ASTM C1279-13(2019)
(Test Method)Standard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully Tempered Flat Glass
Standard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully Tempered Flat Glass
SIGNIFICANCE AND USE
5.1 The strength and performance of heat-strengthened and fully-tempered glass is greatly affected by the surface and edge stress induced during the heat-treating process.
5.2 The edge and surface stress levels are specified in Specification C1048, in the Engineering Standards Manual3 of GANA Tempering Division and in foreign specifications.
5.3 This test method offers a direct and convenient way to non-destructively determine the residual state of stress on the surface and at the edge of annealed and heat-treated glass.
SCOPE
1.1 This test method covers the determination of edge stresses and surface stresses in annealed, heat-strengthened, and fully tempered flat glass products.
1.2 This test method is non-destructive.
1.3 This test method uses transmitted light and is, therefore, applicable to light-transmitting glasses.
1.4 The test method is not applicable to chemically-tempered glass.
1.5 Using the procedure described, surface stresses can be measured only on the “tin” side of float glass.
1.6 Surface-stress measuring instruments are designed for a specific range of surface index of refraction.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.9 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.
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Buy Standard
Standards Content (Sample)
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: C1279 − 13 (Reapproved 2019)
Standard Test Method for
Non-Destructive Photoelastic Measurement of Edge and
Surface Stresses in Annealed, Heat-Strengthened, and Fully
Tempered Flat Glass
This standard is issued under the fixed designation C1279; 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 2. Referenced Documents
1.1 This test method covers the determination of edge 2.1 ASTM Standards:
stresses and surface stresses in annealed, heat-strengthened, C158Test Methods for Strength of Glass by Flexure (De-
and fully tempered flat glass products. termination of Modulus of Rupture)
C162Terminology of Glass and Glass Products
1.2 This test method is non-destructive.
C770Test Method for Measurement of Glass Stress—
1.3 This test method uses transmitted light and is, therefore,
Optical Coefficient
applicable to light-transmitting glasses.
C1048Specification for Heat-Strengthened and Fully Tem-
pered Flat Glass
1.4 The test method is not applicable to chemically-
E691Practice for Conducting an Interlaboratory Study to
tempered glass.
Determine the Precision of a Test Method
1.5 Using the procedure described, surface stresses can be
2.2 Other Documents:
measured only on the “tin” side of float glass.
Engineering Standards Manual
1.6 Surface-stress measuring instruments are designed for a
“Surface and Edge Stress in Tempered Glass”
specific range of surface index of refraction.
3. Terminology
1.7 The values stated in SI units are to be regarded as
3.1 Definitions:
standard. No other units of measurement are included in this
3.1.1 analyzer—a polarizing element, typically positioned
standard.
between the specimen being evaluated and the viewer.
1.8 This standard does not purport to address all of the
3.1.2 polarizer—an optical assembly that transmits light
safety concerns, if any, associated with its use. It is the
vibrating in a single planar direction, typically positioned
responsibility of the user of this standard to establish appro-
between a light source and the specimen being evaluated.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
3.1.3 retardation compensator—an optical device, variants
1.9 This international standard was developed in accor-
of which are used to quantify the optical retardation produced
dance with internationally recognized principles on standard-
in transparent birefringent materials: typically positioned be-
ization established in the Decision on Principles for the
tween the specimen being evaluated and the analyzer.
Development of International Standards, Guides and Recom-
3.2 For definition of terms used in this test method, refer to
mendations issued by the World Trade Organization Technical
Terminology C162.
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This test method is under the jurisdiction of ASTM Committee C14 on Glass Standards volume information, refer to the standard’s Document Summary page on
and Glass Products and is the direct responsibility of Subcommittee C14.08 on Flat the ASTM website.
Glass. Available from GlassAssociation of NorthAmerica (GANA), 800 SWJackson
Current edition approved Aug. 1, 2019. Published August 2019 Originally Street, Ste 1500, Topeka, Kansas 66612–1200. http://www.glasswebsite.com
approved in 1994. Last previous edition approved in 2013 as C1279–13. DOI: Redner, A. S. and Voloshin, A. S., Proceedings of the Ninth International
10.1520/C1279-13R19. Conference on Experimental Mechanics, Denmark, 1990.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1279 − 13 (2019)
4. Summary of Test Methods compensator, W , generates a visible set of fringes or lines of
c
constant retardation R where
4.1 Two test methods are described in this standard:
4.1.1 Procedure A—describes a test method for measuring R 5 R 1R (1)
s c
surface stress using light propagating nearly parallel to the
Sincethespecimen-inducedretardationisproportionaltothe
surface.
surface stress, S, and the path, t, we have:
4.1.2 Procedure B—describes a test method for measuring
R 5 C·S·t 5 C·S·ax (2)
edge-stressusinglightpropagatinginthedirectionperpendicu-
s
lar to the surface.
where:
4.2 In both methods, the fundamental photoelastic concept
R = is the relative retardation,
is used. As a result of stresses, the material becomes optically
C = stress-optical constant (see Note 2),
anisotropic or birefringent. When polarized light propagates
S = surfacestressinthedirectionperpendiculartothepath,
throughsuchanisotropicmaterials,thedifferencesinthespeed t
of light rays vibrating along the maximum and minimum t = path of light traveling between the entrance and exit
principal stress introduce a relative retardation between these
points 1, 2 (Fig. 1),
a = Geometrical factor, (depending upon the prism design)
rays. This relative retardation is proportional to the measured
stresses,andcanbeaccuratelydeterminedusingcompensators. a=t⁄x . This constant is determined by the manufac-
turer.
