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The strength distribution of Zerodur® (Schott AG) prepared by grinding and etching is determined through statistical analysis to be best described as normal. Although a 3-parameter Weibull appearance is exhibited when multiple specimen batches are combined and analyzed together, this appearance is the result of mixing of components of variation. As a result of the particular type of flaw population, little effect of scale is exhibited. Caution is advised when assuming a 3-parameter Weibull distribution and associated threshold solely based on distribution shape. Fractography to understand the flaw distribution should be employed.

International audience; We evaluate Weibull's approach and the coupled criterion (CC) ability to reproduce bending failure stress variations as a function of specimen size in two porous materials, namely gypsum or zinc oxyde. Whereas both approaches well reproduce the size effect in gypsum specimens, only the CC succeeds in correctly predicting the failure stress variation for ZnO specimens. We thus question the basis assumption of increasing critical flaw size with increasing specimen size associated to Weibull's approach. Employing the CC to predict prematurate specimen failure initiating from a critical pore, we determine a relation between the failure force decrease and the corresponding possible critical pore size range. For the three tested gypsum types, we estimate the critical pore size range to lie around 50-250 microns, not retrieving increasing critical pore size with increasing specimen size, as assumed in Weibull's approach, but rather a constant critical pore size range for all specimens.

The previous Measurement of fracture toughness test by using bright indentation for precracked beam method (ASTM C1421) was found difficult to be carried out due to difficulty in precrack generation and measurement of the crack size. In this research single edge precrack V-notch beam (SEPVNB) is introduced as an alternative to solve the problem from previous standardized method. A real crack that can created with referred size is recognized as the best condition for fracture toughness test. The material prepared for this purpose was silicon nitride (Si3N4) produced by CeramTec (Plochingen, Germany) under the name SL200 B. It is a gas pressure sintered ceramic containing 3 wt.% Al2O3 and 3 wt.% Y2O3. The V Notch was prepared by using razor blade with diamond paste following ISO/FDIS 23146 standard preparation with more addition on precrack introduction. The precrack was introduced by so called opposite roller loading. The fracture toughness test was carried out by following procedure in ISO/FDIS 23146 . The result then was compared for validation with both single edge V-notch beam standard (ISO/FDIS 23146 ) and Surface crack in Flexure SCF (ASTM C 1421). The result of fracture toughness by using method that is introduce in this research is found 5.8270.275 MPa1/2 which is close to the result of SCF (5.335 0.222 MPa1/2). Meanwhile the value of fracture toughness by using V-notch beam is 4.9130.098 MPa1/2

The fused filament fabrication (FFF) of ceramics enables the additive manufacturing of components with complex geometries for many applications like tooling or prototyping. Nevertheless, due to the many factors involved in the process, it is difficult to separate the effect of the different parameters on the final properties of the FFF parts, which hinders the expansion of the technology. In this paper, the effect of the fill pattern used during FFF on the defects and the mechanical properties of zirconia components is evaluated. The zirconia-filled filaments were produced from scratch, characterized by different methods and used in the FFF of bending bars with infill orientations of 0&deg, &plusmn, 45&deg, and 90&deg, with respect to the longest dimension of the specimens. Three-point bending tests were conducted on the specimens with the side in contact with the build platform under tensile loads. Next, the defects were identified with cuts in different sections. During the shaping by FFF, pores appeared inside the extruded roads due to binder degradation and or moisture evaporation. The changes in the fill pattern resulted in different types of porosity and defects in the first layer, with the latter leading to earlier fracture of the components. Due to these variations, the specimens with the 0&deg, infill orientation had the lowest porosity and the highest bending strength, followed by the specimens with &plusmn, infill orientation and finally by those with 90&deg, infill orientation.

