部分安定化ジルコニアのR曲線及び疲労き裂進展挙動

抄録

It has been known that the high fracture toughness of partially stabilized zirconia (PSZ) is due to a process zone caused by stress induced transformation at a crack tip. The process zone will give an effect not only on fracture toughness but also on fatigue crack growth behavior. In this paper, the R-curve and fatigue crack growth behavior were investigated by some methods using indentation crack specimens. The toughening exponent obtained by the indentation-strength-in-bending method, where beam strength measurements were made using the specimens with indented loads in the range of 48-294 N, is almost zero, that is, a flat R-curve behavior. On the other hand, K_I-V curve obtained by constant stress rate tests using indented beam specimens indicated that there was much subcritical crack growth, i.e. a small crack propagation parameter of n=7.4. Moreover, the static and cyclic fatigue tests conducted under static four-point bending and under cyclic reversed plane bending respectively, indicated that there was an appreciable degradation in fatigue lifetime due to cyclic loading, and the crack propagation parameter (n=9.3) obtained from cyclic fatigue test was obviously smaller than that (n=23.9) obtained from static fatigue tests. X-ray diffraction patterns measured in fracture surfaces showed a sign that a part of tetragonal phase is transformed to the monoclinic phase by static and cyclic loading. As the results, it is considered that the high fracture toughness is caused by transformation toughening in process zone, and the degradation of lifetime in cyclic fatigue is due to micro cracks induced by cyclic loading in the process zone.