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Journal ArticleDOI

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

01 Sep 1995-Experimental Mechanics (Kluwer Academic Publishers)-Vol. 35, Iss: 3, pp 193-200
TL;DR: A new full-field method for the automatic analysis of isochromatic fringes in white light, named RGB photoelasticity, which makes it possible to determine retardations uniquely in the range of 0–3 fringe orders.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: In this article, a method is proposed to evaluate the stress intensity factor using isochromatics multiplied and extracted from original is-ochromatic images obtained from a white light photoelastic experiment.
Abstract: In this study, a method is proposed to evaluate the stress intensity factor using isochromatics multiplied and extracted from original isochromatic images obtained from a white light photoelastic experiment. This method utilizes R, G and B isochromatics which are captured using a color CCD camera and an image processor in dark- and light-field circular polariscopes. First, for each color, the dark-field intensities are subtracted from the light-field intensities so that each point of the resultant image has a positive, zero or negative value. The isochromatics are then obtained as a series of points whose value is zero, which are extracted using image processing. The fringe orders are assigned to the extracted isochromatics using calibration curves. The stress intensity factor is determined using the Irwin method. This method is applied to an epoxy resin plate with a crack formed at one side under 3-point bending. The result shows that the stress intensity factor is accurately evaluated using R, G and B isochromatics near the crack tip.

3 citations

Journal ArticleDOI
TL;DR: By integrating the white light photoelasticity and spectrometry, a novel method called transmissivity extremities theory for determining the state of stress has been recently developed.
Abstract: By integrating the white light photoelasticity and spectrometry, a novel method called transmissivity extremities theory of photoelasticity to determine the state of stress has been recently develo...

3 citations


Cites background from "Towards RGB photoelasticity: Full-f..."

  • ...Therefore, it can be seen that all TELEs can be obtained by substituting the Al+B determined in the material calibration procedure into equation (14)....

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Journal ArticleDOI
04 Feb 2021
TL;DR: In this article, optical anisotropy effects in architectural glass are evaluated using digital image processing, and the results of textural features of different glass panes are compared using reference areas and geometry factors.
Abstract: In the present paper, optical anisotropy effects in architectural glass are evaluated using digital image processing. Hereby, thermally toughened glass panes were analyzed quantitatively using a circular polariscope. Glass subjected to externally applied stresses or residual stresses becomes birefringent. Polarized light on birefringent materials causes interference colors (iridescence), referred to as anisotropies, which affect the optical appearance of glass panes in building envelopes. Thermally toughened glass, such as toughened safety glass or heat strengthened glass, show these iridescences due to thermally induced residual stress differences. RGB-photoelastic full-field methods allow the quantitative measurement of anisotropies, since the occurring interference colors are related to the measured retardation values. By calibrating the circular polariscope, retardation images of thermally toughened glass panes are generated from non-directional isochromatic images using computer algorithms. The analysis of the retardation images and the evaluation of the anisotropy quality of the glass is of great interest in order to detect and sort out very low quality glass panes directly in the production process. Therefore, in this paper retardation images are acquired from different thermally toughened glass panes then different image processing methods are presented and applied. It is shown that a general definition of exclusion zones, e.g. near edges is required prior to the evaluation. In parallel, the limitations in the application of first-order statistical and threshold methods are presented. The intend of the investigation is the extension of the texture analysis based on the generation of Grey Level Co-occurrence Matrices, where the spatial arrangement of the retardation values is considered in the evaluation. For the first time, the results of textural features of different glass pane formats could be compared using reference areas and geometry factors. By reduction of the original image size, the computation time of textural analysis algorithms could be remarkably speeded up, while the textural features remained the same. Finally, the knowledge gained from these investigations is used to determine uniform texture features, which also includes the pattern of anisotropy effects in the evaluation of thermally toughened glass. Together with a global evaluation criterion this can now be implemented in commercial anisotropy measurement systems for quality control of tempered architectural glass.

2 citations

01 Dec 2002
TL;DR: In this article, a plan polariscope with two different filters was used to calculate the photoelastic parameters, and to compensate the influence of the isochromatic fringes on the isoclinic pattern.
Abstract: A new approach for whole-field digital determination of isoclinic angle and total isochromatic fringe order is presented. The method uses a plan polariscope with two different filters to calculate the photoelastic parameters, and to compensate the influence of the isochromatic fringes on the isoclinic pattern. The method allows for the determination of whole-field fringe orders without zero-order fringes.

