1988 Volume 37 Issue 419 Pages 904-909
Three-point bending and fracture toughness tests were conducted on two kinds of hot-pressed silicon nitride. To examine the size effect on the bending strength, two kinds of different sized specimens were prepared. The bending strength of the large specimens showed a lower mean value and a larger scatter than that of the small specimens. The Weibull statistics gave insufficient explanation on these differences because the Weibull modulus differed depending on the specimen size.
For the fracture thoughness tests, a semicircular crack with a radius of 50μm to 1400μm long was introduced on each specimen surface with the aid of Knoop indentation. The relationship between the fracture stress σF and the Knoop-indented crack length 2c was examined. Up to a critical value of 2cc, σF was insentive to 2c. For 2c>2cc, σF decreased with increasing 2c. The value of 2cc differed depending on the materials. The fractographic observation revealed that for 2c<2cc, the fracture origin was mostly located either at inclusions or defects.
