Towards RGB photoelasticity: Full-field automated photoelasticity in white light
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.
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TL;DR: An overdeterministic least-squares phase-stepping method for automated photoelasticity and a three-wavelength approach to determination of the absolute value of the isochromatic parameter are shown to give reliable results.
Abstract: An overdeterministic least-squares phase-stepping method for automated photoelasticity is described. Problems associated with isochromatic–isoclinic interaction are solved by use of a three-wavelength method to calculate the value of the isochromatic parameter and the isoclinic angle. The ramped isoclinic phase map can now be unwrapped to give the orientation of the principal stresses with respect to a reference axis of the polariscope unambiguously. A three-wavelength approach to determination of the absolute value of the isochromatic parameter is shown to give reliable results also.
70 citations
TL;DR: A technique for full-field evaluation of the isoclinic parameter is presented that combines the phase-shifting method with true color imaging technology and can be suitably integrated with known methods for evaluating the retardations.
Abstract: In this paper, a technique for full-field evaluation of the isoclinic parameter is presented. It combines the phase-shifting method with true color imaging technology to minimize the interaction between isoclinic and isochromatic fringes. The paper also shows how the proposed technique can be suitably integrated with known methods for evaluating the retardations. Furthermore, a digital algorithm for filtering wrapped phase data obtained by general phase-shifting techniques is presented.
68 citations
TL;DR: The article presents an overview of the main characteristics of RGB photoelasticity that is influence of the quarter-wave plate error, number of acquisitions, type of light source, determination of low and high fringe orders, methods for searching the retardation, scanning procedures, calibration on a material different from that under test, combined use of the RGB and phase shifting methods.
66 citations
Cites background or methods from "Towards RGB photoelasticity: Full-f..."
...The light intensity acquired by the colour camera, using a dark field circular polariscope with a white light source, can instead be expressed as [43]:...
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...The light source initially proposed [43,45] and currently widely used is the common incandescent lamp which has a continuous A....
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...The method was first introduced at the University of Palermo since 1990 [42] and subsequently published (1995) in an international context [43,44]....
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...(4) is not easy so that a procedure based on a database search approach has been developed [42,43]....
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TL;DR: In the last three decades of developments, the technique of Digital Photoelasticity (DP) has been stabilized and the methodology has successfully applied to a wide range of problems in Engineering and science as discussed by the authors.
56 citations
TL;DR: In this paper, the authors proposed a new algorithm for processing wrapped isochromatic and isoclinic fringe data generated by the digital photoelastic technique of phase-stepping.
Abstract: New algorithms are proposed for processing wrapped isochromatic and isoclinic fringe data generated by the digital photoelastic technique of phase-stepping. The new algorithms are based on processing areas rather than individual pixels to demodulate the data and employing quality assessments of the data to guide the unwrapping procedure. Comparisons with an existing algorithm, manual analysis, and finite element modeling have been used to demonstrate that the new approach is more robust without any reduction in speed or accuracy. This represents a significant advance for processing complex fringe patterns, which are often encountered in the aerospace industry from which the exemplars are drawn.
53 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
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
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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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
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