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StaticsStaticsofdeformablebodyChapter12
CombinedDeformation
12.1Introduction12.2Skewbending12.3Combinedaxialloadandbending12.4CombinedbendingandtorsionContentscombineddeformationthedeformationoftenincludesmorethantwobasicdeformations.Forexample:purlinsontheroofframe(skewbending)Fyzq2.Theapproximatestepsforstrengthcalculationofcombineddeformationdecompose+superimposeSimplifyordecomposetheexternalforceintoseveralsimpleforms.Drawtheinternalforcesforeachbasicdeformationandfindthelocationofpossibledangeroussections.Thengivethedistributionofstressesinthedangeroussections.Findthelocationofthedangerpointbasedonthestressdistributioninthedangersection.Usetheprincipleofsuperpositiontodeterminethestressstateatthedangerpoint.Accordingtothestressstateofthedangerpoint,establishthecorrespondingstrengthconditionsandperformstrengthcalculations.12.2SkewbendingPurlinsplaceddiagonallyontheroofframezFjyyjzFyF(b)yzxo(c)oyzxzFTakeacantileverbeamwithrectangularsectionasanexample1.InternalforcesandstressesdecomposeFalongtheprincipalaxisbendingmoments:whereM=F(l-x)isthetotalbendingmomentoftheforceFtothecross-sectionm-m.ommyyFzjz(a)lxxThedirectionsofMyandMz
areshowninFigurea.normalbendingstresscausedbyMz:normalbendingstresscausedbyMy:(b)yzs¢Mz(a)xmyDyMzMm(c)yzs¢¢MyCausedbyCausedby(12-1)Whetherσ'andσ''aretensileorcompressivestressescanbedeterminedbythedeformationofthebarinthespecificproblem.Atthesamepointthesetwostressesareco-linearvectors,andbythesuperpositionprinciplewehave(b)yzs¢Mz(a)xmyDyMzMm(c)yzs¢¢MyCausedbyCausedbyExampleThecantileverbeamshownintheFigureissubjectedtohorizontalforceF1andaperpendicularforceF2attwodifferentsections.F1=800N,F2=1650N,l=2m.Trytofindthemaximumnormalstressinthebeamanditslocationforthefollowingtwocases.(1)Thebeamhasarectangularcrosssectionwithawidthandheightofb=9cm,h=18cm.(2)Thebeamhascircularsectionofdiameterd=13cm.2F1m1md1Fzy2F1m1m1FxObhsolution
RectangularcrosssectioncausedbyF2causedbyF1
Superimposetwostresseszy2F1m1m1FxObh(2)Circularcrosssection:Letthecoordinatesofthedangerpointbe(y,z)causedbyF2causedbyF1
2F1m1md1FSuperimposetwostressesfrom
,wehaveso2.MaximumnormalstressandstrengthconditionsThelinewherethenormalstressiszeroiscalledtheneutralaxisonthesection.Ifthepointontheneutralaxisismarkedas(y0,z0),thenlet
σ=0,Theequationfortheneutralaxiscanbeobtainedas(12-2)yzjp1D2D(a)neutralaxisItcanbeseenthattheneutralaxisisalinepassingthroughthecentroidofthesection.Thepointfurthestfromtheneutralaxishasthehighestnormalstressandisthedangerpoint(D1
andD2).yzjF1D2D(a)neutralaxisThestepsforstrengthcalculation:Determiningthelocationofdangeroussectionsbasedoninternalforces.Calculatethemaximumnormalstressonthedangeroussection,thatis,thenormalstressatthedangerouspoint.Determinationofthedangerpoint:thepointonthedangeroussectionfarthestfromtheneutralaxis.
aFzy(b)2D1DNote:(1)Thelocationofthedangerpointiseasytodetermineforacross-sectionwithangleslikearectangle.(2)Forirregularsections,thepositionoftheneutralaxisshouldbedeterminedfirst,andthenatangentlineparalleltotheneutralaxisshouldbemadeontheperimeterofthesection,andthetangentpointisthedangerpoint.
