Optical Retardation and Analyzing Stress in Glass ASTM F218
ASTM F218 provides the analysis of Stress in glass using a polarimeter. Stress is estimated as a function of optical retardation, expressed as the angle of rotation of an analyzing polarizer that affects extinction in the glass. In Procedure, a Polarimeter is used for analyzing Stress & optical retardation.
The ASTM F218 test method helps in quantitative evaluation of stresses. The performance of different glass products may be affected by the presence of residual process stresses, differential thermal expansion between cast components and inclusions
In ASTM F218, the Stress field is evaluated by observing the sample, then directions and signs of stresses are identified by rotating the sample at 45° and 90°. For a larger retardation analyzer, rotation is used to determine stress signs & the tinted plate is removed, and a monochromator filter is placed in the field to measure retardation at any point. To reveal a color pattern, the polarimeter is used When the full-wave plate is introduced.
Specimen Size :
The sample specimen should have uniform thickness.
The optical retardation is calculated by:
R= λ·α /180=f λ
where: R is the optical retardation, α =is the measured analyzer rotation, λ is the wavelength of monochromatic light used in the polarimeter, and f is the fractional order, f = α/180.
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