Residual Stresses In Glass Research Articles

Research advancements and challenges in the design and fabrication of glass to metal seals for solar receiver tube applications

Glass to metal joints has a vast field of application due to their distinct properties in terms of electric conductivity, thermal conductivity, transparency, emissivity, reflectivity, coefficient of thermal expansion (CTE), density, wettability, and crystalline structure. This wide diversification in their properties makes this combination very useful in the special application like concentrated solar receiver tubes (CSRT), etc. This challenge becomes bigger when such seals are subjected to a very high temperature range, ultra-high pressure or ultra-high vacuum, and also gone through repeated number of thermal cycling over a period of time, as may be in the case of CSRT. Hence how to prepare a good quality seal for such constraints, study and control of many factors are essential covered such as seal design, oxidation layer thickness, manufacturing method, temperature and time of making seal, surface roughness, and residual stresses in glass, etc., that has been vigorously studied and reported in this paper.

Research advancements and challenges in the design and fabrication of glass to metal seals for solar receiver tube applications

Glass to metal joints has a vast field of application due to their distinct properties in terms of electric conductivity, thermal conductivity, transparency, emissivity, reflectivity, coefficient of thermal expansion (CTE), density, wettability, and crystalline structure. This wide diversification in their properties makes this combination very useful in the special application like concentrated solar receiver tubes (CSRT), etc. This challenge becomes bigger when such seals are subjected to a very high temperature range, ultra-high pressure or ultra-high vacuum, and also gone through repeated number of thermal cycling over a period of time, as may be in the case of CSRT. Hence how to prepare a good quality seal for such constraints, study and control of many factors are essential covered such as seal design, oxidation layer thickness, manufacturing method, temperature and time of making seal, surface roughness, and residual stresses in glass, etc., that has been vigorously studied and reported in this paper.

A validated modelling technique for incorporating residual stresses in glass structural design

This paper presents the development of a simple parabolic residual stress depth profile model for characterising residual stresses in construction-sector glass. The proposed model requires only the knowledge of the surface residual stress, which is usually available from glass manufacturers. Unlike the complex computational techniques reported in the literature, such as modelling physical, microstructural and mechanical phenomena of glass at different temperatures during manufacturing, the proposed model obviates the need for modelling multi-physics phenomenon of the generation of residual stresses. The proposed model also eliminates the need of sophisticated experimental equipment, such as Scattered-Light-Polariscopes (SCALP), which are usually not available among practicing engineers, in order to characterise the residual stresses. Residual stress predictions from the proposed parabolic model were validated against experimental results reported in the literature. Using the concept of eigenstrains, the paper also extends the results of the proposed parabolic residual stress depth profile model for incorporating the effects of residual stresses in stress analysis of glass structures.

Residual Stress in Glass Components

Architectural trends have tended towards curved glass envelopes and maximised transparency by reducing solid fixing areas. One approach towards transparent glass connections is a heat bonding process based on the principles of welding. This paper investigates the level of residual stress in soda lime silica and borosilicate glass caused by a heat-based connection or forming process. Nominal levels of residual stress prior to heat impact, directly after heat impact and after annealing will be measured on small-scale samples, utilizing a scattered light polariscope (SCALP). Material properties the large temperature range required for the heat bonding process have been identified to allow subsequent numerical modelling to verify the results obtained in this study.

Digital photoelasticity of glass: A comprehensive review

The recent advances in digital photoelasticity have made it possible to use it conveniently for the stress analysis of articles and components made of glass. Depending on the application, the retardation levels to be measured range from a few nanometres to several thousand nanometres, which necessitates different techniques and associated equipments. This paper reviews the recent advances in the photoelasticity of glass with a focus on the techniques/methods developed in the last decade. A brief introduction to the residual stress in glass is provided initially to bring out its tensorial nature. The subsequent sections are organised thematically rather than chronologically, for better readability and easy access of information.