aFzy(b)2D1DIfthecoordinatesofthedangerpointsare(y1,z1)and(y2,z2),andthetensileandcompressivestrengthsofthematerialareequal,thestrengthconditioncanbeexpressedas
(12-3)Forsectionssuchasrectangles,thestrengthconditioncanbesimplyexpressedas
(12-4)aFzy(b)2D1DNote:(1)Whenthetensileandcompressivestrengthofthematerialaredifferent,themaximumtensilestressandmaximumcompressivestressshouldbecalibratedrespectively.(2)Whenselectingsections,sinceWy,Wzintheintensityconditionsareunknown,theintensityconditionscannotbeusedtodeterminethevaluesofWy,Wzatthesametime.SoweneedtosetavalueofWy/Wz
accordingtoexperienceandthentosolvebytrialcalculation.3.DeflectionofskewbendingThedeflectionofabeaminskewbendingcanalsobecalculatedusingthemethodofsuperposition.FirstlythedeflectionfycausedbyFyandthedeflectionfz
causedbyFzarecalculatedaccordingtotheofplanebending.Andthenthemagnitudeanddirectionofthetotaldeflectionfcanbederivedbyvectorsynthesis.Nowwetakearectangularsectionbeamasanexampletoillustratethemethodoffindingthedeflectionofskewbending.Example
Findthedeflectionofthefreeendofthecantileverbeamshowninthefigure.Byreferringtothetable,wecanget
thetotaldeflectionofthefreeendis
Lettheangleofthetotaldeflectiontothez-axisbe,then
ommyyFzjxzfyyfz
fz12.3CombinedaxialloadandbendingThebarwillbedeformedinacombinationoftension(compression)andbendingundertwotypesofloading:
eccentricaxialloads;
combinedaxialandtransverseloads.FNF1.StrengthchecksTheinternalforcesMy,Mz,andNinthecrosssectionoftherodcorrespondtothepositivestresses.(1)accordingtoMy,MzandN,determinethelocationofthedangeroussectionofthebar;(2)thenbasedontheactualdirectionofMy,MzandN,thelocationofthedangerouspointinthedangeroussectionisdetermined;(3)bysuperimposingthestressescorrespondingtoeachinternalforcecomponentatthedangerpoint,thestressvalueatthedangerpointisobtained.Forrectangularsectionrods,wehavewherethesignisdeterminedbywhethertheactualstressatthedangerpointisintensionorcompression.thedangerouspointonthememberisinauniaxialstressstate,anditsstrengthconditionis
Example
TheshortcolumnshownissubjectedtoloadsFandH.TrytofindtheinternalforcesatthecornerpointsA,B,CandDonthefixedendsection.SolutionInternalforcesinfixedendsection(2)
Geometricparametersofcross-sectionH=5kNF=25kNAyzBCD150100255075(3)CalculatestressesH=5kNF=25kNAyzBCD150100255075AyzBCD150100255075H=5kNF=25kN2.CoreofSectionFigure(a)showsashortcolumnsubjecttoeccentriccompression.Asshowninfigure(b),simplifyFtowardstheaxis:yxzFOpyPzA(a)(b)zxOyzMyM(b)zxOyzMyMInthecrosssectionshown,atpointB(y,z),thestresscomponentscorrespondingtothethreedeformationsareWherethenegativesignindicatesthatthestressiscompressivestress.ByzyzpBythemethodofsuperpositionandtakingintoaccounttherelationshipbetweenIz=Aiz2
andIy=Aiy2,thenormalstressatpointBis
Ifwesee(y0,z0)asthecoordinatesofanypointontheneutralaxis,sincethestressontheneutralaxisisequaltozero,theequationoftheneutralaxiscanbeobtainedasTheneutralaxisineccentriccompressionisastraightlinethatdoesnotpassthroughthesection'scentroid
let
theinterceptsoftheneutralaxisontheyaxisandzaxiscanbeobtainedasByzyzNeutralaxisayazNote:(1)ayandyP,azandzPareofoppositesign.SotheneutralaxisandtheactionpointAoftheexternalforceFareoneachsideofthesectioncentroid.
