Toughening of Glass

11 Key excerpts on "Toughening of Glass"

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  73rd Conference on Glass Problems, Volume 34, Issue 1

  • S. K. Sundaram(Author)
  • 2013(Publication Date)
  • Wiley-American Ceramic Society

    (Publisher)

  Stronger glass means lighter products. Transportation costs are reduced. Storage is more efficient. Stronger and lighter products multiply the uses available for the products and make them more efficient. One well known strengthening technology is chemical treatment of glass and chemically tempered smartphone cover glass is becoming more common. But, chemical treatment is limited to certain applications, and because of the length of time it takes and other expenses, it isn't commercially viable for many uses. Thermal tempering of glass is used to produce safety windows in buildings and automobiles and allows us to walk 4000 ft above the floor of the Grand Canyon. Another way to strengthen glass is through lamination, such as bulletproof glasses. Still, these technologies are inadequate. Although they increase the usable strength of some glass articles, they have not addressed the fundamental, root cause of low glass strength. Wouldn't it be better to understand the nucleation of flaws in glass, before applying these advanced tempering techniques to them? So the concept behind the UGSC is to develop a qualitatively stronger base of glass, and then, use strengthening techniques. This would make an enormous leap up in total glass strength. THE CASE FOR AN INDUSTRY COALITION Progress has been slow and it is becoming increasingly apparent, that fundamental breakthroughs will not come from individual company research. Historical gains have been small. Industry has closed over half of North American manufacturing plants in the past 20 years. Members of the scientific and industrial community believe we are at a tipping point where we can exploit advancing fundamental laboratory techniques to better understand the origins of glass strength and engineer new products and processes to advance usable glass strength. We have witnessed advancements in the experimental techniques university researchers can use to study glass strength.

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  Materials NQF3 SB

  TVET FIRST

  • Sparrow Consulting(Author)
  • 2013(Publication Date)
  • Macmillan

    (Publisher)

  Fully tempered glass is stressed to be able to handle pressures of above 65 MPa. It also has three to five times the bending and impact strength of ordinary annealed glass of the same thickness. Heat-strengthened glass is only twice as strong as annealed glass, and is stressed to be able to handle pressures from 40 MPa to 55 MPa. Chemically toughened glass Chemical toughening results in a tougher glass than heat-treated toughening and can be applied to complex glass shapes, whereas only flat glass can be heat treated. In the chemical process a chemical reaction is started when the glass is submerged into a bath of molten potassium nitrate. The chemical reaction that takes place replaces sodium ions on the surface of the glass with larger potassium ions, which forces the surface layer of the glass into compression. It is this chemical condensing process that strengthens the glass. Glass that is treated by heat or chemicals in order to be stronger will still break in the same way as annealed glass. It will, however, be able to carry more weight or stress than standard annealed glass before breaking. Glass should usually be shaped or cut before it is put through the toughening process as it cannot be re-worked afterwards. This also applies to drilling into the glass as it will shatter if drilled after it has been toughened. However, chemically toughened glass can be cut much more easily after it has been through the chemical process than heat-treated glass, as long as it is only cut once. Toughened glass is most commonly used where strength, thermal resistance and safety is important, for example in unframed assemblies in buildings, such as frameless doors, structurally loaded applications, and any other application that would become dangerous in the event of human impact. stressed: set to a certain tension Words & Terms Figure 7.5: Toughened glass Assessment activity 7.3 Individual activity 1. In your own words, describe how toughened glass is manufactured.

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  Forensic Examination of Glass and Paint

  Analysis and Interpretation

  • Brian Caddy(Author)
  • 2001(Publication Date)
  • CRC Press

    (Publisher)

