Photoelastic force measurements in granular materials

Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local of stress. In this review paper, we summarize past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus

Digital photoelasticity in biomedical sensing.

This research investigates on the use of digital photoelasticityin biomedical sensing applications with a particular emphasis on assessment of diabetic foot ulceration. One of the main causes of foot ulceration in diabetic patients is excessive pressure at the sole of the foot, which involves vertical as well as shear forces. Precise role of these forces in predisposing the foot to ulceration is not very well understood, however, a general consensus is that the combined effect of vertical and shear forces is much more harmful than the vertical force alone. Whilst the vertical force can be measured relatively easily,it is difficult to decouple the shear force from the combined force,which is considered to be of more clinical relevance in assessment of diabetic foot ulceration. The major impediment in achieving this objective is lack of suitable shear force measuring devices and limitation of the existing systems that can simulate the actual conditions of foot loading. In this research a photoelastic material has been used to develop a prototype-sensing device, which develops coloured fringes due to foot loading. Intelligent image processing techniques have been employed to analyse and obtain relevant load information from these fringes. The research surveys the existing sensing devices that are commonly used in diabetic foot clinics. It highlights the need for a new sensor design that can be used for pressure-induced pathologies. To meet these requirements and develop a sensor based on the principle of photoelasticity, conventional techniques of RGB photoelasticity and Phase-shifting methods have been fully investigated. This led to identify suitable optical elements for the system design and applicability of these techniques for the intended application. This resulted in devising an experimental set up that can provide coloured image of foot per se actual conditions of foot loading. However, the conventional technique of stress analysis cannot be directly applied in the present case, since the photoelastic effect is induced due to the material deformation as opposed to the usual component loading in photoelastic experiments with coatings. Also, in the current application the applied load has to be estimated from the fringe patterns (i.e. inverse problem) under varying environmental conditions with different loading situations for each subject. As it is difficult to develop analytical models under these conditions and the related inverse might have infinite number of solutions, the use of neural networks has been proposed to overcome these complexities. The network has been trained with direct image data which provides input load information under controlled experimental conditions of vertical as well as shear forces. The prototype sensor also provides qualitative whole-field data of the actual foot loading, which can be used for quick differentiation of foot with or without callus. This may also find use in haptics, pattern recognition and other biomedical sensing applications such as pressure sore assessment for disabled subjects or patients with numbness. With further enhancement in image processing technique this can be developed into a clinically viable system capable of providing complete foot analysis from early stage detection to prevention of ulceration

Full-field modelling of crack tip shielding phenomena

The application of fracture mechanics to engineering design has provided significant advances in understanding of the causes and mechanisms of failure and crack growth. Despite this, there are still some aspects that remain incompletely understood, such as the crack closure/crack shielding effect. The presence of crack closure/shielding acts to reduce . The mechanisms of crack closure/shielding are complicated, and have not been fully understood. This work focuses on the plasticity-induced crack tip shielding mechanism and presents a novel approach to characterise the elastic stress fields under the influence of the plastic enclave surrounding the crack tip. The model is successfully applied to determine the four stress parameters experimentally using full-field photoelastic stress analysis on polycarbonate CT specimens, following studies of the effect of the crack tip position and the valid data collection zone giving the best fit between the model predictions and the experimental data. The predicted values from the model demonstrate good data repeatability, and exhibit sensible trends as a function of crack length and load ratio that are interpretable in terms of physically meaningful changes to the plastic enclave. In addition, the model is proven to describe the stress field around a crack more accurately than classic Williams‟ stress solution. The model is also extended to AL 2024-T3 specimens using a full-field displacement measurement technique, digital image correlation. Using the Sobel edge detection method to identify the crack tip from the displacement fields with a rectangular shaped data collection zone employed in the current study, reasonable trends were again demonstrated in the experimental results as a function of crack length

Automated reflection photoelasticity : digital data acquisition and use.

