Some comments on indentation tests on glass

JSGT 1958 V42 T145-T157

Indentation tests made by pressing lightly a diamond pyramid into glass give results which can only be interpreted as flow phenomena, but when a spherical indenter is used circular cracks have always been found. By examining the theories of the viscosity of glass it is possible to predict that flow could occur if the applied stress reached a certain critical value which turns out to be about one-tenth of the theoretical strength of glass. In order to observe these indentations, therefore, it is necessary to have a system in which the glass will not suffer brittle fracture until stresses very near to the theoretical breaking strength are applied. Deductions from experimental and theoretical evidence suggest that with a spherical indenter it may be impossible with many glasses to satisfy the conditions for flow. From the theoretical predictions it can be shown that the diamond pyramid hardness of a given glass should decrease as the fictive temperature increases, and that among different glasses those which have the higher viscosity at some given elevated temperature should show the greater DPH; experimental evidence appears to support these suggestions.

R. W. Douglas