We discuss "hidden damage" of glass by the rolling process, which results in heterogeneous distribution of microcracks on the edge surface of glass element, which are the fracture source deteriorating glass element strength. It is shown that removal of this damage on the edges of glass elements increases the engineering strength of float glass significantly. Using the "hidden damage" approach, we provide strength determination for the weakest specimens that is statistically relevant and is based on a reliable engineering parameter.Keywords: glass, rolling process, hidden damage, engineering parameter, microcracks. Introduction.Glass is commonly used in engineering, when a transparent, durable, and stiff material is needed. Although glass is not usually used for load-bearing applications, in most cases glass still carries a load. In an automobile, the front and rear windows are an integral part of the structure responsible for a significant part of the total stiffness and resisting the considerable forces generated by the air pressure at high-speed driving.However, the reliable value for the glass strength is an open issue: the tensile strength cannot easily be determined, since glass in direct tensile test will break at the grip. In certain cases, bending test results provide scattered values of the bending strength with a spread of 30 to 50% of the mean strength.For glass fibers and cast glass the distribution can be adequately described using the Weibull statistic approach leading to a probabilistic strength for the glass used [1,2]. For the more common float glass this is more complicated. Results of bending experiments by various authors suggest that the processing and specimen size influence the results and suggest systematic data deviation from the Weibull statistic distribution [3][4][5].A likely explanation for this is that the usual processing of float glass results in multiple types of defects which provides a multilinear Weibull plot [6,7].To investigate this systematically it was decided to look in more depth at the effect of processing on float glass strength. The initial step, which is described in this paper, deals with the effect of cutting and breaking quality on the strength of processed float glass.Float glass is produced as 6´3.21 m "jumbo" plates. These are cut into the required size and the cut edges are usually grinded and polished. The cutting is usually done by scratching the glass with a glazier's diamond or rolling it with a tungsten carbide roller producing a cut on the upper surface. This is schematically shown in Fig. 1. By bending the plate slightly, as is shown by the arrows, tension is generated at the cut resulting in an unstable crack-cut growing down on the figured straight arrow through the thickness separating the glass parts. Surface damages such as crumbled arrises and cross microcracks are forming on edges of both glass parts under the contact cutter action.The depth values of these specific cross microcracks are larger than those of the initial surface microcracks which ...
The paper considers the determining factors in the structural strength of heat-strengthened glass. The values of residual stress were obtained experimentally at different points on the surface of specimens of heat-strengthened, HS-glass and fully tempered, FT-glass using a SCALP-4 laser scanning polarimeter. The distribution of residual stresses is determined over the area of specimens. It is found that the standard methods of in-process control of the degree of strengthening that involve the determination of compressive stresses only at several points, give a fairly rough estimate of the average level of induced residual stresses in glass structural elements and do not determine their real significance near the fracture origins. The necessity of using the methods of more comprehensive in-process control of residual stresses is justified in order to obtain data on their statistical distribution and optimize the heat strengthening modes according to special requirements to building glazing and new engineering products. As shown by the analysis of the investigations dealing with determination of "the strengthening effect" during heat treatment of glass, it does not exceed the level of residual stresses. Due to the statistical nature of glass strength and residual stresses, the determination of the empirical coefficient, which traditionally considers the increasing contribution of residual stresses to the strength value of heat-strengthened glass, presents great difficulties. Based on the sampling of the bending strength values for glass in the initial, as-received state and after heat strengthening using the mathematical statistics methods, the calculated distribution of stress values that characterize the strengthening effect is determined. The influence of the combination of heat strengthening and etching on the strength characteristics of glass is investigated. It is found that the enhancement of the strength of glass and glass products strengthened using combined techniques is accompanied by a significant increase in the scatter of the tensile strength values, and the influence of the combined treatment on the tensile strength of glass is not additive.
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