ASTM C978-04(2009)
(Test Method)Standard Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures
Standard Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures
SIGNIFICANCE AND USE
The quality and performance of an article of glassware may be affected not only by the presence of residual stresses due to heat treatment above the strain point in the ware, but also by additional residual stresses caused by differences in thermal expansion between the glass substrate, and either cord, fired-on vitreous enamel, or ACL decoration.
The effects of those additional residual cord, enamel, or ACL stresses and the resulting performance of such items may be evaluated by performance test procedures. Such evaluations of enamel or ACL stresses may also be accomplished through the determination of appropriate physical properties of the decoration and matrix glass, or by analytical methods.
This test method offers a direct and convenient means of determining the magnitudes and spatial distributions of residual stress systems in glass substrates. The test method is simple, convenient, and quantitatively accurate.
This test method is useful in evaluating the degree of compatibility between the coefficient of thermal expansion of an enamel or ACL applied to a glass substrate.
SCOPE
1.1 This test method covers the determination of residual stresses in a transparent glass matrix by means of a polarizing microscope using null or retardation compensation procedures.
1.2 Such residual stress determinations are of importance in evaluating the nature and degree of residual stresses present in glass matrixes due to cord, or the degree of fit, or suitability of a particular combination of glass matrix and enamel, or applied color label (ACL).
1.3 The retardation compensation method of optically determining and evaluating enamel or ACL residual stress systems offers distinct advantages over methods requiring physical property measurements or ware performance tests due to its simplicity, reproducibility, and precision.
1.4 Limitations—This test method is based on the stress-optical retardation compensation principle, and is therefore applicable only to transparent glass substrates, and not to opaque glass systems.
1.5 Due to the possibility of additional residual stresses produced by ion exchange between glasses of different compositions, some uncertainty may be introduced in the value of the stress optical coefficient in the point of interest due to a lack of accurate knowledge of chemical composition in the areas of interest.
1.6 This test method is quantitatively applicable to and valid only for those applications where such significant ion exchange is not a factor, and stress optical coefficients are known or determinable.
1.7 The extent of the ion exchange process, and hence the magnitudes of the residual stresses produced due to ion exchange will depend on the exchange process parameters. The residual stress determinations made on systems in which ion exchange has occurred should be interpreted with those dependencies in mind.
1.8 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: C978 − 04(Reapproved 2009)
Standard Test Method for
Photoelastic Determination of Residual Stress in a
Transparent Glass Matrix Using a Polarizing Microscope
and Optical Retardation Compensation Procedures
This standard is issued under the fixed designation C978; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope residual stress determinations made on systems in which ion
exchangehasoccurredshouldbeinterpretedwiththosedepen-
1.1 This test method covers the determination of residual
dencies in mind.
stresses in a transparent glass matrix by means of a polarizing
1.8 The values stated in SI units are to be regarded as the
microscopeusingnullorretardationcompensationprocedures.
standard. The values given in parentheses are for information
1.2 Such residual stress determinations are of importance in
only.
evaluating the nature and degree of residual stresses present in
1.9 This standard does not purport to address all of the
glass matrixes due to cord, or the degree of fit, or suitability of
safety concerns, if any, associated with its use. It is the
aparticularcombinationofglassmatrixandenamel,orapplied
responsibility of the user of this standard to establish appro-
color label (ACL).
priate safety and health practices and determine the applica-
1.3 The retardation compensation method of optically de-
bility of regulatory limitations prior to use.
termining and evaluating enamel or ACL residual stress sys-
tems offers distinct advantages over methods requiring physi-
2. Referenced Documents
calpropertymeasurementsorwareperformancetestsduetoits
2.1 ASTM Standards:
simplicity, reproducibility, and precision.
C162Terminology of Glass and Glass Products
1.4 Limitations—This test method is based on the stress-
C770Test Method for Measurement of Glass Stress—
optical retardation compensation principle, and is therefore
Optical Coefficient
applicable only to transparent glass substrates, and not to
E691Practice for Conducting an Interlaboratory Study to
opaque glass systems.
Determine the Precision of a Test Method
F218Test Method for Measuring Optical Retardation and
1.5 Due to the possibility of additional residual stresses
Analyzing Stress in Glass
produced by ion exchange between glasses of different
compositions,someuncertaintymaybeintroducedinthevalue
3. Terminology
of the stress optical coefficient in the point of interest due to a
lack of accurate knowledge of chemical composition in the 3.1 Definitions:
areas of interest. 3.1.1 For additional definitions of terms used in this test
method, refer to Terminology C162.