  Glass has an extremely high strength in compression and when it breaks it does so because of tension at the surface. Glass can be thermally strengthened by inducing compressive stresses in the outer surfaces. In order to break toughened or tempered glass, the surface compression has to be exceeded and additional tension applied. Toughening is carried out by reheating the glass uniformly to a temperature just above that at which deformation could take place and then rapidly cooling the surfaces. The cooling chills the outer layers while the centre of the glass is still hot. As the centre of the glass cools it contracts, pulling the rigid surface layers into compression which is balanced by the tension in the inner layers. This method of strengthening can be applied to flat glass or simple shapes such as curved windscreens or tumblers. The thickness of the glass must be uniform and not too thin. The shape of the article must be such that all surfaces can be cooled uniformly. Bottles do not satisfy these conditions and they cannot be toughened in this way. Thermally toughened glass cannot be further processed since any damage to the surface will expose the central layer, which is in tension, and the glass will break. The shattering of a car windscreen is a good example of this phenomenon. It is possible to toughen certain articles chemically by ion exchange. The article is immersed in a molten potassium salt. The potassium ions replace sodium ions at the surface and, being larger, create a thin layer of compression. This method is limited to specialised products.

  2.6.2 Coating

  The coating of glass surfaces has been practised for centuries. Mirrors are a good example of this art. Coating processes have been developed in recent years for decoration, protection and strengthening, and for technical reasons, for example to control the transmission of light and other radiation through the glass. Most glass containers are coated to assist in the preservation of strength and to improve the handling of the product in manufacture and filling. Coatings are applied at two stages. Hot end coatings are applied immediately the containers have left the forming machine and before they enter the annealing lehr. Cold end coatings are applied just before the annealed containers leave the lehr. The coatings are applied either as a vapour or by spray. Examples of hot end coating materials are compounds of tin or titanium, while corresponding cold end coating compounds include organic waxes, polyethylene emulsions, polyethylene glycols and their fatty acid esters or fatty acids such as oleic acid. All cold end coatings must satisfy the health regulations of the countries where the containers are filled and distributed. Modern flat glass products are often coated. Patterned or textured glass carries a coating on the smooth side to protect the glass during handling and transportation. Advanced glazing products for solar control and for thermal insulation purposes are coated using both on-line and off-line processes. On-line pyrolytic coatings are applied during manufacture on the float line and consist usually of tin oxide doped with elements such as indium to obtain the selected transmission and reflection of wavelengths in the visible and infrared parts of the spectrum. Off-line coatings are applied by vacuum sputtering using a range of metallic and semiconductor materials. The coatings may be multilayer and many of the products are also tinted either in the body of the glass or in the coating.

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  Ceramic Processing

  Industrial Practices

  • Debasish Sarkar(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Prior to manufacturing toughened glass, one has to pick up the annealed float glass, as has already been discussed in Section 8.6. Chemical toughening is another process to toughen the glass; however, in this chapter, we will discuss the temperature-assisted toughening process only. As well as annealing, the following five steps are usually adopted to manufacture toughened glass.

  Step I: It is mandatory to cut the annealed glass sheet to the targeted size before toughening, as annealed glass cannot be cut after toughening. This is followed by grinding the edges to avoid any uneven appearance on the edge, which is responsible for enhancing the chance of failure during toughening. At this stage, holes are drilled if required in the final product.

  Step II: The processed glass sheet is heated above the glass transition temperature but below the softening point. A schematic representation of the manufacturing protocol is illustrated in Figure 8.12 . The bottom surface or tin-side of float glass may contain micro-flaws due to contact with lift off rolls and annealing lehr rolls during manufacturing. This flaw can be identified by fluorescence when illuminated with UV rays. Thus, it is preferable to place the tin-side annealed glass in contact with rolls that enable the closing of micro-cracks under compression and the manufacture of flawless toughened glass.

  FIGURE 8.12 Schematic representation of toughened glass manufacturing protocol.

  Step III: The glass sheet may either be maintained optically flat or formed into a definite curvature or any other desired shape. By preference, the deforming temperature for flat and curved glass is maintained at 625 ± 15 and 650 ± 10°C, respectively. A typical history of temperature-dependent stress and temperature profile distribution is shown in Table 8.5 and Figure 8.13

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  Elasticity and Strength in Glasses

