The Diamond Pyramid Hardness of Glass in Relation to the Strength and Structure of Glass. Part I. An Investigation of the Diamond Pyramid Hardness Test Applied to Glass

JSGT 1954 V38 T479-T500

The most fundamental property of glass, which varies as its composition is changed, is the bonding of the constituent atoms. Valuable direct evidence on the structure of glass should therefore be obtained if the variation of its bond strength with composition could be measured. It is not possible to do this by making conventional strength tests on large specimens, because, as is well known, the variations in the measured strengths of seemingly identical specimens of the same glass are often greater than the differences between the mean values of large numbers of tests on glasses of widely varying compositions. Furthermore, the observed strengths are very much less than the “ultimate strength” to be expected from theory. It is now generally accepted that the variability and the low observed values are both explained by the presence in the glass structure of inherent flaws. Griffith worked out the theory of the effect of elliptically shaped flaws and showed that flaws about 5 μm in length must be postulated in order to explain the order of breaking strengths observed in glass.

L. Ainsworth