An Improved Two-Load Method for Whole-Field Complete Photoelastic Fringe Analysis

Abstract

An improved two-load method for whole-field complete determination of photoelastic parameters is presented. The dark-field isoclinic images are used to determine the isoclinic angles. Using two isoclinic maps obtained from two different loads effectively compensates the indeterminable points. The use of dark-field and light-field photoelastic images for normalization extends the two-load method to analyze dark-field photoelastic fringe patterns and avoids model movement. Larger errors on the determined fringe orders are further reduced by a least-squares quadric fitting. The results are compared well to the theoretical ones. Further comparison of the improved two-load method and the two-wavelength method are given.

Data Acquisition Techniques in Photoelasticity

“…Another possibility for obtaining the phase‐shifted images necessary in the “phase shifting” technique was to use different loads (load stepping) 79–85 or different wavelengths (wavelength stepping) 86–90 instead of employing different positions of the filters (phase stepping). These new approaches surmounted the limitations of the unwrapping process in “phase stepping.” Research continues to this day in order to improve all these “phase shifting” techniques, which offer great potential: the use of white light and the combination with other techniques (as will be seen later on) substantially improve the technique.…”

“…Another possibility for obtaining the phase-shifted images necessary in the ''phase shifting'' technique was to use different loads (load stepping) [79][80][81][82][83][84][85] or different wavelengths…”

Numerical Methods for the Photoelastic Technique using Phase Shifting

“…To compare the new equations for calculating the phase, nine sets of photos with Step set to 3,4,6,7,10,11,16,19, and 31 were generated. In each set, the angle  of the analyzer is varied (): 45, 30, 18, 15, 10, 9, 6, 5, and 3, respectively.…”

“…Usually, the phase of the reference light beam is altered in known steps. Photoelasticity falls into a special category, in that the two light beams cannot be treated separately, but rather always go together [4]. This means a phase shift introduced in one light beam will also introduce a corresponding phase shift in the other beam.…”