  Glass: Science and Technology

  • D Uhlmann(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  C H A P T E R 5 Thermal Tempering of Glass Robert Gardon ENGINEERING AND RESEARCH STAFF FORD MOTOR COMPANY DEARBORN, MICHIGAN I. What is Tempered Glass? 146 A. Fracture of Tempered Glass 149 II. Tempering and Tempered Glass 152 A. Nature of Temper Stresses 152 B. Dependence of Temper Stresses on Process Parameters 153 C. Transient Stresses during Tempering 157 D. Influence of Tempering on Physical Properties of Glass 158 III. Physics of the Thermal Tempering Process 159 A. The Simplest View: Thermoelastic and Permanent Stresses in Glass 160 B. Refinements: Thermal History, Temperature Equalization Stresses, Solidification Stresses, and Structural Effects 163 IV. Theories of Tempering 167 A. General Considerations 167 B. Temperature Equalization Stresses 169 C. Instant Freezing Theories 170 D. Viscoelastic Theory 174 E. Structural Theory 180 F. Assessment of Theories 186 V. Nonuniform Tempering 189 A. Membrane Stresses in Tempered Glass 189 B. Edge Stresses 193 C. Applications of Nonuniform Tempering 193 VI. Tempering Practice 196 A. Historical Overview 196 B. Flat Glass 199 C. Other Tempered Glass Products 205 VII. Standards and Measurement Methods 205 A. Standards for Tempered Glass Products 205 B. Measurement of Temper Stresses 206 References 213 145 Copyright © 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-706705-1 146 ROBERT GARDON I. What Is Tempered Glass? 'Tempered glass is so called by analogy to tempered steel. Both are strengthened by tempering, a process that entails heating the material to a critical temperature and then rapidly quenching it. Here the analogy ends, for the immediate effects of this heat treatment are very different for the two materials. In steel, a new balance of hardness and toughness is pro-duced by the precipitation of carbides.

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  Modern Materials

  Advances in Development and Applications

  • Bruce W. Gonser(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  A process of this nature is utilized in the production of 96% silica glass. Glass-ceramic bodies are available with characteristics of chemical durability similar to those of the more resistant glasses. IV. Engineering of Glass The technical requirements of glasses and their application are con-stantly becoming more exacting, not only for new uses but for estab-lished uses as well. In many new and proposed applications the condi-tions imposed may be unusually severe. This necessitates more elaborate engineering procedures in the design and evaluation of glass products. An adequate knowledge of the characteristics of glass, the effects of various treatments, and the properties of various compositions are all essential for the engineering treatment of these problems. New manu-facturing methods and procedures may alter the characteristics of glass products materially. This must be taken into consideration. Technical methods must be used, not only in the design of products, but in the planning of experiments to evaluate products and in the anal-ysis of results obtained. For structural applications problems associated with probability of failure must be handled in a realistic manner (12). 278 ERROL B. SHAND STRUCTURAL DESIGN OF BRITTLE MATERIALS The general subject of structural design of brittle materials has re-ceived considerable attention in recent years, not only because of prob-lems resulting from the brittle fracture of steels, but also because of the need of structures for temperatures exceeding the limits of metals. Fre-quently, for the conditions imposed, only brittle materials will meet the requirements (12, 13). The characteristics of brittle bodies are found to a pronounced degree in glass. Thus: 1. Glass is strong in compression but generally limited in its tensile strength. It always fails from components of tensile stress even when loaded in compression.

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  Processing, Properties, and Applications of Glass and Optical Materials

  • Arun K. Varshneya, Helmut A. Schaeffer, Kathleen. R. Richardson, Marlene Wightman, L. David Pye, Arun K. Varshneya, Helmut A. Schaeffer, Kathleen. R. Richardson, Marlene Wightman, L. David Pye(Authors)
  • 2012(Publication Date)
  • Wiley-American Ceramic Society

    (Publisher)

  Additionally, improved competitiveness is important. These demands again trigger certain demands on the process-side: More reproducible and robust production, higher strength of the glass articles and lesser energy consumption and emissions during production. STRENGTHENING AND LIGHT-WEIGHTING, HOW? Focusing on the demand for higher strength of glass-containers, there are basically three ways to increase the strength of glass: a) Preserving the initial strength Preserving the initial glass strength can be achieved by protecting the glass (e.g. with a coating) or by optimizing the process to avoid damages. As strength of glass is a question of surface damage, all of this certainly has to be done before the glass has been damaged. b) Restoring the initial strength The initial glass-strength can be restored by e.g. defect healing through tempering near Tg or by coatings. Logically, this approach has to be applied after the glass has been damaged. c) Applying an additional strength In this case the glass strength is increased by giving additional strength to the article (e.g. by ion exchange, coatings, thermal toughening, etc). Applying an additional strength can be done before or after the glass has been damaged. Improvements to strengthening and light-weighting of glass-containers in the past have mainly been achieved through more robust and reproducible processes. These improvements aimed largely on preserving the initial strength. No active strengthening methods have been applied permanently in real manufacturing. Various techniques are applied in production and help, as mentioned, to preserve the initial glass strength and enable a significant light-weighting of containers. There are such revolutionary developments as the independent section machine (IS machine) in 1924 and the narrow-neck press-blow process in 1976.

