Influence of R-curve behaviour on strength degradation due to Hertzian indentation

The influence of R-curve behavior on the post-indentation strength of two types of alumina was studied using Hertzian indentation. Soda-lime glass was included in the study to represent a material which does not exhibit R-curve behavior. Specimens were indented with a spherical indentor with increasing loads. Post-indentation strength was measured in a mineral-oil environment using a ring-on-ring biaxial fixture. Indentation of the two aluminas produced the typical Hertzian crack pattern with a somewhat greater statistical variability than that found in soda-lime glass. The strength of the two aluminas fell into four different regimes that were separated by the onset of ring, cone, and radial crack formation, respectively. R-curve behavior did not appreciably influence the results, undoubtedly due to the relatively narrow range of indentation loads where failure was controlled by ring and cone cracks. These strength results contrast to those obtained for soda-lime glass. In soda-lime glass, ring and cone crack nucleation occurred simultaneously with a sudden drop in strength followed by a gradual decline in strength with increasing indentation load, in accordance with the prediction of Hertzian cone-crack theory.