ASTM C978-04(2019)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C978 − 04 (Reapproved 2019)
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, health, and environmental practices and deter-
1.3 The retardation compensation method of optically de-
mine the applicability of regulatory limitations prior to use.
termining and evaluating enamel or ACL residual stress sys-
1.10 This international standard was developed in accor-
tems offers distinct advantages over methods requiring physi-
dance with internationally recognized principles on standard-
calpropertymeasurementsorwareperformancetestsduetoits
ization established in the Decision on Principles for the
simplicity, reproducibility, and precision.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.4 Limitations—This test method is based on the stress-
Barriers to Trade (TBT) Committee.
optical retardation compensation principle, and is therefore
applicable only to transparent glass substrates, and not to
2. Referenced Documents
opaque glass systems.
2.1 ASTM Standards:
1.5 Due to the possibility of additional residual stresses
C162Terminology of Glass and Glass Products
produced by ion exchange between glasses of different
E691Practice for Conducting an Interlaboratory Study to
compositions,someuncertaintymaybeintroducedinthevalue
Determine the Precision of a Test Method
of the stress optical coefficient in the point of interest due to a
F218Test Method for Measuring Optical Retardation and
lack of accurate knowledge of chemical composition in the
Analyzing Stress in Glass
areas of interest.
3. Terminology
1.6 Thistestmethodisquantitativelyapplicabletoandvalid
3.1 Definitions:
onlyforthoseapplicationswheresuchsignificantionexchange
3.1.1 For additional definitions of terms used in this test
is not a factor, and stress optical coefficients are known or
method, refer to Terminology C162.
determinable.
3.1.2 cord—an attenuated glassy inclusion possessing opti-
1.7 The extent of the ion exchange process, and hence the
calandotherpropertiesdifferingfromthoseofthesurrounding
magnitudes of the residual stresses produced due to ion
glass.
exchangewilldependontheexchangeprocessparameters.The
3.2 Definitions of Terms Specific to This Standard:
3.2.1 analyzer—a polarizing element, typically positioned
between the specimen being evaluated and the viewer.
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 Feb. 1, 2019. Published February 2019. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1987. Last previous edition approved in 2014 as C978-04 (2014). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/C0978-04R19. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C978 − 04 (2019)
3.2.2 applied color label (ACL)—vitrifiable glass color microscope heads frequently contain a second, separate polar-
decoration or enamel applied to and fused on a glass surface. izingelementintendedtominimizeinternalreflections.Ifsuch
a binocular microscope is used, care should be taken to ensure
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.
3.2.4 residual stress—permanent stress that is resident in a
For retardation determinations using rotating compensation
glassy matrix. Such residual stress may result either from heat
methods, the polarizing microscope must be equipped with a
treatment above the strain point of the glass, or from differ-
rotatableanalyzerelement,havingascalegraduatedindegrees
ences in thermal expansion between the glass matrix and a
of rotation, capable of being read to at least 1°, and a
cord, applied enamel, or ACL decoration.
quarter-wave plate, properly indexed.
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
homogenizing the glass melt, or by removal of a fired-on strain-free objective lenses yielding overall magnifications
ceramic or glass decoration. Residual stress caused by ion ranging typically from 25 to 100×.
exchange may only be relieved by either reexchanging the
6.3 Iris Diaphragm, enabling collimation of the light beam
glass to its original state, removing the exchanged glass from
transmitted through the specimen being evaluated.
the matrix, or by remelting the exchanged glass and homog-
enizing the resulting glass melt. 6.4 Compensator, fixed full-wave retardation, commonly
referred to as a sensitive tint plate, full-wave plate, or gypsum
3.2.5 retardation compensator—an optical device, variants
plate, having a fixed retardation value centered on 565-nm
of which are used to quantify the optical retardation produced
wavelength.
in transparent birefringent materials, typically positioned be-
tween the specimen being evaluated and the analyzer.
6.5 Compensator, appropriate variable retardation, used to
null or compensate, and thereby determine, the magnitude of
4. Summary of Test Method
the stress-optical retardation effect produced by the residual
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.
Compensation is achieved by producing a retardation null or ingthickness,typicallymadeofcrystallinequartz,calibratedto
extinction in the specimen using either rotating (11.2), bire- yieldretardationvaluesdirectlyandcoveringarangeoffourto
fringent quartz wedge (11.3), or tilting (11.4) optical retarda- six orders of retardation, or approximately from 2200 to
tion compensators. 3300-nm total retardation.
6.5.2 Tilting Compensator, typically capable of allowing
5. Significance and Use
determination of five orders of retardation.
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.
5.2 The effects of those additional residual cord, enamel, or 6.6 Data Conversion Tables—The latter two tilting and
ACLstresses and the resulting performance of such items may
rotating variable compensator types provide raw data in the
beevaluatedbyperformancetestprocedures.Suchevaluations form of angles of rotation, from which retardation data may be
of enamel orACL stresses may also be accomplished through
obtained through the use of conversion tables provided by the
the determination of appropriate physical properties of the
manufacturer, specific to the particular rotating compensator
decoration and matrix glass, or by analytical methods.
being used.
