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.

Krishnamurthi Ramesh

Department of Applied Mechanics

T. Kasimayan

AMBIENT LIGHT

COLOUR ADAPTATION

REFLECTION PHOTOELASTICITY

RGB PHOTOELASTICITY

THREE FRINGE PHOTOELASTICITY

Calibration

ELASTICITY

Light reflection

Photoelasticity

Physical optics

Color

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IIT Madras

Optics and Lasers in Engineering

Colour adaptation techniques are useful in extending the method of Twelve fringe photoelasticity (TFP) to solve problems in industries as one can minimise the generation of specific calibration tables for each experiment. Image artefacts affect the functioning of existing colour adaptation techniques affecting the fringe order demodulation. Colour transfer is a popular technique in the field of image processing to tackle colour mismatch between images. In this paper, the applicability of two colour transfer strategies in the context of TFP is explored, of which one is selected for implementation. A comparative study on the performance of the novel colour transfer method with the existing colour adaptation techniques is carried out. © 2019 Elsevier Ltd

Applicability of colour transfer techniques in Twelve fringe photoelasticity (TFP)

In white light photoelasticity, the fringe order at a point of interest is resolved by comparing the colour data with a calibration table recorded experimentally. Recently, some researchers have proposed the idea of using a theoretical calibration table if the application image is normalized. In this paper, a systematic study is carried out on the applicability of the theoretical calibration table for fringe demodulation. A novel normalization methodology is developed to improve the dynamic range in both low and high fringe density areas. It is demonstrated that while applying normalization for stretching the dynamic range, one has to use only an experimentally recorded calibration table that is normalized. Specific advantages of using colour adaptation or fringe demodulation by normalization in certain practical situations is discussed. © 2018 Elsevier Ltd

An improved normalization technique for white light photoelasticity

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. © Institution of Mechanical Engineers 2015.

Journal of Strain Analysis for Engineering Design

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

Three-fringe photoelasticity (TFP) is a single isochromatic color image based technique to obtain full-field fringe order data, by comparing RGB intensities of a pixel in the image with the calibration table. Fringe order resolving methods based on neighboring resolved pixels, like refined TFP (RTFP) or window search method, are used to ensure spatial continuity of the fringe order. It is shown in this paper that simple image scanning schemes used to implement such resolving methods result in some unresolved zones for complex shaped specimens that are frequently encountered in industrial applications. A new scanning method termed as advancing front scanning proposed in this paper progresses by resolving a pixel having maximum number of resolved neighboring pixels, starting from a seed point or multiple seed points. Advantages of the advancing front scanning method in conjunction with two-point color adaptation are demonstrated through two examples of specimens of which one is obtained by reflection photoelasticity and the other is a complex simply connected stress frozen slice. © 2013 Elsevier Ltd. All rights reserved.

Advancing front scanning approach for three-fringe photoelasticity

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.

Digital photoelasticity - A comprehensive review

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.

Experimental Techniques

Colour adaptation in three fringe photoelasticity using a single image

A simple five-step approach to obtain whole-field isoclinics and isochromatics using a conventional reflection polariscope is discussed. The only additional requirement was a suitable digital camera, while the benchmark problem of a ring under diametral compression was selected to show the effectiveness of the method. The methodology was extended to obtain the isoclinic and isochromatic fields for the problem of an angle bracket and for fields in a flange joint due to assembly stresses. The first four arrangements of the five-step method corresponded to a plane polariscope based polarization stepping algorithm and these optical arrangements were easily set in the commercially available reflection polariscope. The fifth optical arrangement was the conventional dark field circular polariscope arrangement that was easily set in the polariscope.

Digital reflection photoelasticity using conventional reflection polariscope

Transient thermal stresses of a bimaterial specimen with interface edge cracks subjected to heating along an edge is analysed by refined three-fringe photoelasticity (RTFP). Whole-field, noise-free, fringe order estimation using a single colour image is made possible using RTFP combined with colour adaptation. The stress intensity factors (SIFs) of the interface crack are determined through a multiparameter overdeterministic system of equations by a least-squares approach using experimental data collected automatically. The transient SIFs are found to peak to a higher value than in steady state, and the opening mode is found to be dominant. An increase in thermal load causes the crack to propagate, and this is easily visualized on the basis of quantitative fringe order data available for the whole field. The SIFs of a propagating crack are found to be low. The study shows that the crack propagates easily when the opening mode is dominant. © 2009 IMechE.

Fulltext

Transient thermal stress intensity factors of bimaterial interface cracks using refined three-fringe photoelasticity

In photoelasticity, the method of obtaining the individual values of principal stresses/normal stresses separately is referred to as stress separation. Shear difference is one of the widely used techniques for stress separation in photoelasticity and one needs the value of fringe order and the isoclinic angle free of noise at every pixel over the domain. For accurate parameter determination, a ten-step phase shifting approach which uses a plane polariscope for isoclinic determination and a circular polariscope for isochromatic determination is proposed. A new quality guided approach for isoclinic unwrapping is developed. Isochromatic phasemap free of ambiguous zones is obtained by a new methodology and is unwrapped by a quality-guided approach. Whole field evaluation of stress components and its representation is then presented. The models used in this study are intentionally subjected to moderate loads showing a high level of isochromatic-isoclinic interaction. In view of this, the isoclinic data has several kinks which is found to cause streak formation in the whole field representation of separated stress components. An outlier smoothing algorithm is proposed for getting a smooth variation of the digital photoelastic parameters over the domain. Use of such smoothed data for stress separation has removed the streaks and has also greatly improved the accuracy of the separated stress components. © 2007 Elsevier Ltd. All rights reserved.

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

Three-fringe photoelasticity (TFP) can give the total fringe order from a single-colour isochromatic fringe field. In TFP, the total fringe order is obtained by comparing the colour of the unknown photoelastic fringe pattern with a calibration specimen. Comparison is conventionally done by minimising the colour difference error. This leads to misidentification of fringe order in some regions. A new colour difference formula is proposed with an additional term, which ensures continuity of fringe order data over the domain. Fringe order data obtained by conventional TFP is used as base data and the new method is termed as refined TFP. To enable applicability of the technique to arbitrary geometries, a new boundary identification technique is developed. The new method is demonstrated with some example problems. © 2006 Elsevier Ltd. All rights reserved.

Journal	Optics and Lasers in Engineering
ISSN	01438166
Open Access	No