Moisture assisted crack growth in ceramics

A method is described to study subcritical crack growth in ceramic materials. Large, macroscopic size cracks were used and quantitative crack velocity measurements were made on glass and sapphire as a function of applied force, temperature and environment. The measured crack velocity was a complex function of stress and water vapor concentration in the environment and portions of the data could be adequately explained by the stress corrosion theory of Charles and Hillig.

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Zusammenfassung Es wird über eine Methode zum Studium der unterkritischen Bruchfortpflanzung berichtet. Grosse, makroskopische Bruchspalten wurden beobachtet and zwar wurde die Bruchgeschwindigkeit in Glas and Saphir als Funktion von angewandter Kraft, Temperature and Atmosphäre gemessen. Die gefundene Bruchfortpflanzungsgeschwindigkeit stand in komplexer Beziehung zu Zugspannung und Wasserdampfkonzentration und ein Teil der Daten liess rich auf Grund der Spannungskorrosionstheorie von Charles und Hillig erklären.

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Résumé Une methode est decrite pour l'étude de la croissance des fentes sous-critiques dans des materiaux ceramiques. Des fentes longues de grandeur macroscopique ont été utilizées et les mesures quantitatives de la vitesse de formation des fentes ont été pris dans du verre et du saphir comme une fontion de la force appliquéé, de la temperature et de Fentourage. La vitesse mesurée de la formation des fentes est une fonction complexe de la force et de la concentration de vapeur dans l'entourage, et part des resultats pouvait être expliquée suffisamment bien avec la théorie de Charles et Hillig pour la corrosion par la contrainte.

     

A fracture-mechanical theory of subcritical crack growth in ceramics

An interpretation is proposed of the power law describing the relation between subcritical crack growth rate and stress intensity factor. It is based on the idea that thermal transients both break and re-establish bonds. The effects, which occur during these processes are mathematically described using a Morse potential. Already the rough model employed provides enough information on bond breaking to understand the principle of subcritical crack growth.     

Fatigue crack propagation in aluminum nitride ceramics under cyclic compression

Room temperature fatigue crack growth characteristics under cyclic compressive loads were investigated in pure and 3 wt % yttria doped hot pressed aluminum nitride ceramics. A single edge-notch specimen geometry was used to induce a stable Mode I fatigue crack under cyclic compressive loads. The fatigue crack growth occurred in three stages, where the first stage is dominated by microcrack nucleation, coalescence and slow growth within the notch root. During the second stage, the crack growth is accelerated and finally, the crack growth deceleration and arrest occurred in third stage. The fatigue crack growth occurred predominantly by intergranular fracture. Insights gained from the experimental results and microscopic observations are discussed.     

Fatigue crack growth from indentation flaw in ceramics

The indentation crack has been used as a model of surface flaw in the strength tests of ceramics. In evaluation of such strength properties, some attention should be paid to the residual stress effect due to indentation. In this study, fatigue crack growth behavior from indentation flaws was investigated in sintered silicon nitride and alumina. In both materials, a V-shaped behavior was observed in the relation between the maximum stress intensity factor and the crack growth rate. The indentation crack was analyzed using fracture mechanics procedure to take account of the residual component. On the basis of the analysis, an effective stress intensity factor for crack growth was successfully evaluated. The growth rate was correlated to the effective stress intensity factor, and the relation was found to be approximated by a power law equation. The fatigue crack growth property obtained as above was applied to an estimation of fatigue life of specimens without artificial flaws. The flaw size effect was also discussed based on the result.

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Résumé On utilise une fissure d'indentation comme modèle de défaut de surface dans les essais de résistance des céramiques. Au cours de l'évaluation de telles propriétés, il faut accorder quelqu'attention à l'effet de contrainte résiduelle associée à l'indentation. Dans cette étude, on a examiné le comportement de croissance d'une fissure en fatigue à partir de défauts d'indentation dans du nitrure de silicium et dans de l'alumine frittés. Dans les deux matériaux, on a observé un comportement en formed de Vé dans la relation entre le facteur d'intensité de contraintes maximum et la vitesse de croissance de la fissure. La fissure d'indentation a été analysée à l'aide d'un protocole de mécanique de rupture pour tenir compte de la composante résiduelle. Sur base de l'analyse, un facteur d'intensité de contrainte effectif pour la croissance d'une fissure a été évalué avec succès. On a établi une corrélation entre la vitesse de croissance et le facteur d'intensité des contraintes effectif, et on a trouvé que cette relation pouvait être rendue de manière approchée par une loi parabolique. On a appliqué les propriétés de croissance des fissures de fatigue obtenues comme ci-dessus à l'estimation de la vie en fatigue d'éprouvettes ne comportant pas de défauts artificiels. On discute également de l'effet de la taille du défaut en se basant sur le résultat de l'étude.

     

Evaluation of slow crack growth parameters for silicon carbide ceramics

The difficulties in applying existing dynamic and static fatigue theories to model the slow crack growth behaviour of silicon carbide ceramics are examined critically. The assumptions that the geometric parameter,Y, in the fracture mechanics relation remains unchanged for all flaws during crack growth and that the final crack size is much larger than the initial crack size are found to be unrealistic for silicon carbide ceramics.     

Test method for the determination of crack growth rates and crack growth resistance under cyclic loading

Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 3, pp. 137–152, May – June, 1994.     

A plasticity-corrected stress intensity factor for fatigue crack growth in ductile materials under cyclic compression

For prediction of the fatigue crack growth (FCG) behavior under cyclic compression, a plasticity-corrected stress intensity factor (PC-SIF) range ΔK_pc is proposed on the basis of plastic zone toughening theory. The FCG behaviors in cyclic compression, and the effects of load ratio, preloading and mean load, are well predicted by this new mechanical driving force parameter. Comparisons with experimental data showed that the proposed PC-SIF range ΔK_pc is an effective single mechanical parameter capable of describing the FCG behavior under different cyclic compressive loading conditions.     

Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, K, for fracture initiation in cyclic tension is as low as 3.4 M Pam^1/2, and catastrophic failure occurs at K=6.6 M Pam^1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of static failure modes. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y₂O₃. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.     

R-curve and subcritical crack growth in lead zirconate titanate ceramics

R-curves and subcritical crack growth curves (V–K_I) were determined for undoped, K doped (PKZT) and Nb doped (PNZT) lead zirconate titanate (PZT) ceramics and the results are discussed including the effect of doping, grain size and the polarisation state. A pronounced crack growth resistance was observed in the soft PNZT ceramic, which is attributed to ferroelastic domain switching. Subcritical crack growth in the studied PZT materials is governed by both environmentally stress-induced corrosion at the crack tip and the crack shielding due to domain switching. Increasing domain switching capacity by structural modification of the material (i.e. by donor doping or by increasing the grain size) or by poling sifts the V–K_I curve to higher values of the stress intensity factor.     

Prediction of part-through crack growth under cyclic loading

A comparative analysis is performed of the applicability of the local, averaged and effective stress intensity factor ranges as part-through crack growth criteria under cyclic loading. The effective stress intensity factor range is found to be preferable for the purpose of part-through crack growth prediction. The surface and corner crack shape variations under cyclic loading are predicted and fatigue lives of cracked specimens are estimated. Part-through crack growth prediction results are compared with experimental data.