Study on static and dynamic fatigue of dental ceramics in water environment

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The objectives of this study were to propose the fatigue crack propagation test of dental ceramics under the control of the stress intensity factor (K-value) and to investigate the static and/or dynamic fatigue of dental ceramics. The rectangular specimens (29x5x3mm), which were introduced the precrack using bridge compression method before testing, was employed. Fatigue crack propagation tests were accomplished under the control of K-value (frequency 10Hz, load ratio R=0.1, 0.5, 0.8) with the electro-hydraulic fatigue testing machine (EHF-FD01, Shimadzu). Fatigue crack growth rates (da/dN) of three commercial ceramics and two commercial composites were obtained as a function of the stress intensity factor range (△K) in the air and water environment. Then a fatigue crack growth threshold (△KィイD2thィエD2) was determined according to ASTM E 647-91. After testing we examined microstructural features of fatigue fracture surfaces by SEM, and then also evaluated the degradation of the marginal area and crack propagation of fired ceramic inlay restorations consisting of a feldspathic porcelain system after 8years in vivo. In the water environment, △KィイD2thィエD2 of ceramics were lower than that in the air, although △KィイD2thィエD2 of composites in the water were higher than in the air. Dynamic fatigue dominated the fatigue of the dental ceramics in the air, and static fatigue was accelerated in the water environment. Microstructural examination of fatigue fracture surfaces showed that the fatigue surfaces were more rough than unstable-fractured surfaces. Crack deflection and crack bridging might increase fatigue resistance of ceramics. Fatigue crack propagation tests could be carried out stably with the precise control of K-value. The m of dental ceramics was very high, and the range of △K in which fatigue crack propagated stably was extremely narrow. Therefore, it was hard to propagate a stable fatigue crack in dental ceramics without control of K-value.