Fracture toughness and residual stress measurements in tempered glass by Hertzian indentation

Hertzian indentation was used to assess the fracture toughness (KIc) and the surface residual stresses of tempered soda lime glass. Although such measurements are usually performed on chemically strengthened glasses, where the thickness of the layer under compression is limited to several micrometres, our study considers the most useful case of thermally quenched glass, widely used in industry and where the compressed layer is several hundred micrometres thick. Measurements were performed on annealed and thermally quenched samples and residual stresses were calculated using two different models of the literature. Results were compared to stress measurements performed by a conventional photoelasticimetry technique. It is shown that Hertzian indentation gives qualitatively correct results which are comparable for each of the literature models used, provided that interfacial friction between the spherical indenter and substrate is correctly taken into account in the calculations. It is shown that a scratch tester used to perform sliding indentation onto the surface of the samples is a suitable tool for assessing the friction coefficient value.