Strength of glass under contact micropressure

Abstract

1. The microstrength of a glass in compression and tension is considerably higher than its macrostrength determined by testing standard specimens in uniaxial compression and symmetrical bending under conditions of contact loading. Thus polished sheet glass in the original state (supplied by industry) is able to withstand local contact pressures to 600 kgf/mm2without disturbing the macrostrength until microdefects are present. 2. The strengthening of polished sheet glass by the ion exchange method results in an increase of 1.24 times the level of pressures which cause the formation of local defects on the contact surface of the glass in the form of circular microcracks. The effectiveness of strengthening the glass by ion exchange to increase the serviceability and reliability of contact joints in multielement composite structures is evaluated by increasing the loads transmitted with an indenter on glass by 1.8-2 times and also the lower values of the maximum pressures in the glass under the indentor which correspond to the moment of the appearance of a circular microcrack. 3. The defects in the polished surface of a glass in the form of round microcracks obtained during local contact compression and lying in a tensile zone result in the loss of the strenthening effect in the glass by ion exchange during symmetrical bend tests. 4. In comparing different brands of glasses and other varieties of glassy materials intended for compression supporting structures in which contact joints between individual elements are inevitable it is expedient to use as the criterion of preferable usefulness the value for the maximum pressure under the ball indeater which corresponds to the formation of a microcrack in local contact compression.