Evaluation of uncertainty in the measurement of the stress-optic coefficient

In this study, a method for evaluating the uncertainty of stress-optic coefficient measurements of photoelastic materials on a uniaxial tension/compression specimen is presented. The same problem is also approached in other previously used methods, in which multiple data fitted with ordinary least squares are considered. However, only the repeatability contribution to uncertainty is considered in those other methods, which are therefore not consistent with the ISO–Guide to the expression of Uncertainty in Measurement. All possible contributions to uncertainty can be taken into account using the General Least Squares-Lagrange Multipliers (GLS-LM) method presented here. The application of the method is illustrated with an example, from which it can be seen that uncertainty has been underestimated in the other methods used to date. As well as the estimate of the stress-optic coefficient and its corresponding standard uncertainty, the method also provides a data consistency test and an outlier identification tool.