Assessment of Fringe Pattern Normalisation for Twelve Fringe Photoelasticity

Single colour image based photoelastic techniques have gained importance in the recent years. In twelve fringe photoelasticity (TFP) technique, the whole field fringe order (N) data is obtained by comparing RGB intensities of all the pixels in the image with that in the calibration table. This technique is suitable for problems where multiple acquisitions of the model are difficult. Initially, the methods were proposed to demodulate a maximum fringe order of 3 since the colours tend to merge after this. There have been efforts to push this limit by using advanced imaging hardware and fluorescent light sources. Recent research in this field explored the use of fringe pattern normalisation in conjunction with the use of theoretically generated calibration table for isochromatics modulation. However, the method was demonstrated using models having almost uniform fringe gradients. This work assesses the performance of fringe pattern normalisation for twelve fringe photoelasticity in problems having different fringe gradients. Studies are carried out in the benchmark problem of a circular disc under diametral compression. The isochromatic results obtained are compared with the corresponding values obtained analytically.