For additional background see “Surface and Edge Stress in
Tempered Glass” .
6.1.3 The compensator adds its own retardation. It is lin-
early variable along its length y and is calculated as
5. Significance and Use
R 5 b·y (3)
c
5.1 The strength and performance of heat-strengthened and
fully-temperedglassisgreatlyaffectedbythesurfaceandedge
Where bisaconstant,determinedbythemanufacturerofthe
stress induced during the heat-treating process.
compensator. The observer sees in the compensator plane a
total retardation R.
5.2 The edge and surface stress levels are specified in
Specification C1048, in the Engineering Standards Manual of R 5 R 1R 5 a·C·S·x1b·y (4)
s c
GANA Tempering Division and in foreign specifications.
6.1.4 The fringes (lines of R=Constant) are, therefore,
5.3 This test method offers a direct and convenient way to
tilted lines. (See Fig. 2). The angle θ is the tilt of these fringes
non-destructively determine the residual state of stress on the
relative to a plane containing the light path of Figs. 1 and 2.
surface and at the edge of annealed and heat-treated glass.
The measured stress is proportional to the tangent of the tilt
angle θ, measured using a goniometer, and to an instrument
6. Principles of Operation
calibration constant, K MPa, determined by the manufacturer.
6.1 Procedure A: Measuring Surface Stress:
a·C·S
6.1.1 Measurement of surface stresses requires an optical
tan θ 5 and (5)
b
apparatus that permits the injection of polarized light rays
b
propagatinginathinlayeradjacenttothesurface(seeNote1).
Stress 5 · tan θ 5 K· tan θ
Ca
A prism is usually used for this purpose. The rays emerge at
critical angle i .The photoelastic retardation due to the surface
c
In the actual procedure (see 15.1 below) the operator
stresses, (see Fig. 1), is measured using a wedge-compensator.
measures the tilt angle θ of the observed set of fringes.
6.1.2 The incident light beam should be arriving at the
NOTE 1—The surface-stress measuring apparatus described in this
criticalanglei andpolarizedat45°totheentranceoftheprism
c
section is manufactured by StrainopticTechnologies, Inc. in NorthWales,
edge. A quartz wedge-compensator, W , placed in the path of
c
Pennsylvania.
emerging light adds a retardation, R , to the retardation R
c s
NOTE 2—The stress constant of float glass is typically 2.55 to 2.65
induced by stresses in the surface of the specimen. The
Brewsters. Calibration can be performed using one of the test methods
analyzer, A, placed between the eyepiece, E, and the wedge- described in Test Methods C770.
FIG. 1 Apparatus For Measuring Surface Stress
C1279 − 13 (2019)
FIG. 1 Apparatus For Measuring Surface Stress (continued)
FIG. 2 Fringes Observed in the Plane of the Compensator
6.2 Procedure B: Measuring Edge Stress:
6.2.1 Measurement of edge stress is accomplished using a
polarimeter equipped with a wedge-compensator, as shown
schematically in Fig. 3.
6.2.2 The angle between the polarizer and the edge of the
specimen must be 45° (see Fig. 3a), and the analyzer must be
perpendicular to the polarizer. The overall magnification
should be at least 20× to permit clear visibility of the reticle,
and of photoelastic fringes near the edge. The reticle placed
adjacent to the specimen must have graduations of 0.1 mm
(0.004 in.) or smaller. The resolution of the compensator
should be at least 5 nm, and the compensator should be
calibrated by the manufacturer at 565 nm wavelength with
FIG. 3 Schematic of the Instrument for Measuring Edge Stress
results of calibration expressed in nm/div.
C1279 − 13 (2019)
6.2.3 The compensator used could be of linear wedge type 9. Test Specimens and Loading Schemes
(Babinet) or uniform-field type (Babinet-Soleil). The linear-
9.1 Twoloadinggeometriescanbepracticed:cantileverand
wedge type requires a reticle placed adjacent to the compen-
four-point bending.
sator wedge and a linear-motion scale, or lead screw, locating
9.2 Cantilever-Beam Specimen (Fig. 4)—the dimensions of
the wedge position with reference to the reticle.
the specimen used for cantilever loading should be selected
6.2.4 The uniform field does not require a reticle, and must
within limits shown below:
beequippedwithaleadscrewmeasuringtherelativemotionof
Thickness (t): 6 mm (0.22 in.) minimum,
its wedges.
