Measurement of Rising R ‐Curve Behavior in Toughened Silicon Nitride by Stable Crack Propagation in Bending

A simple procedure for measuring the R ‐curve properties of ceramics by a stable fracture test in three‐point bending is described. As a typical case, data are displayed for a Si 3 N 4 material toughened by the presence of acicular grains in situ grown during the sintering process. The fracture mechanics specimen was a single‐edge double‐notched beam (SEDNB), whose notch was sharpened to a radius of <10 μm in order to reduce the amount of elastic energy stored at its root prior to crack extension. Furthermore, a stabilizer, specially designed for the bending geometry, was used to control crack stability. During stable extension, the crack could be easily arrested at selected locations of the load‐displacement curve, the load quickly released, and the stable crack extension directly measured by the die‐penetration technique. The crack resistance, K R , of the material was calculated from the measured crack extent and the onset load value before unloading. This method enabled us to precisely monitor the critical load value at which the load‐displacement curve deviated from linear behavior, as well as crack extensions from a few tens of micrometers to about 1 mm. As an application of this method, the fracture resistance of a Si 3 N 4 material with rising R ‐curve behavior was measured and found to increase from about 5.5 to 9.0 MPaμm 1/2 within a 0.8‐mm extension.