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  Architectural Graphic Standards

  • Keith E. Hedges(Author)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  Most manufacturers heat‐treat the glass using a horizontal process that can introduce warpage, kinks, and bowing into the finished product, which may create aesthetic or technical concerns. A vertical process may still be available that produces tong marks or depressions into the glass surface near the suspended edge. Vertical processing may produce large amounts of warping and distortion. The heat treatment quenching pattern on the surface of the glass can become visible as a pattern of light and dark areas at certain oblique viewing angles and with polarized light. This effect can be more pronounced with thicker glass and may be an aesthetic consideration. Refer to ASTM C1048 for allowable tolerances and other properties.

  Heat‐strengthened glass is generally two to three times stronger than annealed glass. It cannot be cut, drilled, or altered after fabrication. Unlike tempered glass, it breaks into large, sharp shards similar to broken annealed glass. Heat‐strengthened glass is not acceptable for safety glazing applications.

  TEMPERED GLASS

  Tempered glass is generally four to five times stronger than annealed glass. It breaks into innumerable small, cube‐shaped fragments. It cannot be cut, drilled, or altered after fabrication; the precise size required and any special features (such as notches, holes, edge treatments, and so on) must be specified when ordering.

  Tempered glass can be used as a safety glazing material provided it complies with the ANSI and CPSC references listed in the “Laminated Glass” section, below. Tempered glass can be used in insulating and laminated assemblies and in wired, patterned, and coated processes. All float and sheet glass 1/8 in. or thicker may be tempered.

  ULTRACLEAR GLASS

  The high clarity and high visible light transmittance that characterize ultraclear glass come from the special soda lime mixture it is made from, which minimizes the iron content that normally gives a slight greenish color to clear flat glass. Ultraclear glass is generally available in thicknesses from 1/8 to 3/4 in. It can be heat‐strengthened, tempered, sandblasted, etched, or assembled into laminated glass. Ultraclear glass is used for commercial display cases, museum cases, display windows, frit‐coated spandrel glass, aquariums, mirrors, shelving, security glass, and other uses in which clarity and better color transmittance are required.

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  Glass

  Mechanics and Technology

  • Eric Le Bourhis(Author)
  • 2014(Publication Date)
  • Wiley-VCH

    (Publisher)

  et al. (1967) approach for thermally tempered glasses can be generalized to chemically tempered glasses (Bouyne and Gaume, 2001).

  The consequence of the above results is that toughened glass cannot be cut or otherwise modified in shape as this would disturb the system of pre-stresses, and with fracture beginning at any point in the glass, the whole sheet would shatter immediately into small (although harmless) fragments. Hence, glass tempering is carried out offline after cutting, drilling and edge-grinding operations (Chapters 8 , 10 ).

  7.7.4 Impact-Induced Fracture

  There has been little experimental work dedicated to the way an impact crack generates and further enters the tensile region, causing fracture. Figure 7.11 is hence of much interest as it shows a high-speed photographic sequence in the cross section of a thermally tempered soda–lime–silica glass impacted by a hard sphere thrown at 150 m s−1 towards its surface. The dark fringe corresponds to zero stress (these are photoelastic images; see also Chapter 11 ). As the sphere impacts the block, the stress field changes, as revealed by the photoelastic survey of the zone (frames 1–3). Interestingly between frames 4 and 5 the cracks enter the region of tensile stress and this leads to the initiation of catastrophic failure of the sample. The cracked area is observed as a dark and growing surface as time increases. The velocity of the cracks depends on the direction they propagate. The velocity of the cracks moving towards the surface of the impact is much smaller (200–300 m s−1 ) than the velocity of the cracks spreading along the loading axis. In frames 7–12, cracks further spread and bifurcate, the images showing chaotic features. In fact, finite element analysis (FEA) happens to be of great help in understanding impact failure (Appendix K; Brajer, Hild and Roux, 2010) while we further discuss crack initiation under contact loading in Chapter 8

