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3.2.4residualstress—permanentstressthatisresidentinaglassymatrix.Suchresidualstressmayresulteitherfromheattreatmentabovethestrainpointoftheglass,orfromdiffer-encesinthermalexpansionbetweentheglassmatrixandacord,appliedenamel,orACLdecoration.

3.2.4.1Discussion—Theresidualstressmaybemodi edeitherbyheattreatmentabovethestrainpoint,remeltingandhomogenizingtheglassmelt,orbyremovalofa red-onceramicorglassdecoration.Residualstresscausedbyionexchangemayonlyberelievedbyeitherreexchangingtheglasstoitsoriginalstate,removingtheexchangedglassfromthematrix,orbyremeltingtheexchangedglassandhomog-enizingtheresultingglassmelt.

3.2.5retardationcompensator—anopticaldevice,variantsofwhichareusedtoquantifytheopticalretardationproducedintransparentbirefringentmaterials,typicallypositionedbe-tweenthespecimenbeingevaluatedandtheanalyzer.4.SummaryofTestMethod

4.1Thistestmethodprovidesforthequantitativedetermi-nationofresidualstressesintransparentglassmatrixesbymeansofphotoelasticretardationcompensationprocedures.Compensationisachievedbyproducingaretardationnullorextinctioninthespecimenusingeitherrotating(11.2),bire-fringentquartzwedge(11.3),ortilting(11.4)opticalretarda-tioncompensators.

5.Signi canceandUse

5.1Thequalityandperformanceofanarticleofglasswaremaybeaffectednotonlybythepresenceofresidualstressesduetoheattreatmentabovethestrainpointintheware,butalsobyadditionalresidualstressescausedbydifferencesinthermalexpansionbetweentheglasssubstrate,andeithercord, red-onvitreousenamel,orACLdecoration.

5.2Theeffectsofthoseadditionalresidualcord,enamel,orACLstressesandtheresultingperformanceofsuchitemsmaybeevaluatedbyperformancetestprocedures.SuchevaluationsofenamelorACLstressesmayalsobeaccomplishedthroughthedeterminationofappropriatephysicalpropertiesofthedecorationandmatrixglass,orbyanalyticalmethods.

5.3Thistestmethodoffersadirectandconvenientmeansofdeterminingthemagnitudesandspatialdistributionsofre-sidualstresssystemsinglasssubstrates.Thetestmethodissimple,convenient,andquantitativelyaccurate.

5.4ThistestmethodisusefulinevaluatingthedegreeofcompatibilitybetweenthecoefficientofthermalexpansionofanenamelorACLappliedtoaglasssubstrate.

6.Apparatus

6.1Microscope,monocularorbinocularpolarizing,havingarotating,andpreferablygraduated,samplestage.Binocularmicroscopeheadsfrequentlycontainasecond,separatepolar-izingelementintendedtominimizeinternalre ections.Ifsuchabinocularmicroscopeisused,careshouldbetakentoensurethattheantire ectionpolarizingelementisremovedfromthe eldofview.Aneyepiececontainingmutuallyperpendicularorotherwiseeasilyreferencedcrosshairsshouldbeprovided.Forretardationdeterminationsusingrotatingcompensation

methods,thepolarizingmicroscopemustbeequippedwitharotatableanalyzerelement,havingascalegraduatedindegreesofrotation,capableofbeingreadtoatleast1°,andaquarter-waveplate,properlyindexed.

6.2WhiteLightSourceshouldbeprovided,togetherwithstrain-freeobjectivelensesyieldingoverallmagni cationsrangingtypicallyfrom25to1003.

6.3IrisDiaphragm,enablingcollimationofthelightbeamtransmittedthroughthespecimenbeingevaluated.

6.4Compensator, xedfull-waveretardation,commonlyreferredtoasasensitivetintplate,full-waveplate,orgypsumplate,havinga xedretardationvaluecenteredon565-nmwavelength.

6.5Compensator,3appropriatevariableretardation,usedtonullorcompensate,andtherebydetermine,themagnitudeofthestress-opticalretardationeffectproducedbytheresidualstressinducedintheglasssubstrate.Variablecompensatorsmaybeused.

6.5.1Wedge,graduatedbirefringent,ofcontinuouslyvary-ingthickness,typicallymadeofcrystallinequartz,calibratedtoyieldretardationvaluesdirectlyandcoveringarangeoffourtosixordersofretardation,orapproximatelyfrom2200to3300-nmtotalretardation.

6.5.2TiltingCompensator,4typicallycapableofallowingdeterminationof veordersofretardation.

6.5.3RotatingCompensator,5typicallyallowingadeter-minationofretardationofoneorderoronewavelengthinmagnitudetobedetermined.Amonochromatizing lterisusuallyprovidedbytherotatingcompensatormanufacturer.Careshouldbetakentousetheappropriatematching lterfortheparticularrotatingcompensatorbeingused.

6.6DataConversionTables—Thelattertwotiltingandrotatingvariablecompensatortypesproviderawdataintheformofanglesofrotation,fromwhichretardationdatamaybeobtainedthroughtheuseofconversiontablesprovidedbythemanufacturer,speci ctotheparticularrotatingcompensatorbeingused.

6.7GlassImmersionDish,strain-free, atbottomed,ofsufficientdiametertoconveniently tonthemicroscopestage.Theimmersiondishshouldnot,inandofitself,addanysigni cantopticalretardationtothe eldofview.Thedishshouldbeofsufficientdepthtoenablethespecimensectionbeingevaluatedtobecompletelyimmersedinanindexofrefractionmatchingimmersion uid.

6.8SuitableImmersionFluid,havinganindexofrefractionmatchingthatoftheglasssubstratebeingevaluated,generallytowithin60.01unitsinrefractiveindexasmentionedinTestMethodF218.

6.9SampleHolder,toorientandmaintaintheplanesofstressatthepointofinterest(POI),paralleltotheopticalcolumnofthemicroscope,ifthegeometryofthespecimensectionissuchthattheplanesofstresstobeexamineddonotinitiallyparalleltheopticalaxisofthemicroscope.

3

CompensatorsofSenarmont,Berek,andFriedeltypeandgraduatedquartzcompensatorshavebeenfoundsuitableforthispurpose.4

AcompensatoroftheBerektypehasbeenfoundsatisfactoryforthispurpose.5

CompensatorsoftheFriedelorSenarmonttypehavebeenfoundsatisfactoryforthis

purpose.