ALBERT

Notes: LaCoO3 and La0.8Ca0.2CoO3 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 La0.8Ca0.2CoO3 measured using single-edge notched beam and single-edge V-notched beam methods coincide and are equal to 2.2 MPa·m1/2 at room temperature and decrease to ∼1 MPa·m1/2 at temperatures 〉300°C. A decrease in fracture toughness is consistent with ferroelastic behavior, because the rhombohedral distortion decreases with increasing temperature.

Notes: The work reported was conducted to provide a basis for a number of structural ceramic mechanical property standardization activities in the United States, Germany, Japan, and Sweden. A comparison of key property values of a commercial silicon nitride determined in a number of laboratories was a major objective. The work reported was conducted by 10 U.S. laboratories on GN-10 silicon nitride, and represented the U.S. work within an International Energy Agency program including the United States, Germany, Japan, and Sweden. Fracture location analyses showed that fracture location within the inner span often was not a linear function of location within the span. Some of this behavior was explained by random sampling effects based upon simulation predictions, but some was apparently dependent upon friction within the fixtures in spite of efforts to minimize it. Flexural strengths were measured at 25° and 1250°C in air and were analyzed using the two-parameter Weibull model in terms of m and σΘ using both linear regression (LR) and maximum likelihood (ML) methods. Under the measurement conditions for the 10 room-temperature strength sets, the value of the ML estimator for m varied by as much as 36%, while the value for the σΘ parameter estimator varied only 3.3%. The LR estimator for m varied by about 54%. For the high-temperature specimens, the ML estimator for m varied by 48% while the LR estimator varied by 38%. Ranked fracture location analysis showed that the high-temperature fracture locations were more random than those in the room-temperature specimens, and was probably due to friction in the high-temperature fixtures. There was little pin rolling ability in many of the high-temperature fixtures used. Monte Carlo and one-way analysis-of-variance (ANOVA) methods provided insight into the consistency of the strength values. Monte Carlo predictions showed that for room-temperature strength, the maximum likelihood estimator m for all 10 laboratories fit within the 10% and 90% confidence bounds for 30 specimen sets. The dispersion of the high-temperature data was such that the m estimator satisfied the model only at the 1% and 99% confidence levels for the 15 specimen sets. ANOVA results showed that for the room-temperature flexural strength, data from all 10 laboratories were not distinguishable for this evaluator at the 95% confidence level and that scatter within individual data sets was a larger effect than was the variation between the data sets. For the high-temperature data, the results from one laboratory were clearly outside the allowable range at this confidence level.

Notes: The work reported was conducted to provide a basis for structural ceramic mechanical property standardization activities under way in the United States, Germany, Japan, and Sweden. All measurements reported here were conducted by 10 U.S. groups on GN-10 silicon nitride within an International Energy Agency program including the United States, Germany, Japan, and Sweden. This cooperative work included tensile strength studies of two geometries of button-head tensile specimens. The authors conducted some of the measurements and performed data analyses and interpretation. The tensile fracture behavior of GN-10 silicon nitride was studied at room temperature. A total of 150 strain-gaged button-head tensile specimens were measured. One hundred of a straight collet design and 50 of a tapered collet design were fractured. All specimens were highly strain gaged and the outputs for each were measured during loading to fracture. Bending moments were calculated. Each participating laboratory group fractured 15 tensile specimens, 10 of the straight collet design and 5 of the tapered collet design under rigorously controlled testing conditions. Of 100 straight collet specimens 75 broke in the gage section. Of 50 tapered collet specimens 34 broke within the gage section. Analysis of the Weibull m and σΘ estimators at upper and lower confidence bounds of 95% and 5% did not indicate a clear choice between the two designs. For specimens which fractured in the gage section, the unbiased maximum likelihood Weibull estimators for m and σΘ were 12.5 and 730 and 10.4 and 716, for the straight and tapered collet configurations, respectively. These are not statistically different at the 95% and 5% confidence levels. Strengths were also analyzed in terms of a three-parameter Weibull model. The straight collet specimen data fitted the three parameter model well with a threshold stress estimator γ of 506 MPa, while the tapered collet specimens provided a poorer fit to the model and had a threshold stress estimator of 432 MPa, a difference of about 15%. Regression analysis indicated that the straight collet grip provided less bias of strength as a function of bending moment. The straight collet specimens showed essentially little dependence of tensile strength upon bending moment in the range of 0% to 6%, while the tapered collet specimens showed a decrease in strength as the bending moment increased from 0% to 4%. However, the regression parameter was low and no significant statistical conclusion could be made regarding the superiority of either of the grip designs.

Notes: A thermal-shock strength-testing technique has been developed that uses a high-resolution, high-temperature infrared camera to capture a specimen's surface temperature distribution at fracture. Aluminum nitride (AlN) substrates are thermally shocked to fracture to demonstrate the technique. The surface temperature distribution for each test and AlN's thermal expansion are used as input in a finite-element model to determine the thermal-shock strength for each specimen. An uncensored thermal-shock strength Weibull distribution is then determined. The test and analysis algorithm show promise as a means to characterize thermal shock strength of ceramic materials.

Notes: Abstract Dynamic fatigue data as a function of temperature for indented-annealed and indented-aged soda-lime glass was analysed using an exponential crack velocity equation based on stress corrosion theory. Agreement in the crack velocity parameters as derived from the two sets of data indicates that the residual contact stress of the indented-aged samples was properly accounted for. However, the predicted macroscopic crack velocity curves based on the indentation fatigue data did not agree with those based on double cantilever beam measurements and possible reasons are discussed.

Notes: Abstract Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained ∼90% α-SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10 × 10−10 s−1) at temperatures ≤1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (∼0.8 to 3 × 10−9 s−1) at stresses below a threshold value of ∼190 MPa. At stresses 〉190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n = 17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature ≤1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.