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  Sustainability of Construction Materials

  • Jamal Khatib(Author)
  • 2016(Publication Date)
  • Woodhead Publishing

    (Publisher)

  Fig. 5.10C ). The PVB interlayer has a certain degree of tensile strength, and this strength may be utilised during a severe breakage where broken glass pieces can be locked in compression as a result of the arching action.

  Fig. 5.10 Postfracture behaviour of (A) float, (B) fully tempered and (C) laminated glass.

  5.10 Glass structural design criteria

  The increasing use of glass as a load-bearing material has led to the development of design standards, technical guidelines and recommendations in recent years, for example, IStructE (2014) . A comprehensive overview of all current design standards and design methodologies is beyond the scope of this chapter. A brief overview of the limit state design methodology recommended in the draft Eurocode, BS EN 1288-2 (2000) , is described here. Readers interested in in-depth reviews of design codes and design methodologies should refer to subject-specific text (eg, IStructE, 2014 ).

  The ultimate limit state design adopted in BS EN 1288-2 (2000) compares the applied tensile stress with the design glass strength. The applied stresses may be determined using standard methods of structural analysis that are based on loads multiplied by partial factors as defined in current Eurocodes: (1) ‘Basis of Structural Design’ (BS EN 1990:2002, 2010 ) and (2) ‘Actions on Structures’ (BS EN 1991:1-1:2002, 2010 ). The design strength of glass is determined by applying a series of partial factors to the characteristic strength of glass (the characteristic strength values of different glass types are presented in Table 5.4 ). These partial factors usually account for: (1) load duration, (2) surface profile of glass, (3) material partial safety factor, (4) partial safety factor for surface prestress (if any), (5) partial safety factor for method of prestressing, etc. The typical values of the material partial factors can be found in BS EN 1288-2 (2000)

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  Sustainability and Innovation

  The Next Global Industrial Revolution

  • Salah M. El-Haggar(Author)
  • 2016(Publication Date)
  • The American University in Cairo Press

    (Publisher)

  CHAPTER 6 Innovation in the Glass Industry: Upcycle of Glass Waste: Foam Glass

  Dina Abdel Alim and Salah M. El-Haggar

  Glass Industry

  Glass is a non-crystalline, inorganic, nonmetallic ceramic material. It is made from inorganic materials that are fused at high temperature, then cooled to a rigid condition without crystallization, forming an amorphous structure. As a non-crystalline material, the glass molecules are not arranged in a repetitive long-range order. Its molecules change their orientation in a random manner (Smith, 1996). Glass, as a ceramic material, has a wide range of properties that make it indispensable for many engineering designs. It is hard and transparent at room temperature, along with excellent corrosion resistance to most substances in the normal working environment; it is also an electrical insulator. These properties make glass widely used in many fields, such as construction and the automotive industry, where it is used mainly as vehicle glazing. Tempered glass and laminated glass (safety glass) are used for automobile windows and windshields. The electronics and electrical industries use glass extensively because it is an insulating material that provides a vacuum-tight enclosure for electron tubes and lamps. High-tech glass is used for display screens and monitors in mobile phones, computers, and televisions. The chemical and pharmaceutical industries use glass because of its high chemical and corrosion resistance. Glass is used in laboratory apparatus and in liners for pipes and reaction vessels. In the household, glass is used for containers and tableware.

  Table 6.1. Composition and properties of common commercial glasses (Smith, 1996)

  Most glass is constituted of a network of ionically covalently bonded silica (SiO2 ) tetrahedra. Other oxides are added to glass to give it a range of properties that suit many applications. For example, the addition of Na2 O, K2 O, CaO, and MgO oxides to glass modifies the basic silica network and lowers the glass melt viscosity, which makes glass more workable and easier to form (Smith, 1996). Some of the common types of commercial glass are listed in table 6.1

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