Brookfield DV3T Rheometer Manual do Utilizador descarregar pdf (Página 52)

Brookeld Engineering Labs., Inc. Page 52 Manual No. M13-2100

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.

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