(1.2)-2.2 压缩与弯曲-英文字幕

1

Helloeveryone。

Let'sreviewthemaincontentoflastlesson.

wetalkedaboutaveryimportantconceptofstressstatesoftcoefficient:theratioofmaximumshearstresstomaximumnormalstress

Itsmeaningistorepresentthedegreeofhardnessandsoftnessoftheexperiment。

2

Inthislesson，wewillstudytwokindsofstaticloadtests:Compressionandbending。

Itincludesfouraspects:Compressiontest、Characteristicsofcompressiontest、Bendingtest、Characteristicsofbendingtest。

3

Theprocessis.Axialpressureisappliedtothesample,then,Measurethemechanicalpropertiesofthematerialsuchasstrengthandplasticity.

SampleshapesincludeCylindricalorcube

Thesizeofthesample:Thevalueofheightofsampledividedbydiametrebetweenonetothree.

Onethingtopayattentionhere:Thefrictionresistancebetweentheendfaceofthespecimenandtheindenterhasgreatinfluenceonthetestresults。

Inordertoreducetheendfriction,bothendsofthesamplemustbesmoothandparalleltoeachother,lubricatingoilorgraphitepowdercanbecoatedwhennecessary,ortheuseofconicalpressureplate.

ThetestreferstothenationalstandardGBT7314Metallicmaterials-Testmethodforcompressionatroomtemperature。

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Ontheleftoftheslide,weseetheTensileandcompressioncurveofbrittlematerials,Itcanbeseenthatthecompressivepropertyofthematerialisbetterthanthetensileproperty.

Theindex(Forbrittlematerials)ofcompressiontestis

ReHcUpperyieldcompressivestrength

ReLcLowweryieldcompressivestrength

Rmccompressivestrength

εcCompressiondeformationrate

ψcSectionexpansionrate

OneofthemostimportantindicatorsisRmc。

Thecompressivestrengthisequaltoforcedividedbyarea

Therearetwocurvesinthediagram,curve1representsplasticmaterialcompression.curve2representsbrittlematerialcompression.

Thedeformationofplasticmaterialismuchlargerthanthatofbrittlematerial,itsplasticisbetter

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Inthefigure,wecanseethespecimensofplasticandbrittlematerialsbeforeandaftercompression

Thecharacteristicsofcompressiontestsinclude:

Firstly,Thesoftcoefficientofstressstateincompressiontestisequaltotwo,so,Materialsthatexhibitbrittlefractureundertensileloadalsoexhibitcertainplasticityundercompression.

Sencondly,itisusedtodeterminethemechanicalpropertiesofbrittlematerials.Itiscutalongtheaxisat45°.

Thirdly，MaterialswithgoodplasticitywillshowBarrelingphenomenonwhentheyarecompressed，SimilartoastretchedNecking。Thereasonofthisphenomenonisthefrictioneffectbetweenindenterandspecimen。

Materialswithlesspoorplasticityoftenhavecompressioncurvessimilartotensilecurves,whilethoseofbrittlematerialsareofteninconsistentwithtensilecurves.

Finally，Thecompressivestrengthofbrittlematerialsisgenerallyhigherthanitstensilestrength.

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Bendingtestprocedure:Thecylindricalorrectangularsampleisplacedonasupportofacertainspantocarryoutthree-pointorfour-pointbendingloading.

ByrecordingtherelationcurvebetweenbendingforceFanddeflectionf,themechanicalpropertiesofthematerialunderbendingforceweredetermined.

SampleshapesincludeCylinderorrectangle.

Seeslidepageforsamplesize

TheexperimentalconditionisthatthespanLsofthesampleis16timesofthediameterDortheheightH.

Thestressdistributiononthecrosssectionofthestressedrodisnotuniformandshowsalineardistributionlaw,thatis,thesurfaceismaximumandthecenteriszero.Thestressonthewholecrosssectionisnormalstress.

Itsmainindexisbendingstrength,itequaltoMbdividedbyW，WhereMbisthemaximumbendingmoment，Wisthebendingsectioncoefficient。

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Thecharacteristicsofbendingtestareasfollows：

Firstofall，Thebendingtestisnotaffectedbythedeflectionofthespecimen,andcanstablymeasurethebendingstrengthofbrittlematerialsandlowplasticmaterials,andcanclearlyshowtheplasticityofbrittleandlowplasticmaterials,suchascastiron、toolsteelandceramics。

Foranother,Inthebendingtest,thestressdistributiononthespecimensectionisnotuniform,andthesurfacestressisthelargest,sothesurfacedefectofthematerialcanbereflectedsensitivelytocheckthesurfacequalityofthematerial

Finally，Bendingtestiscommonlyusedtomeasuremechanicalpropertiesofmechanicalpartsunderbendingload,suchasshaftplate、spring,asabasisfordesignormaterialselection

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Ok,solet'ssumitup