Evaluation of crack propagation in ceramics by double-torsion.

Abstract

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.

Practical Methods for Crack Length Measurement and Fatigue Crack Initiation Detection Using Ion-Sputtered Film and Crack Growth Characteristics in Glass and Ceramics

“…Considering that the n values range from 530 to 580 in region I, and from 17 to 25 in region II for the alumina ceramics, the crack growth rate increases markedly owing to the small increase in stress intensity factor, particularly in region I. The n values obtained by Wakai et al (1986) using double-torsion test pieces range from 84 to 130 in the crack growth rate range of 2x10 -7 to 1x10 -3 , which is different from those obtained in this experiment. Considering that the crack lengths in this experiment are below 1mm, much shorter than those in the experiments of Wakai et al, the difference in n might be due to the fact that the crack growth characteristics depend on crack length.…”

“…Many researches have been performed to clarify the relationship between the crack growth rate and the stress intensity factor for glass and ceramics using fatigue tests (Wakai et al, 1986, Takahashi et al, 1991, Sawaki et al, 1992, Aze et al, 2002, Yoda, 1989, Kimura et al, 1987. However, most of these researches used standard test pieces, such as compact test pieces or double-torsion test pieces, where the crack length is usually over several millimeters; thus, the crack growth characteristics known up to now are considered appropriately restricted to the large cracks that are impractical in the structure components made of glass and ceramics.…”