Static Stress Fatigue and Strength of a Vitreous Silicate

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Abstract
Various types of static loading stresses were applied to freshly cleaved surfaces formed on glass laths. With increasing applied force the number of linear flaws increased. At high stresses the number per unit area was the same order of magnitude as predicted for the pile up of surface flaws to form a microcrack. The diameter of loop flaws, originating at etch pit sources, increased with stress in agreement with a theoretical prediction based on a vacancy diffusion mechanism. It was postulated that with applied loads the vacancies diffuse into the loop source region and increase the local stress field. This stress is partially relieved by the loop expansion. Higher flaw densities were found by etching during actual stress application than by loading in air followed by placing in the etchant. This difference was explained for samples loaded in air by the release of the stress causing some of the flaws to disappear before placing in the etchant, whereas loading under the etchant displays the flaws formed during the loading period. Probe experiments showed that the flaw patterns are not the result of a stress release effect in a surface film created by the etching solution. Samples subjected to localized forces, applied under the etchant, disclosed an increase in local flaw concentration in direct proportion to the load. The results appeared to be linear until the typical Hertz cone fracture occurred. At fracture there was a sudden decrease in the flaws formed at the load region. This decrease in flaw concentration at fracture was believed to indicate the combining of flaws to form a microcrack. The importance of surface energy changes on the flaw formation was also shown in quantitative measurements. With decreasing surface energy the flaw length Fl and flaw number Fn also decreased. On aged surfaces the flaw interaction is less extensive and this may explain the reported higher indentation strengths on cleavage surfaces that have been aged. The buildup of flaws and the formation of microcracks also offers a possible explanation for the maximum measured strength of glass being less than one-half the theoretical value.

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