ASTM C978-87(1996)

Standard Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures

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.

General Information

• Status: Historical
• Publication Date: 09-Apr-2002
• ICS: 81.040.10 - Raw materials and raw glass
• Technical Committee: C14 - Glass and Glass Products
• Drafting Committee: C14.10 - Glass Decoration

Relations

• Replaced By: ASTM C978-02 - Standard Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures
• Effective Date: 10-Apr-2002
• Referred By: ASTM C1426-14(2020) - Standard Practices for Verification and Calibration of Polarimeters
• Effective Date: 10-Apr-2002
• Referred By: ASTM C1048-18 - Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass
• Effective Date: 10-Apr-2002
• Referred By: ASTM F218-13(2020) - Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass
• Effective Date: 10-Apr-2002
• Referred By: ASTM C1422/C1422M-20a - Standard Specification for Chemically Strengthened Flat Glass
• Effective Date: 10-Apr-2002

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: C 978 – 87 (Reapproved 1996)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
1.1 This test method covers the determination of residual
priate safety and health practices and determine the applica-
stresses in a transparent glass matrix by means of a polarizing
bility of regulatory limitations prior to use.
microscope using null or retardation compensation procedures.
1.2 Such residual stress determinations are of importance in
2. Referenced Documents
evaluating the nature and degree of residual stresses present in
2.1 ASTM Standards:
glass matrixes due to cord, or the degree of fit, or suitability of
C 162 Terminology of Glass and Glass Products
a particular combination of glass matrix and enamel, or applied
C 770 Test Method for Measurement of Glass Stress-
color label (ACL).
Optical Coefficient
1.3 The retardation compensation method of optically de-
F 218 Test Method for Analyzing Stress in Glass
termining and evaluating enamel or ACL residual stress sys-
tems offers distinct advantages over methods requiring physi-
3. Terminology
cal property measurements or ware performance tests due to its
3.1 Definitions—For additional definitions of terms used in
simplicity, reproducibility, and precision.
this test method, refer to Terminology C 162.
1.4 Limitations—This test method is based on the stress-
3.1.1 cord—an attenuated glassy inclusion possessing opti-
optical retardation compensation principle, and is therefore
cal and other properties differing from those of the surrounding
applicable only to transparent glass substrates, and not to
glass. C 162
opaque glass systems.
3.2 Definitions of Terms Specific to This Standard:
1.5 Due to the possibility of additional residual stresses
3.2.1 residual stress—permanent stress that is resident in a
produced by ion exchange between glasses of different com-
glassy matrix. Such residual stress may result either from heat
positions, some uncertainty may be introduced in the value of
treatment above the strain point of the glass, or from differ-
the stress optical coefficient in the point of interest due to a lack
ences in thermal expansion between the glass matrix and a
of accurate knowledge of chemical composition in the areas of
cord, applied enamel, or ACL decoration.
interest.
3.2.1.1 Discussion—The residual stress may be modified
1.6 This test method is quantitatively applicable to and valid
either by heat treatment above the strain point, remelting and
only for those applications where such significant ion exchange
homogenizing the glass melt, or by removal of a fired-on
is not a factor, and stress optical coefficients are known or
ceramic or glass decoration. Residual stress caused by ion
determinable.
exchange may only be relieved by either reexchanging the
1.7 The extent of the ion exchange process, and hence the
glass to its original state, removing the exchanged glass from
magnitudes of the residual stresses produced due to ion
the matrix, or by remelting the exchanged glass and homog-
exchange will depend on the exchange process parameters. The
enizing the resulting glass melt.
residual stress determinations made on systems in which ion
3.2.2 applied color label (ACL)—vitrifiable glass color
exchange has occurred should be interpreted with those depen-
decoration or enamel applied to and fused on a glass surface.
dencies in mind.
3.2.3 polarizer—an optical assembly that transmits light
1.8 The values stated in SI units are to be regarded as the
vibrating in a single planar direction, typically positioned
standard. The values given in parentheses are for information
between a light source and the specimen being evaluated.
only.
3.2.4 retardation compensator—an optical device, variants
1.9 This standard does not purport to address all of the
of which are used to quantify the optical retardation produced