Width (W): 8t ≤ W ≤ 12t,
Length (L): 6W minimum,
7. Sampling
Distance to the point of measurement (L ): 4W, and
O
7.1 ProcedureA: Measuring Surface Stress—Thenumberof
Clamped length: 1.5W
points to be measured are determined by either the product
9.2.1 A heat-strengthened or tempered specimen, with pol-
specification or by the following protocol described in Speci-
ishededgesispreferred,butannealedspecimenscanbeusedif
fication C1048.
the range of stress is less than 24.13 MPa (3500 psi).
7.2 Procedure B: Measuring Edge Stress—Readings must
9.3 Four-Point Bending Specimen (Fig. 5)—The four-
be obtained at the mid-span point of every edge.
point bending specimen should be preferred since it has
uniform stress in the central loading zone. The dimensions of
8. Calibration and Standardization
the specimen should be selected within the following limits:
8.1 A test bar is subjected to bending using traceably
Thickness (t): minimum 2 mm (0.079 in.),
certified deadweights or calibrated load-cells to introduce
Width (W): 8t ≤ W ≤ 12t (see Note 3),
surface stresses that can be calculated from the specimen
Length L : 6W minimum,
O
geometry and forces applied. At a point in which the stresses
Gage length L sction: 3W,
C
are calculated, those same stresses also are measured using the
Minimum overall length L: 12W, and
instrumenttobecalibratedorverified.Sinceboththespecimen
Edges: Polished, no chips in the gage section, bevel less
dimension and the applied forces can be established accurately
than 0.1t.
usingtraceable(primary)standards,themethodpermitsafully
traceable calibration of the stress-measuring instrument. NOTE3—Whenthethickness tislessthan6mm,andwidthofthebeam
exceeds 12t, instead of the beam bending, plate bending equations should
8.2 The instrument to be calibrated is placed on the surface
be used to calculate surface stress, or suitable corrections are required in
5 6
of the calibration specimen. Stresses at a point where the
the equations in 11.1. Barata and Ashwell show the correction proce-
dures.
instrument is placed are calculated using expressions shown in
Section 11. To increase the precision of measurement, several
levels of stress are produced by applying forces incrementally.
Barata, F. I., “When Is a Beam a Plate?” American Ceramic Society
Measurement of stress using the instrument to be calibrated is
Communications, May 1981.
repeated for each stress level and these measurements are used 6
Ashwell, D. G., “TheAnticlastic Curvature of Rectangular Beams and Plates,”
to calibrate the instrument. Journal of Aeronautics, Vol 54, 1950, pp. 708-715.
FIG. 4 Cantilever Beam Loading
C1279 − 13 (2019)
FIG. 5 Calibration Using Four-Point Bending
9.4 Application of Forces—Forces required must be calcu- eliminate possible twisting action, the knife edge should be
lated to eliminate possible breakage. Stresses must be esti- narrow, or a steel ball used to center the point of application of
mated first using the equations in 9.1. A tempered specimen force.
may be subjected to stress levels up to 10000 psi (69 MPa). 9.4.2 Four-Point Bending Specimen—In the case of four-
Using annealed specimens, the stress should remain at a safe point bending, the force must be applied equally at two points,
level, typically below 3500 psi (24 MPa). and two articulated knives or roller supports are required to
9.4.1 Cantilever Specimen—Thespecimenmustbeclamped ascertain accurately the length L and L . Particular precau-
0 c
securely using wood, plastic, or rubber-lined metal clamping tions are required to insure that the end supports do not
surfaces, with rounded edges, as shown in Fig. 6. The forces introduce a twist in the specimen, as a result of nonparallel
can be applied using a calibrated testing machine or dead support surfaces or nonflatness of the specimen itself. Fig. 5
weights,bymeansofknifeedges,toinsureexactpositioningof illustrates the setup for application of forces to obtain tension
the line-of-loading B-B. The pad used for load application can and compression on the upper face. Test Methods C158
be secured from slipping using high-friction materials. To provides a description of support design.
FIG. 6 Clamping of a Cantilever Beam
C1279 − 13 (2019)
10. Calibration Procedure 11.3 Trace “best fit” straight line, to establish the instru-
ment constant K for the surface polarimeter:
10.1 Whencalibratingasurfacepolarimeter,applyforcesin
five equal increments, using a testing machine or dead weight. ∆Stress
K 5 (8)
Whencalibratingacriticalanglemeasuringinstrument,atleast ∆tanθ
ten increments are needed, and a tempered specimen must be
11.4 Since the relation between the measured distance
...