The testing of fracture toughness becomes problematic when only limited amount of material is available that hinders the production of typical beam specimens to be tested in bending. Here we explore fracture toughness testing methodologies that allow for small discs and plates having surface cracks to be tested in biaxial flexure using the Ball-on-3-balls (B3B) set-up, or sawed notches as in the Compact Tension geometry. The B3B-KIc test has shown to be versatile and account for a very small overestimation of the KIc-value in the order of 0.8–1.25% due to in-plane crack mispositioning, and a maximum of 4% if a worst-case scenario of additional out-of-plane mispositioning is assumed. The geometrical factor in the standard SCF method, derived by Newman and Raju, resulted in an overestimation of ∼8% of the KIc-value compared to the new calculation by Strobl et al. for materials with Poisson’s ratio

A new method for fracture toughness determination of ceramic balls is presented. The starter crack is introduced into the surface of the ball by a Knoop indentation followed by grinding off the deformed zone. The loading through surface tensile stresses is realized by water quenching, i.e. dropping the heated ball into water. The temperature difference is stepwise increased to find the critical temperature difference for the initiation of crack growth. The geometric factor is calculated in a parametric finite element study, whereas the temperature distribution in the ball was previously determined by using the Biot concept. Combining experimentally measured critical temperature differences for different cracksizes and ball diameters with numerical results of the geometric factor, the fracture toughness of the silicon nitride balls is evaluated. For the evaluation, the knowledge of several material properties (e.g. the CTE) and other parameters is necessary, which have influence on the precision of the measurement. The overall measurement uncertainty is estimated to be about ± 10 %, what roughly corresponds to the value determined with standard measurement procedures. There is an excellent agreement with published fracture toughness results of these balls determined by the modified Surface Crack in Flexure procedure.

In high performance hybrid bearings the balls are conventionally made of silicon nitride ceramics. There are some disadvantages such as costs or the higher stiffness of silicon nitride compared to steel. Therefore, alternative materials are under investigation. The surface strength is one of the most important criteria for the qualification of the spherical components in the application. It has to be evaluated for each new material (or new surface finish). The recently standardised Notched Ball Test (NBT) enables one to determine the surface strength (tensile strength) of balls, which is strongly influenced by the surface finish and volume flaw populations. In this paper the NBT strength of five candidate materials for rolling elements (silicon carbide, silicon nitride, alumina, zirconia and zirconia toughened alumina) is investigated. Fractography is performed to evaluate the direct correlation between the defects found at the surface and the measured strength.

Zerodur ® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability under changing temperature. For example, it is used for mirrors of space telescopes. Due to the large amount of material that has to be removed during fabrication of the mirrors a rather coarse grit is used for grinding. Strength limiting defects introduced by this procedure are eliminated by removing the damaged surface layer by etching to achieve a high strength. Through a large test campaign comprising a high number of strength specimens (>160) of various types and sizes, the validity of the Weibull strength statistics was investigated. The purpose was to accurately determine the parameters of the Weibull law for Zerodur ® when treated in the same conditions as mirrors. As a rule of thumb the average strength of Zerodur ® is assumed to be around 10 MPa. The measured strength is much higher. The strength distributions showed unexpected results, especially the absence of the expected size effect. Through a thorough fractographic analysis it was shown what kind of defects were responsible for failure of etched Zerodur ® and why the prerequisites for the validity of the Weibull theory were not fulfilled in the investigated specimens.

In this study, fatigue life predictions were made for silicon nitride components undergoing hybrid ceramic-steel rolling contact. Three-dimensional stress fields were extracted using a bespoke finite element model and subsequently used to calculate stress intensity factors by means of the weight function method. Crack-growth rates were estimated by assuming crack growth to follow a Paris power-law expression whose material parameters were obtained from four-point bending fatigue experiments under fluctuating tensile load (R = 0.1) and fully-reversed alternating load (R = −1). The crack-growth behavior obtained from the calculations was compared with experimental results obtained from twin-disk-type RCF experiments. The calculated lifetime predictions were in accordance with the experimental results. Crack formation and propagation was found more likely to occur in locations undergoing alternating load rather than those undergoing pure fluctuating tensile load.

Objective Strength is one of the preferred parameters used in dentistry for determining clinical indication of dental restoratives. However, small dimensions of CAD/CAM blocks limit reliable measurements with standardized uniaxial bending tests. The objective of this study was to introduce the ball-on-three-ball (B3B) biaxial strength test for dental for small CAD/CAM block in the context of the size effect on strength predicted by the Weibull theory. Methods Eight representative chairside CAD/CAM materials ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Specimens were prepared with highly polished surfaces in rectangular plate (12 × 12 × 1.2 mm3) or round disc (O = 12 mm, thickness = 1.2 mm) geometries. Specimens were tested using the B3B assembly and the biaxial strength was determined using calculations derived from finite element analyses of the respective stress fields. Size effects on strength were determined based on results from 4-point-bending specimens. Results A good agreement was found between the biaxial strength results for the different geometries (plates vs. discs) using the B3B test. Strength values ranged from 110.9 MPa (Vitablocs Mark II) to 1303.21 MPa (e.max ZirCAD). The strength dependency on specimen size was demonstrated through the calculated effective volume/surface. Significance The B3B test has shown to be a reliable and simple method for determining the biaxial strength restorative materials supplied as small CAD/CAM blocks. A flexible solution was made available for the B3B test in the rectangular plate geometry.