A novel method for the evaluation of stress-optic coefficient of commercial float glass

The measurement of residual stress in float glass is important in glass industries as they affect their cutting quality. Photoelasticity can be used for residual stress analysis of float glass as they exhibit stress induced birefringence. Stress measurement using photoelasticity necessitates the knowledge of the stress-optic coefficient of float glass. Carrier fringe method in conjunction with digital photoelasticity is used for the photoelastic calibration of float glass. The conventional method of photoelastic calibration is improved to calibrate float glass with residual stress which eliminates the need for annealing them. The new method developed is self consistent as the results are verified by the reconstruction of the composite fringe pattern. The use of the improved method is recommended for calibration of float glass as it takes into account the residual stress in glass sample apart from its potential for automation.

A direct approach for modeling ion-exchange residual stress distribution in glass

In this article, we propose a direct approach to model the residual stress in glass due to ion exchange. Rather than employing the traditional thermal strain analogy approach in literature, the nominal residual strain distribution due to ion exchange is addressed with direct analysis of ion-exchange volume change first. Then, based on the principle of superposition, the residual stress in ion-exchanged glass is then formulated with a direct approach in the framework of elasticity, and a complete solution for residual stress distribution is obtained with consideration of the asymmetric ion molar concentration distribution. Finally, a simple example is provided to demonstrate the validity of the fundamental formulations proposed in this article.

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.

Effect of residual stresses in float-glass on cutting quality

The main reasons for the appearance of rejects during glass cutting are presented: off-cut breakage and breakage during cutting. Attention is focused mainly on the residual stresses in the glass; equipment-related reasons for rejects are briefly discussed. The existing method of monitoring the residual stresses in glass and its drawbacks are examined. These studies have made it possible to clarify the reasons for problems arising during glass cutting. Explanations are given for the results obtained.

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.

Measurement of edge residual stresses in glass by the phase-shifting method

Control and measurement of residual stress in glass is of great importance in the industrial field. Since glass is a birefringent material, the residual stress analysis is based mainly on the photoelastic method. This paper considers two methods of automated analysis of membrane residual stress in glass sheets, based on the phase-shifting concept in monochromatic light. In particular these methods are the automated versions of goniometric compensation methods of Tardy and Sénarmont. The proposed methods can effectively replace manual methods of compensation (goniometric compensation of Tardy and Sénarmont, Babinet and Babinet–Soleil compensators) provided by current standards on the analysis of residual stresses in glasses.

Residual stresses in glass after molding and its influence on optical properties

Compression molding of glass optics is gradually becoming a viable manufacturing process for high precision optical lenses. However, during cooling, glass properties undergo a small amount of changes due to structural relaxation. As a result, residual stresses are trapped inside the molded glass lenses. In this research, compression molded lenses were studied using the birefringence method assisted by the finite element analysis. The experimental and simulation results showed that compression molding process can be controlled to a very high precision providing that the proper cooling is applied during cooling. In addition, numerical analysis using finite element analysis also provides a better understand of the cooling process and offers an economic solution for process optimization for low cost glass lens production.

Photoelastic tomography as hybrid mechanics

Photoelastic tomography is a non-destructive method of 3D stress analysis. It permits determination of normal stress distribution in an arbitrary section of a 3D test object. In case of axial symmetry also the shear stress distribution can be determined directly from the measurement data. To determine also the other stress components one can use equations of the theory of elasticity. Such a combined application of experimental measurements and numerical handling of the equations of the theory of elasticity is named hybrid mechanics. It is shown that if stresses are due to external loads, the hybrid mechanics algorithm is based on the equations of equilibrium and compatibility. In the case of the measurement of the residual stress in glass the compatibility equation can not be applied. In this case a new relationship of axisymmetric thermoelasticity, the generalized sum rule can be applied.