1.6 Thistestmethodisquantitativelyapplicabletoandvalid
3.1.2 cord—an attenuated glassy inclusion possessing opti-
onlyforthoseapplicationswheresuchsignificantionexchange
calandotherpropertiesdifferingfromthoseofthesurrounding
is not a factor, and stress optical coefficients are known or
glass.
determinable.
3.2 Definitions of Terms Specific to This Standard:
1.7 The extent of the ion exchange process, and hence the
3.2.1 analyzer—a polarizing element, typically positioned
magnitudes of the residual stresses produced due to ion
between the specimen being evaluated and the viewer.
exchangewilldependontheexchangeprocessparameters.The
3.2.2 applied color label (ACL)—vitrifiable glass color
decoration or enamel applied to and fused on a glass surface.
This test method is under the jurisdiction of ASTM Committee C14 on Glass
and Glass Products and is the direct responsibility of Subcommittee C14.10 on
Glass Decoration. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2009. Published September 2009. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1987. Last previous edition approved in 2004 as C978-04. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C0978-04R09. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C978 − 04 (2009)
3.2.3 polarizer—an optical assembly that transmits light that the antireflection polarizing element is removed from the
vibrating in a single planar direction, typically positioned field of view. An eyepiece containing mutually perpendicular
between a light source and the specimen being evaluated. or otherwise easily referenced crosshairs should be provided.
For retardation determinations using rotating compensation
3.2.4 residual stress—permanent stress that is resident in a
methods, the polarizing microscope must be equipped with a
glassy matrix. Such residual stress may result either from heat
rotatableanalyzerelement,havingascalegraduatedindegrees
treatment above the strain point of the glass, or from differ-
of rotation, capable of being read to at least 1°, and a
ences in thermal expansion between the glass matrix and a
quarter-wave plate, properly indexed.
cord, applied enamel, or ACL decoration.
3.2.4.1 Discussion—The residual stress may be modified
6.2 White Light Source should be provided, together with
either by heat treatment above the strain point, remelting and
strain-free objective lenses yielding overall magnifications
homogenizing the glass melt, or by removal of a fired-on
ranging typically from 25 to 100×.
ceramic or glass decoration. Residual stress caused by ion
6.3 Iris Diaphragm, enabling collimation of the light beam
exchange may only be relieved by either reexchanging the
transmitted through the specimen being evaluated.
glass to its original state, removing the exchanged glass from
the matrix, or by remelting the exchanged glass and homog-
6.4 Compensator, fixed full-wave retardation, commonly
enizing the resulting glass melt.
referred to as a sensitive tint plate, full-wave plate, or gypsum
plate, having a fixed retardation value centered on 565-nm
3.2.5 retardation compensator—an optical device, variants
wavelength.
of which are used to quantify the optical retardation produced
in transparent birefringent materials, typically positioned be-
6.5 Compensator, appropriate variable retardation, used to
tween the specimen being evaluated and the analyzer.
null or compensate, and thereby determine, the magnitude of
the stress-optical retardation effect produced by the residual
4. Summary of Test Method
stress induced in the glass substrate. Variable compensators
4.1 This test method provides for the quantitative determi-
may be used.
nation of residual stresses in transparent glass matrixes by
6.5.1 Wedge, graduated birefringent, of continuously vary-
means of photoelastic retardation compensation procedures.
ingthickness,typicallymadeofcrystallinequartz,calibratedto
Compensation is achieved by producing a retardation null or
yieldretardationvaluesdirectlyandcoveringarangeoffourto
extinction in the specimen using either rotating (11.2), bire-
six orders of retardation, or approximately from 2200 to
fringent quartz wedge (11.3), or tilting (11.4) optical retarda-
3300-nm total retardation.
tion compensators.
6.5.2 Tilting Compensator, typically capable of allowing
determination of five orders of retardation.
5. Significance and Use
6.5.3 Rotating Compensator, typically allowing a determi-
5.1 The quality and performance of an article of glassware
nation of retardation of one order or one wavelength in
may be affected not only by the presence of residual stresses
magnitude to be determined. A monochromatizing filter is
due to heat treatment above the strain point in the ware, but
usually provided by the rotating compensator manufacturer.
also by additional residual stresses caused by differences in
Care should be taken to use the appropriate matching filter for
thermalexpansionbetweentheglasssubstrate,andeithercord,
the particular rotating compensator being used.
fired-on vitreous enamel, or ACL decoration.