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: C1279 − 13 (Reapproved 2019)
Standard Test Method for
Non-Destructive Photoelastic Measurement of Edge and
Surface Stresses in Annealed, Heat-Strengthened, and Fully
Tempered Flat Glass
This standard is issued under the fixed designation C1279; 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 2. Referenced Documents
1.1 This test method covers the determination of edge 2.1 ASTM Standards:
stresses and surface stresses in annealed, heat-strengthened, C158 Test Methods for Strength of Glass by Flexure (De-
and fully tempered flat glass products. termination of Modulus of Rupture)
C162 Terminology of Glass and Glass Products
1.2 This test method is non-destructive.
C770 Test Method for Measurement of Glass Stress—
1.3 This test method uses transmitted light and is, therefore,
Optical Coefficient
applicable to light-transmitting glasses.
C1048 Specification for Heat-Strengthened and Fully Tem-
pered Flat Glass
1.4 The test method is not applicable to chemically-
tempered glass. E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
1.5 Using the procedure described, surface stresses can be
2.2 Other Documents:
measured only on the “tin” side of float glass.
Engineering Standards Manual
1.6 Surface-stress measuring instruments are designed for a
“Surface and Edge Stress in Tempered Glass”
specific range of surface index of refraction.
3. Terminology
1.7 The values stated in SI units are to be regarded as
3.1 Definitions:
standard. No other units of measurement are included in this
3.1.1 analyzer—a polarizing element, typically positioned
standard.
between the specimen being evaluated and the viewer.
1.8 This standard does not purport to address all of the
3.1.2 polarizer—an optical assembly that transmits light
safety concerns, if any, associated with its use. It is the
vibrating in a single planar direction, typically positioned
responsibility of the user of this standard to establish appro-
between a light source and the specimen being evaluated.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. 3.1.3 retardation compensator—an optical device, variants
1.9 This international standard was developed in accor-
of which are used to quantify the optical retardation produced
dance with internationally recognized principles on standard- in transparent birefringent materials: typically positioned be-
ization established in the Decision on Principles for the tween the specimen being evaluated and the analyzer.
Development of International Standards, Guides and Recom-
3.2 For definition of terms used in this test method, refer to
mendations issued by the World Trade Organization Technical
Terminology C162.
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This test method is under the jurisdiction of ASTM Committee C14 on Glass Standards volume information, refer to the standard’s Document Summary page on
and Glass Products and is the direct responsibility of Subcommittee C14.08 on Flat the ASTM website.
Glass. Available from Glass Association of North America (GANA), 800 SW Jackson
Current edition approved Aug. 1, 2019. Published August 2019 Originally Street, Ste 1500, Topeka, Kansas 66612–1200. http://www.glasswebsite.com
approved in 1994. Last previous edition approved in 2013 as C1279 – 13. DOI: Redner, A. S. and Voloshin, A. S., Proceedings of the Ninth International
10.1520/C1279-13R19. Conference on Experimental Mechanics, Denmark, 1990.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1279 − 13 (2019)
4. Summary of Test Methods compensator, W , generates a visible set of fringes or lines of
c
constant retardation R where
4.1 Two test methods are described in this standard:
R 5 R 1R (1)
4.1.1 Procedure A—describes a test method for measuring
s c
surface stress using light propagating nearly parallel to the
Since the specimen-induced retardation is proportional to the
surface.
surface stress, S, and the path, t, we have:
4.1.2 Procedure B—describes a test method for measuring
R 5 C·S·t 5 C·S·ax (2)
edge-stress using light propagating in the direction perpendicu-
s
lar to the surface.
where:
4.2 In both methods, the fundamental photoelastic concept
R = is the relative retardation,
is used. As a result of stresses, the material becomes optically
C = stress-optical constant (see Note 2),
anisotropic or birefringent. When polarized light propagates
S = surface stress in the direction perpendicular to the path,
through such anisotropic materials, the differences in the speed
t
of light rays vibrating along the maximum and minimum t = path of light traveling between the entrance and exit
principal stress introduce a relative retardation between these points 1, 2 (Fig. 1),
rays. This relative retardation is proportional to the measured a = Geometrical factor, (depending upon the prism design)
a = t ⁄x . This constant is determined by the manufac-
stresses, and can be accurately determined using compensators.
For additional background see “Surface and Edge Stress in turer.
Tempered Glass” .
6.1.3 The compensator adds its own retardation. It is lin-
early variable along its length y and is calculated as
5. Significance and Use
R 5 b·y (3)
c
5.1 The strength and performance of heat-strengthened and
fully-tempered glass is greatly affected by the surface and edge
Where b is a constant, determined by the manufacturer of the
stress induced during the heat-treating process.
compensator. The observer sees in the compensator plane a
total retardation R.
5.2 The edge and surface stress levels are specified in
Specification C1048, in the Engineering Standards Manual of
R 5 R 1R 5 a·C·S·x1b·y (4)
s c
GANA Tempering Division and in foreign specifications.
