Nondestructively determining local strength and residual stress of glass by Hertzian indentation

Hertzian indentation was used to evaluate the local strength of glasses and to estimate residual stresses in strengthened glasses. It was found that initiation of Hertzian crack in glasses depends on the mean stress within a surface thickness related to material properties, but independent of the peak stress. The maximum mean stress for crack initiation was a constant and was defined as the local strength of the specimen at the contact position. Indentation experiments, using alumina spheres with various sizes as indenters, were performed on unstrengthened and strengthened glasses, respectively. The results showed a good stability in the local strength and a good agreement between the residual stresses measured by the present method and by a polarizing device. In addition, the potential merits of this method are: a) Nondestructive and in-situ measurement of strength can be performed without using a standard specimen and a rupture test; b) Strength proof test and strength comparison can be easily preformed.