ASTM F218 Optical Retardation and Analyzing Stress in Glass
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 this test procedure, a Polarimeter is used for analyzing stress & optical retardation.
The ASTM F218 test method helps in the 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 stress are identified by rotating the sample at 45° and 90°. For a larger retardation analysis, rotation is used to determine the stress signs & the tinted plate is removed. A monochromator filter is placed in the field to measure retardation at any point. The polarimeter is used to reveal a color pattern.
Specimen Size :
The sample specimen should have a uniform thickness.
For ASTM F218, the optical retardation is calculated by:
R= λ·α /180=f λ
R – optical retardation,
α – measured analyzer rotation
λ – wavelength of monochromatic light used in the polarimeter, and
f – fractional order, f = α/180
The ASTM F218 test method helps in the quantitative evaluation of stresses.
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