(2)iftheactionpointofthepressureFgraduallyapproachesthecentroid,theinterceptwillgraduallyincrease.TheneutralaxiswillgraduallymoveawayfromthecentroidofthesectionyxzFopypzA(a)Thereisaareaaroundthecentroid,Whenthepressureisappliedwithinthisarea,theneutralaxiswillnotcrossthecross-section,andtherewillbenotensilestressesinthecross-section.Thisareaiscalledthecoreofsection.yz1ya1234515321za43Determinetheboundaryofthecoreofanarbitrarysection.Theline①canberegardedastheneutralaxis.Letitsinterceptsontheprincipalcentroidalinertiaaxesyandzbeay1,
az1.Thepoint1ofexternalforcecorrespondingtothisneutralaxiscanbecalculatedasfollows(*)yz1ya1234515321za43Similarly,otherlines②③...tangenttotheperimeterofthesectioncanberegardedasneutralaxes.Thecoordinatesofcorrespondingpoints2,3...ontheboundaryofthecoreofsectioncanbeobtainedbytheabovemethodinturn.Connectthesepointstoobtainaclosedcurve,whichistheboundaryofthecoreofsection.yz1ya1234515321za4312.4CombinedbendingandtorsionMostofthecommonshaftsinengineeringaresubjectedtobothtorsionalandbendingdeformation,whichiscalledthecombineddeformationofbendingandtorsion.F2zyxF11.Deformationofcircularshaftssubjectedtobendingandtorsion(1)Strengthconditionsdrawthebendingmoment(My
andMz)diagramsandtorque(Mn)diagram.findthetotalbendingmoment
fromthesyntheticdiagramofbendingmomentandthetorque,thepositionofthedangeroussectionofthecircularshaftcanbedetermined.zy1D2DozMyMwM(a)Action
planeThemaximumtorsionalshearstresscorrespondingtothetorqueMninthedangeroussectionoccursattheedgeofthecrosssectionwiththevalue (a)ThenormalstressinbendingcorrespondingtothesyntheticbendingmomentMWismaximumatpointsD1andD2,anditsvalueis(b)1D2Dontntwsws(b)wsws1Dntnt1D(c)wswszy1D2DozMyMwMwM(a)Action
planeThedistributionofshearstressesandnormalstressesarealongthediameterD1D2ofthesectionshowninFigure(b).BothD1
andD2aredangerouspoints.AnelementiscutatpointD1,ThestressesineachsectionoftheelementareshowninFigure(c).zy1D2DozMyMwMwM(a)Action
plane1D2Dontntwsws(b)wsws1Dntnt1D(c)wswsThedangerpointisinaplanestressstate,sostrengthconditionsshouldbeestablishedaccordingtotheoriesofstrength.TheprincipalstressatpointD1is
Accordingtothethirdtheoryofstrength,thestrengthconditionisAftersubstitutingtheprincipalstress,weget
wswswsws1Dntnt1D(c)Accordingtothefourththeoryofstrength,thestrengthconditionisSimilarly,aftersubstitutingtheprincipalstress,wecanget
(2)Strengthconditionsexpressedintheformof“calculatedbendingmoment”
(12-18)Forcircularsections,thereisWn=2W.Thealternativeformulationofthestrengthconditioncanbeobtained.thethirdtheoryofstrength:thefourththeoryofstrength:
(12-19)(12-20)(12-21)
、,
arecalledthe"calculatedbendingmoments"correspondingtothethirdandfourthstrengththeories,respectively.Obviously,afterintroducingtheconceptof"calculatedbendingmoment",Wejustneedtocalculatethevalueofthebendingmomentandtorqueatthedangersection.Andthenthestrengthcheckcanbecarriedoutdirectly.wswswsws1Dntnt1DEquations(12-20)and(12-21)applyonlytocombineddeformationsofbendingandtorsionofcircularorhollowcircularsection.Equations(12-18)and(12-19)aremorewidelyapplicable,Itisapplicableaslongasthedangerouspointisintheplanestressstateshown.(3)CombinedbendingandtorsionaldeformationofacircularshaftwithaxialforcesPropulsionshaftofshipForthestrengthcalculationofthistypeofrod,theformula(12-15)or(12-16)isstillavailable,butσwshouldbereplacedbythesumofthenormalstressinaxialcompression(tension)andthenormalbendingstress.However,fortheCombinedbendingandtorsionaldeformationofacircularshaftwithaxialforces.Theformula(12-15)or(12-16)isunavailable.TeMAB1F2FeMTwswsnt1DExample
AsteelcircularshaftisfittedwithtapewheelsAandB,asshowninFigure(a).BothwheelshavethesamediameterD=1mandweightF=5kN.ThetensioninthetapeonwheelAishorizontalandthetensioninthetapeonwheelBisintheplumbdirection.Theallowablestressforthecircularshaftis[σ]=80Mpa.Trytofindtherequireddiameteroftheshaftaccordingtothethirdtheoryofstrength.
