RGB Photoelasticity Research Articles

Method for extracting the full-field dispersion data of birefringent objects based on the digital RGB photoelasticity

In this paper, we are presenting a new photoelastic pattern analysis technique to find the point-wise distribution of the dispersion data of birefringent materials. This technique can be applied to profile the dispersion data in three dimensions during static, online, real-time, and dynamic investigations using the well-known digital photoelasticity technique. This method is applied to characterize the dispersion parameters of monofilament isotactic polypropylene fibers. The photoelastic patterns are included for illustration.

Scanning schemes in white light Photoelasticity – Part I: Critical assessment of existing schemes

The use of white light based Three Fringe Photoelasticity (TFP)/RGB Photoelasticity has gained importance in the recent years. With recent advances in TFP, it is possible to resolve fringe orders upto twelve. The main advantage of this technique is that it requires only a single image for isochromatic demodulation, which makes it suitable especially for problems where recording multiple images is difficult. The accuracy of isochromatic data obtained using TFP/RGB Photoelasticity is dependent on the scanning scheme used to refine the data, which is necessary to incorporate fringe order continuity. In this paper, the existing scanning schemes are critically evaluated for their ability to scan the entire model domain, influence of seed point selection and noise propagation. The scanning schemes are assessed using four problems of increasing level of geometric complexity – Circular disc under compression (simply connected), bi-axially loaded cruciform specimen with an inclined crack, a thick ring subjected to internal pressure and a finite plate with a hole (multiply connected).

Non-uniqueness of the Color Adaptation Techniques in RGB Photoelasticity

Color adaptation in RGB photoelasticity takes care of the tint variations in a calibration specimen and the analysis image, whereby an existing calibration look-up table (LUT) is modified to adapt to the changed tint environments. The theoretical justification available for color adaptation ignores important practical aspects such as the modulation of intensities and dispersion of spectral signatures. In this paper, these nuances of color adaptation have been verified through experiments and a realistic theoretical explanation for color adaptation is provided. The experimental and theoretical analysis show that color adaptation technique produces non-unique LUTs, which is the prime reason behind the uncertainties due to color adaptation seen in the literature. This paper proposes a new strategy to overcome the non-uniqueness problem, which is validated and applied to demodulate actual fringe maps. The validation results show that the fringe maps are sufficiently smooth with 0.02 mean absolute deviation (MAD) in fringe orders. Moreover, common industrial uncertainties which can be resolved through the proposed methodology are highlighted.

Red-green bicolor photoelasticity using a single-color image

Conventional RGB or tricolor photoelasticity processes the intensity information of the red, green, and blue color planes to demodulate the photoelastic fringe orders (or isochromatics). We explore the possibility of isochromatic demodulation using the intensity data of only the red and green color planes drawn from a single-color image. This would help in avoiding the uncertainties caused due to the blue color plane in RGB photoelasticity. Moreover, a new procedure is proposed for isochromatic demodulation using a synthetically generated look-up table (LUT) to address the tint and intensity variations between the calibration and analysis images. The proposed method has been validated and extended to analyze live models and stress frozen slices. The results show that the proposed bicolor technique has the potential for demodulating higher fringe orders using generic white light sources with acceptable accuracy. Moreover, the bicolor algorithm consumes 13% less computational time than the tricolor algorithm.

Novel calibration and color adaptation schemes in three-fringe RGB photoelasticity

Isochromatic demodulation in digital photoelasticity using RGB calibration is a two step process. The first step involves the construction of a look-up table (LUT) from a calibration experiment. In the second step, isochromatic data is demodulated by matching the colors of an analysis image with the colors existing in the LUT. As actual test and calibration experiment tint conditions vary due to different sources, color adaptation techniques for modifying an existing primary LUT are employed. However, the primary LUT is still generated from bending experiments. In this paper, RGB demodulation based on a theoretically constructed LUT has been attempted to exploit the advantages of color adaptation schemes. Thereby, the experimental mode of LUT generation and some uncertainties therein can be minimized. Additionally, a new color adaptation algorithm is proposed using quadratic Lagrangian interpolation polynomials, which is numerically better than the two-point linear interpolations available in the literature. The new calibration and color adaptation schemes are validated and applied to demodulate fringe orders in live models and stress frozen slices.

A critical assessment of automatic photoelastic methods for the analysis of edge residual stresses in glass

The measurement of residual stresses is of great importance in the glass industry. The analysis of residual stresses in the glass is usually carried out by photoelastic methods since the glass is a photoelastic material. This article considers the determination of membrane residual stresses of glass plates by digital photoelasticity. In particular, it presents a critical assessment concerning the automated methods based on gray-field polariscope, spectral content analysis, phase shifting, RGB photoelasticity, “test fringes” methods and “tint plate” method. These methods can effectively automate manual methods currently specified in some standards.

Photoelastic Analysis of Edge Residual Stresses in Glass by the Automated Tint Plate Method

The analysis of residual stress in glass is usually carried out by means of photoelastic methods. This article considers the automation of the white light photoelastic method based on the use of a full-wave plate placed behind the glass plate. In particular, the method in based on the use of RGB photoelasticity in white light in conjunction with a full wave plate. The proposed method have been applied to the analysis of membrane residual stresses in tempered glass, showing that it can effectively replace manual methods of photoelastic analysis of residual stresses in glass when a low photoelastic retardation is present.

