ASTM F218-13 (2020) pdf free.Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass.
1.1 To measure the retardation at any given point, remove the tint plate, place the monochromatic filter in the field of view, and rotate the analyzer with respect to its initial position until maximum extinction (darkness) occurs at the POI. The angle a through which the analyzer must be rotated to the left or the right is a measure of the retardation at the point.
1.2 In white light, the color of the fringe moving toward the POI will keep changing. To eliminate possible errors and to increase the contrast, the monochromatic filter, B, must be inserted for this operation, or the monochromatic lamp must be used.
2.1 When the retardation is required to be measured in a given area or section where several extinction points may exist,rotate the analyzer (CW or CCW) until the maximum extinction is achieved at each selected point. Use the procedure previously described in this section to measure retardation at those points, and the sequence of the observed colors described in 8.3 to differentiate between tensile or compressive stress.
2.2 When a maximum value is specified and the specimens are of a uniform thickness it is necessary only to set the analyzer at the angle specified and then observe whether any unclosed loop-shaped fringes are present in the stress pattern. If not, it may be concluded that the maximum retardation that is present is less than the specified maximum. If any are present, then the retardation is greater than the specified maximum. To determine the exact magnitude of the retardation, use the method outlined in 8.2 and 8.4.
2.3 When the full-wave plate (also called the“tint plate”) is introduced, the polarimeter can be used to reveal a color pattern. White light must be used for this observation, and the analyzer must be set in standard position (perpendicular to thepolarizer). Table 2 shows the color distribution that may be expected together with the associated magnitude of the retardation and tension-compression indicated.ASTM F218 pdf download.