6.6 Data Conversion Tables—The latter two tilting and
5.2 The effects of those additional residual cord, enamel, or
rotating variable compensator types provide raw data in the
ACLstresses and the resulting performance of such items may
form of angles of rotation, from which retardation data may be
beevaluatedbyperformancetestprocedures.Suchevaluations
obtained through the use of conversion tables provided by the
of enamel orACL stresses may also be accomplished through
manufacturer, specific to the particular rotating compensator
the determination of appropriate physical properties of the
being used.
decoration and matrix glass, or by analytical methods.
6.7 Glass Immersion Dish, strain-free, flat bottomed, of
5.3 Thistestmethodoffersadirectandconvenientmeansof
sufficient diameter to conveniently fit on the microscope stage.
determining the magnitudes and spatial distributions of re-
The immersion dish should not, in and of itself, add any
sidual stress systems in glass substrates. The test method is
significant optical retardation to the field of view. The dish
simple, convenient, and quantitatively accurate.
should be of sufficient depth to enable the specimen section
5.4 This test method is useful in evaluating the degree of
being evaluated to be completely immersed in an index of
compatibility between the coefficient of thermal expansion of
refraction matching immersion fluid.
an enamel or ACL applied to a glass substrate.
6.8 Suitable Immersion Fluid, having an index of refraction
matching that of the glass substrate being evaluated, generally
6. Apparatus
to within 60.01 units in refractive index as mentioned in Test
6.1 Microscope, monocular or binocular polarizing, having
F218.
Method
a rotating, and preferably graduated, sample stage. Binocular
microscope heads frequently contain a second, separate polar- 6.9 Sample Holder, to orient and maintain the planes of
izingelementintendedtominimizeinternalreflections.Ifsuch stress at the point of interest (POI), parallel to the optical
a binocular microscope is used, care should be taken to ensure column of the microscope, if the geometry of the specimen
C978 − 04 (2009)
section is such that the planes of stress to be examined do not procedure. The microscope eyepiece should contain a pair of
initially parallel the optical axis of the microscope. mutually perpendicular or otherwise easily referenced
crosshairs.
6.10 Means of Preparing the Section Containing the POI to
9.2 Orienttheeyepiecesuchthatoneorbothoftheeyepiece
be Analyzed, such as an abrasive or diamond-impregnated
cutoff wheel, or a hot wire bottle-cutting apparatus. Care crosshairs parallel the 45° diagonal positions in the field of
view. The crosshairs will be used to orient the sections for
should be taken to ensure that the section is not heated during
which retardation determinations are to be made.
cutting so as to affect the residual stress distribution in the
specimen section.
9.3 The microscope polarizing element should be oriented
intheopticalcolumnat0°orinanEast-West(E-W)alignment,
6.11 Means of Physically Measuring the Optical Path
while the analyzer should be set in the field of view at 90° or
Length, paralleling the stress planes through the thickness of
a North-South (N-S) alignment, perpendicular to the polarizer.
the section containing the POI to within 0.03 mm (0.001 in.).
The microscope field of view should be at maximum darkness
7. Sampling
or extinction at this point if the polarizing elements are
properly oriented, that is, mutually perpendicular to one
7.1 Thetestspecimensmaybesectionscutfromappropriate
another with no compensator installed.
locations containing areas of interest to be evaluated in
production sampled articles of commerce, fired decorated or 9.4 If the field of view should not be at maximum darkness
enameled ware, or laboratory specimens especially prepared
or extinction, the less-than-dark or brightened field indicates
for evaluation. that the polarizing elements are not mutually perpendicular.
The East-West alignment of the polarizer should be checked
8. Test Specimens
and then the analyzer should be rotated to a mutually perpen-
dicular alignment with the polarizer, a position where the field
8.1 Ensure that the test specimen is appropriately annealed,
of view is at its darkest, extinction position.
in that retardation due to inappropriate annealing could affect
the retardation due to the stress systems being evaluated at the
9.5 Oninsertionofafixed,sensitivetintplateorafull-wave
POI.
retardation plate in the microscope accessory slot, which plate
isalignedat45°betweenproperlycrossedpolarizingelements,
NOTE 1—To ensure proper annealing, determine the stress-optical
the darkened extinction field of view should then become
retardationinacomparablereferenceareaofthetestspecimenawayfrom
the POI, free of ACL and other residual stress sources. Proper annealing
reddish-purple or magenta in color.
shouldresultinminimalretardationduetoannealingstressintheselected
10. Calibration and Standardization
reference area.