zyxABCD5kN2kN2kN300500500(a)5kNsolution:Simplifytheexternalforcestowardtheaxis.Makethetorquediagram(Mn)andthebendingmomentdiagrams(My
Mz).ThesyntheticbendingmomentsofsectionsCandBarezyxABCD2kN2kN3005005005kN7kN1.5kNm12kNnMxxzMyxzyM5kN5kN1.5kNm1.5kNm2.1kNm2.25kNm1.5kNmSyntheticbendingmomentdiagramMwisshowninFigure.ItcanbeprovedthatthecurveofdiagramMwisconcave.Obviously,sectionCisadangeroussection.(3)Accordingtothethirdtheoryofstrength:Bysubstitutingthedata,thereareTherequireddiameterisABCD2kN5kNxwM2.58kNm2.48kNm2kN5kNExample
Figure(a)showsapropulsionshaftofacargoship.ThepowerofthemainengineisknownasN=7277kW,speedn=119r/min,effectivepropulsiveforceT=767kN,theweightofthepropellerbladesF1=180kN,thetotalweightoftheshaftoutreachF2=45kN,diameteroftheshaftd=51.5cm.Andthereisa1=1.9m,a2=1.2m.Thematerialishighqualitycarbonsteelwithayieldstressofσs=250MPa,allowablefactorofsafety[n]=4.CheckthestrengthofsectionAofthepropulsionshaftaccordingtothefourththeoryofstrength.TMAB1F2F2a1aMT(a)d(2)InternalforcesinsectionA
FN=T=767kNyzMnMaFNAsectionTeMAB1F2F2a1aeMT(a)dsolution(1)Torsionalcouplemoment(3)stressesatthepointofdangerNormalstressesduetobendingmomentsandaxialforcesareshowninFigure.Thecompressivestressisgreatestatpointaofthelowersectionedge,withavalueofThemaximumtorsionalshearstressoccursattheedgeofthecircularsectionandthevalueofpointaisBythefourththeoryofStrength,wecanobtainWsNsspointzMnMyaFNAsection(4)CalculatethefactorofsafetyThefactorofsafetyofashaftrepresentsthesafetyreserveoftheshaftinoperationandisequaltothebreakingstressdividedbytheworkingstress.
Thereforetheshaftissafe.PleaseconsiderthefollowingquestionIfthethirdtheoryofstrengthisadopted,canthestrengthconditioninthisquestionbewrittenasPleasegiveyourreasons.2.DeformationofrectangularbarssubjectedtobendingandtorsionForthecombinedbendingandtorsionaldeformationofarectangularsectionrod,thestrengthcalculationprocedureisapproximatelythesameasforacircularshaft,butwiththefollowingimportantdifferences.1.Forrectangularsectionbars,whentherearebothbendingmomentsMyandMzactingonthesection,theycannotbesynthesizedintoatotalbendingmomentMWasinthecaseofcircularaxes.BecausetheactionsurfaceofMW
isnottheprincipalinertiasurface,itisnotaplanebendingaftersynthesis.weshouldcalculatethenormalstressescausedbyMyandMzseparately,andthensuperimposethematthesamepoint.2、Deformationofrectangularbarssubjectedtobendingandtorsion2.Theshearstressisamaximumatthemidpointofthelongsideoftherectangularsection,alocalextremeatthemidpointoftheshortside,andzeroatthecorner.Careshouldbetakenindeterminingthedangerpoint.3.Whenarectangularsectionbarundergoesbendingandtorsionaldeformationsimultaneously,thestressstateofthedangerouspointcanhavetwotypes:(1)rectangularcornerpoint,onlynormalstress,noshearstress,inauniaxialstressstateandthesamestrengthconditionsasbending;(2)themidpointofthelongorshortsideoftherectangularsection,bothbendingstressandtorsionalshearstress,thesamestressstateascombinedbendingandtorsionaldeformationofthecircularaxis.Therefore,thestrengthconditions(12-18)and(12-19)isstillapplicable,buttheexpression"calculatedbendingmoment"cannotbeused.
