Whole field evaluation of stress components in digital photoelasticity—Issues, implementation and application

Digital photoelasticity has rapidly progressed in the last few years and has matured into an industry-friendly technique. This review thematically classifies all the developments in digital photoelasticity and highlights the relative merits and drawbacks of the various techniques. The overall objective is to provide enough information and guidance to allow an end-user to make an informed choice on the type of technique to be used in a particular situation.

Digital photoelasticity – A comprehensive review

With advancements in digital image processing and data acquisition, a separate branch of photoelasticity namely digital photoelasticity came into existence. Here, intensity information of the acquired image is used for the evaluation of whole-field photoelastic parameters. Digital photoelasticity provides only wrapped phasemaps of isoclinics and isochromatics and they have to be unwrapped in different ways for getting the continuous-phase values. In the case of the isochromatic phasemap, ambiguity removal prior to unwrapping is essential. In this paper, a 10-step phase-shifting methodology is proposed and a new strategy for obtaining the isochromatic phasemap free of ambiguity is demonstrated. Isoclinic unwrapping is performed by a new adaptive quality guided algorithm. Adaptive in the sense that isoclinic phase unwrapping is done autonomously even in the presence of isotropic points/π jumps occurring in the isoclinic phasemap. The isochromatic phasemap is also unwrapped using the quality guided path follower. The methodology is validated for the problem of a ring under diametral compression and later shown for three other models which have complex stress fields. Wherever possible, the parameters obtained by the new methodology are compared with analytical or numerical methods and the comparison is quite good.

Adaptive Quality Guided Phase Unwrapping Algorithm for Whole-Field Digital Photoelastic Parameter Estimation of Complex Models

Branch cutting algorithm for unwrapping photoelastic phase map with isotropic point

Photoelasticity is the only whole-field experimental technique which can analyse both 2-D and 3-D elasticity problems. In digital photoelasticity one gets two phase maps, one corresponding to principal stress direction (isoclinic) and the other corresponding to principal stress difference (isochromatic). The phase maps for both isoclinics and isochromatics are to be unwrapped differently for obtaining continuous phase values. Autonomous phase unwrapping is one of the challenging issues and this paper focuses on recent advances on isochromatic phase map unwrapping. A comparative study of different autonomous phase unwrapping algorithm is done by solving a bench mark problem and a stress frozen slice with cut-outs. The need for domain delimiting in addition to domain masking is brought out while using autonomous phase unwrapping algorithms.

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Assessment of autonomous phase unwrapping of isochromatic phase maps in digital photoelasticity

Phasemaps obtained by phase-shifting techniques contain ambiguous zones. To obtain total fringe order by phase unwrapping, the phasemap should be corrected for ambiguity. Each available ambiguity removal method can resolve ambiguity for only a class of problems. Appropriate selection of these algorithms and their combination for a specific case can give a better result. A comprehensive ambiguity removal methodology is demonstrated in this paper. Disturbance in the intensity pattern of the phase shifted images due to lines and marks on the models lead to noise in phasemaps. This in turn affects the ambiguity removal and unwrapping procedures. A methodology has also been proposed to remove the lines and marks from the phase-shifted images.

P. L. Prashant

Papers

New initiatives in phase unwrapping in digital photoelasticity