Browsing by Author "Rawson, Harold"

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    Dependence of emissivity on angle for coated and uncoated glass and the calculation of radiant energy exchange in double-glazed units
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1989) Rawson, Harold
    The calculation of heat transfer by radiation between the inner surfaces of a double-glazed unit requires information on the variation of emissivity with angle for each surface. It is difficult and very time-consuming to obtain the information experimentally. Current practice is to obtain the normal emissivity indirectly by a reflectivity measurement and to multiply this by a factor to obtain the hemispherical emissivity. The factor increases with decreasing normal emissivity. Although there is some theoretical justification for the factors used, they need to be examined in some detail, especially when coated surfaces are involved. Using equations for the optics of multilayer thin films and published values for the optical constants of the layer materials, the angular dependence of emissivity has been calculated for glass coated in various ways, including some which simulate commercial coatings. It is shown that the currently used factors are likely to be satisfactory for the current commercial coatings. However, for sputtered coatings containing a silver layer, much higher factors are predicted if the silver layer were to have the same optical constants as carefully prepared, vacuum-evaporated silver. As new coatings and different methods of manufacture are developed, the current factors may need to be revised.
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    Matrix algebra method for deriving oxide factors and its relationship to the principle of least squares
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Rawson, Harold
    Multiple regression analysis of glass property-composition data involves the derivadon of 'normal equations', one for each oxide factor. The number of these is, in general, much smaller than the number of initial equations, which, for each composition, relate the property to the glass composition. The normal equations are readily derived by a matrix multiplication process. It is shown, by the use of a simple numerical example, that the method minimizes the sum of squares of the residuals in the initial set of equations. The method is used to derive oxide factors for calculating thermal expansion coefficients of glasses in a number of systems. Good agreement between calculated and measured expansion coefficients was obtained, the best being for a large group of alkaline earth aluminoborosilicate glasses, data for which is given in a recent patent. For oxides of both group I and group II elements, there is a linear dependence of the oxide factor on z/a, where z is the valency and a is the cation-oxygen distance. The method and results are compared with earlier work.
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    Radiative heat transfer in glass manufacture - One- and two-dimensional problems
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Rawson, Harold
    The one-dimensional radiation-conduction model, originally developed by Gardon, has been used to calculate: firstly, the rate of reheat of glass surfaces after forming in metal moulds, and secondly, the rate of temperature changes in glass in a forehearth following a step change in roof temperature. The extent of reheat at short times ( < 2 s) is not greatly dependent on the absorption properties of the glass but is much more so when the reheat time is long e.g. in hand working and in forming large articles. The forehearth temperature changes calculated by the "correct" radiation-conduction method are more rapid than those calculated by the "effective conductivity" method. A two-dimensional radiation-conduction model has been developed and used to calculate the rates of temperature change in glass in a forehearth. These calculations also show that the "effective conductivity" method is inadequate, especially when applied to time-dependent problems.
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    The effect of coatings in reducing the damage of glass surfaces
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1973) Rawson, Harold; Turton, Geoffrey
    Previously etched microscope slides have been coated with films of octadecylamine and tin oxide. The films were applied both singly and in combination. Friction measurements were made in which the coated slide was mounted on a trolley, a spherical glass indenter was pressed by a measured force against the coated surface and the force required to draw the trolley carrying the slide at a constant speed under the indenter was measured. After removing the coatings the slides were subjected to the Ernsberger lithium ion exchange process to reveal the surface flaws produced by the sliding indenter. It was found that the oxide films alone do not have a marked effect on the coefficient of friction, but do reduce the surface damage. The octadecylamine films markedly reduce both the coefficient of friction and the surface damage. Large normal loads are required to break down these films. The most resistant films are those in which a film of octadecylamine is applied on top of a tin oxide film. Such a composite film is extremely resistant to breakdown. The results are discussed in terms of what is known about mechanisms of surface damage and are compared with results of similar experiments carried out on glass containers.
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    Theory of stress distribution caused by side wall impact on glass containers
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Rawson, Harold
    The stresses and strains in a cylinder subjected to point loads applied at each end of a diameter can be calculated from shell theory if the load distribution is represented by a double Fourier series. The analysis is applied to the calculation of stresses produced by impact on the side wall of a glass container. In doing so, it is assumed that the impact is a relatively slow event and that all the kinetic energy of impact is converted into strain energy in the container at the instant of maximum deformation. The experimental information available is not sufficiently detailed to make possible an exact comparison with the calculated stresses. Nevertheless the degree of agreement is good enough to suggest that the method which has been followed is satisfactory. The computer program which has been developed is a simple one and has small storage requirements.
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    Why do we make glass so weak? A review of research on damage mechanisms
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Rawson, Harold
    The review covers the state of knowledge of damage mechanisms and relates information obtained in laboratory studies with that obtained from studies of the strength of glassware. It also considers the significance of this information in the context of glass manufacturing processes. Recent research has shown that above Tg significant flaw healing occurs. One healing mechanism is flaw blunting by surface tension. However several studies have shown that applied stresses cause changes in the shape and orientation of flaws in such a way as to reduce their stress-raising effect. Studies of the fracture of visco-elastic polymers are referred to. It is suggested that they may give guidance on the experiments that should be carried out and on theoretical analyses that may be appropriate when studying the fracture of glass above Tg. Sticking of glass to mould materials and the transfer of foreign materials to the glass surface are well-known causes of damage. It is important to continue with efforts to reduce this problem. Research on room-temperature damage by indenters shows the effectiveness of surface compressive stresses in reducing this damage. However compressive surface coatings are effective in increasing strength only if the coating thickness is comparable to the depth of the most severe flaw present. Results are presented illustrating the effectiveness of both low-modulus and high-modulus coatings in reducing stresses at the glass surface under indenter loading. Polymer coatings are effective in maintaining the strength of high strength silica fibres, partly by reducing fatigue effects. There is some evidence that these coatings can also increase or maintain the strength of soda-lime glass. However it is unlikely that this is due to the polymer coating acting as a barrier to water permeation.