TheEndStaticsStaticsofdeformablebodyChapter13
EnergyMethod
13.1Strainenergyofbars13.2Mohrtheorem13.3DiagrammultiplicationmethodforMohrintegration13.4Castigliano'stheoremContentsU=W(13-1)Principleofstrainenergymethod:theworkdonebytheexternalforceatthecorrespondingdisplacementisnumericallyequaltothestrainenergystoredinthedeformedbody.Thestrainenergymethod:Themethodsofsolvingproblemsusingtheoremsandprinciplesrelatedtotheconceptofstrainenergy.External:workWdonebyexternalforceInternal:potentialenergydeformationenergyU13.1Strainenergyofbars1.
StrainenergyforbasicdeformationThecalculationofthestrainenergyunderseveralbasicdeformationsisnowexamined.(1)AxialtensionorcompressionForaxialtensionorcompressionofstraightrodsofequalcross-section,theexternalforceislinearlyrelatedtotheaxialdeformationofthebarwithintheelasticrange.ABoFl
D
(a)FlD
l(b)FThestrainenergyofthebarcanthusbewrittenas
a.Iftheinternalforcevariescontinuouslyalongtheaxisofthebar,i.e.FN=FN(x),Thestrainenergyofthebarcanbe
b.Iftheinternalforcesvaryinsteps,
wheremisthenumberoftensionandcompressionbars.Thestrainenergy(strainenergydensityorspecificenergy)perunitvolumeofatension(compression)baris
ABoM
j(2)Torsionofcircularshafts(a)lMjjMWorkdonebytorque:(b)
AccordingtoequationU=W,thisworkisequaltothetorsionalstrainenergystoredinthecircularshaft.Whenthecircularshaftissubjectedtoexternalforcecouplesatbothendsonly,wehaveThus,thetorsionalstrainenergyofthecircularshaftcanbewrittenasDiscussion:Iftheinternalforcecouplemomentvariescontinuouslyalongtheaxisofthecircularshaft,i.e.Mn=Mn(x),thestrainenergyforthewholecircularshaftisIftheinternalcouplemomentvariesinstepsalongtheaxis,wehave
Thestrainenergyperunitvolumeofthecircularshaft,i.e.thestrainenergydensityinthepureshearstate,is
(3)PlanebendingPurebendingofstraightcantileverbeamsofequalsection.Astheconcentratedcouplemomentgraduallyincreasesfromzerotoitsfinalvaluem,theangleofrotationatthefreeendofthecantileverbeamalsograduallyincreasesfromzerotoitsfinalvalueθ(Fig.a).(b)ABoql(a)
workdonebyMcanbeexpressedintermsoftheareaofthetriangleOAB,i.e
Discussion:strainenergyofthepurelybendingbeamstrainenergyofthestraightbeamintransverseforcebending
Intherangeoflinearelasticityandunderstaticload,thestrainenergyofabarcanbeexpresseduniformlyas
F:generalizedforceδ:generalizeddisplacementF:force
δ:displacement;F:forcecouplemoment
δ:angular-displacement2.CharacteristicsofelasticstrainenergyThedeformationenergiescannotsimplybesuperimposedingeneral.Note:If
M1andM2denotethebendingmomentscausedbythetwoexternalforce(F1F2)actingalonerespectively,whentheyacttogether,thebendingmomentsofthebeamshouldbeM1+M2.Thestrainenergyofthebeamis(2)theelasticstrainenergyisindependentoftheorderofloadinganddependsentirelyonthefinalvalueoftheloadanddisplacement.(3)whenthecross-sectionalchangesorinternalforcesarerepresentedbydifferentfunctions,thedeformationenergyshouldbecalculatedinseparatesections.(4)therodisalinearelastomersatisfyingHooke'slaw,forthenon-linearelastomer,thedeformationenergywillbecome3、TheClapeyron’stheorem-------UniversalexpressionsfordeformationenergyδidenotesthegeneralizeddisplacementofthegeneralizedforceFiatthepointofactionalongitsdirectionofaction.δi
canbewrittenas
whereδi1representsthegeneralizeddisplacementatthepointofFialongitsactiondirection.ItiscausedbythegeneralizedforceF1.Therestaresimilar.β1⋯βm
areconstantsrelatedtothestructure.1F2F1d2dmFmd…..ThesumoftheworkdonebyeachloadisnumericallyequaltothestrainenergyofthestructureThisconclusioniscalledClapeyron’stheorem.Itcanbedescribedasthesumofthedeformationenergyofalinearelastomerequaltoone-halfoftheproductofeachexternalforceanditscorrespondingdisplacement.4.Strainenergyforcombineddeformation
Usingthegeneralexpressionforstrainenergy,thestrainenergyofabarsubjectedtothecombinedactionofbending,torsionandaxialtensioncanbeobtained.