Automation of reflection photoelasticity has simplified the stress and strain analysis of real engineering components and reduced analysis time. However, images obtained from automated reflection photo elasticity contain noise and accuracy of the analysis will be affected by the degraded intensity images. In the present study, major sources of noise in automated reflection photoelasticity have been found to be the photoelastic coating and the electronic instrumentation. An automated reflection polariscope PSIOS developed by Patterson and Wang (1998) for the simultaneous observation and capture of four phase-stepped photo elastic images was used as an example. The majority of the noise is in the high spatial frequency domain. The zero-phase, low pass Butterworth filter was found to be the most effective and flexible smoothing method for reducing the effect of noise in the intensity images. Results from experiments performed for assessing the ability of the PSIOS indicated that it is capable of yielding accurate results for the stress analysis of real components in both static and dynamic conditions and that it is fast and easy to use. Full-field experimental methods are often used to validate the stress distribution generated from numerical analysis. A common practice is to plot data along a line across the maps and to include both experimental and numerical results on the same axes. This approach is used widely and usually a reasonable, quantitative conclusion can be made. However, it cannot obtain more information about the relationship between the stress maps. Another method is to compare hot spots on experimental maps to the numerical maps. If the hot spots on the two maps match well, the numerical method is considered valid. However, when designs are being optimised for weight or crack paths are being investigated, comparison of the positions of the hot spots alone will not be enough and the correlation elsewhere in the data field should be taken into account. It has been shown that fit between the stress map from an experimental method and the stress map from the numerical analysis can be represented by a statistical parameter, the scaled standard deviation. An evaluation of the method was performed using stress maps from transmission photoelasticity, thermoelasticity and the finite element method as examples. The results from experiments using a curved tiebar, a circular ring and a real engineering component in this case, a race car hub carrier indicated that the scaled standard deviation represents the fitness between the two stress maps. If the scaled standard deviation is smaller than 0.1, then the experimental map and the numerical map can be considered to be in good agreement

Thermoelastic and photoelastic full-field stress measurement

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, non-contact nondestructive evaluation techniques allows the individual stress components to be measured. One of 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 combined thermoelastic and photoelastic stress measurements: PMMA and polycarbonate.;An anisotropic electromagnetic boundary value model was developed to understand 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 of measurement errors on the calculated stress tensor to be quantified. One significant source of error was the retro-reflective backing, which depolarized the light and degraded the resulting photoelastic fringes. A quantitative analysis of the degraded fringes, to be used as a rating scheme for reflective backing materials, 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 components. Progress in the understanding of the depolarization at the reflective backing allows the specification of new materials that will minimize this effect, as well as the development of robust computer algorithms to correct for any remaining depolarization

Thermoelastic and photoelastic full-field stress measurement

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, non-contact nondestructive evaluation techniques allows the individual stress components to be measured. One of 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 combined thermoelastic and photoelastic stress measurements: PMMA and polycarbonate.;An anisotropic electromagnetic boundary value model was developed to understand 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 of measurement errors on the calculated stress tensor to be quantified. One significant source of error was the retro-reflective backing, which depolarized the light and degraded the resulting photoelastic fringes. A quantitative analysis of the degraded fringes, to be used as a rating scheme for reflective backing materials, 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 components. Progress in the understanding of the depolarization at the reflective backing allows the specification of new materials that will minimize this effect, as well as the development of robust computer algorithms to correct for any remaining depolarization

Review of RGB photoelasticity

Automatic methods of photoelasticity have had a significant progress with the development of automatic acquisition and image processing methods. This article concerns RGB photoelasticity, which allows the determination of the photoelastic retardation using, usually, a single acquisition of the isochromatic fringes in white light by a colour camera. In particular, 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. A short section on the applications of RGB photoelasticity completes the article