5.3 Thistestmethodoffersadirectandconvenientmeansof
6.7 Glass Immersion Dish, strain-free, flat bottomed, of
determining the magnitudes and spatial distributions of re-
sufficient diameter to conveniently fit on the microscope stage.
sidual stress systems in glass substrates. The test method is
The immersion dish should not, in and of itself, add any
simple, convenient, and quantitatively accurate.
significant optical retardation to the field of view. The dish
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
6. Apparatus
matching that of the glass substrate being evaluated, generally
6.1 Microscope, monocular or binocular polarizing, having to within 60.01 units in refractive index as mentioned in Test
a rotating, and preferably graduated, sample stage. Binocular Method F218.
C978 − 04 (2019)
6.9 Sample Holder, to orient and maintain the planes of properly centered. The objectives should be relatively low
stress at the point of interest (POI), parallel to the optical powered, 2.5 to 10× being used during the initial examination
column of the microscope, if the geometry of the specimen procedure. The microscope eyepiece should contain a pair of
section is such that the planes of stress to be examined do not mutually perpendicular or otherwise easily referenced
initially parallel the optical axis of the microscope. crosshairs.
6.10 Means of Preparing the Section Containing the POI to
9.2 Orienttheeyepiecesuchthatoneorbothoftheeyepiece
be Analyzed, such as an abrasive or diamond-impregnated
crosshairs parallel the 45° diagonal positions in the field of
cutoff wheel, or a hot wire bottle-cutting apparatus. Care
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
9.3 The microscope polarizing element should be oriented
specimen section.
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
or extinction at this point if the polarizing elements are
7. Sampling
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
9.4 If the field of view should not be at maximum darkness
production sampled articles of commerce, fired decorated or
or extinction, the less-than-dark or brightened field indicates
enameled ware, or laboratory specimens especially prepared
that the polarizing elements are not mutually perpendicular.
for evaluation.
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
retardationinacomparablereferenceareaofthetestspecimenawayfrom
the darkened extinction field of view should then become
the POI, free of ACL and other residual stress sources. Proper annealing
reddish-purple or magenta in color.
shouldresultinminimalretardationduetoannealingstressintheselected
reference area.
10. Calibration and Standardization
8.2 Cut a section, of generally not less than 2.0 mm (0.08
10.1 For microscopes and compensators that are not
in.) and not more than 30.0 mm (1.18 in.) in optical path
factory-standardized to determine the optical sign of stresses,
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
residual stress in the POI being evaluated.
NOTE 2—The calibration section used should have stress-optical retar-
8.3 If the sections being cut contain high magnitudes of dation characteristics similar to the section being evaluated.
retardation at the POI, the cut section thickness may be
10.2 Orienttheouteroriginalsurfaceofthesection,directly
decreased proportionately from the thickness values listed in
opposite the kerf, to lie parallel to the diagonal Northeast-
8.2 to decrease the magnitude of retardation to be measured at
Southwest (NE-SW) direction in the field of view as seen in
the POI.
Fig. 1(a).
9. Preparation of Apparatus
10.3 Gently squeeze the ring section across a diameter
paralleling the NE-SW diagonal to produce a tensile stress on
9.1 Ensure that the microscope optical system is properly
aligned and the objectives to be used in the examination are the original outside section surface at the region of interest
(POI) at Point A. A simultaneous compressive stress will be
generatedontheinsidesectionsurfacenearthePOIatPoint B,
“Polariscopic Examination of Glass Container Sections,” Journal of the
American Ceramic Society, Vol 27, No. 3, March, 1944. directly opposite Point A on the tensile surface.
C978 − 04 (2019)
10.10 TypicalspecimensectionPOIorientations,relativeto
the particular compensator slow-wave direction necessary to
provide proper blue- and orange-position locations for the
variablecompensatorsdescribedinthistestmethod,areshown
in Table 1.
10.11 The particular diagonal specimen POI orientation
correspondingtothebluepositioninaspecificquadrantwithin
the field of view may vary for different microscopes, depend-
ing on the particular orientation of the polarizing elements and
compensators being used. Therefore the specimen-section
positioningproceduresoutlinedin10.2through10.8shouldbe
(a) Blue Position
periodically checked and reaffirmed.
11. Procedure
11.1 Specimen Orientation—Rotate the graduated micro-
scope stage containing the specimen section so that the POI in
the specimen section to be evaluated is in a N-S orientation.
11.1.1 The specimen section POI containing the residual
stresssystemtobeanalyzedshouldbeuniformlydark,withthe
fixed compensator removed from the field, as should the
background field of view exterior to the specimen. The
specimen POI is said to be in the EXTINCTION position in
(b) Orange Position
this orientation.
FIG. 1 Split Ring Section Used in Establishing Stress Sense and
Proper Specimen Orientation
TABLE 1 Retardation Color Equivalents With and Without
Sensitive Tint Plate (Observed Color in Flint Soda-Lime-Silica
A
Glass Only)
10.4 Note and record the specimen POI orientation relative
NOTE 1—Letters a through o indicate the most distinctive colors for
to the two diagonal positions, and the retardation color pro-
various ranges. When using the tint plate in the orange position, if the
duced on the outside tensile surface of the section at Point A
colorappearstofallbetween cand e,reorientthePOItotheblueposition,
with the sensitive tint plate installed in the microscope.
and verify that the retardation color at the POI is indeed d.
