1996 Volume 45 Issue 10 Pages 1090-1095
Fatigue crack initiation and propagation in sintered silicon nitride have been investigated with plain specimens. The crack initiation was observed by the in situ observing system equiped with a scanning laser microscope. The crack propagation behavior of the plain specimens was compared with those of the specimens with long cracks and semi-elliptical surface cracks.
The main results obtained are as follows;
(1) The cracks on the plain specimens can propagate with the low stress intensity factor which is far lower than the threshold stress intensity of long cracks.
(2) In some cases, a crack stops propagation or propagates very slowly after crack initiation. In these cases, the crack propagation rate is not determined uniquely by the stress intensity factor, but varies very much.
(3) At the final stage of fatigue life, the relationship between stress intensity factor and crack propagation rate of the planin specimen approaches that of the small semi-elliptical surface cracks. This means, for the most part of fatigue life, the power low (da/dN=C·KImaxn) is not applicable.