2 citations

DOI
01 Oct 1999
Abstract: Photoelasticity is an optical technique that measures the difference of the principal stresses plus the principal stress direction. A complementary technique is thermoelasticity which measures the sum of the principal stresses. Combining these two full-field, noncontact nondestructive evaluation techniques allows the individual stress components to be measured. One o f the main difficulties in merging these two measurement systems is in identifying an appropriate surface coating. Thermoelasticity demands a highly emissive surface, while photoelasticity requires a thick, stress-birefringent, transparent coating with a retro-reflective backing. Two coatings have been identified that can be used for com­ bined thermoelastic and photoelastic stress measurements: PMM A and polycarbonate. An anisotropic electromagnetic boundary value model was developed to under­ stand more fully the mechanisms through which photoelastic stress patterns are produced. This model produced intensity contour maps which matched the fringe patterns observed in the laboratory, and allowed the effect o f measurement errors on the calculated stress tensor to be quantified. One significant source o f error was the retro-reflective backing, which depolarized the light and degraded the resulting photoelastic fringes. A quantitative analysis o f the degraded fringes, to be used as a rating scheme for reflective backing mate­ rials, showed that the isoclinic lines shift position as a result of the backing roughness and oblique incidence. This is a concern when calculating the stress components through the combination of photoelasticity and thermoelasticity because the data maps are integrated at the pixel level. Small shifts in the photoelastic fringes result in incorrect information being assigned to some pixels and hence lead to uncertainties in the stress tensor compo­ nents. Progress in the understanding o f the depolarization at the reflective backing allows the specification of new materials that will minimize this effect, as well as the develop­ ment of robust computer algorithms to correct for any remaining depolarization.

2 citations

References
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Book
01 Jan 1987
TL;DR: The "Handbook on Experimental Mechanics" as mentioned in this paper is a comprehensive reference in the field of experimental mechanics and has been used extensively in the past 50 years for a wide range of applications.
Abstract: The "Handbook on Experimental Mechanics" is a comprehensive reference in the field. Since 1950, new experimental techniques, such as holography, laser speckle interferometry, optical heterodyning and modal analysis, have emerged as practical tools in the broader field of experimental mechanics. The emergence of new materials and new disciplines, such as composite materials and fracture mechanics, resulted in the evolution of traditional experimental techniques to new fields such as orthotropic photoelasticity and experimental fracture techniques. The new revised edition of the handbook includes, among other things, one new chapter on digital image processing; key sections of the handbook have been entirely rewritted or updated. The handbook should be useful to any mechanical engineer or anyone interested in stress analysis of materials.

504 citations

Journal ArticleDOI
01 May 1991-Strain
TL;DR: The design of a automated system for photoelastic analysis of complex components and the potential of the system for providing detailed data over the full field of view is demonstrated by the analysis of a slice from a model of a bolt.
Abstract: The design of a automated system for photoelastic analysis of complex components is described, and an outline of the theory used in its operation is given. The potential of the system for providing detailed data over the full field of view is demonstrated by the analysis of a slice from a model of a bolt.

221 citations

Journal ArticleDOI
TL;DR: In this paper, the half-fringe photoelasticity (HFP) method is proposed for whole-field stress analysis based on a symbiosis of two techniques, namely classical photo elasticity and modern digital image analysis.
Abstract: This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’. Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis. This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.

126 citations


"Towards RGB photoelasticity: Full-f..." refers background or methods in this paper

  • ...The intensity eqs ( 9 ) and (10) can be used to evaluate A only if the Fi functions, the transfer function of the digital board (relationship between the RGB values and intensity values L Ig and Ib), the dispersion of birefringence and the isoclinic angle ~ are known....

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  • ...The equations of polariscope ( 9 ) and (10) show that the influence of errors e of optical retarders depends on the isoclinic angle cx; there is maximum error where c~ = 0 deg (principal stresses parallel to polarizers), whereas no error occurs where a -- 45 deg (principal stresses parallel to retarders)....

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  • ...For lower orders, using eqs ( 9 ) and (10) allows us to evaluate the unknown retardation A (or 8) by processing of the acquired RGB levels....

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Book ChapterDOI
01 Jan 1986
TL;DR: In the following a method is proposed, which enables the complete extraction of photoelastic information at local picture elements (pixel) from series of related images of the same stress state, and was modified to meet the special requirements ofphotoelastic patterns.
Abstract: Computer-aided methods for evaluation of photoelastic patterns use video technique and digital image processing. They are based on localization of fringe centers [ 1, 2, 3, 4]. Neighbourhood operations are needed to reduce the influence of nonuniform illumination, inhomogeneous optical components and models, etc. Fractional orders of the relative retardation and of the isoclinic parameter at points between the fringes are computed by spline- functions, if the components of the plane stress state have to be derived from photoelastic data. In the following a method is proposed, which enables the complete extraction of photoelastic information at local picture elements (pixel) from series of related images of the same stress state. For this purpose the well-established phase-shifting technique (see e. g. [ 5,6]) was modified to meet the special requirements of photoelastic patterns.

78 citations

Journal ArticleDOI
TL;DR: In this paper, a new method of photoelastic measurement has been developed, where the light emerging from a polariscope is spectrally separated and projected on a photodiode array.
Abstract: A new method of photoelastic measurement has been developed. The light emerging from a polariscope is spectrally separated and projected on a photodiode array. It is shown that the relative retardation can be retrieved from light intensity measured at several wavelengths. Key parameters affecting the precision of this approach are discussed and evaluated.

76 citations