Digital determination of photoelastic birefringence using two wavelengths

Abstract

ABSTRACT--A new approach for digital determination of photoelastic birefringence is proposed. The relationships between the intensity values and the fringe orders of two wavelengths are derived. This scheme allows for automatic determination of fringe orders of a full-field photoelastic fringe pattern without using zero-order fringes in the fringe pattern. The usefulness of this method was demonstrated on two experimental fringe patterns with different wavelengths.

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.…”

“…(wavelength stepping) [86][87][88][89][90] instead of employing different positions of the filters (phase stepping). These new approaches surmounted the limitations of the unwrapping process in ''phase stepping.''…”

A temporal phase unwrapping algorithm for photoelastic stress analysis

“…Moreover, the extrapolation procedure requires isolating the object from the background. Alternatively either two levels of load [7][8][9] or two or more monochromatic light sources [10][11][12][13][14] can be used. In this way, as shown by Eq.…”

<title>Complete analysis of photoelastic fringe patterns using two wavelengths</title>

“…(4), the fringe order is multiple-valued as N = n + sin (IL/lu)"2 lit, for n = 0, 1, 2, and N = n -sin_I (IL/iu)2 Irt, for n = 1, 2, 3, ... (8) If the maximum fringe order in the field of interest does not exceed 1/2, the fringe order can be determined without ambiguity. However, ifthe maximum fringe order is greater than 1/2, two patterns having different wavelength are required to determine the total fringe orders.…”

“…However, in cases where the maximum fringe order is greater than 1 ,errors in the digitized value mean that exact fringe orders may not be those with a minimum difference. Thus a procedure that uses a 5x5 cross-shaped points was adopted to find the correct total fringe order, N, ofthe central point for one 8 from the multiple-valued fringe orders obtained from the generated isochromatic fringe patterns. After the correct total fringe order of a point was determined, this point was used as a seed point to find the total fringe orders of the rest of the points.…”