The present work focuses on the determination of the effective thermoelastic properties and the statistical characterization of stress fluctuations in silicon nitride's local phases. For that purpose, full field finite element solutions have been considered, based on 3D electron backscatter diffraction (EBSD) data of silicon nitride. A second-order mean field homogenization scheme, consisting in Hashin–Shtrikman bounds, has been also considered. Ab-initio simulations have been performed in order to determine the temperature-dependent elastic properties of the local phases. The isotropic material microstructure has been checked based on both experimental results and full field solutions. The effective thermoelastic properties have been assessed with the newly obtained experimental results. The stress fluctuations within silicon nitride's local phases have been examined under mechanical and thermal loadings. It has been shown that the amorphous phase is the most vulnerable to fracture and to micro-cracks initiation.

A method for fracture toughness measurement of ceramics using small disks and plates is presented. Similar to the surface-crack-in-flexure (SCF) method a semielliptical surface crack is introduced centrally into one plane side of the specimen which is fractured in a ball-on-three-balls test. Finite element simulations are used to evaluate the stress intensity factor (SIF) for this loading geometry for a range of crack sizes and crack geometries. Empirical formulae for the geometric function are provided for evaluation of the test. The effect of position uncertainties is investigated using FEM and experiments. Other sources which may contribute to the measurement error are identified and quantified, resulting in recommendations for the practical realization of the test. A determination of the fracture toughness within ±10% measurement uncertainty is possible with specimens larger than 8 mm in diameter and thicker than 0.5 mm. With larger specimens an uncertainty comparable to other fracture toughness tests can be achieved. For precise measurements it is important to position the crack within ±120 μm of the stress maximum, to know Poisson's ratio exactly and to test cracks that have the maximum SIF at their deepest point. A method how this can be achieved is presented.

Silicon nitride rolling elements for hybrid bearings (rollers, two sizes of balls and plate material as a reference) made from the same material lot and using the same sintering route are investigated in regard to their microstructure and their mechanical properties. For the first time, a direct comparison of the surface strength of the components as well as of their fracture toughness is enabled by recently developed testing methods for balls and rollers. It was found that the microstructure of these material variants influences the fracture toughness up to about 10%. In addition, due to the different surface finish, the surface strength of the different rolling elements (made of the same material) is very different. The differences may reach up to 100% and more.

The “Surface Crack in Flexure” method is widely used for fracture toughness (KIc) determination of ceramics. In part I of the paper we developed the theoretical fundamentals to apply this procedure to ceramic balls by using the stress application as developed for the so-called “Notched ball test”. The new test (SCF-NB) can be used to test spherical components without the need to cut out special specimens such as bending bars. In this work the practical part is presented including suggestions for crack introduction and specimen preparation and possible measurement errors are discussed. It is concluded that a measurement error less than ±5% is possible. Experiments on balls and bars made from the same silicon nitride ceramic indicate that SCF-NB delivers the same KIc-values as standardised measurements on bars. Additionally, KIc-values obtained for silicon carbide, alumina and zirconia ceramics are presented. They coincide with KIc-data from the literature.

A new fracture toughness test for discs and plates is presented, which can be applied to small specimens (>5 mm diameter). A semi-elliptical surface crack is made into the centre of the top plane using a Knoop intender. Then the layer containing the plastically deformed zone is ground off and the crack is loaded in tension using the Ball-on-3-Balls test. Applied to five different ceramic materials the results gained with the new method agree well with those of standardised methods.

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small pre-crack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical pre-crack shape. In this work fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt. % Al2O3 and 3 wt. % Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters the amount of material removed from the surface. Short specimens with size 3 x 4 x >25 mm were prepared for this purpose. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws.