6.1.4 The fringes (lines of R = Constant) are, therefore,
5.3 This test method offers a direct and convenient way to
tilted lines. (See Fig. 2). The angle θ is the tilt of these fringes
non-destructively determine the residual state of stress on the
relative to a plane containing the light path of Figs. 1 and 2.
surface and at the edge of annealed and heat-treated glass.
The measured stress is proportional to the tangent of the tilt
angle θ, measured using a goniometer, and to an instrument
6. Principles of Operation
calibration constant, K MPa, determined by the manufacturer.
6.1 Procedure A: Measuring Surface Stress:
a·C·S
6.1.1 Measurement of surface stresses requires an optical
tan θ 5 and (5)
b
apparatus that permits the injection of polarized light rays
b
propagating in a thin layer adjacent to the surface (see Note 1).
Stress5 · tan θ 5 K· tan θ
Ca
A prism is usually used for this purpose. The rays emerge at
critical angle i . The photoelastic retardation due to the surface
c
In the actual procedure (see 15.1 below) the operator
stresses, (see Fig. 1), is measured using a wedge-compensator.
measures the tilt angle θ of the observed set of fringes.
6.1.2 The incident light beam should be arriving at the
NOTE 1—The surface-stress measuring apparatus described in this
critical angle i and polarized at 45° to the entrance of the prism
c
section is manufactured by Strainoptic Technologies, Inc. in North Wales,
edge. A quartz wedge-compensator, W , placed in the path of
c
Pennsylvania.
emerging light adds a retardation, R , to the retardation R
c s
NOTE 2—The stress constant of float glass is typically 2.55 to 2.65
induced by stresses in the surface of the specimen. The
Brewsters. Calibration can be performed using one of the test methods
analyzer, A, placed between the eyepiece, E, and the wedge- described in Test Methods C770.
FIG. 1 Apparatus For Measuring Surface Stress
C1279 − 13 (2019)
FIG. 1 Apparatus For Measuring Surface Stress (continued)
FIG. 2 Fringes Observed in the Plane of the Compensator
6.2 Procedure B: Measuring Edge Stress:
6.2.1 Measurement of edge stress is accomplished using a
polarimeter equipped with a wedge-compensator, as shown
schematically in Fig. 3.
6.2.2 The angle between the polarizer and the edge of the
specimen must be 45° (see Fig. 3a), and the analyzer must be
perpendicular to the polarizer. The overall magnification
should be at least 20× to permit clear visibility of the reticle,
and of photoelastic fringes near the edge. The reticle placed
adjacent to the specimen must have graduations of 0.1 mm
(0.004 in.) or smaller. The resolution of the compensator
should be at least 5 nm, and the compensator should be
calibrated by the manufacturer at 565 nm wavelength with
FIG. 3 Schematic of the Instrument for Measuring Edge Stress
results of calibration expressed in nm/div.
C1279 − 13 (2019)
6.2.3 The compensator used could be of linear wedge type 9. Test Specimens and Loading Schemes
(Babinet) or uniform-field type (Babinet-Soleil). The linear-
9.1 Two loading geometries can be practiced: cantilever and
wedge type requires a reticle placed adjacent to the compen-
four-point bending.
sator wedge and a linear-motion scale, or lead screw, locating
9.2 Cantilever-Beam Specimen (Fig. 4)—the dimensions of
the wedge position with reference to the reticle.
the specimen used for cantilever loading should be selected
6.2.4 The uniform field does not require a reticle, and must
within limits shown below:
be equipped with a lead screw measuring the relative motion of
Thickness (t): 6 mm (0.22 in.) minimum,
its wedges.
Width (W): 8t ≤ W ≤ 12t,
7. Sampling Length (L): 6W minimum,
Distance to the point of measurement (L ): 4W, and
O
7.1 Procedure A: Measuring Surface Stress—The number of
Clamped length: 1.5W
points to be measured are determined by either the product
9.2.1 A heat-strengthened or tempered specimen, with pol-
specification or by the following protocol described in Speci-
ished edges is preferred, but annealed specimens can be used if
fication C1048.
the range of stress is less than 24.13 MPa (3500 psi).
7.2 Procedure B: Measuring Edge Stress—Readings must
9.3 Four-Point Bending Specimen (Fig. 5)—The four-
be obtained at the mid-span point of every edge.
point bending specimen should be preferred since it has
uniform stress in the central loading zone. The dimensions of
8. Calibration and Standardization
the specimen should be selected within the following limits:
8.1 A test bar is subjected to bending using traceably
Thickness (t): minimum 2 mm (0.079 in.),
certified deadweights or calibrated load-cells to introduce
Width (W): 8t ≤ W ≤ 12t (see Note 3),
surface stresses that can be calculated from the specimen
Length L : 6W minimum,
O
geometry and forces applied. At a point in which the stresses
Gage length L sction: 3W,
C
are calculated, those same stresses also are measured using the
Minimum overall length L: 12W, and
instrument to be calibrated or verified. Since both the specimen
Edges: Polished, no chips in the gage section, bevel less
dimension and the applied forces can be established accurately
than 0.1t.
using traceable (primary) standards, the method permits a fully
traceable calibration of the stress-measuring instrument. NOTE 3—When the thickness t is less than 6 mm, and width of the beam
exceeds 12t, instead of the beam bending, plate bending equations should
8.2 The instrument to be calibrated is placed on the surface
be used to calculate surface stress, or suitable corrections are required in
5 6
of the calibration specimen. Stresses at a point where the
the equations in 11.1. Barata and Ashwell show the correction proce-
instrument is placed are calculated using expressions shown in dures.
