An extensive experimental study of the factors affecting the tensile strength and static fatigue of bulk glass has been conducted. To minimize the effects of past history of the specimens, all specimens were subjected to a controlled, reproducible surface abrasion. Time, temperature, and chemical environment were subject to control and systematic variation during the period from abrasion to test and during the strength test itself. Specimens consisted of flat laths tested in cross bending with the abraded spot in the center of the tension face. An electronically controlled electromagnetic tester permitted applications of pulse loads or constantly increasing loads with controllable durations from about 0.0025 second up to any desired value. The apparatus and methods for producing the abrasions, controlling the environment, and performing the tests are described in this paper. A brief review of the experimental background on the strength of glass is also presented as an introduction to the aims and concepts of this study.
A controlled grit blast was found to be a reproducible method of producing standardized damage to a glass surface. The effects of grit size, blast pressure, and amount of grit on the strength of the resulting specimens are reported. Static fatigue curves (strength vs. load duration) were obtained for specimens immersed in room-temperature distilled water and in liquid nitrogen (77OK.) after the specimens had been subjected to various abrasion treatments. The low-temperature strength was independent of load duration, and for surface damage of simple geometry it was inversely proportional to the square root of the initial crack depth, consistent with the Griffith theory. Abrasions of different geometry produced Mering static fatigue curves at room temperature, and in one case curves actually crossed. If, however, the strength values for each abrasion were divided by the low-temperature strength for that abrasion and plotted vs. a reduced time coordinate, all the data could be fitted to a single "universal fatigue curve." This analysis led to a clear distinction between "linear" and "point" flaws, the former being flaws (such as emery scratches) which have an extension in a direction perpendicular to the applied stress and the latter being of a more localized character. Linear flaws fatigue more rapidly than point flaws by a factor of W y and for each type of damage the fatigue rate is inversely proportional to the exponential of the initial flaw depth. A detailed analysis of the data in terms of several static fatigue theories from the literature shows that none of them provides a complete and adequate explanation of these results.
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