Brookfield DV3T - Page 51

Brookeld Engineering Labs., Inc. Page 51 Manual No. M13-167-A0415
Pre-Shear Speed and Time
AnoptionalPre-Shearstepcanbeincludedinthetestparameters.Theusermustsupplythepre-
shearspeed(0.01-250rpm)andpre-sheartime(1second-5minutes59seconds).
Pre-shearing istheshearingofsamplebeforemeasuringitsyieldproperties.Thisprocessbreaks
downthesample’sstructure.Itisparticularlyusefuliftheinvestigatorwantstoeliminateprevious
shearhistory(e.g.,bumping,transferring)ofthesamplebeforetestingandobservethestructural
rebuildingofthesample.Thismaysimulatethefollowing:ketchuppumpedoutofabottlewill
rebuildafteritcomestorestontheFrenchFries.Therearematerialswhosemeasuredyieldstress
willbelowerafterpre-shearingthaniftestedwithoutpre-shearing.Thismaybeusedtocompare
therateatwhichdifferentmaterialsrebuild.Theyieldstressmeasuredinapre-shearedsampleis
the“dynamicyield”,whiletheyieldstressmeasuredforanoriginallyundisturbedmaterialisthe
“staticyield”.Zeroingafterpre-shearingisperformedonthesampleishighlyrecommendedfor
everytest.Thenextsectionexplainshowtodothis.
TIP:SetPre-ShearSpeedandPre-ShearTimetozeroifnoPre-Shearisrequired.
Zero
Anoptional,buthighlyrecommended,torqueZerostepcanbeincludedinthetestparameters.A
zerospeedwillcausetherheometertorotatethemotorintheappropriatedirectionuntil0%torque
isachieved.
Thismaybenecessarybecausethespindlesometimestwistsasmallamountduringinsertioninto
thesample.Thisresultsinasmall,althoughpossiblysignicant,torqueappliedtothesample.
Slowzeroingspeedsareusedtoeliminatethisinitialtorqueandminimizeeffectsonthesample’s
structurebeforethetestisstarted.
Zeroingisanessentialstepafterpre-shearing.
Azerostepensuresaconsistentstartingpointforeachtest.Aspeedforthestepmustbesupplied
intheappropriatebox(0.01-0.50).Fasterspeedsachievea“zero”quickerbutmaycauseamore
variablestartingpoint.The DV3Twillresetzerospeedto0.01rpmwhenthe%Torquevalue
reaches0.20%Torque.
Wait Time
AnoptionalWaitTimestepcanbeincludedinthetestparameters.WaitTimecreatesatimedelay
afterzeroingbeforetheactualtestrunbegins.Duringthisdelay,themotorshaftwillbeatzero
(0)RPM.
Wait Time isthetimethesampleisallowedtorestbetweenthecompletionofzeroingandthestart
oftheyieldmeasurement.Somesamplesrebuildstructuremoreslowlythanothersaftershearing,
suchasduringhandling,pouringsampleintoabeaker,etc.Certainlow-viscositypaintsmayalso
havealowyieldstress.Waiting30seconds,forexample,afterimmersingthespindlemayallow
thesampletorebuild,producingamoreconsistenttestmethod.
Run Speed
Run SpeedisthemotorspeedfortheDV3Tat which the material is tested.  It is common for
materialstoappearstifferwhentestedathigherspeeds.Thatis,theslopeofthestress-vs.-strain
curveincreaseswithincreasingspeed.Thisisbecausethematerialstructurehaslesstimeinwhich
toreacttodissipatetheappliedstress.Increasingthespeedwill,inmostcases,increasetheyield
stressmeasuredbytheinstrument.Mostyieldtestsareconductedatrelativelylowspeeds(<1rpm)
tominimizeanyinertialeffectswhenusingvanespindles.Therangeofacceptablerunspeedsis
0.01-5.0RPM.