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 enduser to make an informed choice on the type of technique to be used in a particular situation.

Krishnamurthi Ramesh

Department of Applied Mechanics

T. Kasimayan

ambiguity

DIGITAL POLARISCOPE

INCONSISTENCY

INTEGRATED PHOTOELASTICITY

ISOCHROMATICS

ISOCLINICS

PHASE MAPS

PHASE UNWRAPPING

PHASE-SHIFTING

REFLECTION PHOTOELASTICITY

STRESS SEPARATION

ELASTICITY

Photoelasticity

Stress analysis

Physical optics

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IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Digital photoelasticity – A comprehensive review

Journal of Strain Analysis for Engineering Design

The effect of an interacting internal crack on the edge crack in a transient thermal stress field is evaluated using digital photothermoelastic experiments and finite element (FE) analysis. Initially, a transient thermal stress field is simulated using FE analysis for which the thermal boundary conditions are set based on the experimental isochromatic fringes. In this simulated thermal stress field, single edge crack with and without an internal short crack interacting at different configurations are modeled and the non-dimensional Stress Intensity Factor (SIF) values are evaluated at different time intervals. It is observed that the asymmetric and collinear configurations tend to increase the SIF of the edge crack whereas a parallel crack tends to decrease the SIF of the edge crack. For experimental evaluation, specimens with a single edge crack and asymmetric configuration of interacting cracks are considered and the non-dimensional SIF values are evaluated under a thermal stress field generated by edge heating and cooling. The results are compared with those obtained using the FE analysis. © 2019, Society for Experimental Mechanics.

Experimental Mechanics

Study of Crack Interaction Effects Under Thermal Loading by Digital Photoelasticity and Finite Elements

This paper discusses the method for evaluating the unknown contact parameters by post processing the whole field fringe order data obtained from reflection photoelasticity in a nonlinear least squares sense. Recent developments in Twelve Fringe Photoelasticity (TFP) for fringe order evaluation from single isochromatics is utilized for the whole field fringe order evaluation. One of the issues in using TFP for reflection photoelasticity is the smudging of isochromatic data at the contact zone. This leads to error in identifying the origin of contact, which is successfully addressed by implementing a semi-automatic contact point refinement algorithm. The methodologies are initially verified for benchmark problems and demonstrated for two application problems of turbine blade and sheet pile contacting interfaces. © 2018 Elsevier Ltd

Optics and Lasers in Engineering

Analysis of contact zones from whole field isochromatics using reflection photoelasticity

Earlier studies on photoplastic behaviour of polycarbonate are limited to the strain at which the localized yielding (necking) initiates (about 6% strain). In this study, the photoplastic behaviour of polycarbonate is studied using digital image correlation (DIC). Two cases are considered: one in which no localized yielding is observed and the other in which initiation and propagation of the localized yielding is observed. For the first case, a modified dog bone specimen is proposed to evaluate the photoplastic constant. In the second case, a conventional tensile test specimen is used. Both photoplasticity (transmission and reflection) and DIC experiments are performed on the specimens to determine the relation between fringe order and principal strain difference for strains of up to 60%. It is observed that the photoplastic constant is a function of strain. Finally, the behaviour of the tensile test specimen is numerically simulated. The true stress-strain curve of polycarbonate needed for FE simulation is determined using DIC experiments on the standard tensile test specimen. By post-processing the FE results, isochromatic fringes are plotted and compared with experimental results. © 2018, Society for Experimental Mechanics.

Study of Photoplastic Behaviour of Polycarbonate using Digital Image Correlation

Experimental studies to exploit photoelastic data of conformal geometries to extract contact parameters are non-existent because closed-form stress field equations were not available until recently. In this paper, the explicit equations recently reported in the literature for a flat punch with rounded edges are generalized so that a single set of equations can be used for a flat punch with rounded edges and Hertzian contacts with arbitrary radii of curvatures. The generality of the governing equations is verified by plotting isochromatics for conformal and non-conformal contact situations. A generic method to evaluate unknown contact parameters from the whole-field isochromatic data for conformal and non-conformal geometries is implemented. The methodology is initially verified using theoretically generated isochromatic data and is then used to experimentally evaluate two contact situations. In view of high-fringe gradient zones, the suitability of various digital photoelastic methods is compared. A novel four-step phase shifting technique is proposed in which isochromatic and isoclinic data can be evaluated using the minimum number of images. © 2018, Society for Experimental Mechanics.

