Engineering ceramics have two opposite aspects in their mechanical properties; low fracture toughness and high compressive strength together with excellent wear properties. The aim of this study is to develop a method of determining the compressive fracture strength of engineering ceramics precisely by using a simple loading device. In compressive fracture tests, attention must be paid not to introduce tensile stress at fracture originating locations, since ceramics are typically about 10 times stronger in compression than in tension, fracture may occur by tensile stress even in compressive loading when intensity of tensile stress field reach some critical condition.
In the experiments, investigated was the effect of some factors, such as thin steel sheet inserted between an anvil and specimen and the size of chamfer at the corner of cylindrical specimen, on compressive strength and fracture mode. The results on two engineering ceramics, Si3N4 and SiC, indicate the following features on fracture strength and fracture mode: (1) When radial and tangential tensile stresses are generated in the end face layer of specimen, vertical cracking type fracture occurs, and such stress distributions are due to the elastic deformation of the thin steel sheet with low elastic modulus in comparison with the cramics studied. (2) When the above stress condition can be avoided, the specimen shows crushing type fracture following cleavage cracking of small flakes at the side face near the corner edge of specimen where tensile radial stress of high intensity is generated, and such a stress condition is attained when specimen end faces are directly loaded compressively without the thin sheet so as to constrain the radial deformation of specimen end face not to occur.
