L 7Properties of MechTest methods 01.docx
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L 7Properties of MechTest methods 01.docx
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L7PropertiesofMechTestmethods01
Hardness
Anothermechanicalpropertythatmaybeimportanttoconsiderishardness,whichisameasureofamaterial’sresistancetolocalisedplasticdeformation(e.g.asmalldentorascratch).[Callister,Jr.W.D.,]
Hardnessisapropertythatisoftenrelatedtoabroadrangeofmechanicalandevenphysicalproperties.Theengineeringdefinitionofhardnessis“amaterial’sresistancetopermanentindentationunderstaticordynamicforces”–thetestsusedtodeterminethehardnessofamaterialarecalledindentationhardnesstests.
Theindentationhardnessisbyfarthemostoftenusedmethodformeasuringhardness.Therearethreestandardisedindentationhardnesstests:
▪TheBrinellhardnesstest
▪TheVickershardnesstest
▪TheRockwellhardnesstest
Thereareothertypesofhardnesstests:
▪Scratchhardnesstestswhichutilisematerialsofknownhardnessto‘scratch’unknowntestpiecesorspecimens.
▪Wearhardnesswhichisameasurementofresistancetowearunderspecificconditions.
▪Reboundhardnesswhichismeasureasenergyabsorbedunderimpactloads.
[EngineeringMaterials]-11HardnessandTensileTesting-p208
Thehardnessofmetalscanbedefinedasresistancetoplasticdeformationandtestmethodsarebasedonforcinganindenterintothesurfaceunderaknownload.
(i)Brinelltest
Ahardsteelballindenterisused.Indenterdiametersmaybe1,2,5or10mm.
Hardnessnumber,HB=
WhereF=indentingload(kgf),D=diameterofindenter(mm)andd=diameterofimpression(mm).
(ii)Vickerdiamondhardnesstest
Asquarepyramidaldiamondindenter,withanincludedangleƟ=136o,betweenoppositefacesisforcedintothesurfaceofametalunderaconstantloadfor15s.
Hardnessnumber,HD=
WhereF=indentingforce(kgf)andd=meanlengthofimpressiondiagonals(mm)
Example11.1–p212
Indentationhardnesstestsaremadeonasampleofmetalandthefollowingresultsrecorded:
(1)usingVickersdiamondtestwith30kgloadthemeanlengthsofdiagonalswere:
1stimpression–0.527mm,2ndimpression–0.481mm,3rdimpression–0.497mm;
HD=1.854F/d2.Forthefirstimpression,HD=(1.854×30)/0.5272=200.ForthesecondimpressionHD=240,forthethirdHD=225.TheaverageofthreeimpressionsgivesHD=221.7.
(2)usingBrinelltestwith10mmballand3000kgloadthediameterofimpressionwas4.01mm.
CalculatethediamondandBrinellhardnessnumbersforthematerialandexplainanyvariationsintheresults.
HB=(2×3000)÷{10π(10-
)}=228
ThesmallsizeoftheindenterusedintheVickerstestmeansthatinanalloyanimpressionmaybeonanon-representativehardorsoftspot.Anumberofdeterminationsmayhavetobemadetoobtainatrueaveragevalue.Usinga10mmdiameterballindentertheBrinelltestgivesaresultmorerepresentativeofthemeanhardnessofthematerial.
Example11.1
TheindentingloadsnormallyusedinconnectionwithVickersdiamondtestsare:
1kg,2.5kg,5kg,10kg,20kg,30kgand50kg.Ifdeterminationsaremostaccuratewhenimpressiondiagonalsareapproximately0.5mm,whatindentingloadsshouldbeselectedforthetestingof:
(i)aluminiumsampleswithhardnessoftheorderofHD=20,
(ii)brasssampleswithhardnessoftheorderofHD=60,and
(iii)SteelsampleswithhardnessoftheorderofHD=200?
Example11.3
Samplesofpurecopperinboththeannealedandcold-workedconditionsweresubjectedtoBrinellhardnesstests,usinga1mmdiameterballindenter,withvariousloads.Thetestdataaregivenbelow.
(1)CalculatetheBrinellhardnessofthecoppersamples.
(2)CompleteaMeyeranalysisanddeterminetheMeyerconstants.
Material
Indentingload(kg)
Indentationdiameter(mm)
Annealedcopper
5
10
15
0.386
0.540
0.636
Coldworkedcopper
10
20
30
0.375
0.527
0.632
Example11.1
(iii)Meyerhardnessanalysis
TheMeyerrelationshipfortheBrinelltestisF=adnwhereFistheload(kgf),disthediameteroftheindentationandaandnareconstantsofthematerialanditscondition.Theaisrelatedtotheresistancetoindenterpenetrationandnistheworkhardeningindex.Therelationshipcanbewritten:
logF=loga+nlogd
Usingaballindenteroffixeddiameterandaseriesofloads,thedatarecordedcanbeplottedaslogFagainstlogd.Astraightlineshouldbeobtainedandthevaluesofaandndetermined.
