ASTM C770-98

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: C 770 – 98
Standard Test Method for
Measurement of Glass Stress—Optical Coefficient
This standard is issued under the fixed designation C 770; 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 designed for the wavelength of the light being used. The
polarizing axes of the polarizer and analyzer shall be set at
1.1 This test method covers procedures for determining the
right angles to each other with each being located at an angle
stress-optical coefficient of glass, which is used in photoelastic
of 45° with the horizontal and vertical. The analyzer, however,
analyses. In Procedure A the optical retardation is determined
shall be mounted in a rotatable mount having a scale graduated
for a glass fiber subjected to uniaxial tension. In Procedure B
on either side from 0 to 180°. The quarter-wave plate shall be
the optical retardation is determined for a beam of glass of
fixed to give maximum extinction when the polarizer and
rectangular cross section when subjected to four-point bending.
analyzer are crossed at right angles; that is, when its polarizing
In Procedure C, the optical retardation is measured for a beam
axes are set at 45° and 135° to the horizontal and vertical. In
of glass of rectangular cross-section when subjected to uniaxial
place of the immersion cell E, a means of supporting and
compression.
loading a glass specimen shall be provided, either in air (Fig.
1.2 This standard does not purport to address all of the
3(a)) or in an immersion liquid (Fig. 3(b)). In this arrangement
safety concerns, if any, associated with its use. It is the
the optical elements of the polarimeter between light source
responsibility of the user of this standard to establish appro-
and telescope have been reversed and a large scale graduated in
priate safety and health practices and determine the applica-
2-nm divisions is employed with the rotatable analyzer I.
bility of regulatory limitations prior to use.
4.1.1.1 Fig. 3 illustrates the fiber-stressing and optical
2. Referenced Documents
arrangement used in Procedure A. Figure 3(a) shows the fiber
mounted vertically, positioned, and supported by two brass
2.1 ASTM Standards:
collars with swivel handles so that the kilogram weight may be
C 336 Test Method for Annealing Point and Strain Point of
applied to load the fiber. A light shield having entrance and exit
Glass by Fiber Elongation
slits surrounds the fiber providing a degree of collimation to the
C 598 Test Method for Annealing Point and Strain Point of
light passing through the fiber and also helping to eliminate
Glass by Beam Bending
stray light.
F 218 Test Method for Analyzing Stress in Glass
4.1.1.2 In Fig. 3(b) the fiber is stressed while immersed in a
3. Significance and Use
liquid which matches the refractive index of the fiber. This
arrangement provides more satisfactory viewing of the fiber.
3.1 Stress-optical coefficients are used in the determination
4.1.2 Procedure B:
of stress in glass. They are particularly useful in determining
4.1.2.1 The apparatus for the beam-bending procedure is
the magnitude of thermal residual stresses for annealing or
shown in Fig. 4(a). Radiation from a white-light source passes
pre-stressing (tempering) glass. As such, they can be important
through the following components and in this sequence: a
in specification acceptance.
diffusing plate, an adjustable aperture, a polarizer whose axis is
4. Apparatus
at 45° to the vertical, the glass specimen, a Babinet compen-
sator, a polarizer whose axis is at 90° to that of the first
4.1 Stressing Equipment and Polarimeter:
polarizer, and a telescope of modest power.
4.1.1 Procedure A— Figs. 1 and 2 illustrate a polarimeter
4.1.2.2 The loading scheme is shown in Fig. 4(b). Metal
employing a quarter-wave plate and rotatable analyzer, de-
fixtures shall be provided to subject the specimen to four-point
scribed in Test Method F 218. The quarter-wave plate shall be
bending. A support span of 115 mm and a moment arm, a, of
45 mm are recommended. Dimensions within 5 % of these
This test method is under the jurisdiction of ASTM Committee C-14 on Glass
values are acceptable. Symmetrical loading is essential, and
and Glass Products and is the direct responsibility of Subcommittee C14.04 on
requires careful centering of the upper loading block. The knife
Physical and Mechanical Properties.
edges shall be finished to approximately 5-mm radius. Loading
Current edition approved Oct. 10, 1998. Published January 1999. Originally
published as C 770 – 73 T. Last previous edition C 770 – 95. can be accomplished through a yoke, which rests in a V-groove
Annual Book of ASTM Standards, Vol 15.02.
in the upper loading block, and a weight pan as shown.
Annual Book of ASTM Standards, Vol 10.04.
However, any convenient loading scheme at the center of the
Goranson and Adams, “Measurement of Optical Path Differences,” Journal of
upper block may be used.
