ASTM F218 Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass

ASTM F218 is a standard test method for the quantitative evaluation of residual stress in glasses. Residual stresses develop in glasses due to process, differential thermal expansion between fused components, and inclusions. The performance of different glass products may be affected by the presence of residual process stresses, differential thermal expansion between cast components, and inclusions. The ASTM F218 (Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass) test method guides the analysis of stress in glass using a polarimeter, and the test procedure is based on principles developed by Jessop and Friedel.                                                                                                           

The test sample should have a uniform thickness. 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. To reveal a color pattern, the polarimeter is used when the full-wave plate is introduced. Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass

The ASTM F218 test method helps in the quantitative evaluation of stresses. The residual stress in glass samples is evaluated as a function of optical retardation. Optical retardation is expressed as the angle of rotation of an analyzing polarizer that causes extinction in the glass.

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