Nowinterceptamicro-segmentoflengthdxinthebar,iftheaxialforce,bendingmomentandtorqueinthecrosssectionareFN(x),M(x)andMn(x)(forthemicro-sectiondx,FN(x),M(x)andMn(x)shouldberegardedasexternalforces).Therelativeaxialdisplacement,rotationangleandtorsionanglebetweenthetwoendcrosssectionsared(Δl),dθanddφ,respectivelySincethedeformationscausedbyeachofFN(x),M(x)andMn(x)areindependentofeachother,thestrainenergywithinthemicro-segmentdxshouldbeThen,thedeformationenergyoftheentirecombineddeformedbarcanbeobtainedbyintegratingtheaboveequation.Example1:TrytofindthestrainenergyofthesquaretrussstructureandfindtherelativedisplacementsatpointsAandC.ItisknownthateachbarhasthesametensileandcompressiverigidityEA.solution:Axialforces:Deformationenergy:BACDFFlWorkdonebyexternalforceFromU=W,
weobtainThen,wecangetBACDFFlExample1:TrytofindthestrainenergyofthesquaretrussstructureandfindtherelativedisplacementsatpointsAandC.ItisknownthateachbarhasthesametensileandcompressiverigidityEA.Example2:Rightfigureshowsaplanerigidframe.ThebendingrigidityandtensilerigidityoftheframeareknowntobeEIandEA,respectively.trytofindtheverticaldisplacementδAofendA.SolutionSectionAB:SectionBC:Deformationenergy:FBACaDeformationenergy:VerticaldisplacementofsectionA:Ifa=landcrosssectiondiameterisd(l=10d),thenExample2:Rightfigureshowsaplanerigidframe.ThebendingrigidityandtensilerigidityoftheframeareknowntobeEIandEA,respectively.trytofindtheverticaldisplacementδAofendA.FBACathen:Thesecondterminbracketsislessthan0.05%.So,theeffectofaxialforcescangenerallybeneglectedwhensolvingfordeformationsordisplacementsinbendingresistantbarstructures.Example2:Rightfigureshowsaplanerigidframe.ThebendingrigidityandtensilerigidityoftheframeareknowntobeEIandEA,respectively.trytofindtheverticaldisplacementδAofendA.FBACaExample3:Aplanecurvedbarwithasemi-circularaxisisshown.AconcentratedforceperpendiculartotheplaneinwhichtheaxisislocatedisactingatthefreeendA.TrytofindtheverticaldisplacementofsectionA.solution:Itcanbeseenfromfigure(b)thatthetorsionandbendingonthecrosssectionm-nare
AFROjdjFAmmndj(b)Deformationenergy:Deformationenergyofthewholerod:mndj(b)Example3:Aplanecurvedbarwithasemi-circularaxisisshown.AconcentratedforceperpendiculartotheplaneinwhichtheaxisislocatedisactingatthefreeendA.TrytofindtheverticaldisplacementofsectionA.LettheverticaldisplacementofAbe.Duringthedeformation,theworkdonebytheexternalforceisnumericallyequaltothestrainenergyofthecurvedbar,i.e.Therefore:mndj(b)Example3:Aplanecurvedbarwithasemi-circularaxisisshown.AconcentratedforceperpendiculartotheplaneinwhichtheaxisislocatedisactingatthefreeendA.TrytofindtheverticaldisplacementofsectionA.13.2Mohr’stheoremMohr’stheoremisaneffectivetoolfordeterminingdisplacementatanypointinanydirection.TheconceptandpropertiesofstrainenergyarenowusedtoderiveMohrtheorem,usingabeamasanexample.SupposethebeamisbentanddeformedundertheactionofanexternalforceF1,F2......,asshowninFigure(a).WecalculatethedeflectionδatanypointConthebeamundertheactionoftheaboveexternalforce.C1F2FABd(a)thestrainenergycausedbyM(x)canbefound….oneunitforceF
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