抄録
To clarify the validity of evaluation of the threshold value of fracture toughness Kth on the surface layer of ceramics by sphere indentation test, sphere indentation tests with indenters of various diameters 2R were performed using Si3N4, specimens made by HIP-sintering and the stress intensity factor K1 for surface crack under ball on plate contact loading was numerically obtained. The crack length ci was estimated from experimental results using K1, where ci is the length of the crack leading to a ring crack and the conditions for ring crack initiation were assumed to be K1> Kth. It was shown that the average values of ci increased with increases in 2R in the case of small 2R, but the averages of ci gradually approached a constant value in cases with a large 2R. The constant value of ci was estimated as 8-9μm using Kth=5.3 MPa·m1/2 and was almost equivalent to the grain size of the test material.