RGB photoelasticity applied to the analysis of membrane residual stress in glass

The measurement of residual stresses is of great relevance in the glass industry. The analysis of residual stress in glass is usually made by photoelastic methods because glass is a photoelastic material. This paper considers the determination of membrane residual stresses in glass plates by automatic digital photoelasticity in white light (RGB photoelasticity). The proposed method is applied to the analysis of membrane residual stresses in some tempered glass. The proposed method can effectively replace manual methods based on the use of white light, which are currently provided by some technical standards.

Photoelastic Analysis of Edge Residual Stresses in Glass by Automated “Test Fringes” Methods

Since the glass is a birefringent material, the analysis of residual stress in glass is usually carried out by means of photoelastic methods. This paper considers the automation of the “test fringes” method which is based on the use of a Babinet compensator or of a beam subjected to bending. In particular, two automated methods are proposed: the first one is based on the use of the centre fringe method in monochromatic light and the second one is based on the use of RGB photoelasticity in white light. The proposed methods have been applied to the analysis of membranal residual stresses in some tempered glasses, showing that they can effectively replace manual methods of photoelastic analysis of residual stresses in glass.

DEFENSE HOLE DESIGN FOR UNIAXIAL DOMINANT HYBRID LOAD FOR INFINITE PLATE

A baseline data for designing optimum Defense Hole System (DHS) for tension dominant hybrid load (Tensile/Shear > 25%) is obtained. Maximum stress reduction and optimum DHS parameters are achieved. The maximum stress reduction achieved range from 15% up to nearly 18% based on the principle stress ratio (25% < Tensile/Shear < 100%). This reduction is available by introducing auxiliary circular holes, i.e., DHS, along the principal stress direction. A two-DHS is shown to be the optimum for tensile dominant loaded plate. Two major goals are achieved by introducing such defense system: maximum stress reduction and material reduction. Redesign optimization method (iterative numerical optimization technique) is utilized to investigate this problem. Parametric optimization technique is also utilized in producing the routs for reaching those optimum cases. Finite element analysis is used to optimize the size and the location of the DHS. Selected optimum cases are verified experimentally using RGB photoelasticity.

RGB Photoelasticity: Review and Improvements

Abstract: This paper considers the main developments of RGB photoelasticity with reference to the maximum measurable retardation. In this paper, a new procedure based on the standard error function evaluated on a subset of the calibration array is also proposed and experimentally tested. The experiments show that the filament lamp makes it possible to find retardations until approximately 4 fringe orders while the fluorescent lamp makes it possible to determine higher fringe orders (12 fringe orders in this paper) owing to the discrete spectrum of the source. The paper shows that, by using the incandescent lamp, the primary limiting factor is the lack of modulation of the R, G and B signals whereas, by using the fluorescent lamp, the limitation of the maximum fringe order derives mainly from the gradient of the fringes and the procedure of search of the retardation.

Phase shifting photoelasticity in white light

The availability of image acquisition systems has led to the development of digital photoelasticity both in monochromatic and white light. In particular white light has been used mainly with the following methods: Spectral Content Analysis, RGB photoelasticity and phase shifting photoelasticity. Phase shifting photoelasticity in the colour domain has been used effectively for the determination of the isoclinic parameter in order to reduce the influence of the isochromatic fringes. The method has been also proposed for the determination of the isochromatic fringe order. This paper concerns the general characteristics of phase shifting photoelasticity in the colour domain. Special attention is drawn towards the influence of spectral content of the light source, the spectral response of the colour camera filters, the dispersion of the birefringence and the error of quarter wave plates.

Developments in RGB Photoelasticity

In this paper the combined use of both RGB and phase stepping photoelasticity is proposed. The method is characterised by the following features: maximum measurable order greater than that of the RGB method, ability to determine the total fringe order without necessity of unwrapping.

Complete Fringe Order Determination in Digital Photoelasticity using Fringe Combination Matching

Abstract: A novel approach is presented for obtaining the complete (integer and fractional) value of isochromatic parameters in digital imaging photoelasticity. This takes the form of a new variant on the existing technique of RGB photoelasticity, involving the matching of combinations of isochromatic fringe value measured on the three colour planes or channels rather than the usual RGB method of matching colour combinations directly. The new variant of RGB photoelasticity avoids the need for calibration for specific materials. This approach is demonstrated with real photoelastic data and compared with results from a conventional unwrapping process and from manual readings.

Multiplication and Extraction of Isochromatics in RGB Photoelasticity Using Image Processing.

A method is proposed for multiplying and extracting isochromatics in RGB photoelasticity and assigning fringe orders to them. The method uses R, G and B isochromatics, which are taken using a color CCD camera in dark-and light-field circular polariscopes with a white light. Light intensities in the dark field image are subtracted from those in the light field image, and the points with zero values are extracted. The fringe orders are assigned to the multiplied and extracted fringes using calibration curves, which show the relationships between the fringe orders obtained using a single wavelength and those using R, G and B filters in the CCD camera. In order to investigate the accuracy of the method, a computer simulation is used for the theoretical isochromatics in a circular disk subjected to a concentrated load. The result shows that error of the assignment of fringe orders to multiplied and extracted isochromatics is about 1.5%. This method is applied to an epoxy resin plate with a crack at one side under 3-point bending and the stress intensity factor is determined. The result shows that the stress intensity factor is accurately determined.

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results.