ASTM C770-16 (2020) pdf free.Standard Test Method for Measurement of Glass Stress—Optical Coefficient.
1.1 Mount the fiber specimen vertically by the beaded end in the test fixture so that approximately the midlength is in the polariscope light beam and the fiber image is clearly in focus.
1.2 Adjust the light shield or aperture so that the slits are in the line of sight when viewing the fiber through the telescope.
1.3 Rotate the polarimeter analyzer until a bright area or line is visible, centered in the fiber Cross section and parallel to its sides (the “image” of the light source that the cylindrical fiber“lens” tends to form).
1.4 Rotate the analyzer until the bright line becomes darkest or reaches extinction. Record the retardation indicated on the polarimeter scale either in degrees or nanometres.
2.1 The polarimetry apparatus for the beam-bending procedure is shown in Figs. 4 and 5. Radiation from a whitelight source passes through the following components and in this sequence: a diffuser, a polarizer whose axis is at 45° to the vertical, the glass specimen, a quarter-wave plate, and a second polarizer (analyzer) whose axis is at 90° to that of the first polarizer.
2.2 A schematic of the loading scheme is shown in Fig.
3. Metal fixtures shall be provided to subject the specimen to four-point bending. A support span of 115 mm and a moment arm, a, of 45 mm are recommended. Dimensions within5 % of these values are acceptable. Symmetrical loading is essential, and requires careful centering of the upper loading block. The knife edges shall be finished to approximately 5 mm radius.Loading can be accomplished through a yoke, which rests in Vgroove in the upper loading block, and a weight pan as shown. However, any convenient loading scheme at the center of the upper block may be used.
4. Select a mass of the glass to be tested that has good optical quality with no heavy cords or striae. By conventional lamp-working methods, draw 0.6 to 0.9 m (2 to 3 ft) of fiber from the glass, suficient to provide five specimens 76 to 102 mm (3 to 4 in.) long with taper (variation in diameter along the length) less than 0.025 mm (0.001 in.) and diameters in the range 0.635 mm (0.025 in.) to 0.760 mm (0.030 in.). ASTM C770 pdf download.