New initiatives in single-colour image-based fringe order estimation in digital photoelasticity

Abstract

The use of single-colour image for determining the fringe orders in digital photoelasticity has gained importance in recent years, and in this, the fringe orders are obtained by comparing the colour information at each pixel of the model image using a calibration table. They were originally used for identifying fringe orders of up to 3, and there have been attempts to extend this to higher fringe orders. The methods proposed were tested only for the special case of the application specimen where the fringe signature is similar to that in the calibration specimen for fringe orders beyond 3. In this article, various colour difference formulae are studied, and a new colour difference formula is proposed to handle fringes beyond 3. The advantage of the new colour difference formula is illustrated for the problem of a thick ring subjected to internal pressure and a finite plate with a hole. A new multi-directional smoothing methodology is proposed to smooth the fringe order obtained. The results are compared with those obtained by 10-step phase shifting technique. Advancing front scanning approach coupled with multiple seed points is used to solve a variety of problems in which both the geometry and the fringe signature are quite different from the calibration specimen. The influence of seed point selection is brought out by solving the problem of a plate with a hole having up to 15 fringe orders. New approaches to handle noisy industrial images are also dealt with.