Franklin Institute, Vol 216, 1933, p. 475.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
C 770
FIG. 1 Polarimeter
FIG. 2 Orientation of Polarimeter in Standard Position
4.1.2.3 A Babinet compensator is positioned so as to pro- 4.3 Weights that are known to an accuracy of 61%.
duce vertical fringes (Fig. 4(c)). The neutral fringe must fall
5. Test Specimen
near the center of the support span. Recommended fringe
spacing is 1000 6 200 nm of retardation per centimeter. In 5.1 Procedure A:
actual practice the compensator is placed very close to the 5.1.1 Select a mass of the glass to be tested that has good
specimen inside the loading yoke. optical quality with no heavy cords or striae. By conventional
4.1.2.4 A telescope is mounted in a rotating collar equipped lamp-working methods, draw 0.6 to 0.9 m (2 to 3 ft) of fiber
with an angular scale which can be read to 0.1° by a vernier. from the glass, sufficient to provide five specimens 76 to 102
The cross hairs in the eyepiece are used to measure the tilt mm (3 to 4 in.) long with taper (variation in diameter along the
angle of the neutral fringe as shown in Fig. 4(c). An 80-mm length) less than 0.025 mm (0.001 in.) and diameters in the
objective lens and 103 eyepiece are adequate components for range 0.635 mm (0.025 in.) to 0.760 mm (0.030 in.). The
the telescope. difference in mutually perpendicular diameters at any point
4.1.2.5 The adjustable aperture is set at the smallest diam- along the specimen length shall be less than 0.0076 mm
eter that permits suitable viewing. As with the fiber apparatus, (0.0003 in.).
this provides some collimation and helps to eliminate stray 5.1.2 Bead both ends of each specimen by holding the end
in a flame with the fiber vertical until a bead of two to three
light.
4.1.3 Procedure C: fiber diameters forms.
5.1.3 Anneal the specimens together so as to remove most
4.1.3.1 Polarimeter as described in Test Method F 218.
4.1.3.2 Force application frame, shown in Fig. 5 must of the lamp-working stress (Annex A2).
include: 5.2 Procedure B:
a) A strain-gage load cell and load cell indicator, capable of 5.2.1 Select a mass of glass to be tested that has good optical
measuring the force applied within 1 % accuracy. quality with no heavy cords or striae. By conventional grinding
b) Hydraulic or mechanical means of applying constant methods, prepare a beam of rectangular cross section. The
force and maintaining the force during the measuring time. width of the beam shall be within the range 20 to 30 mm (0.8
c) Swivel-mounted loading blocks, offering at least two to 1.2 in.), the thickness within the range 6 to 10 mm (0.25 to
degrees of swivel freedom, to avoid the loading on the edge. 0.40 in.), and the length within the range 120 to 130 mm (4.75
4.2 Micrometer Caliper, for measuring specimen dimen- to 5.10 in.). Use a fine grind for the upper and lower surfaces
sions to 0.0025 mm (0.0001 in.). (as the beam sits on the loading fixture) and polish the viewing
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
C 770
(a) Fiber in Air (Top View, Optical Elements)
(b) Fiber Immersed
A—Light Source J—Telescope
C—Optical cell and index liquid K—Brass collars
E—Polarizer P—Pulley system
G—Quarter-wave plate S—Shield and slits
I—Rotatable analyzer
FIG. 3 Optical and Fiber-Stressing Polarimeter Arrangement
surfaces. The ends need not be finished and a simple saw cut width, but not longer than 603 thickness, to avoid buckling
will suffice. The four major surfaces shall be flat and parallel to
failures.
within 0.050 mm (0.002 in.).
5.3.4 Both ends must be ground flat and parallel, within 0.1
5.2.2 Before final finishing, fine anneal the glass (Annex
mm (0.004 in.).
A2) to such a degree that when the specimen is placed in the
fixture unloaded there is very little curvature to the portion of
6. Procedure
the neutral fringe that appears within the specimen.
6.1 Procedure A:
5.3 Procedure C:
6.1.1 Mount the fiber specimen vertically by the beaded end
5.3.1 The thickness of the specimen (see Fig. 6) should be
3 in the test fixture so that approximately the midlength is in the
no less than 5 mm ( ⁄16in.).
polariscope light beam and the fiber image is clearly in focus.
5.3.2 The width should be no less than 10 mm ( ⁄8 in.).
5.3.3 The length of the specimen should be larger than 43 6.1.2 Adjust the light shield or aperture so that the slits are
...