Abstract
In order to clarify the crack size effect on fatigue strength, static and cyclic fatigue tests on sintered Si3N4 with a small crack were carried out. The small semi-elliptical surface crack was introduced by Knoop indentation. The residual stress attributed to indentation was eliminated by removing the surface layer. The surface removal was about 7 times of the net depth of indentation. In order to compare strictly the fatigue life with that estimated from a long crack, fatigue speciments were cut out from CT specimen with which the crack propagation rate was measured. As the fatigue life under cyclic load (R=0.1, f=60Hz) was shorter than that under static load, it was clear that the effect of cyclic load existed. The fatigue life obtained in the present work was compared with the estimated life which was derived from crack propagation data of CT specimen. It was concluded that the fatigue life in the present work was longer than the estimated fatigue life. It was thought that the difference between these fatigue lives was attributed to the difference in crack propagation rate due to crack size. On the fracture surface, there was no debris nor wear mark which was a characteristic mark seen on the fatigue fracture surface of CT specimen. It was considered that the crack opening displacement of small semi-elliptical surface crack was too small (maximum C.O.D. was about 0.1μm) to put Si3N4-crystals between the crack surfaces.
