Measurement of Glass Stress—Optical Coefficient ASTM C770

Measurement of Glass Stress—Optical Coefficient ASTM C770

ASTM C770 test method includes procedures for measuring the stress-optical coefficient of glass used in photoelastic analysis. The standard units of the stress-optical coefficient are mm²/N.


    ASTM C770 is useful in determining the magnitude of residual thermal stresses for pre-stressing (tempering) or annealing glass. This standard test method is also helpful for deciding optical retardation for a beam of a glass of rectangular cross-section when subjected to uniaxial tension, four-point bending, and uniaxial compression. Stress-optical coefficients mainly determine stress in the glass. 

    Test Procedure:

    Test MethodSection
    Procedure A (Uniaxial tension)4.1.1
    Procedure B (Beam Bending)4.1.2
    Procedure C (uniaxial compression)4.1.3

    Specimen Size : 

    The stress optical coefficients K are calculated at a wavelength of λ = 0.5893 µm and temperature of 21°C. The measurement accuracy is ± 3% or ± 0.06·10–6 mm²N–1.


    n = refractive index. 

    E = Young’s modulus.

    µ = Poisson number.


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