Crack initiation at microindentations on thermally toughened and untoughened glass

The use of microindentations to produce controlled cracks for the study of the fracture properties for ceramic and glasses is now a well established technique. The extent of the radial microcrack that are produced at the corners of the impressions depend upon the applied load, the material, the state of stress at the surface, and the envrionment. The effects of thermal toughening or tempering the glass have been fully analysed by this technique. The present results from microhardness testing thermally toughened and untoughened soda -- lime glass, using a standard commercial Zwick hardness tester are shown. The threshold contact load for radial crack initiation is clearly greater for toughened than for untoughened glass: i.e. the toughened glass is more resistant to this initiation process, which would suggest that the compressive stresses associated with toughening inhibit initiation as well as propagation.