ASTM F218 Optical Retardation and Analyzing Stress in Glass

Scope:

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.                                   

Test Procedure:

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.

Data:

For ASTM F218, the optical retardation is calculated by:

 R= λ·α /180=f λ

where: 

R – optical retardation, 

α – measured analyzer rotation 

λ – wavelength of monochromatic light used in the polarimeter, and 

f – fractional order, f = α/180

Conclusion:

The ASTM F218 test method helps in the quantitative evaluation of stresses.