RESISTANCE OF GLASS TO THERMAL STRESSES*

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Abstract
The theory of stresses introduced in glass by thermal shocks is a matter of fundamental interest to the glass manufacturer. Analysis is complicated by the existence in many cases of unknown degrees of stress relief or transference by flexure. General treatment of these cases is extremely difficult because of irregular or variable shapes. There is, however, a large group of thermal‐shock phenomena in which flexure of the piece is not a factor. Complete immersion shocks fall in this category; so also do thermal shocks administered to tubes and pipes, since the cylindrical shape prevents flexure. (There is a small end effect accompanied by flexure, but in practice only a small fraction of breaks originate in the end region.) Many examples of breakage from shocking one side only of a piece are also of this simple type, since maximum tension occurs immediately from pure “stretch” and is greater than subsequent stresses accompanied by flexure.Analysis of the stresses resulting from stretch incurred by thermal gradients has been carried out by many writers for plates and cylinders, and various applications have been made of the results.1 The calculation of maximum fiber tension, however, is not in itself enough to determine thermal endurance. The duration of the stress also plays an important role. The simplest example of this is the well‐known fact that in immersion tests breakage frequently does not occur for an appreciable interval, sometimes several seconds. Yet maximum tension on the cooled surface is almost immediate and falls off rapidly. The importance of duration is pointed out by Schönborn,1(c) but he does not attempt any quantitative treatment. It is the purpose of the subsequent analysis to incorporate the time of stressing into the discussion of thermal endurance.

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