In
34、c. Thomson Learning is a trademark used herein under license.Thestress-strainbehaviorofbrittlematerialscomparedwiththatofmoreductilematerialsMechanicalBehaviorofMaterialsMechanicalBehaviorofMaterialsChapter 4, mechanical properties of metalsToughness:Toughness: is is defined defined as as the the to
35、tal total area area under under the the stress stress strain strain curve curve up up to to fracture fracture (in.lb./in.(in.lb./in.3 3). ). It It is is a a measure measure of of the the total total amount amount of of energy energy that that can can be be absorbed absorbed prior to fracture. Brittl
36、e materials are not tough.prior to fracture. Brittle materials are not tough.NoteNote: :It It is is not not possible possible to to make make this this integration integration unless unless we we have have some some mathematical mathematical function function that that describes describes the the re
37、lationship relationship between between stress stress and and strain strain up up to to fracture fracture ( ( = = EeEe only only describes describes the the relationship relationship during during elastic elastic deformation, deformation, not not plastic plastic deformation). deformation). Some Some
38、 possible possible mathematical mathematical models models will will be be described described in in the the following following section. section. As As an an approximation, approximation, toughness toughness can can be be estimated estimated as as the the area area under under the the curve curve u
39、sing using the the combined combined areas areas of of simple simple shapes shapes such such as as rectangles rectangles and and triangles.triangles.MechanicalBehaviorofMaterialsGiven Given the the true true stress stress strain strain curve curve =KK n n , , the the toughnesstoughness (the (the spe
40、cific specific energy energy (in.lb./in(in.lb./in3 3) ) dissipated dissipated up up to to fracture) fracture) can can be be calculated calculated by by integrating integrating with with respect respect to to strain up to the strain at fracture strain up to the strain at fracture ( ( f f) ) Then usin
41、g the true stress strain modelThen using the true stress strain model =K=K n n MechanicalBehaviorofMaterialsExampleProblemMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Figure 6.10 The stress-strain cur
42、ve for an aluminum alloy from Table 6-1MechanicalBehaviorofMaterialsExampleProblemMechanicalBehaviorofMaterialsExampleProblemYoungsModulusofAluminumAlloyFromthedatainExample6.1,calculatethemodulusofelasticityofthealuminum alloy. Use the modulus to determine the length afterdeformationofabarofinitial
43、lengthof50in.Assumethatalevelofstressof30,000psiisapplied.Example6.3SOLUTIONMechanicalBehaviorofMaterialsDuctilityofanAluminumAlloyThealuminumalloyinExample6.1hasafinallengthafterfailureof2.195in.andafinaldiameterof0.398in.atthefracturedsurface.Calculatetheductilityofthisalloy.Example6.4SOLUTIONMech
44、anicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.The effect of temperance (a) on the stress-strain curve and (b) on the tensile properties of an aluminum alloyMechanicalBehaviorofMaterialsTrueStressandTrueStr
45、ainCalculationCompareengineeringstressandstrainwithtruestressandstrainforthealuminumalloyinExample6.1at(a)themaximumloadand(b)fracture.Thediameteratmaximumloadis0.497in.andatfractureis0.398in.Example6.5SOLUTIONMechanicalBehaviorofMaterialsSOLUTION(Continued)MechanicalBehaviorofMaterialsCompression:C
46、ompression: ManyMany manufacturingmanufacturing processesprocesses suchsuch asas forging,forging, rolling,rolling,extrusion,extrusion,areareperformedperformedwithwiththetheworkworkpiecepiecesubjectedsubjectedtotocompressivecompressiveforces.forces. CompressionCompression test,test, inin whichwhich t
