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LaCoO 3 and La 0.8 Ca 0.2 CoO 3 ceramics show a nonelastic stress-strain behavior during four-point bending experiments where hysteresis loops are observed during loading-unloading cycles. Permanent strain is stored in the material after unloading, and a mechanism related to ferroelastic domain switching in the rhombohedral perovskite is proposed. Domain switching in the materials has been confirmed using X-ray diffractometry. Fracture toughnesses of La 0.8 Ca 0.2 CoO 3 measured using single-edge notched beam and single-edge V-notched beam methods coincide and are equal to 2.2 MPa⅐m 1/2 at room temperature and decrease to ϳ1 MPa⅐m 1/2 at temperatures >300°C. A decrease in fracture toughness is consistent with ferroelastic behavior, because the rhombohedral distortion decreases with increasing temperature.

539.4Modern methods of determining crack resistance of ceramics are analyzed and it is shown that fracture of the specimens can be divided into the three stages: formation of a stress concentrator, nucleation and propagation of a crack. Note that standard methods provide determining at the second and third stages but the EF method effects measurements at all the three stages, with testing small-size specimens. The flaking resistance F R was estimated by the Rockwell indentation of the specimens of oxide and nonoxide linear elastic and inelastic ceramics as well as particulate ceramic composites. It was established that F R values for homogeneous linear elastic ceramics were invariant (independent of flaking loads and chip scar sizes). It was demonstrated on the flaking resistance-critical stress intensity factor diagram that the values for ceramics with enhanced damage resistance were found to the left of the base line, while those of homogeneous linear elastic ceramics fell on this line. The values for ceramics with enhanced crack propagation resistance are to the right of this line. Crack propagation resistance diagrams (R-lines) are proposed, they are similar to known R-curves but they are plotted on the basis of results obtained with flaking off the edges of ceramic specimens. For homogeneous linear elastic ceramics these lines are flat, which is indicative of their surface energy invariance.

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