Fracture Mechanics Study of Ring Crack Initiation in Ceramics by Sphere Indentation

In our previous papers, we showed that the ring crack initiation strength (microfracture strength) of gas-pressure-sintered silicon nitride, evaluated by the sphere indentation method, is not related to the conventional flexural strength of the same material, using the concept of effective area based on the Weibull distribution function. In the present study, to clarify the reason for this, a model for initiation of a ring crack emanating from a surface microcrack was introduced. Using this model, the microfracture strength was discussed from the viewpoint of fracture mechanics. The dependence of the ring crack initiation load on the indenter radius was also examined. As a result, the essential difference between the microfracture and flexural strengths was well interpreted using this model.