47、hethe specimenspecimen isis subjectedsubjected toto compressivecompressive load,load,givesgivesinformationinformationusefulusefulforforthesetheseprocesses.processes.WhenWhenthetheresultsresultsofofcompressioncompressionteststests andand tensiontension teststests onon ductileductile metalsmetals area
48、re compared,compared, thethe truetrue stress-truestress-truestrainstraincurvescurvesforforthethetwotwoteststestscoincide.coincide. ThisThiscomparabilitycomparabilitydoesdoesnotnotholdholdtruetrueforfor brittlebrittle materials,materials, whichwhich areare generallygenerally strongerstronger andand m
49、oremore ductileductile inincompressionthanintensioncompressionthanintensionMechanicalBehaviorofMaterials. Factor of safety, NOften N isbetween1.2 and 5Example:Calculateadiameter,d,toensurethatyielddoesnotoccurinthe1045carbonsteelrodbelow.Useafactorofsafetyof5.DesignorSafetyFactors51045 plain carbon
50、steel: y = 310 MPa TS = 565 MPaF = 220,000NdLod = 0.067 m = 6.7 cmMechanicalBehaviorofMaterialsBend Test for MaterialsBendTestforBrittleMaterials1.Bendtest-Applicationofaforcetothecenterofabarthatissupportedoneachendtodeterminetheresistanceofthematerialtoastaticorslowlyappliedload.2.Flexuralstrength
51、-Thestressrequiredtofractureaspecimeninabendtest.3.Flexuralmodulus-Themodulusofelasticitycalculatedfromtheresultsofabendtest,givingtheslopeofthestress-deflectioncurve.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein un
52、der license.Thebendtestoftenusedformeasuringthestrengthofbrittlematerials,and(b)thedeflectionobtainedbybendingBendTestforBrittleMaterialsMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Stress-deflectionc
53、urveforMg0obtainedfromabendtestBendTestforBrittleMaterialsMechanicalBehaviorofMaterialsBendingBending(Flexure):(Flexure): TheTheBendBendtesttestisiscommonlycommonlyusedusedforforbrittlebrittlematerials.materials. ItItusuallyusuallyinvolvesinvolvesa aspecimenspecimenthatthathashasa arectangularrectan
54、gularcross-section.cross-section. TheTheloadloadisisappliedapplied vertically,vertically, atat eithereither oneone pointpoint oror two:two: asas a a result,result, thesethese teststests arearereferredreferredtotoasasthree-pointthree-pointandandfourfourpointpointbend,bend,respectively.respectively. T
55、heThelongitudinallongitudinalstressesstressesininthesethesespecimensspecimensarearetensiletensileatattheirtheirlowerlowersurfacessurfacesandandcompressivecompressiveattheiruppersurfaces.attheiruppersurfaces.ThestressatfractureinbendingisknownastheThestressatfractureinbendingisknownasthetransverserup
56、turestrength.transverserupturestrength.BendTestforBrittleMaterialsMechanicalBehaviorofMaterialsHardness of MaterialsHardness is a measure of the materials resistance to localized plastic deformation (e.g. dent or scratch). In general, hardness usually implies a resistance to deformation, and for met
57、als the property is a measure of their resistance to permanent or plastic deformation. To a person concerned with the mechanics of materials testing, hardness is most likely to mean the resistance to indentation.HardnessofMaterialsMechanicalBehaviorofMaterialsSteelSteel isis harderharder thanthan al
58、uminum,aluminum, andand aluminumaluminum isis harderharder thanthan lead.lead. SeveralSeveralmethodshavebeendevelopedtomeasurethehardnessofmaterials.methodshavebeendevelopedtomeasurethehardnessofmaterials.HardnessofMaterialsMechanicalBehaviorofMaterialsHardnessHardness andand StrengthStrength: : Stu
59、diesStudies havehave shownshown thatthat (in(in thethe samesame units)units) thethehardnesshardness ofof a a cold-workedcold-worked metalmetal isis aboutabout threethree timestimes itsits yieldyield stress:stress: forforannealedannealed metals,metals, itit isis aboutabout fivefive timestimes thethe
60、yield.yield. AA relationshiprelationship hashas beenbeenestablishedestablished betweenbetween thethe ultimateultimate tensiletensile strengthstrength (UTS)(UTS) andand thethe BrinellBrinellhardness(HB)forsteels.InSIunits,hardness(HB)forsteels.InSIunits,UTSUTS=3.5*(HB),3.5*(HB),wherewhereUTSUTSisisin