Digital photoelasticity in biomedical sensing

This research investigates on the use of digital photoelasticityin biomedical sensing applications with a particular emphasis on assessment of diabetic foot ulceration. One of the main causes of foot ulceration in diabetic patients is excessive pressure at the sole of the foot, which involves vertical as well as shear forces. Precise role of these forces in predisposing the foot to ulceration is not very well understood, however, a general consensus is that the combined effect of vertical and shear forces is much more harmful than the vertical force alone. Whilst the vertical force can be measured relatively easily,it is difficult to decouple the shear force from the combined force,which is considered to be of more clinical relevance in assessment of diabetic foot ulceration. The major impediment in achieving this objective is lack of suitable shear force measuring devices and limitation of the existing systems that can simulate the actual conditions of foot loading. In this research a photoelastic material has been used to develop a prototype-sensing device, which develops coloured fringes due to foot loading. Intelligent image processing techniques have been employed to analyse and obtain relevant load information from these fringes. The research surveys the existing sensing devices that are commonly used in diabetic foot clinics. It highlights the need for a new sensor design that can be used for pressure-induced pathologies. To meet these requirements and develop a sensor based on the principle of photoelasticity, conventional techniques of RGB photoelasticity and Phase-shifting methods have been fully investigated. This led to identify suitable optical elements for the system design and applicability of these techniques for the intended application. This resulted in devising an experimental set up that can provide coloured image of foot per se actual conditions of foot loading. However, the conventional technique of stress analysis cannot be directly applied in the present case, since the photoelastic effect is induced due to the material deformation as opposed to the usual component loading in photoelastic experiments with coatings. Also, in the current application the applied load has to be estimated from the fringe patterns (i.e. inverse problem) under varying environmental conditions with different loading situations for each subject. As it is difficult to develop analytical models under these conditions and the related inverse might have infinite number of solutions, the use of neural networks has been proposed to overcome these complexities. The network has been trained with direct image data which provides input load information under controlled experimental conditions of vertical as well as shear forces. The prototype sensor also provides qualitative whole-field data of the actual foot loading, which can be used for quick differentiation of foot with or without callus. This may also find use in haptics, pattern recognition and other biomedical sensing applications such as pressure sore assessment for disabled subjects or patients with numbness. With further enhancement in image processing technique this can be developed into a clinically viable system capable of providing complete foot analysis from early stage detection to prevention of ulceration.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Study of Mixed Mode stress intensity factors by two- and three-dimensional photoelasticity

An experimental study was performed to investigate the stress intensity factors from two-dimensional inclined edge cracked models and three-dimensional inclined semi-circular surface cracks that penetrate part-way through a thick plate;To improve the accuracy of photoelastic data collection, fringe multiplication and fringe sharpening technique with a digital image processing system were used. Fracture coefficients, together with the exact origin of the crack tip, were extracted from data sets that produced an overdeterministic system of equations solved by an iterative least squares method. Power series type Williams equations were used in the analysis. The accuracy of the experimental results were evaluated qualitatively and quantitatively. For qualitative visual assessment, regenerated fringes were plotted by using the fracture equations and coefficients estimated from the data sets. For quantitative accuracy evaluation, statistical parameters, such as standard deviation and correlation coefficients between theoretical and observed fringes at the data points were introduced;The accuracy evaluation indicated that the first 4 terms of Williams equations are sufficient to describe the stress field in the vicinity of the crack tip for both Mode I and Mixed Mode cases. It was found that the modified Westergaard equations or the first two terms of Williams equations cannot be used to extract Mixed Mode fracture parameters;The average values of the new experimental results, K(,I)/K(,Io) and K(,II)/K(,I), for the two-dimensional inclined edge cracked plates lie within the range of numerical solutions and previous experimental values. Variation of stress intensity factor along a circumference of a semi-circular surface crack in pure Mode I loading is similar to that predicted by a finite element solution, but at higher numerical values. New experimental solutions for K(,I)/K(,Io) and K(,II)/K(,I) at the maximum depth of the three-dimensional inclined semi-circular cracks are presented