Section 11. To increase the precision of measurement, several
levels of stress are produced by applying forces incrementally.
Barata, F. I., “When Is a Beam a Plate?” American Ceramic Society
Measurement of stress using the instrument to be calibrated is
Communications, May 1981.
repeated for each stress level and these measurements are used 6
Ashwell, D. G., “The Anticlastic Curvature of Rectangular Beams and Plates,”
to calibrate the instrument. Journal of Aeronautics, Vol 54, 1950, pp. 708-715.
FIG. 4 Cantilever Beam Loading
C1279 − 13 (2019)
FIG. 5 Calibration Using Four-Point Bending
9.4 Application of Forces—Forces required must be calcu- eliminate possible twisting action, the knife edge should be
lated to eliminate possible breakage. Stresses must be esti- narrow, or a steel ball used to center the point of application of
mated first using the equations in 9.1. A tempered specimen force.
may be subjected to stress levels up to 10 000 psi (69 MPa). 9.4.2 Four-Point Bending Specimen—In the case of four-
Using annealed specimens, the stress should remain at a safe point bending, the force must be applied equally at two points,
level, typically below 3500 psi (24 MPa). and two articulated knives or roller supports are required to
9.4.1 Cantilever Specimen—The specimen must be clamped ascertain accurately the length L and L . Particular precau-
0 c
securely using wood, plastic, or rubber-lined metal clamping tions are required to insure that the end supports do not
surfaces, with rounded edges, as shown in Fig. 6. The forces introduce a twist in the specimen, as a result of nonparallel
can be applied using a calibrated testing machine or dead support surfaces or nonflatness of the specimen itself. Fig. 5
weights, by means of knife edges, to insure exact positioning of illustrates the setup for application of forces to obtain tension
the line-of-loading B-B. The pad used for load application can and compression on the upper face. Test Methods C158
be secured from slipping using high-friction materials. To provides a description of support design.
FIG. 6 Clamping of a Cantilever Beam
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10. Calibration Procedure 11.3 Trace “best fit” straight line, to establish the instru-
ment constant K for the surface polarimeter:
10.1 When calibrating a surface polarimeter, apply forces in
ΔStress
five equal increments, using a testing machine or dead weight.
K 5 (8)
When calibrating a critical angle measuring instrument, at least Δtanθ
ten increments are needed, and a tempered specimen must be
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM 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: C1279 − 13 C1279 − 13 (Reapproved 2019)
Standard Test Method for
Non-Destructive Photoelastic Measurement of Edge and
Surface Stresses in Annealed, Heat-Strengthened, and Fully
Tempered Flat Glass
This standard is issued under the fixed designation C1279; 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 edge stresses and surface stresses in annealed, heat-strengthened, and fully
tempered flat glass products.
1.2 This test method is non-destructive.
1.3 This test method uses transmitted light and is, therefore, applicable to light-transmitting glasses.
1.4 The test method is not applicable to chemically-tempered glass.
1.5 Using the procedure described, surface stresses can be measured only on the “tin” side of float glass.
1.6 Surface-stress measuring instruments are designed for a specific range of surface index of refraction.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.9 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.
2. Referenced Documents
2.1 ASTM Standards:
C158 Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture)
C162 Terminology of Glass and Glass Products
C770 Test Method for Measurement of Glass Stress—Optical Coefficient
C1048 Specification for Heat-Strengthened and Fully Tempered Flat Glass
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
2.2 Other Documents:
Engineering Standards Manual
“Surface and Edge Stress in Tempered Glass”
3. Terminology
3.1 Definitions:
3.1.1 analyzer—a polarizing element, typically positioned between the specimen being evaluated and the viewer.
3.1.2 polarizer—an optical assembly that transmits light vibrating in a single planar direction, typically positioned between a
light source and the specimen being evaluated.
This test method is under the jurisdiction of ASTM Committee C14 on Glass and Glass Products and is the direct responsibility of Subcommittee C14.08 on Flat Glass.
