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: In this article, a photoelastic stress compensation method for measurement verification along with fuzzy theory was used to reorganize a set of processes that can be used to evaluate the residual stress of a product.
Abstract: The phenomenon of residual stress in optical lens injection molding affects the quality of optical devices, with the refractive errors that are caused by geometric errors being the most serious, followed by the reduced accuracy and function of optical components; it is very important to ensure that the lens geometry remains intact and that the refractive index is reduced. This paper uses a photoelastic stress compensation method for measurement verification along with fuzzy theory to reorganize a set of processes that can be used to evaluate the residual stress of a product, whereby the use of corresponding theoretical formulas can effectively quantify and measure the residual stress of the product. A mold flow simulation is used to analyze the molded optical components and determine the feasibility of evaluating the quality of the lens. Through the measurement of the refractive stress value of the optical components, the molding quality of the lens can be improved, and its force distribution effects can be investigated. Geometric analysis and shear stress affect the performance of optical components, and these errors may also cause irreparable problems during secondary processing. Therefore, it is crucial to reduce the residual stress of optical components. When the stress distribution is uniform and the internal melting pressure is reasonably configured, the product’s shrinkage rate can be controlled; the method for determining the residual stress is the core theme of this research.
5 citations
TL;DR: A novel method is proposed, which can automatically perform the phase- unwrapping process directly based on the result of phase-shifting of the status of ambiguity and no prior knowledge is needed of the total fringe order for at least one point.
Abstract: In phase shifting in photoelasticity, phase unwrapping is one of the crucial steps in the processing of determining the total fringe or- ders. This task is tedious, particularly when ambiguity exists in an irregu- lar phase map. Currently the phase-unwrapping process is usually per- formed line by line with prior knowledge of the total fringe order for at least one point after eliminating the isoclinic ambiguity. In this paper, a novel method is proposed, which can automatically perform the phase- unwrapping process directly based on the result of phase-shifting inde- pendent of the status of ambiguity and no prior knowledge is needed of the total fringe order for at least one point. The basic idea of this method is to exploit the relationships between phase retardations caused by three different loads applied on a specimen to find directly the total fringe order at each point. Experiments on a 'C' shape specimen under com- pression are provided as demonstrations. © 2000 Society of Photo-Optical In- strumentation Engineers. (S0091-3286(00)00910-7)
5 citations
TL;DR: In this paper, the effect of stretching rate on necking initiation was studied and it was shown that fast stretching causes faster arrangement of molecular chains and hence decreases the time required for necking.
Abstract: Abstract The digital photoelastic technique is used to characterise the necking behaviour of isotactic polypropylene (iPP) fibres. The effect of stretching rate on necking initiation is studied. The birth of necking is observed using photoelastic patterns of the stretched fibres to understand how the localised difference between the principle stresses grows to form a necking region. Finally, the formation of multiple necking regions is characterised photoelastically. These multiple necks are initiated using the same formation mechanism and conditions as if there is only a single necking region. It was evident that, fast stretching causes faster arrangement of molecular chains and hence decreases the time required for necking initiation. Recommendations are suggested for optimum mechanical processing conditions of iPP fibres to avoid failure by necking. Photoelastic patterns are given for illustration.
5 citations
TL;DR: In this paper, constrained least squares (LSSOL) techniques are used to express the contact conditions as linear inequality constraints upon the displacement solution, and a variant of Tikhonov regularization is also used to penalize against oscillatory traction distributions on the unknown boundary.
Abstract: Existing work by the present team has involved the development of an inverse boundary element approach for fitting unknown boundary conditions to full-field stress data obtained using photoelasticity, treating the displacements on the unknown region as the solution variables. However, the recovered boundary conditions sometimes show unrealistic features such as negative contact pressures or shear tractions which exceed realistic frictional values; noisy or incomplete input data make these problems worse and can make the results oscillatory. The work described here exploits available constrained least-squares techniques in order to express the contact conditions as linear inequality constraints upon the displacement solution. A variant of Tikhonov regularization is also used to penalize against oscillatory traction distributions on the unknown boundary. The chosen implementation, solved using the LSSOL algorithm, can handle the issue of rigid-body motion without the need for manual imposition of restraints...
5 citations
TL;DR: This technique could embed the numeric data of the particle forces in the digital photoelastic images which provide comprehensive information for a quantitative analysis of the progressive evolution of the soil arching phenomena in a particulate media.
Abstract: This paper explains a newly developed technique for identifying particles belonging to the contact force chains in a model particle assembly. This was accomplished through digital image analysis of the color images obtained from photoelastic measurements. Descriptive statistics for RGB color intensities of pixels in a central square region inside the particle were digitally measured to determine the particle force, as opposed to the traditional method of counting fringe orders. The relationships between the descriptive statistics of the RGB color intensities and the applied forces were analyzed. The developed image analysis technique was applied to digital photoelastic images of an assembly of stacked particles. This technique could embed the numeric data of the particle forces in the digital photoelastic images which provide comprehensive information for a quantitative analysis of the progressive evolution of the soil arching phenomena in a particulate media.
5 citations
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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