Abstract
In order to investigate the effects of crack size on the crack propagation behavior of a sintered silicon nitride, crack propagation tests were carried out under cyclic loads and static load. Two kinds of small semielliptical surface crack were introduced by Knoop indentor. By polishing the surface layer, the residual stress around the Knoop indentation was completely removed. The crack propagation rate of small cracks (350µm, 200µm) was lower than that of long cracks obtained with a CT specimen. On the other hand, by measuring and analyzing the crack opening displacement (COD) of 350µm crack, we were able to determine the quantity of the true stress intensity factor K Itip. K Itip was defined by the following equation K Itip K Imax- K S in which K Imax is the stress intensity factor calculated by the applied stress and the crack length. K s is the stress intensity factor shielded by bridging effects. Furthermore, we found that COD after cyclic loading was larger than that before cyclic loading. It was evident that the increase in COD after cyclic loading was attributed to the reduction of bridging force with cyclic fretting at the bridging sites. Under the cyclic loads, the reduction of bridging force resulted in an increase in K Itip which, in turn, accelerated the crack propagation rate.
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