Evaluation of Crack Propagation in Ceramics by Double-Torsion

In ceramics, delayed failure occurs due to slow growth of preexisting cracks by stress corrosion. Such subcritical crack growth in high performance ceramics should be characterized for their life prediction based on fracture mechanics. In the present study, the effect of environment on the stress intensity factor (K)-crack velocity (V) relationship was studied for various high performance ceramic materials. The double torsion (DT) method that utilizes the load relaxation technique was employed for the determination of K-V characteristics. Three regions were distinctly observed in the K-V diagrams not only for oxide ceramics, such as alumina and yttria-partially stabilized zirconia (PSZ), but also for non-oxide ceramics, sintered and hot-pressed silicon nitride. In water, straight lines corresponding to region 1 were obtained. The position of region 2, in which crack velocity is controlled by diffusion of corrosive species, depended on the amount of water in air and toluen. Stress corrosion of silicon nitride by water is believed to be caused by the presence of residual oxinitride glass phase at grain boundaries. The mechanism of stress corrosion by water was discussed on the basis of chemical bond rupture model. Acoustic emission caused by crack propagation was also studied for PSZ and sintered silicon nitride. The amplitude of acoustic emission event was dependent on crack velocity for sintered silicon nitride, but independent for PSZ.