Current edition approved Oct. 1, 2013Aug. 1, 2019. Published October 2013August 2019 Originally approved in 1994. Last previous edition approved in 20092013 as
C1279C1279 – 13.-09. DOI: 10.1520/C1279-13.10.1520/C1279-13R19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from Glass Association of North America (GANA), 800 SW Jackson Street, Ste 1500, Topeka, Kansas 66612–1200. http://www.glasswebsite.com
Redner, A. S. and Voloshin, A. S., Proceedings of the Ninth International Conference on Experimental Mechanics, Denmark, 1990.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1279 − 13 (2019)
3.1.3 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 For definition of terms used in this test method, refer to Terminology C162.
4. Summary of Test Methods
4.1 Two test methods are described in this standard:
4.1.1 Procedure A—describes a test method for measuring surface stress using light propagating nearly parallel to the surface.
4.1.2 Procedure B—describes a test method for measuring edge-stress using light propagating in the direction perpendicular to
the surface.
4.2 In both methods, the fundamental photoelastic concept is used. As a result of stresses, the material becomes optically
anisotropic or birefringent. When polarized light propagates through such anisotropic materials, the differences in the speed of light
rays vibrating along the maximum and minimum principal stress introduce a relative retardation between these rays. This relative
retardation is proportional to the measured stresses, and can be accurately determined using compensators. For additional
background see “Surface and Edge Stress in Tempered Glass” .
5. Significance and Use
5.1 The strength and performance of heat-strengthened and fully-tempered glass is greatly affected by the surface and edge
stress induced during the heat-treating process.
5.2 The edge and surface stress levels are specified in Specification C1048, in the Engineering Standards Manual of GANA
Tempering Division and in foreign specifications.
5.3 This test method offers a direct and convenient way to non-destructively determine the residual state of stress on the surface
and at the edge of annealed and heat-treated glass.
6. Principles of Operation
6.1 Procedure A: Measuring Surface Stress:
6.1.1 Measurement of surface stresses requires an optical apparatus that permits the injection of polarized light rays propagating
in a thin layer adjacent to the surface (see Note 1). A prism is usually used for this purpose. The rays emerge at critical angle i .
c
The photoelastic retardation due to the surface stresses, (see Fig. 1), is measured using a wedge-compensator.
6.1.2 The incident light beam should be arriving at the critical angle i and polarized at 45° to the entrance of the prism edge.
c
A quartz wedge-compensator, W , placed in the path of emerging light adds a retardation, R , to the retardation R induced by
c c s
stresses in the surface of the specimen. The analyzer, A, placed between the eyepiece, E, and the wedge-compensator, W , generates
c
a visible set of fringes or lines of constant retardation R where
R 5 R 1R (1)
s c
Since the specimen-induced retardation is proportional to the surface stress, S, and the path, t, we have:
R 5 C·S·t 5 C·S·ax (2)
s
where:
R = is the relative retardation,
C = stress-optical constant (see Note 2),
S = surface stress in the direction perpendicular to the path, t
t = path of light traveling between the entrance and exit points 1, 2 (Fig. 1),
a = Geometrical factor, (depending upon the prism design) a = t ⁄x . This constant is determined by the manufacturer.
a = Geometrical factor, (depending upon the prism design) a = t ⁄x . This constant is determined by the manufacturer.
FIG. 1 Apparatus For Measuring Surface Stress
C1279 − 13 (2019)
FIG. 1 Apparatus For Measuring Surface Stress (continued)
6.1.3 The compensator adds its own retardation. It is linearly variable along its length y and is calculated as
R 5 b·y (3)
c
Where b is a constant, determined by the manufacturer of the compensator. The observer sees in the compensator plane a total
retardation R.
R 5 R 1R 5 a·C·S·x1b·y (4)
s c
6.1.4 The fringes (lines of R = Constant) are, therefore, tilted lines. (See Fig. 2). The angle θ is the tilt of these fringes relative
to a plane containing the light path of Figs. 1 and 2. The measured stress is proportional to the tangent of the tilt angle θ, measured
using a goniometer, and to an instrument calibration constant, K MPa, determined by the manufacturer.
a·C·S
tan θ5 and (5)
b
b
Stress 5 · tan θ5 K· tan θ
Ca
In the actual procedure (see 15.1 below) the operator measures the tilt angle θ of the observed set of fringes.
FIG. 2 Fringes Observed in the Plane of the Compensator
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NOTE 1—The surface-stress measuring apparatus described in this section is manufactured by Strainoptic Technologies, Inc. in North Wales,
Pennsylvania.
NOTE 2—The stress constant of float glass is typically 2.55 to 2.65 Brewsters. Calibration can be performed using one of the test methods described
in Test Methods C770.
6.2 Procedure B: Measuring Edge Stress:
6.2.1 Measurement of edge stress is accomplished using a polarimeter equipped with a wedge-compensator, as shown
schematically in Fig. 3.
6.2.2 The angle between the polarizer and the edge of the specimen must be 45° (see Fig. 3a), and the analyzer must be
perpendicular to the polarizer. The overall magnification should be at least 20× to permit clear visibility of the reticle, and of
photoelastic fringes near the edge. The reticle placed adjacent to the specimen must have graduations of 0.1 mm (0.004 in.) or
smaller. The resolution of the compensator should be at least 5 nm, and the compensator should be calibrated by the manufacturer
at 565 nm wavelength with results of calibration expressed in nm/div.
