Three-point bending and cyclic bending fatigue tests were conducted on two kinds of hot-pressed silicon nitride. To examine the relationship between the defect size 2c and the static and fatigue strength σ_F, σ_w respectively a semicircular crack with a radius of 70μm to 310μm long was introduced on each specimen surface with the aid of a Knoop indentation. The result shows that up to a critical value of 2c_c, both σ_F and σ_w were insensitive to 2c. For 2c > 2c_c, both σ_F and σ_w decreased with increasing 2c. Both materials had almost the same value of 2c_c. The fatigue strength of the materials with a small crack was found to be about 40% of the static bending strength.
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