8.2 Cut a section, of generally not less than 2.0 mm (0.08 10.1 For microscopes and compensators that are not
factory-standardized to determine the optical sign of stresses,
in.) and not more than 30.0 mm (1.18 in.) in optical path
length, from the portion of the ware containing the POI. The thesenseofthestressesbeingevaluated,thatis,theirtensileor
sectionmaythenconsistofabar,aring,orotherappropriately compressive nature, must be established for the particular
shaped section. microscope being used with either a sensitive tint plate or
8.2.1 In the case of ring section specimens, especially those full-wave fixed retardation compensator installed in the micro-
usedforcord,vitreousenamel,orACLstressevaluations,open scope column accessory slot between crossed polarizers. This
the ring section with a vertical saw cut to form a narrow kerf, may be accomplished, for instance, by positioning a well-
relieving whatever architectural stresses may be present in the annealed split ring section, containing a saw cut or kerf, in the
section. field of view as shown in Fig. 1. A bar section, or other
8.2.2 Care should be taken to ensure that both cut section calibration section, may be similarly bent producing an iden-
surfaces are parallel to each other, and are perpendicular to the tical effect.
optical path length of the section paralleling the planes of
NOTE 2—The calibration section used should have stress-optical retar-
residual stress in the POI being evaluated.
dation characteristics similar to the section being evaluated.
8.3 If the sections being cut contain high magnitu
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:C978–02 Designation:C978–04 (Reapproved 2009)
Standard Test Method for
Photoelastic Determination of Residual Stress in a
Transparent Glass Matrix Using a Polarizing Microscope
and Optical Retardation Compensation Procedures
This standard is issued under the fixed designation C 978; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of residual stresses in a transparent glass matrix by means of a polarizing
microscope using null or retardation compensation procedures.
1.2 Suchresidualstressdeterminationsareofimportanceinevaluatingthenatureanddegreeofresidualstressespresentinglass
matrixes due to cord, or the degree of fit, or suitability of a particular combination of glass matrix and enamel, or applied color
label (ACL).
1.3 The retardation compensation method of optically determining and evaluating enamel orACLresidual stress systems offers
distinct advantages over methods requiring physical property measurements or ware performance tests due to its simplicity,
reproducibility, and precision.
1.4 Limitations—This test method is based on the stress-optical retardation compensation principle, and is therefore applicable
only to transparent glass substrates, and not to opaque glass systems.
1.5 Due to the possibility of additional residual stresses produced by ion exchange between glasses of different compositions,
some uncertainty may be introduced in the value of the stress optical coefficient in the point of interest due to a lack of accurate
knowledge of chemical composition in the areas of interest.
1.6 This test method is quantitatively applicable to and valid only for those applications where such significant ion exchange
is not a factor, and stress optical coefficients are known or determinable.
1.7 The extent of the ion exchange process, and hence the magnitudes of the residual stresses produced due to ion exchange
will depend on the exchange process parameters. The residual stress determinations made on systems in which ion exchange has
occurred should be interpreted with those dependencies in mind.
1.8 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
C 162 Terminology of Glass and Glass Products
C 770 Test Method for Measurement of Glass Stress-Optical Coefficient
E 691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
F 218 TestMethodforAnalyzingStressinGlass TestMethodforMeasuringOpticalRetardationandAnalyzingStressinGlass
3. Terminology
3.1 Definitions—For additional definitions of terms used in this test method, refer to Terminology C 162.
3.1.1 cord—an attenuated glassy inclusion possessing optical and other properties differing from those of the surrounding glass.
C162
3.2 Definitions of Terms Specific to This Standard:
3.2.1 residual stress—permanent stress that is resident in a glassy matrix. Such residual stress may result either from heat
This test method is under the jurisdiction of ASTM Committee C14 on Glass and Glass Products and is the direct responsibility of Subcommittee C14.10 on Glass
Decoration.
Current edition approved Apr. 10, 2002. Published May 2002. Originally published as C978-87. Last previous edition C978-87 (1996).