6.2.3 The compensator used could be of linear wedge type (Babinet) or uniform-field type (Babinet-Soleil). The linear-wedge
type requires a reticle placed adjacent to the compensator wedge and a linear-motion scale, or lead screw, locating the wedge
position with reference to the reticle.
6.2.4 The uniform field does not require a reticle, and must be equipped with a lead screw measuring the relative motion of its
wedges.
7. Sampling
7.1 Procedure A: Measuring Surface Stress—The number of points to be measured are determined by either the product
specification or by the following protocol described in Specification C1048.
7.2 Procedure B: Measuring Edge Stress—Readings must be obtained at the mid-span point of every edge.
8. Calibration and Standardization
8.1 A test bar is subjected to bending using traceably certified deadweights or calibrated load-cells to introduce surface stresses
that can be calculated from the specimen geometry and forces applied. At a point in which the stresses are calculated, those same
stresses also are measured using the instrument to be calibrated or verified. Since both the specimen dimension and the applied
forces can be established accurately using traceable (primary) standards, the method permits a fully traceable calibration of the
stress-measuring instrument.
FIG. 3 Schematic of the Instrument for Measuring Edge Stress
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8.2 The instrument to be calibrated is placed on the surface of the calibration specimen. Stresses at a point where the instrument
is placed are calculated using expressions shown in Section 11. To increase the precision of measurement, several levels of stress
are produced by applying forces incrementally. Measurement of stress using the instrument to be calibrated is repeated for each
stress level and these measurements are used to calibrate the instrument.
9. Test Specimens and Loading Schemes
9.1 Two loading geometries can be practiced: cantilever and four-point bending.
9.2 Cantilever-Beam Specimen (Fig. 4)—the dimensions of the specimen used for cantilever loading should be selected within
limits shown below:
Thickness (t): 6 mm (0.22 in.) minimum,
Width (W): 8t ≤ W ≤ 12t,
Length (L): 6W minimum,
Distance to the point of measurement (L ): 4W, and
O
Clamped length: 1.5W
9.2.1 A heat-strengthened or tempered specimen, with polished edges is preferred, but annealed specimens can be used if the
range of stress is less than 24.13 MPa (3500 psi).
9.3 Four-Point Bending Specimen (Fig. 5)—The four-point bending specimen should be preferred since it has uniform stress
in the central loading zone. The dimensions of the specimen should be selected within the following limits:
Thickness (t): minimum 2 mm (0.079 in.),
Width (W): 8t ≤ W ≤ 12t (see Note 3),
Length L : 6W minimum,
O
Gage length L sction: 3W,
C
Minimum overall length L: 12W, and
Edges: Polished, no chips in the gage section, bevel less than 0.1t.
NOTE 3—When the thickness t is less than 6 mm, and width of the beam exceeds 12t, instead of the beam bending, plate bending equations should
5 6
be used to calculate surface stress, or suitable corrections are required in the equations in 11.1. Barata and Ashwell show the correction procedures.
9.4 Application of Forces—Forces required must be calculated to eliminate possible breakage. Stresses must be estimated first
using the equations in 9.1. A tempered specimen may be subjected to stress levels up to 10 000 psi (69 MPa). Using annealed
specimens, the stress should remain at a safe level, typically below 3500 psi (24 MPa).
FIG. 4 Cantilever Beam Loading
Barata, F. I., “When Is a Beam a Plate?” American Ceramic Society Communications, May 1981.
Ashwell, D. G., “The Anticlastic Curvature of Rectangular Beams and Plates,” Journal of Aeronautics, Vol 54, 1950, pp. 708-715.
C1279 − 13 (2019)
FIG. 5 Calibration Using Four-Point Bending
9.4.1 Cantilever Specimen—The specimen must be clamped securely using wood, plastic, or rubber-lined metal clamping
surfaces, with rounded edges, as shown in Fig. 6. The forces can be applied using a calibrated testing machine or dead weights,
by means of knife edges, to insure exact positioning of the line-of-loading B-B. The pad used for load application can be secured
from slipping using high-friction materials. To eliminate possible twisting action, the knife edge should be narrow, or a steel ball
used to center the point of application of force.
9.4.2 Four-Point Bending Specimen—In the case of four-point bending, the force must be applied equally at two points, and two
articulated knives or roller supports are required to ascertain accurately the length L and L . Particular precautions are required
0 c
to insure that the end supports do not introduce a twist in the specimen, as a result of nonparallel support surfaces or nonflatness
of the specimen itself. Fig. 5 illustrates the setup for application of forces to obtain tension and compression on the upper face.
Test Methods C158 provides a description of support design.
10. Calibration Procedure
...
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