.au/wa/applications/test_types/hardness/brinell.aspx?
ref=.au/search
BrinellHardnessTest
Dr.J.A.BrinellinventedtheBrinelltestinSwedenin1900.Theoldestofthehardnesstestmethodsincommonusetoday,theBrinelltestisfrequentlyusedtodeterminethehardnessofforgingsandcastingsthathaveagrainstructuretoocourseforRockwellorVickerstesting.Therefore,Brinelltestsarefrequentlydoneonlargeparts.Byvaryingthetestforceandballsize,nearlyallmetalscanbetestedusingaBrinelltest.Brinellvaluesareconsideredtestforceindependentaslongastheballsize/testforcerelationshipisthesame.
IntheUSA,Brinelltestingistypicallydoneonironandsteelcastingsusinga3000Kgtestforceanda10mmdiametercarbideball.Aluminumandothersofteralloysarefrequentlytestedusinga500Kgtestforceanda10or5mmcarbideball.ThereforethetypicalrangeofBrinelltestinginthiscountryis500to3000kgwith5or10mmcarbideballs.InEuropeBrinelltestingisdoneusingamuchwiderrangeofforcesandballsizes.It'scommoninEuropetoperformBrinelltestsonsmallpartsusinga1mmcarbideballandatestforceaslowas1kg.TheselowloadtestsarecommonlyreferredtoasbabyBrinelltests.
Standards
BrinellTestmethodsaredefinedinthefollowingstandards:
ASTME10
ISO6506
BrinellTestMethod
AllBrinelltestsuseacarbideballindenter.Thetestprocedureisasfollows:
Theindenterispressedintothesamplebyanaccuratelycontrolledtestforce.
Theforceismaintainedforaspecificdwelltime,normally10-15seconds.
Afterthedwelltimeiscomplete,theindenterisremovedleavingaroundindentinthesample.
Thesizeoftheindentisdeterminedopticallybymeasuringtwodiagonalsoftheroundindentusingeitheraportablemicroscopeoronethatisintegratedwiththeloadapplicationdevice.
TheBrinellhardnessnumberisafunctionofthetestforcedividedbythecurvedsurfaceareaoftheindent.Theindentationisconsideredtobesphericalwitharadiusequaltohalfthediameteroftheball.TheaverageofthetwodiagonalsisusedinthefollowingformulatocalculatetheBrinellhardness.
TheBrinellnumber,whichnormallyrangesfromHB50toHB750formetals,willincreaseasthesamplegetsharder.Tablesareavailabletomakethecalculationsimple.AtypicalBrinellhardnessisspecifiedasfollows:
356HBW
Where356isthecalculatedhardnessandtheWindicatesthatacarbideballwasused.Note-PreviousstandardsallowedasteelballandhadanSdesignation.Steelballsarenolongerallowed.
Applications
BecauseofthewidetestforcerangetheBrinelltestcanbeusedonalmostanymetallicmaterial.Seetheapplicationguide.Thepartsizeisonlylimitedbythetestinginstrument'scapacity.
Strengths
1.Onescalecoverstheentirehardnessrange,althoughcomparableresultscanonlybeobtainediftheballsizeandtestforcerelationshipisthesame.
2.Awiderangeoftestforcesandballsizestosuiteveryapplication.
3.Nondestructive,samplecannormallybereused.
Weaknesses
1.ThemaindrawbackoftheBrinelltestistheneedtoopticallymeasuretheindentsize.Thisrequiresthatthetestpointbefinishedwellenoughtomakeanaccuratemeasurement.
2.Slow.Testingcantake30secondsnotcountingthesamplepreparationtime.
http:
//www.gordonengland.co.uk/hardness/vickers.htm
VickersHardnessTest
TheVickershardnesstestmethodconsistsofindentingthetestmaterialwithadiamondindenter,intheformofarightpyramidwithasquarebaseandanangleof136degreesbetweenoppositefacessubjectedtoaloadof1to100kgf.Thefullloadisnormallyappliedfor10to15seconds.Thetwodiagonalsoftheindentationleftinthesurfaceofthematerialafterremovaloftheloadaremeasuredusingamicroscopeandtheiraveragecalculated.Theareaoftheslopingsurfaceoftheindentationiscalculated.TheVickershardnessisthequotientobtainedbydividingthekgfloadbythesquaremmareaofindentation.
F=Loadinkgf
d=Arithmeticmeanofthetwodiagonals,d1andd2inmm
HV=Vickershardness
WhenthemeandiagonaloftheindentationhasbeendeterminedtheVickershardnessmaybecalculatedfromtheformula,butismoreconvenienttouseconversiontables.TheVickershardnessshouldbereportedlike800HV/10,whichmeansaVickershardnessof800,wasobtainedusinga10kgfforce.Severaldifferentloadingsettingsgivepracticallyidenticalhardnessnumbersonuniformmaterial,whichismuchbetterthanthearbitrarychangingofscalewiththeotherhardnesstestingmethods.TheadvantagesoftheVickershardnesstestarethatextremelyaccuratereadingscanbetaken,andjustonetypeofindenterisusedforalltypesofmetalsandsurfacetreatments.Althoughthoroughlyadaptableandveryprecisefortestingthesoftestandhardestofmaterials,undervaryingloads,theVickersmachineisafloorstandingunitthatismoreexpensivethantheBrinellorRockwellmachines.
VickersHardnessNumberCalculator
TopofForm
Force(kgf)
Entervalue
Meandiagonallengthd(mm)
Entervalue
HV
Result
BottomofForm
ThereisnowatrendtowardsreportingVickershardnessinSIunits(MPaorGPa)parti
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