The failure of ceramic materials is caused by the extension of small flaws. Therefore, linear-elastic fracture mechanics can be applied to describe the failure behaviour. The main problem in the application of the simple fracture mechanics relation is the existence of a rising crack growth resistance curve, which is caused by crack bridging forces behind the advancing crack tip or by transformations in front of the crack tip. The increasing crack growth resistance leads to problems in the transformation of results from specimens with macrocracks to components with natural cracks.

Fracture Mechanics of Ceramics

The paper reviews the current state of art of the fracture of brittle ceramic materials. Typical loading situations (thermal shock, contact damage) are analysed and the resulting fracture modes are discussed. In focus of the paper are the brittle fracture and the resulting probabilistic aspects. The delayed failure of brittle materials (sub critical crack growth and cyclic fatigue) is also discussed.

Fracture of Ceramics

The subcritical growth of fatigue cracks under (tension-tension) cyclic loading is demonstrated for ceramic materials, based on experiments using compact C(T) specimens of a MgO-partially-stabilized zirconia (PSZ), heat-treated to vary the fracture toughness Kc from ∼3 to 16 MPa·m1/2 and tested in inert and moist environments. Analogous to behavior in metals, cyclic fatigue-crack rates (over the range 10−11 to 10−5 m/cycle) are found to be a function of the stress-intensity range, environment, fracture toughness, and load ratio, and to show evidence of fatigue crack closure. Unlike toughness behavior, growth rates are not dependent on through0-thickness constraint. Under variable-amplitude cyclic loading, crack-growth rates show transient accelerations following low-high block overloads and transient retardations following high-low block overloads or single tensile overloads, again analogous to behavior commonly observed in ductile metals. Cyclic crack-growth rates are observed at stress intensities as low as 50% of Kc, and are typically some 7 orders of magnitude faster than corresponding stress-corrosion crack-growth rates under sustained-loading conditions. Possible mechanisms for cyclic crack advance in ceramic materials are examined, and the practical implications of such “ceramic fatigue” are briefly discussed.

Cyclic Fatigue-Crack Propagation in Magnesia-Partially-Stabilized Zirconia Ceramics

Tension-tension cyclic loading tests have been conducted on a coarse-grained alumina ceramic that exhibits toughness-curve behavior by grain-interlock bridging Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact-tension specimens A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit A custom-made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process

Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

The ambient-temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-crack experiments using compact C(T) specimens, cyclic fatigue-crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack-tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic-matrix composite materials are discussed.

Cyclic Fatigue-Crack Growth in a SiC-Whisker-Reinforced Alumina Ceramic Composite: Long- and Small-Crack Behavior

The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 Hz, it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

The cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation induced flaw at the center of the reduced gage section, were tested, where the cyclic load was applied by four-point bending up to frequencies of 10Hz using an electrohydraulic testing machine.Sintered silicon nitride showed a susceptibility to cyclic fatigue, and its lifetime decreased with increasing maximum applied stress. An apparent fatigue limit was about 70% of the flexural strength. A couple of crack arrest position marks around the indentation were detected in the fracture surfaces of both flexure and fatigue test specimens, and the size of each mark well corresponded with the analytical result based on a contact residual stress. Frequency did not affect the lowest limit of the fatigue lifetime, although a scatter of the data and the highest limit of the life-time increased as the frequency was lowered from 10Hz to 0.01Hz. High magnification fractography revealed an intergranular dominant fatigue failure with partial transgranular failure at perpendicularly elongated crystals. It suggests that the intergranular fatigue crack can be arrested at grain boundary triplets, and also can be reactivated by subsequent cyclic loadings.The crack growth rate calculated from the fatigue lifetime showed a three region characteristics having a plateau at 70 to 90% of the fracture toughness, which suggests the possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Katsutoshi Komeya

Papers

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

Cyclic fatigue of sintered silicon nitride at room temperature.