61、inMpa.Mpa.OrOr UTSUTS=500*(HB),500*(HB),wherewhereUTSUTSisisininpsiandHBisinkg/mm2,asmeasuredforaloadof3000kg.psiandHBisinkg/mm2,asmeasuredforaloadof3000kg.HardnessofMaterialsMechanicalBehaviorofMaterialsHardness-TestingProceduresHardness-TestingProcedures:Thefollowingconsiderationsmustbetakenfor:Th
62、efollowingconsiderationsmustbetakenforhardnesstesttobemeaningfulandreliable:hardnesstesttobemeaningfulandreliable:1. 1.ThezoneofdeformationundertheindentermustbeallowedtodevelopThezoneofdeformationundertheindentermustbeallowedtodevelopfreely.freely.2. 2.Indentationshouldbesufficientlylargetogivearep
63、resentativehardnessIndentationshouldbesufficientlylargetogivearepresentativehardnessvalueforthebulkmaterial.valueforthebulkmaterial.3. 3.Surfacepreparationisnecessary,ifconductingRockwelltestandotherSurfacepreparationisnecessary,ifconductingRockwelltestandothertests,excepttests,exceptBrinellBrinellt
64、est.test.(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.HardnessofMaterialsMechanicalBehaviorofMaterialsMechanicalBehaviorofMaterialsHardnessofMaterialsTemperature EffectsTemperature Effects: Increasing the temperature generally ha
65、s the following effects : Increasing the temperature generally has the following effects on stress-strain curves:on stress-strain curves:a. a.It raises ductility and toughnessIt raises ductility and toughnessb. b.It lowers the yield stress and the modulus of elasticityIt lowers the yield stress and
66、the modulus of elasticityc. c.It lowers the strain-hardening exponent of most metalsIt lowers the strain-hardening exponent of most metalsMechanical Behavior of MaterialsRate-of-Deformation (Strain Rate) EffectsRate-of-Deformation (Strain Rate) Effects: Deformation (strain) rate is defined as the :
67、Deformation (strain) rate is defined as the speed at which a tension test is being carried out, in units of, say, mm/s.speed at which a tension test is being carried out, in units of, say, mm/s.The strain rate is a function of the specimen length. A short specimen elongates The strain rate is a func
68、tion of the specimen length. A short specimen elongates proportionately more during the same time period than does a long specimen.proportionately more during the same time period than does a long specimen.(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used
69、 herein under license.Whenaductilematerialispulledinatensiletest,neckingbeginsandvoidsformstartingnearthecenterofthebarbynucleationatgrainboundariesorinclusions.Asdeformationcontinuesa45shearlipmayform,producingafinalcupandconefractureMechanical Behavior of MaterialsImpactTestingofMaterialsImpact te
70、st - Measures the ability of a material to absorb thesuddenapplicationofaloadwithoutbreaking.Impact energy - The energy required to fracture a standardspecimenwhentheloadisappliedsuddenly.Impacttoughness-Energyabsorbedbyamaterial,usuallynotched,duringfracture,undertheconditionsofimpacttest.Fracturet
71、oughness-Theresistanceofamaterialtofailureinthepresenceofaflaw.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Theimpacttest:(a)TheCharpyandIzodtests,and(b)dimensionsoftypicalspecimensMechanicalBehavioro
72、fMaterialsDuctiletobrittletransitiontemperature(DBTT)-Thetemperaturebelowwhichamaterialbehavesinabrittlemannerinanimpacttest.Notchsensitivity-Measurestheeffectofanotch,scratch,orotherimperfectiononamaterialsproperties,suchastoughnessorfatiguelife.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a d
73、ivision of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.ResultsfromaseriesofIzodimpacttestsforasuper-toughnylonthermoplasticpolymerMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein un
74、der license.The Charpy V-notch properties for a BCC carbon steel and a FCCstainlesssteel.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Theareacontainedwithinthetruestress-truestraincurveisrelatedtothet