This test method is under the jurisdiction of ASTM Committee C-14 on Glass
and Glass Products and is the direct responsibility of Subcommittee C14.10 on
Glass Decoration. Annual Book of ASTM Standards, Vol 15.02.
Current edition approved July 31, 1987. Published September 1987. Annual Book of ASTM Standards, Vol 10.04.
C 978
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
3.2.5 analyzer—a polarizing element, typically positioned the stress-optical retardation effect produced by the residual
between the specimen being evaluated and the viewer. stress induced in the glass substrate. Variable compensators
may be used.
4. Summary of Test Method
6.5.1 Wedge, graduated birefringent, of continuously vary-
4.1 This test method provides for the quantitative determi-
ing thickness, typically made of crystalline quartz, calibrated to
nation of residual stresses in transparent glass matrixes by
yield retardation values directly and covering a range of four to
means of photoelastic retardation compensation procedures.
six orders of retardation, or approximately from 2200 to
Compensation is achieved by producing a retardation null or
3300-nm total retardation.
extinction in the specimen using either rotating (11.2), bire-
6.5.2 Tilting Compensator, typically capable of allowing
fringent quartz wedge (11.3), or tilting (11.4) optical retarda-
determination of five orders of retardation.
tion compensators. 6
6.5.3 Rotating Compensator, typically allowing a deter-
5. Significance and Use mination of retardation of one order or one wavelength in
magnitude to be determined. A monochromatizing filter is
5.1 The quality and performance of an article of glassware
usually provided by the rotating compensator manufacturer.
may be affected not only by the presence of residual stresses
Care should be taken to use the appropriate matching filter for
due to heat treatment above the strain point in the ware, but
the particular rotating compensator being used.
also by additional residual stresses caused by differences in
6.6 Data Conversion Tables—The latter two tilting and
thermal expansion between the glass substrate, and either cord,
rotating variable compensator types provide raw data in the
fired-on vitreous enamel, or ACL decoration.
form of angles of rotation, from which retardation data may be
5.2 The effects of those additional residual cord, enamel, or
obtained through the use of conversion tables provided by the
ACL stresses and the resulting performance of such items may
manufacturer, specific to the particular rotating compensator
be evaluated by performance test procedures. Such evaluations
being used.
of enamel or ACL stresses may also be accomplished through
6.7 Glass Immersion Dish, strain-free, flat bottomed, of
the determination of appropriate physical properties of the
sufficient diameter to conveniently fit on the microscope stage.
decoration and matrix glass, or by analytical methods.
The immersion dish should not, in and of itself, add any
5.3 This test method offers a direct and convenient means of
significant optical retardation to the field of view. The dish
determining the magnitudes and spatial distributions of re-
should be of sufficient depth to enable the specimen section
sidual stress systems in glass substrates. The test method is
being evaluated to be completely immersed in an index of
simple, convenient, and quantitatively accurate.
refraction matching immersion fluid.
5.4 This test method is useful in evaluating the degree of
6.8 Suitable Immersion Fluid, having an index of refraction
compatibility between the coefficient of thermal expansion of
matching that of the glass substrate being evaluated, generally
an enamel or ACL applied to a glass substrate.
to within 60.01 units in refractive index as mentioned in Test
6. Apparatus
Method F 218.
6.1 Microscope, monocular or binocular polarizing, having
6.9 Sample Holder, to orient and maintain the planes of
a rotating, and preferably graduated, sample stage. Binocular
stress at the point of interest (POI), parallel to the optical
microscope heads frequently contain a second, separate polar-
column of the microscope, if the geometry of the specimen
izing element intended to minimize internal reflections. If such
section is such that the planes of stress to be examined do not
a binocular microscope is used, care should be taken to ensure
initially parallel the optical axis of the microscope.
that the antireflection polarizing element is removed from the
6.10 Means of Preparing the Section Containing the POI to
field of view. An eyepiece containing mutually perpendicular
be Analyzed, such as an abrasive or diamond-impregnated
or otherwise easily referenced crosshairs should be provided.
cutoff wheel, or a hot wire bottle-cutting apparatus. Care
For retardation determinations using rotating compensation
should be taken to ensure that the section is not heated during