Evaluation of Conformal and Non-Conformal Contact Parameters Using Digital Photoelasticity

The use of white light based Twelve Fringe Photoelasticity/RGB Photoelasticity has gained popularity in the recent years. The main advantage of this technique is that it requires only a single image recorded under white light for isochromatic demodulation. This makes it suitable for problems where multiple acquisitions are difficult. Accuracy of fringe order estimation using Twelve Fringe Photoelasticity is dependent on the spatial resolution of the isochromatic fringe pattern. The existing scanning schemes for refining the fringe order data do not take this into account and leads to the propagation of noise from the low resolution zones. In this work, a novel scanning scheme is proposed whose progression is guided by the spatial resolution of the isochromatic fringe pattern. Initially, a method based on the intensity gradients is developed to create a whole field map resembling the resolution of the fringe pattern. This map is used to guide the scanning scheme for refining the fringe order data. The proposed scanning scheme encapsulates the noise within the zones of low spatial resolution and eliminates their propagation. The working of the scanning scheme is demonstrated using the benchmark problem of a circular disc subjected to diameter compression. © The Society for Experimental Mechanics, Inc. 2017.

Conference Proceedings of the Society for Experimental Mechanics Series

Novel scanning scheme for white light photoelasticity

The recent advances in digital photoelasticity have made it possible to use it conveniently for the stress analysis of articles and components made of glass. Depending on the application, the retardation levels to be measured range from a few nanometres to several thousand nanometres, which necessitates different techniques and associated equipments. This paper reviews the recent advances in the photoelasticity of glass with a focus on the techniques/methods developed in the last decade. A brief introduction to the residual stress in glass is provided initially to bring out its tensorial nature. The subsequent sections are organised thematically rather than chronologically, for better readability and easy access of information. © 2016 Elsevier Ltd

Digital photoelasticity of glass: A comprehensive review

Recently, stereolithography, one of the rapid prototyping (RP) techniques, has simplified the process of making three-dimensional (3-D) photoelastic models. One of the issues in stereolithography-made models is the noise due to porosity of the model. This is undesirable for data handling in digital photoelasticity. A preliminary study showed that the thickness of the slice has an influence on the appearance on the noise. In this paper, use of 10-step phase-shifting technique (PST) and refined three-fringe photoelasticity (RTFP) is explored to determine the isochromatic data as accurately as possible. A slice cut from a 3-D model of a spline shaft made of stereolithographic material is used for isochromatic determination. It is found that with suitable postprocessing, the quantitative results obtained from 10-step PST and RTFP are comparable. The relative merits of these two techniques for analysing stereolithographic models are brought out. © 2011, Society for Experimental Mechanics.

Experimental Techniques

Noise-free determination of isochromatic parameter of stereolithography- built models

A new technique used to tune the color code table developed for one specimen in three fringe photoelasticity (TFP) for another specimen that may have some color variation was investigated. A calibration table needs to be generated with RGB values assigned with known fringe orders in TFP/refined three fringe photoelasticity (RGBP). A beam under 4-point bending was conventionally used to generate the calibration table as the variation in fringe order is known to be linear across the depth of the beam. The use of a single calibration table could help to simplify the use of TFP in an industrial environment. It can be done by suitably modifying the RGB variation of the calibration table in such a way that it matches the application specimen. The proposed method finds applicability in analyzing stress-frozen slices and could be extended for transient problems. The validity of the proposed technique was demonstrated using a 4-point bend specimen and the benchmark problem of a disc under diametral compression.

Colour adaptation in three fringe photoelasticity using a single image

In three fringe photoelasticity, a single colour isochromatic image is used to estimate the total fringe order by comparing the colour at each point with that of a calibration table. The generation of the calibration table is critical, and certain nuances involved therein are brought out in this paper. Ideally, the same specimen material and lighting conditions must be used for both the calibration and application experiments, which are often not possible in an industrial scenario, and colour adaptation is a simple way of suitably modifying the calibration table. Till date, only experimental analyses of the colour adaptation procedures are available in the literature; in this paper, the colour adaptation procedures are investigated from an analytical viewpoint. This study has brought out that a two-point colour adaptation scheme accounts not only for tint variation between the calibration and application specimens, but also for ambient illumination. This finding has also been experimentally verified for a benchmark problem, and thereafter extended for reflection photoelastic analyses. © 2010 Elsevier Ltd. All rights reserved.

The influence of ambient illumination on colour adaptation in three fringe photoelasticity

The condition for the occurrence of inconsistent zones when all the stresses are known is found based on the Mohr's circle of stress, and a new concept of inconsistency map is introduced. To verify the appearance of inconsistent zones experimentally, ten-step phase-shifted images are recorded using a monochrome CCD camera. Because of experimental difficulties, the boundaries of the inconsistent zones are not as smooth as in the analytical results. The experimental dark field isochromatics and wrapped isoclinic phasemap reveal that the stress field is very complex making the identification of inconsistent zones very difficult. The inconsistency map is useful in identifying the inconsistent zones and selecting a suitable seed point for unwrapping to obtain the unwrapped experimental isoclinic phasemap, which is then smoothed using the outlier smoothing algorithm.

Journal	Journal of Strain Analysis for Engineering Design
ISSN	03093247
Open Access	No