Current edition approved May 1, 2009. Published September 2009. Originally approved in 1987. Last previous edition approved in 2004 as C 978 - 04.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 15.02.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C978–04 (2009)
treatment above the strain point of the glass, or from differences in thermal expansion between the glass matrix and a cord, applied
enamel, or ACL decoration.
3.2.1.1Discussion—The residual stress may be modified either by heat treatment above the strain point, remelting and
homogenizing the glass melt, or by removal of a fired-on ceramic or glass decoration. Residual stress caused by ion exchange may
only be relieved by either reexchanging the glass to its original state, removing the exchanged glass from the matrix, or by
remelting the exchanged glass and homogenizing the resulting glass melt. analyzer—a polarizing element, typically positioned
between the specimen being evaluated and the viewer.
3.2.2 applied color label (ACL)—vitrifiable glass color decoration or enamel applied to and fused on a glass surface.
3.2.3 polarizer—an optical assembly that transmits light vibrating in a single planar direction, typically positioned between a
light source and the specimen being evaluated.
3.2.4 residual stress—permanent stress that is resident in a glassy matrix. Such residual stress may result either from heat
treatment above the strain point of the glass, or from differences in thermal expansion between the glass matrix and a cord, applied
enamel, or ACL decoration.
3.2.4.1 Discussion—The residual stress may be modified either by heat treatment above the strain point, remelting and
homogenizing the glass melt, or by removal of a fired-on ceramic or glass decoration. Residual stress caused by ion exchange may
only be relieved by either reexchanging the glass to its original state, removing the exchanged glass from the matrix, or by
remelting the exchanged glass and homogenizing the resulting glass melt.
3.2.5 retardation compensator—an optical device, variants of which are used to quantify the optical retardation produced in
transparent birefringent materials, typically positioned between the specimen being evaluated and the analyzer.
3.2.5analyzer—a polarizing element, typically positioned between the specimen being evaluated and the viewer.
4. Summary of Test Method
4.1 This test method provides for the quantitative determination of residual stresses in transparent glass matrixes by means of
photoelastic retardation compensation procedures. Compensation is achieved by producing a retardation null or extinction in the
specimen using either rotating (11.2), birefringent quartz wedge (11.3), or tilting (11.4) optical retardation compensators.
5. Significance and Use
5.1 The quality and performance of an article of glassware may be affected not only by the presence of residual stresses due
to heat treatment above the strain point in the ware, but also by additional residual stresses caused by differences in thermal
expansion between the glass substrate, and either cord, fired-on vitreous enamel, or ACL decoration.
5.2 The effects of those additional residual cord, enamel, orACL stresses and the resulting performance of such items may be
evaluated by performance test procedures. Such evaluations of enamel or ACL stresses may also be accomplished through the
determination of appropriate physical properties of the decoration and matrix glass, or by analytical methods.
5.3 This test method offers a direct and convenient means of determining the magnitudes and spatial distributions of residual
stress systems in glass substrates. The test method is simple, convenient, and quantitatively accurate.
5.4 This test method is useful in evaluating the degree of compatibility between the coefficient of thermal expansion of an
enamel or ACL applied to a glass substrate.
6. Apparatus
6.1 Microscope, monocular or binocular polarizing, having a rotating, and preferably graduated, sample stage. Binocular
microscope heads frequently contain a second, separate polarizing element intended to minimize internal reflections. If such a
binocular microscope is used, care should be taken to ensure that the antireflection polarizing element is removed from the field
of view. An eyepiece containing mutually perpendicular or otherwise easily referenced crosshairs should be provided. For
retardation determinations using rotating compensation methods, the polarizing microscope must be equipped with a rotatable
analyzer element, having a scale graduated in degrees of rotation, capable of being read to at least 1°, and a quarter-wave plate,
properly indexed.
6.2 White Light Source should be provided, together with strain-free objective lenses yielding overall magnifications ranging
typically from 25 to 1003.
6.3 Iris Diaphragm, enabling collimation of the light beam transmitted through the specimen being evaluated.
6.4 Compensator, fixed full-wave retardation, commonly referred to as a sensitive tint plate, full-wave plate, or gypsum plate,
having a fixed retardation value centered on 565-nm wavelength.
6.5 Compensator, appropriate variable retardation, used to null or compensate, and thereby determine, the magnitude of the
stress-optical retardation effect produced by the residual stress induced in the glass substrate. Variable compensators may be used.
