Framework to select refining parameters in Total fringe order photoelasticity (TFP)

Abstract

What makes Total fringe order photoelasticity (TFP) the ideal digital photoelastic technique for industrial applications is the sufficiency of a single colour isochromatic image for fringe order evaluation and the convenience of capturing this image using easily accessible, relatively cheap hardware. In TFP, the colour information in the application image is matched with an experimental colour calibration table. A simple least squares colour matching is insufficient for this due to errors caused by the repetition of colour in the isochromatics. This is rectified through a process called fringe order refinement. Among the existing fringe order refining methods, the combination of – flexible seeding, window search method, and FRSTFP scanning – has proved to be the best choice for solving a variety of problems for the complete model domain. The successful application of this combination requires the right choice of parameter values for its constituent methods. A systematic parametric study of these refining parameters is a lacuna in the literature, which is addressed in this work, by selecting a variety of problems. The study has revealed the interconnection between the refining parameters, which has helped to formulate a set of recommendations for selecting the same. It is found that some of the existing understandings on refining parameters require a relook. The difficulty in refining fringe fields having multiple fringe gradient reversals with a single set of parameters is illustrated, and how this can be overcome by local refinement is brought out. Additionally, a methodology to track the source of refining error is proposed.