Determination of the isoclinic map for complex photoelastic fringe patterns

Most of the existing algorithms used for processing phase-shifted photoelastic data attempt to compute the unambiguous or demodulated isoclinic map in order to obtain the unambiguous or continuous isochromatic map. However, in some cases experiments on engineering components yield isoclinic maps that are severely corrupted due to the interaction between isoclinics and isochromatic. The result is that some of these algorithms fail in the direct demodulation of isoclinic maps from phase-shifted photoelastic data. An indirect way to obtain the isoclinic map by computing first the unambiguous isochromatic map is presented. The employed approach is based on a regularisation process that, by minimising a cost function, selects the appropriate value of the relative retardation angle at each pixel. In this way, an unambiguous map can be straightforwardly unwrapped and calibrated to generate an isochromatic map. The unambiguous isoclinic angle map is then calculated using the regularized isochromatic map. The process has been demonstrated to be robust and reasonably quick for crack tip fringe patterns.