methods, the polarizing microscope must be equipped with a
cutting so as to affect the residual stress distribution in the
rotatable analyzer element, having a scale graduated in degrees
specimen section.
of rotation, capable of being read to at least 1°, and a
6.11 Means of Physically Measuring the Optical Path
quarter-wave plate, properly indexed.
Length, paralleling the stress planes through the thickness of
6.2 White Light Source should be provided, together with
the section containing the POI to within 0.03 mm (0.001 in.).
strain-free objective lenses yielding overall magnifications
7. Sampling
ranging typically from 25 to 100 3 .
6.3 Iris Diaphragm, enabling collimation of the light beam
7.1 The test specimens may be sections cut from appropriate
transmitted through the specimen being evaluated.
locations containing areas of interest to be evaluated in
6.4 Compensator, fixed full-wave retardation, commonly
referred to as a sensitive tint plate, full-wave plate, or gypsum
Compensators of Senarmont, Berek, and Friedel type and graduated quartz
plate, having a fixed retardation value centered on 565-nm
compensators have been found suitable for this purpose.
wavelength.
A compensator of the Berek type has been found satisfactory for this purpose.
Compensators of the Friedel or Senarmont type have been found satisfactory
for this purpose.
C 978
production sampled articles of commerce, fired decorated or that the polarizing elements are not mutually perpendicular.
enameled ware, or laboratory specimens especially prepared The East-West alignment of the polarizer should be checked
for evaluation. and then the analyzer should be rotated to a mutually perpen-
dicular alignment with the polarizer, a position where the field
8. Test Specimens
of view is at its darkest, extinction position.
8.1 Ensure that the test specimen is appropriately annealed,
9.5 On insertion of a fixed, sensitive tint plate or a full-wave
in that retardation due to inappropriate annealing could affect
retardation plate in the microscope accessory slot, which plate
the retardation due to the stress systems being evaluated at the
is aligned at 45° between properly crossed polarizing elements,
POI.
the darkened extinction field of view should then become
reddish-purple or magenta in color.
NOTE 1—To ensure proper annealing, determine the stress-optical
retardation in a comparable reference area of the test specimen away from
10. Calibration and Standardization
the POI, free of ACL and other residual stress sources. Proper annealing
should result in minimal retardation due to annealing stress in the selected
10.1 For microscopes and compensators that are not
reference area.
factory-standardized to determine the optical sign of stresses,
8.2 Cut a section, of generally not less than 2.0 mm (0.08
the sense of the stresses being evaluated, that is, their tensile or
in.) and not more than 30.0 mm (1.18 in.) in optical path
compressive nature, must be established for the particular
length, from the portion of the ware containing the POI. The microscope being used with either a sensitive tint plate or
section may then consist of a bar, a ring, or other appropriately
full-wave fixed retardation compensator installed in the micro-
shaped section. scope column accessory slot between crossed polarizers. This
8.2.1 In the case of ring section specimens, especially those
may be accomplished, for instance, by positioning a well-
used for cord, vitreous enamel, or ACL stress evaluations, open annealed split ring section, containing a saw cut or kerf, in the
the ring section with a vertical saw cut to form a narrow kerf,
field of view as shown in Fig. 1. A bar section, or other
relieving whatever architectural stresses may be present in the calibration section, may be similarly bent producing an iden-
section.
tical effect.
8.2.2 Care should be taken to ensure that both cut section
NOTE 2—The calibration section used should have stress-optical retar-
surfaces are parallel to each other, and are perpendicular to the
dation characteristics similar to the section being evaluated.
optical path length of the section paralleling the planes of
10.2 Orient the outer original surface of the section, directly
residual stress in the POI being evaluated.
opposite the kerf, to lie parallel to the diagonal Northeast-
8.3 If the sections being cut contain high magnitudes of
Southwest (NE-SW) direction in the field of view as seen in
retardation at the POI, the cut section thickness may be
Fig. 1(a).
decreased proportionately from the thickness values listed in
10.3 Gently squeeze the ring section across a diameter
8.2 to decrease the magnitude of retardation to be measured at
paralleling the NE-SW diagonal to produce a tensile stress on
the POI.
the original outside section surface at the region of interest
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 ob
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