10.5 Rotate the section 90° clockwise, such that the POI on
NOTE 2—The retardation colors indicated in the table are referenced
only to transparent colorless flint soda-lime-silica glasses.
the outer original section surface at Point A, opposite the saw
With 565-nm Sensitive Tint Plate
kerf, is now oriented parallel to the Northwest-Southeast
Equivalent
(NW-SE) diagonal in the field of view as seen in Fig. 1(b).
Blue Position Orange Position
Retardation, nm
10.6 Gentlysqueezethesectioninadirectionparallelingthe
violet-red violet-red 0
NW-SEdiagonalandagainnoteandrecordthePOIorientation
violet-blue (a) red (a) 20
(b) dark blue red-orange (b) 35
andtheretardationcolorproducedontheoutsidesurfaceofthe
blue (c) orange (c) 75
section due to the tensile stress at Point A.
(d) blue-green orange-yellow (d) 120
deep green (e) gold-yellow (e) 150
10.7 The blue position is defined as that specimen POI
(f) green yellow (f) 180
orientation parallel to which a planar tensile stress of sufficient
pale green (g) pale yellow (g) 220
magnitude will be revealed by a bluish retardation color, (h) yellowish green yellow-white (h) 255
greenish yellow (i) white (i) 290
between crossed polarizers with a sensitive tint plate or
pale yellow gray-white 330
full-wave compensator installed. A compressive stress, of
sufficient and equal magnitude, will be revealed by an orangy
Without 565-nm Sensitive Tint Plate
retardation color in the same blue specimen position.
Equivalent
Orange position
Retardation, nm
10.8 When the specimen section POI is then rotated 90°
black 0
from the blue position to the position where its outside surface
gray (various shades) up to 255
parallels the diagonal position opposite the blue position, that
gray-yellow 290
same tensile stress will appear as an orangy retardation color,
(j) dirty yellow (j) 330
(k) dirty brown (k) 380
hence the name, orange position. The corresponding compres-
(l) brown-orange (l) 440
sive stress, of sufficient and equal magnitude, will now appear
(m) brown-red (m) 480
as a bluish retardation color in the orange position. (n) violet-red (n) 565
(o) blue-green (o) 675
10.9 Retardation readings should be referenced to the par-
A
“Polariscopic Examination of Glass Container Sections,” Journal of the Ameri-
ticular position, that is, blue or orange position, in which the
can Ceramic Society, Vol 27, Number 3, March 1944.
retardation readings were made.
C978 − 04 (2019)
11.1.2 ThePOIshouldexhibitcompleteextinctiononbeing
rotated to successive 90° positions relative to the initial N-S
POI orientation.
11.1.3 Rotate the microscope stage bearing the specimen
section POI exactly 45° clockwise using either the graduated
rotating stage scale or the eyepiece crosshairs as references
from the N-S orientation achieved in 11.1, to put the specimen
section surface containing the POI to be analyzed in a
DIAGONAL (NE-SW) orientation.
11.1.4 The POI should exhibit its maximum brightness or
highest order retardation color in this position.
11.1.5 Rotate the specimen section 90° counterclockwise to
the opposite diagonal position, that is, paralleling the DIAGO-
NAL (NW-SE) orientation.
11.1.6 Observe and note the orientation and the retardation
color seen in the specimen POI, both with and without the
sensitive tint plate installed, in both orientations.
11.1.7 The complementary retardation colors observed in
the POI when oriented in opposing diagonal positions, both
with and without the tint plate installed, may be used in FIG. 2 Orientation of the Fixed Retardation Compensator Slow-
Wave Vibration Direction
conjunction with Table 1 to qualitatively determine retardation
values corresponding to various retardation colors observed in
the POI in colorless or flint soda-lime-silica glass only.
variable compensator being used. The retardation produced in
11.1.8 Table 1 may be used to obtain an initial estimate of
the POI must then be exactly compensated or nulled by the
the magnitude of retardation present at the POI. This estima-
particular variable retardation compensator being used.
tion procedure also serves as a verification of the respective
11.2 Rotating Compensator Retardation Determination:
quantitative retardation determination procedures detailed in
11.2.1 With the specimen section containing the POI re-
11.1.9 through 11.1.13.
movedfromthefieldofview,insertamonochromatizingfilter,
11.1.9 Note the orientation of the slow-wave reference
appropriate to the particular rotating compensator being used,
direction indicated on the body of the fixed sensitive-tint or
into the microscope optical path below the microscope con-
full-wave compensator.
densinglensassembly.Thisfiltermonochromatizeswhitelight
11.1.10 Insert the fixed compensator into the accessory slot
for the rotating compensator retardation measurement
of the microscope optical column. The slow-wave direction of
procedure, and is closely matched to a specific rotating
the fixed compensator should parallel the NE-SW diagonal
compensator. Rotating compensators may be constructed to
position as shown in Fig. 2.
require either a 5
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