Completely automated determination of two-dimensional photoelastic parameters using load stepping

The new approach to phase-stepping photoelasticity known as "load stepping" is used to determine automatically the isochromatic parameter alpha and the isoclinic angle theta. There is no need for the user to calibrate the results other than to convert the isochromatic parameter into a principal stress difference using the material fringe constant. Four phase-stepped images are collected using a circular polariscope for each of three load steps, which differ by small equal increments. A ramped phase map for the isochromatic parameter is produced in the range -pi<alpha not less than or equal to pi that can be unwrapped using conventional techniques. Calibrated values of alpha can then be calculated using the phase map of partial derivative alpha, which is the incremental change in the isochromatic value between the different steps. The value of the isoclinic angle is automatically determined in the correct range -pi/2<theta less than or equal to pi/2 without unwrapping. The results obtained from experimental testing of a disk-in-compression specimen using transmission photoelasticity presented in comparison with theoretical solutions demonstrate the accuracy of the new approach. In another test performed on a bar in torsion using reflection photoelasticity we demonstrate how determination of the isoclinic angle in the range -pi/2<theta less than or equal to pi/2 gives results superior to those obtained if the isoclinic angle is determined in the wrapped range -pi/4<theta less than or equal to pi/4. (C) 1998 Society of Photo-Optical instrumentation Engineers. [S0091-3286(98)01806-6].