Stress Separation in Digital Photoelasticity Part A - Photoelastic Data Unwrapping and Smoothing

Abstract

The advent of digital photoelasticity saw the development of various techniques to get photoelastic data of isoclinics and isochromatics at every pixel in the domain. Unlike in other interferometric techniques, in photoelasticity one gets phase data of two quantities and their mutual dependence and interaction affect their evaluation. For stress separation studies, one requires both isochromatics and isoclinics accurately free of any kinks in the domain. With this in view, a robust approach is evolved to get these parameters. Initially wrapped isoclinics are obtained using a plane polariscope based phase shifting technique. These are unwrapped by a quality guided approach. However, the result has several kinks due to isochromatic-isoclinic interaction. An outlier smoothing algorithm is proposed for getting a smooth variation of the isoclinics. Six-step phase shifting method using a circular polariscope is used for determining the isochromatic data. The smoothed isoclinic values obtained are used in isochromatic calculation to get isochromatic phasemap free of ambiguities. This phasemap is also unwrapped by a quality guided approach and smoothed by the outlier algorithm. These methodologies are validated for two benchmark problems of a disk under diametral compression and a ring subjected to internal pressure. The models are subjected to moderate loads showing a high level of isochromatic-isoclinic interaction.