In creating the Interlaboratory Study for ASTM C770, which includes three different test methods, Procedure B was selected for use in the ILS. During the course of developing the protocol and testing the procedure, a significant discrepancy was discovered in the equation used to calculate the glass stress-optical coefficient. An attempt to locate the original source documents was unsuccessful. While it is possible to change the equation arbitrarily and force a correct solution, it cannot be proven to be mathematically sound or based on any known principles. In view of this, and also for the sake of employing a simpler, more widely used method, I am proposing that the existing Procedure B be replaced by a method of obtaining the glass stress-optical coefficient using a classic formula for calculating bending stress in conjunction with a suitable 4-point apparatus and the Senarmont method of Analyzer Rotation using a calibrated polarimeter, per ASTM Standard Test Method F218.
KeywordsGlass; Optical coefficient; Optical materials/properties/tests; Stress--glass materials/applications; Stress cracking tests;
Ballot Item Approved as C770-2016 and Pending Publication