Energy analysis of brittle fracture and its application to zirconium-oxide ceramics

Abstract

<p>The most useful properties of ceramics (high temperature strength, chemical inertness and hardness at low density) are accompanied by brittleness. This is still the main factor limiting widespread application of ceramic materials. In the present thesis an energy approach to fracture of ceramics was undertaken and refined to account for a nonelastic behaviour of these materials. A chevron-notched (CN) four-point bend specimen was recognized as effective experimental arrangement for room and elevated temperature tests. Consequently, a number of theoretical studies of the specimen's performance were undertaken. Modelling of the variation of the strain energy release rate with the crack extension revealed that the subcritical crack growth in a CN specimen causes dependence of the measured fracture parameters on the experimental procedure (stressing rate, stiffness of the testing system, crack length). It appears that, as complete fracture is approached (i.e. 100% of the specimen's cross-section), the measured work-of-fracture approaches that required for crack initiation. An electrical potential drop technique for crack length measurement in the CN specimen was developed for elevated temperatures fracture studies in the ionically conducting zirconium oxide ceramics. The resistance-to-fracture versus crack extension was determined for a range of temperatures (25 to 1300℃) for stabilized zirconias and their HfO₂ solid solutions and with second phase p-Al₂O₃ particles dispersed in them. The room temperature results agreed with the literature data and model predictions. Above 1000℃ an energy input of ⁻1 J/m² is required to drive the crack through zirconium oxide ceramics. Viscoelastic effects and crack interaction with p-Al₂O₃ particles result in a total fracture energy dissipation two orders of magnitude higher.</p>