scope:

MIL-HDBK-722(MR) contains technical information and data pertaining to products formed from glass.

As considered here glass is an inorganic noncrystalline material (defined in more detail in the beginning of Chapter 2). Glass which is melted and formed and then transformed to a crystalline body, sometimes called glass-ceramic, is also considered briefly. Neither industry nor any organization has identified compositions and properties of commercial glasses except in a general way, and no glass specifications have been established for this purpose. Compositions of a number of commercial glasses listed in Table I are merely representative of the indicated types. Glass designations of individual glass makers may represent specific compositions, but these have been avoided here. Many glass compositions have been and are being developed for special applications. Except for several types of particular interest for military purposes they are not considered here.

It is anticipated that this handbook will be used by personnel engaged in any of a variety of occupations including engineering design, development, procurement, inspection, manufacturing, supply, maintenance, and disposal of military equipment and materials. To satisfy the wide range of this audience the discussion of glass products ranges from the preparation of raw materials to the forming and finishing of the final products.

Tables are included to indicate representative values of the physical properties of some common commercial glasses, but minimum values of these properties, as are sometimes used as application data for other materials, are not given. Because of the brittle nature of glass the number of significant mechanical properties which can be listed is limited. Furthermore, breaking stresses are not explicitly correlated with composition or type, but rather on methods of forming and treatment. Consequently strength data cannot be tabulated for glasses.

a. Chapter 1. Chapter 1 is a short introductory chapter in which the purpose and scope of the handbook are identified, and a brief review of glass product development and use is presented.

b. Chapter 2. This chapter describes how certain chemical compositions can be cooled from the molten state to form the characteristic structure of the glassy state. It is noted that some unusual properties are associated with this structure. Common glass-forming constituents are noted and examples are given of compositions of a few common glasses. Melting operations are described briefly, as are the common methods used for forming various kinds of products while the glass is still in its viscous condition. Some of the many types of finishing operations which are carried out on the glass after its original forming are mentioned.

c. Chapter 3. This chapter on selection criteria for glass is concerned primarily with its properties. Certain properties, such as viscosity and brittleness, are peculiar to this material so that their treatment has been relatively expanded. The wide use of glass for the transmission of light requires the consideration of its optical properties. All property values given here must be accepted as approximate because compositions of the various types are not definitely fixed.

d. Chapter 4. Chapter 4 includes discussions of several technical matters not considered elsewhere. There is a brief analysis of the thermal treatments of annealing and tempering. Procedures for strength testing are included because types of tests commonly applied to other materials are not readily adaptable to brittle materials. In addition to the conventional methods of determining breaking stresses, the subject of fracture analysis of glass is discussed. One unusual feature is that breaking stresses can usually be estimated from markings on the fracture surface with a fair degree of accuracy. The use of this method is particularly useful in the diagnosis of fractures which occur in service.

The structural design of brittle materials imposes limitations which may be of little importance for materials of a more conventional kind. Not only are the characteristics of mechanical failure different for glass, but the results of failure can be catastrophic. Although design principles are similar to those for other materials, the general philosophy of design and the direction of approach will be modified. Actual design procedures used constitute much more than the substitution of properties, including an assumed breaking stress, than in the procedures used for metals. These distinctions are discussed briefly.

e. Chapter 5. This chapter discusses some representative applications of glass which are of military significance. The discussion attempts to show the engineering principles involved in the various designs rather than structural details. Many of the military uses of glass involve the transmission of light for purposes of glazing and vision through the medium. Such glazing components in widely different fields have been considered. It is shown that when requirements become more varied and more severe, both the type of glass and the structural design of components may be modified greatly.

It is noted that glasses may be required to transmit or absorb radiations in parts of the electromagnetic spectrum beyond the visible range. Glasses of special properties are often used for such purposes. Plastics reinforced with glass fibers may be used to overcome some of the mechanical limitations of massive glass components. This composite material is shown to have many applications.

f. Chapter 6. This chapter deals with the subject of quality assurance. No single means, taken alone, will provide this assurance. The first requisite lies in the ability of the supplier to provide glass material and products which will function satisfactorily in service. Then there should be some criterion embodied in a product specification for verifying the suitability of the product at the time of acceptance. Requirements to be met by the product and test procedures for verification of compliance are both included in such specifications. It is possible for products to meet the requirements of a specification and still not operate satisfactorily in service. This can occur if the requirements and tests of the specification are not related properly to conditions which will be encountered in service. As a consequence, a product specification is subject to review and change. This can result from changes in service conditions or from some development of the product which may affect its performance. These factors should be kept in mind when any specification is being considered.

This chapter lists a number of federal and military specifications related to glass products. These specifications are for products of massive glass, although some are for composite materials. Many other specifications have been issued in special fields, including electronic tubes and components, fibrous glass, and glass reinforced plastics.