Fracture Toughness of Glass Using the Indentation Fracture Technique

Half-penny shaped cracks called median vents are formed about indentations made with sharp indenters in brittle materials. Two fracture mechanics analyses for the formation of median cracks exist and constitute a basis upon which the fracture toughness (K_c) can be obtained from indentation fracture experiments. However, little attention has been directed toward assessing the applicability of indentation fracture as a method for determining K_c in brittle materials.

In this paper, such an evaluation of indentation fracture was performed on various glasses. Initially, indenter load-median crack length data were used to establish that the model of Lawn and Fuller described best the indentation fracture data. Their analysis was used to calculate K_c-values, which were compared to K_c-values obtained with conventional fracture mechanics methods. The difference in K_c-values determined by the two methods was ∼30 percent or less for normal glasses (glasses containing network modifier ions), and good reproducibility of indentation fracture K_c-values was found. A qualitative ranking of K_c by these two methods, however, did not agree. An evaluation of K_c by indentation fracture was not possible in anomalous (silica-rich) glasses due to the presence of cone cracks.