### ASTM WK37341

#### Work Item: ASTM WK37341 -
Revision of C770 - 98(2008) Standard Test Method for Measurement of Glass Stress—Optical Coefficient

(What is a Work Item?)

**Active Standard: C770 - 98(2013)**

Developed by Subcommittee: C14.04 | Committee C14 | Contact Staff Manager

**1. Rationale**

Correction of Mathematical Expression Used to Calculate Stress-Optical Coefficient (ASTM C770, Procedure B) An Interlaboratory Study (ILS) is planned to establish a Precision and Bias statement for ASTM Standard Test Method C770, Measurement of Glass Stress-Optical Coefficient. In the course of developing the testing protocol described for Procedure B (Beam-Bending Method), a significant discrepancy was found between the experimental results and the expected values. Upon investigation, it was determined that there was an apparent error in exponential notation (possibly a typo or editing error) in the equation used for calculating the stress-optical coefficient. The error was revealed by iteration and restatement of the terms in the equation to enable agreement among the various units of measurement in the expression. When the correction was applied, the resulting values were within the expected range. The origin of the published equation is unknown to ASTM. Therefore, in the absence of evidence to the contrary, this proposed revision is assumed to be a correction. Any C14 member with a differing opinion is invited to respond with a clarification. The purpose of this workitem is to obtain approval by ballot of the suggested corrections/revisions in order to proceed with the ILS. In addition to the change in the equation, editorial changes are recommended to replace the measurement unit shown in Figure 7 (kg) with (N) and to simplify an expression (m/m * Pa) to a more preferred equivalent term (1/Pa) in the equation used to calculate the stress-optical coefficient (K). A full description of the proposed revision is included in the collaboration area or available via email from Joel Feingold at stress4@strainoptics.com.

**Keywords**

Glass; Optical coefficient; Optical materials/properties/tests; Stress--glass materials/applications; Stress cracking tests;

Citing ASTM Standards

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