ASTM C1279-94

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Designation: C 1279 – 94
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
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 C 1279; 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 3.1.2 polarizer—an optical assembly that transmits light
vibrating in a single planar direction, typically positioned
1.1 This test method covers the determination of edge
between a light source and the specimen being evaluated.
stresses and surface stresses in annealed, heat-strengthened,
3.1.3 retardation compensator—an optical device, variants
and fully tempered flat glass products.
of which are used to quantify the optical retardation produced
1.2 This test method is non-destructive.
in transparent birefringent materials: typically positioned be-
1.3 This test method uses transmitted light and is, therefore,
tween the specimen being evaluated and the analyzer.
applicable to light-transmitting glasses.
3.2 For definition of terms used in this test method, refer to
1.4 The test method is not applicable to chemically-
Terminology C 162.
tempered glass.
1.5 Using the procedure described, surface stresses can be
4. Summary of Test Methods
measured only on the “tin” side of float glass.
4.1 Two test methods are described in this standard:
1.6 Surface-stress measuring instruments are designed for a
4.1.1 Procedure A describes a test method for measuring
specific range of surface index of refraction.
surface stress using light propagating nearly parallel to the
1.7 This standard does not purport to address all of the
surface.
safety concerns, if any, associated with its use. It is the
4.1.2 Procedure B describes a test method for measuring
responsibility of the user of this standard to establish appro-
edge-stress using light propagating in the direction perpendicu-
priate safety and health practices and determine the applica-
lar to the surface.
bility of regulatory limitations prior to use.
4.2 In both methods, the fundamental photoelastic concept
2. Referenced Documents is used. As a result of stresses, the material becomes optically
anisotropic or birefringent. When polarized light propagates
2.1 ASTM Standards:
through such anisotropic materials, the differences in the speed
C 162 Terminology of Glass and Glass Products
of light rays vibrating along the maximum and minimum
C 770 Test Method for Measurements of Glass Stress-
principal stress introduce a relative retardation between these
Optical Coefficient
rays. This relative retardation is proportional to the measured
C 1048 Specification for Heat-Treated Glass: Kind HS,
stresses, and can be accurately determined using compensators.
Kind FT Coated and Uncoated Glass
For additional background see “Surface and Edge Stress in
F 218 Test Method for Analyzing Stress in Glass
Tempered Glass”.
2.2 Other Documents:
Engineering Standards Manual
5. Significance and Use
“Surface and Edge Stress in Tempered Glass”
5.1 The strength and performance of heat-strengthened and
3. Terminology fully-tempered glass is greatly affected by the surface and edge
stress induced during the heat-treating process.
3.1 Definitions:
5.2 The edge and surface stress levels are specified in
3.1.1 analyzer—a polarizing element, typically positioned
Specification C 1048, in the Engineering Standards Manual of
between the specimen being evaluated and the viewer.
GTA and in foreign specifications.
5.3 This test method offers a direct and convenient way to
This test method is under the jurisdiction of ASTM Committee C-14 on Glass
non-destructively determine the residual state of stress on the
and Glass Products and is the direct responsibility of Subcommittee C14.08 on Flat
surface and at the edge of annealed and heat-treated glass.
Glass.
Current edition approved Aug. 15, 1994. Published October 1994.
6. Principles of Operation
Annual Book of ASTM Standards, Vol 15.02.
Available from Glass Tempering Association, 3310 Harrison, Topeka, Kansas.
6.1 Procedure A: Measuring Surface Stress:
Redner, A. S. and Voloshin, A. S., Proceedings of the Ninth International
6.1.1 Measurement of surface stresses requires an optical
Conference on Experimental Mechanics, Denmark, 1990.
C 1279
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 . The photoelastic retardation due to the surface
c
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
c
edge. A quartz wedge-compensator, W , placed in the path of
c
emerging light adds a retardation, R , to the retardation R
c s
induced by stresses in the surface of the specimen. The
analyzer, A, placed between the eyepiece, E, and the wedge-
FIG. 2 Fringes Observed in the Plane of the Compensator
compensator, W , generates a visible set of fringes or lines of
c
constant retardation R where
angle u, measured using a goniometer, and to an instrument
R 5 R 1 R (1)
s c
calibration constant, K MPa (psi), determined by the manufac-
turer.
Since the specimen-induced retardation is proportional to the
surface stress, S, and the path, t, we have:
a·C·S
tanu5 and (5)
R 5 C·S·t 5 C·S·aX (2) b
s
b
where: Stress 5 · tanu5 K·tan u
C
a
R 5 is the relative retardation,
In the actual procedure (see 9.1 below) the operator mea-
C 5 stress-optical constant (see Note 2),
sures the tilt angle u of the observed set of fringes.
S 5 surface stress in the direction perpendicular to the
path, t
NOTE 1—The surface-stress measuring apparatus described in this
t 5 path of light traveling between the entrance and exit
section is manufactured by Strainoptic Technologies, Inc. in North Wales,
points 1, 2 (Fig. 1),
Pennsylvania.
a 5 Geometrical factor, (depending upon the prism de-
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
sign) a 5 t/X. This constant is determined by the
described in Test Methods C 770.
manufacturer.
6.1.3 The compensator adds its own retardation. It is lin-
6.2 Procedure B: Measuring Edge Stress:
early variable along its length y and is calculated as
6.2.1 Measurement of edge stress is accomplished using a
polarimeter equipped with a wedge-compensator, as shown
R 5 b·y (3)
c
schematically in Fig. 3.
Where b is a constant, determined by the manufacturer of the
6.2.2 The angle between the polarizer and the edge of the
compensator. The observer sees in the compensator plane a
specimen must be 45° (see Fig. 3a), and the analyzer must be
total retardation R.
perpendicular to the polarizer. The overall magnification
R 5 R 1 R 5 a·C·S·x 1 b·y (4)
s c
should be at least 203 to permit clear visibility of the reticle,
6.1.4 The fringes (lines of R 5 Constant) are, therefore, and of photoelastic fringes near the edge. The reticle placed
adjacent to the specimen must have graduations of 0.1 mm
tilted lines. (See Fig. 2). The angle u is the tilt of these fringes
relative to a plane containing the light path of Fig. 1 and Fig. (0.004 in.) or smaller. The resolution of the compensator
2. The measured stress is proportional to the tangent of the tilt should b
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