75、ensile toughness. Although material B has a lower yield strength, itabsorbsagreaterenergythanmaterialA.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Schematicdrawingoffracturetoughnessspecimenswith(a)e
76、dgeand(b)internalflaws(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.The fracture toughness Kc of a 3000,000psi yield strength steel decreases withincreasingthickness,eventuallylevelingoffattheplanestrainfracturetoughnessKlcMechani
77、calBehaviorofMaterialsFatigueofMaterialsReversed cycle of stress i.e. the maximum and minimum stresses are equal. A repeated stress cycle i.e. max (Rmax) and min (Rmin) are not equal. A complicated stress cycle which might be encountered in a part such as an aircraft wing which is subjected to perio
78、dic unpredictable overloads due to gusts.Typical fatigue stress cycles. (a) Reversed stress; (b) repeated stress; (c) irregular or random stress cycleTypeofFatigueStressesThebasicmethodofpresentingengineeringfatiguedataisbymeansoftheS-Ncurve,aplotofstressSagainstthenumberofcyclestofailureN.Thevalueo
79、fstressthatisplottedcanbea,max,ormin.ThemostcommonlyusedparameteristhestressratioisR=(Smin/Smax).Ifthe stresses are fully reversed, then R = -1. If the stresses are partiallyreversed,R=anegativenumberlessthan1.Ifthestressiscycledbetweenamaximumstressandnoload,R=zero.Ifthestressiscycledbetweentwotens
80、ilestresses,R=apositivenumberlessthan1.TheS-Ncurveisdeterminedforaspecifiedvalueofm,R(R=min/max).TheusualprocedurefordetermininganS-Ncurveistotestthefirstspecimenatahighstresswherefailureisexpectedinafairlyshortnumberofcycles,e.g.,atabouttwo-thirdsthestatictensilestrengthofthematerial.Theteststressi
81、sdecreasedforeachsucceedingspecimenuntiloneortwospecimensdonotfailinthespecifiednumbersofcycles.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.TheS-NfatiguecurveforanacetalpolymerMechanicalBehaviorofMat
82、erials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Examplesofstresscycles.(a)Equalstressintensionandcompression,(b)greatertensilestressthancompressivestress,and(c)allofthestressistensileAfatiguefailureisparticularlyinsidiousbecau
83、seitoccurswithoutanyobviouswarning.Thermalfatigue.Thermalcyclingcauseexpansionandcontraction,hencethermalstress,ifcomponentisrestrained.Corrosion fatigue. Chemical reactions induce pits which act as stressraisers.CorrosionalsoenhancescrackpropagationFatiguetestsareusuallymadewithsmooth,polishedspeci
84、mensundercompletelyreversedstressconditions.Fatiguepropertiesarefrequentlycorrelatedwithtensileproperties.Ingeneral,thefatiguelimitofcastandwroughtsteelsisapproximately50percentoftheultimatetensilestrength.Theratioofthefatiguelimit(orthefatiguestrengthat106cycles)tothetensilestrengthiscalledthefatig
85、ueratio.MechanicalBehaviorofMaterialsThe highest stress at which a (non-failure) is obtained is taken as thefatiguelimit.Formaterialswithoutafatigue limit the test is usuallyterminatedforpracticalconsiderationsatalowstresswherethelifeisabout108or5x108cycles.TheS-Ncurveisusuallydeterminedwithabout8to
86、12specimens.Fatiguelimit(endurancelimit)occurs for somematerials(someFeandTialloys).Inthiscase,theS-NcurvebecomeshorizontalatlargeN.Thefatiguelimitismaximumstressamplitudebelowwhichthematerialneverfails,nomatterhowlargethenumberofcycleis.MechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division o
87、f Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.TheS-NcurvesforatoolsteelandanaluminumalloyMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Schematic representation of a
88、 fatiguefracturesurfaceinasteelshaft.Crack initiation at the sites of stressconcentration (microcracks, scratches,indents,interiorcorners,dislocationslipsteps,etc.).Stage I: initial slow propagation alongcrystalplaneswithhighresolvedshearstress. Involves just a few grains, andhasflatfracturesurface.