Residual stress in glass: Indentation crack and fractography approaches

Objective To test the hypothesis that the indentation crack technique can determine surface residual stresses that are not statistically significantly different from those determined from the analytical procedure using surface cracks, the four-point flexure test, and fracture surface analysis. Methods Soda-lime–silica glass bar specimens (4 mm × 2.3 mm × 28 mm) were prepared and annealed at 650 °C for 30 min before testing. The fracture toughness values of the glass bars were determined from 12 specimens based on induced surface cracks, four-point flexure, and fractographic analysis. To determine the residual stress from the indentation technique, 18 specimens were indented under 19.6 N load using a Vickers microhardness indenter. Crack lengths were measured within 1 min and 24 h after indentation, and the measured crack lengths were compared with the mean crack lengths of annealed specimens. Residual stress was calculated from an equation developed for the indentation technique. All specimens were fractured in a four-point flexure fixture and the residual stress was calculated from the strength and measured crack sizes on the fracture surfaces. Results The results show that there was no significant difference between the residual stresses calculated from the two techniques. However, the differences in mean residual stresses calculated within 1 min compared with those calculated after 24 h were statistically significant ( p = 0.003). Significance This study compared the indentation technique with the fractographic analysis method for determining the residual stress in the surface of soda-lime–silica glass. The indentation method may be useful for estimating residual stress in glass.

Investigation on the viscoelasticity of optical glass in ultraprecision lens molding process

Glass molding press is an efficient manufacturing technology for ultraprecision optical elements with complex shapes. In glass molding, viscoelastic property of glass is an essential aspect that determines the glass deformation behavior around the molding temperature. In this paper, viscoelasticity of glass has been measured experimentally by uniaxially compressing cylindrical glass preforms above the glass transition temperature using an ultraprecision glass molding machine. The elastic modulus and viscosity of glass were obtained by curve fitting techniques using the Burgers model and the Maxwell model for creep and stress relaxation, respectively. Based on the thermo mechanical and viscoelastic parameters obtained from experiments, finite element model simulations of the glass molding process were performed, which can be used to visualize the stress/strain distribution and to predict the residual stress in glass.

A Discrete Algorithm of Integrated Photoelasticity for Axisymmetric Problems

By measuring stresses in cylinders with no stress gradient in axial direction, the cross-section of the cylinder may be considered as consisting of concentric rings in each of which the stresses are constant. Axial stress distribution in the cylinder is determined stepwise starting with the outermost ring (the so-called onion-peeling method). This paper generalizes the onion-peeling method for the case of axisymmetric stress distributions when stress gradient in the axial direction is present. Measurement of the integrated isoclinic and optical retardation is carried out in two parallel sections which are perpendicular to the specimen axis, Δz apart from each other. The distributions of the axial stress and shear stress are determined directly from the measurement data, using linear approximation of integrated photoelasticity. Other stress components are determined using the equilibrium equation and the compatibility equation (if stresses are due to external loads) or using the generalized sum rule (if residual stresses in glass are measured). The method is less sensitive to measurement errors than the Abel inversion. In comparison with the polynomial approximation of the stress distributions, the onion-peeling method gives more adequate results if stress distribution is not smooth. The paper is illustrated with several applications.

Microscale computational simulation and experimental measurement of thermal residual stresses in glass–alumina functionally graded materials

Glass–alumina functionally graded materials are new attractive composite materials, that can achieve peculiar mechanical properties due to their gradual compositional variation. Nevertheless, the difference between the coefficients of thermal expansion of the constituent phases may result in significant thermal residual stresses in service or during fabrication. A proper glass formulation can minimize the mismatch in thermo-mechanical properties, thus relevantly reducing the mean value of the resultant thermal stresses. However, it is a crucial requirement to evaluate the effect of microstructural discreteness and randomness on the actual stress distribution in functionally graded materials. With this aim, a computational model which applies the finite element method at the microscale is used. The careful modelling of the real microstructural details enables to accurately predict the local stress values and distribution. In order to verify the reliability of the computational simulations, the residual thermal stresses were also experimentally measured by means of a piezo-spectroscopic technique. The comparison between the numerical and the experimental results validate the microstructure-based model.

ON HYBRID THERMOMECHANICS FOR MULTILAYERED CYLINDERS

The following inverse thermoelasticity problem for a stepwise inhomogeneous cylinder is considered. The axial stress distribution has been measured experimentally but the temperature field is not known. By eliminating temperature from the equations of thermoelasticity in the case of plane deformation, new relationships between the stress components are derived and a closed-form solution of the problem is found. The classical sum rule is generalized for stepwise inhomogeneous cylinders. Because residual stress in glass can be described by the equations of thermoelasticity, the results make it possible to completely determine stresses in step-index optical fibers and fiber preforms. An experimental example illustrates application of the method presented.