Scope:

The strength and performance of heat-strengthened and fully tempered glass are greatly affected by the surface and edge stress induced during the heat-treating process. Residual stresses are stresses that remain in a solid material even after the original cause of the stresses has been removed. ASTM C1279 offers a direct, non-destructive way to determine the residual stress on the surface and at the edge of an annealed and heat-treated glass. 

This test method uses transmitted light and is, therefore, applicable to light-transmitting glasses only. It does not apply to glasses that are chemically tempered. For float glasses, surface stresses can be measured only on the “tin” side. Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin.

Test Procedure:

Two test methods are described in ASTM C1279. Both methods are based on the concept of photoelasticity. Photoelasticity describes changes in the optical properties of a material under stress. It is used to experimentally determine the stress distribution in a material, where it gives a picture of the stress distributions around the discontinuities in materials.

Polarized light is introduced into the tempered glass. The speed of the light changes due to residual stresses. Since residual stresses are not spread evenly across the glass, the change in speed of light is different at different parts. The differences in the speed of light rays vibrating along the maximum and minimum principal stress introduce relative retardation between these rays. This relative retardation is proportional to the measured stresses, and it is measured using compensators.

The following are the two test procedures in ASTM C1279:

Procedure A measures surface stress by using light propagating nearly parallel to the surface. The number of points to be measured is determined by the product specification.

Procedure B measures edge-stress by using light propagating in the direction perpendicular to the surface. Readings are obtained at the midspan point of every edge.

Before the test, the specimen is allowed to reach ambient temperature throughout its entire mass to avoid thermal stresses. 

 Specimen size:

The test specimens used in ASTM C1279 testing should be annealed, heat-strengthened, and tempered flat glass products.

Data:

The following data is calculated in ASTM C1279 testing:

Procedure A

Surface Stress:

S = K tan θ

Where:
S = Surface Stress
K = instrument calibration constant determined by the manufacturer

Procedure B

Edge Stress:

Edge Stress S_e = R_e/T_*C_B

Where:
R_e = the compensator reading
t = the thickness of the glass sample
C_B = stress-optical constant
S_e = stress
b = compensator constant, determined by the manufacturer.

Conclusion:

ASTM C1279 is a non-destructive method to determine the residual stresses at the edge and on the surface of annealed, heat-strengthened, and tempered flat glass products.