89、StageII:fasterpropagationperpendicular to the applied stress.Crackgrowsbyrepetitivebluntingandsharpeningprocessatcracktip.Crackeventually reaches critical dimensionandpropagatesveryrapidly.MechanicalBehaviorofMaterialsMagnitudeofstress(mean,amplitude.)Qualityofthesurface(scratches,sharptransitionsan
90、dedges).Large enough variation or fluctuation in the applied stress, andSufficientlylargenumberofcyclesoftheappliedstress.Other variables include stress concentration, corrosion, temperature,overload, metallurgical structure, residual stresses, and combinedstresses,whichtendtoaltertheconditionsforfa
91、tigue.VariableaffectingFatigueMechanicalBehaviorofMaterialsIntroducingcompressivestressesintothinsurfacelayerby“shotpeening”-firingsmallshotintosurfacetobetreated.Casehardening-createC-orN-richouterlayer.MakesharderouterandalsointroducescompressivestressesUsematerialswithlowthermalexpansioncoefficie
92、ntsDecreasecorrosivenessofmedium,ifpossibleAddprotectivesurfacecoatingAddresidualcompressivestressesPreventthedevelopmentofsurfacediscontinuitiesduringprocessing.Reduceoreliminatetensileresidualstressescausedbymanufacturing.PreventingFatigueFailureMechanicalBehaviorofMaterialsCreepofMaterialsCreepBe
93、haviorCreep is a time-dependent and permanent deformation of materialswhen subjected to a constant load at a high temperature (0.4Tm).Examples:turbineblades,streamgenerators.StagesofCreepCreepTestingMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learni
94、ng is a trademark used herein under license.TheeffectoftemperatureorappliedstressonthecreepcurveCreepBehaviorMechanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.AtypicalcreepcurveSecondary/steady-statecreepi
95、soflongestdurationandisthemostimportantparameterofthecreepbehaviorinlong-lifeapplications=/tStagesofCreepPrimary/transientcreep.Secondary/steady-statecreep.Tertiarycreep.MechanicalBehaviorofMaterialsCreep: Withincreasingstressor temperature,theinstantaneousstrainincreases,thesteady-statecreeprateinc
96、reasesandthetimetorupturedecreases. The stress/temperaturedependence of the steady-statecreepratecanbedescribedbyss=Knexp(-Qc/RT)whereQcistheactivationenergyforcreep,Kandnarematerialconstants.DifferentmechanismsareresponsibleforcreepindifferentmaterialsThemechanismsinclude1.Stress-assistedvacancydif
97、fusion2.Grainboundarydiffusion3.Grainboundarysliding4.DislocationmotionMechanicalBehaviorofMaterialsCreeptest-Measurestheresistanceofamaterialtodeformationandfailurewhensubjectedtoastaticloadbelowtheyieldstrengthatanelevatedtemperature.Climb-Movementofadislocationperpendiculartoitsslipplanebythediff
98、usionofatomstoorfromthedislocationline.Creeprate-Therateatwhichamaterialdeformswhenastressisappliedatahightemperature.Rupturetime-Thetimerequiredforaspecimentofailbycreepataparticulartemperatureandstress.MechanicalBehaviorofMaterials1.Stress-rupturecurve-Amethodofreportingtheresultsofaseriesofcreept
99、estsbyplottingtheappliedstressversustherupturetime.2.Larson-Miller parameter - A parameter used to relate the stress,temperature,andrupturetimeincreep.3.Stress-corrosion- A phenomenon in which materials react withcorrosivechemicalsintheenvironmentleadingtotheformationofcracksandloweringofstrength.Me
100、chanicalBehaviorofMaterials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Resultsfromaseriesofcreeptests.(a)Stress-rupturecurvesforaniron-chromium-nickelalloyand(b)theLarson-MillerparameterforductilecastironCreepBehaviorMechanicalBehaviorofMaterials