6.5.1 Wedge, graduated birefringent, of continuously varying thickness, typically made of crystalline quartz, calibrated to yield
retardation values directly and covering a range of four to six orders of retardation, or approximately from 2200 to 3300-nm total
retardation.
6.5.2 Tilting Compensator, typically capable of allowing determination of five orders of retardation.
6.5.3 Rotating Compensator, typically allowing a determination of retardation of one order or one wavelength in magnitude to
be determined.Amonochromatizing filter is usually provided by the rotating compensator manufacturer. Care should be taken to
C978–04 (2009)
use the appropriate matching filter for the particular rotating compensator being used.
6.6 Data Conversion Tables—The latter two tilting and rotating variable compensator types provide raw data in the form of
anglesofrotation,fromwhichretardationdatamaybeobtainedthroughtheuseofconversiontablesprovidedbythemanufacturer,
specific to the particular rotating compensator being used.
6.7 Glass Immersion Dish, strain-free, flat bottomed, of sufficient diameter to conveniently fit on the microscope stage. The
immersion dish should not, in and of itself, add any significant optical retardation to the field of view. The dish should be of
sufficient depth to enable the specimen section being evaluated to be completely immersed in an index of refraction matching
immersion fluid.
6.8 Suitable Immersion Fluid, having an index of refraction matching that of the glass substrate being evaluated, generally to
within 60.01 units in refractive index as mentioned in Test Method F 218.
6.9 Sample Holder, to orient and maintain the planes of stress at the point of interest (POI), parallel to the optical column of
the microscope, if the geometry of the specimen section is such that the planes of stress to be examined do not initially parallel
the optical axis of the microscope.
6.10 Means of Preparing the Section Containing the POI to be Analyzed, such as an abrasive or diamond-impregnated cutoff
wheel, or a hot wire bottle-cutting apparatus. Care should be taken to ensure that the section is not heated during cutting so as to
affect the residual stress distribution in the specimen section.
6.11 Means of Physically Measuring the Optical Path Length, paralleling the stress planes through the thickness of the section
containing the POI to within 0.03 mm (0.001 in.).
7. Sampling
7.1 The test specimens may be sections cut from appropriate locations containing areas of interest to be evaluated in production
sampled articles of commerce, fired decorated or enameled ware, or laboratory specimens especially prepared for evaluation.
8. Test Specimens
8.1 Ensure that the test specimen is appropriately annealed, in that retardation due to inappropriate annealing could affect the
retardation due to the stress systems being evaluated at the POI.
NOTE 1—To ensure proper annealing, determine the stress-optical retardation in a comparable reference area of the test specimen away from the POI,
free ofACLand other residual stress sources. Proper annealing should result in minimal retardation due to annealing stress in the selected reference area.
8.2 Cut a section, of generally not less than 2.0 mm (0.08 in.) and not more than 30.0 mm (1.18 in.) in optical path length, from
the portion of the ware containing the POI. The section may then consist of a bar, a ring, or other appropriately shaped section.
8.2.1 In the case of ring section specimens, especially those used for cord, vitreous enamel, orACLstress evaluations, open the
ring section with a vertical saw cut to form a narrow kerf, relieving whatever architectural stresses may be present in the section.
8.2.2 Care should be taken to ensure that both cut section surfaces are parallel to each other, and are perpendicular to the optical
path length of the section paralleling the planes of residual stress in the POI being evaluated.
8.3 If the sections being cut contain high magnitudes of retardation at the POI, the cut section thickness may be decreased
proportionately from the thickness values listed in 8.2 to decrease the magnitude of retardation to be measured at the POI.
9. Preparation of Apparatus
9.1 Ensure that the microscope optical system is properly aligned and the objectives to be used in the examination are properly
centered. The objectives should be relatively low powered, 2.5 to 103 being used during the initial examination procedure. The
microscope eyepiece should contain a pair of mutually perpendicular or otherwise easily referenced crosshairs.
9.2 Orient the eyepiece such that one or both of the eyepiece crosshairs parallel the 45° diagonal positions in the field of view.
The crosshairs will be used to orient the sections for which retardation determinations are to be made.
9.3 Themicroscopepolarizingelementshouldbeorientedintheopticalcolumnat0°orinanEast-West(E-W)alignment,while
the anal
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