Scope:

Using a polarizing microscope and optical retardation compensation techniques, ASTM C978 test calculates residual stress in a transparent glass matrix. The magnitudes and local distributions of residual stress systems in glass substrates can be determined using this test method. It’s also used to see how compatible the coefficient of thermal expansion of enamel or applied color label (ACL) applied to a glass substrate is with the coefficient of thermal expansion of the glass substrate. The ASTM C978 test approach is quantitatively acceptable and effective when no substantial ion exchange and optical stress coefficients are known.

Test Procedure:

The ASTM C978 test method utilizes photoelastic retardation compensation procedures to determine residual stresses in clear glass matrices quantitatively. To produce a retardation null or extinction in the test specimen, compensators such as rotating compensators (covering a range of one order of retardation), birefringent quartz wedge compensators (covering a range of four to six orders of retardation), or tilting optical retardation compensators (covering a range of five orders of retardation) are used.

Specimen Size : 

Ring section, bar section, or any other correctly shaped section of the test specimens with suitable annealing are used in ASTM C978 testing.

The length of the optical path in the segment of the test specimen shall not be less than 2.0 mm and not more than 30.0 mm.

Data:

For ASTM C978, the optical retardation value, R, is calculated as :

R = (L × a) /180°

where,
L = wavelength
a = angle of rotation

Conclusion:

ASTM C978 determines residual stress in a transparent glass matrix using a polarizing microscope and optical retardation compensation procedures.