R ‐Curve Determination for the Initial Stage of Crack Extension in Si 3 N 4

In experimental R ‐curve investigations crack development usually starts from notches. The validity of R ‐curves depends on the size of the notch root radius. This influence is ignored in many cases. In this study it will be shown how the notch radius affects the formally computed crack resistance curve and how the initial true R ‐curve can be obtained from the same data. The tests were performed on a silicon nitride with 5 wt% Y 2 O 3 and 2 wt% MgO. The evaluation of load versus displacement measurements during fracture showed a very steep R ‐curve with the saturation reached even after about 10 μm crack extension.