The strength and fracture toughness of small ceramic discs under biaxial flexural load are investigated with two different miniaturized test methods: the small punch test (SPT) and the ball-on-three-balls test (B3B). An alumina ceramic was chosen as sample material. While the specimen geometries are identical for both tests, the experimental set-ups and the stress fields are different. First, the Weibull parameters of strength have been estimated. Second, the fracture toughness is evaluated with an adaption of the surface crack in flexure (SCF) method. The discs are prepared with artificial surface cracks whose sizes have to be measured. The evaluation of stresses and stress intensity factors is based on finite element calculations.

The Newman and Raju formula for the stress intensity factor of a semi-elliptical surface crack loaded in uniaxial tension or in bending has been developed about 30 years ago using an FE-analysis for several geometric parameters and fitting an empirical equation to the data points. The Poisson's ratio analyzed was 0.3. In this paper a reassessment of the Newman and Raju formula is made, where all relevant geometric parameters of crack and specimen and the Poisson's ratio are considered. The deviations of the old formula from the new results are up to 21%, if the full range of Poisson's ratio is taken into account. Furthermore the influence of the crack-surface intersection angle is discussed. The results of this work are important for more precise fracture toughness measurements in brittle materials and give a practical guidance for appropriate specimen preparation for fracture toughness measurements, which is also considered here.

A new toughness test for ball-shaped specimens is presented. In analogy to the “Surface Crack in Flexure”-method the fracture toughness is determined by making a semi-elliptical surface crack with a Knoop indenter into the surface of the specimen. In our case the specimen is a notched ball with an indent opposite to the notch. The recently developed “Notched Ball Test” produces a well defined and almost uniaxial stress field. The stress intensity factor of the crack in the notched ball is determined with FE methods in a parametric study in the practical range of the notch geometries, crack shapes and other parameters. The results correlate well with established calculations based on the Newman–Raju model. The new test is regarded as a component test for bearing balls and offers new possibilities for material selection and characterisation. An experimental evaluation on several ceramic materials will be presented in a consecutive paper.

The residual stress intensity factors at the surface and at the deepest point of the semi-elliptical Knoop indentation crack is determined from the stresses in the damaged zone below the indenter. For this purpose, the weight function approach by Cruse and Besuner was used and wide-range expressions of the geometric function are given. The solution is then applied to a commercial silicon nitride for which all relevant geometrical data are available.

Every brittle failure of a ceramic component is preceded by a certain amount of sub-critical crack growth (SCCG). In order to incorporate lifetime predictions into design considerations, material data on this phenomenon have to be known. Such material data are only available for a few materials. This is primarily due to the fact that the necessary experiments are time consuming and the evaluation of the data may be complicated. In this work two common measurement methods, static bend tests and constant stress-rate tests were used to determine the parameters for sub-critical crack growth, n and B of a commercial alumina ceramic. Several evaluation procedures were applied to the constant stress-rate data. The data evaluation turned out to be straightforward because a clear separation of the inert strength plateau from the SCCG influenced strength could be observed. A large range of stress-rates spanning five orders of magnitude could be used for the evaluation. All evaluation procedures lead to comparable values for n and B, constant stress-rate tests provide more accurate values for the SCCG parameters than the evaluation of the lifetime distribution. While n could be determined with reasonable scatter, the variety in B was much bigger.

The strength and the defects responsible for failure occurring in tubular alumina components were investigated. Strength testing was performed on machined components in 3-point and 4-point bending. Weibull statistics were applied to describe the strength distributions. Fracture surfaces were investigated using stereo-microscopy and SEM. Fractography revealed that three different types of defects were responsible for failure of the 3-point as well as the 4-point bend specimens. Using an iterative procedure, a separate Weibull distribution for each defect type was evaluated using the data of the two different specimen types.

Precision shaped ceramic components can be obtained by an emerging technique called Lithography based Ceramic Manufacturing (LCM). A green part is made from a slurry consisting of a ceramic powder in a photocurable binder with addition of dispersant and plasticizer. Components are built in a layer–by-layer way by exposing the desired cross- sections to light. The parts are subsequently sintered to their final density. It is a challenge to produce ceramic component with this method that yield the same mechanical properties in all direction. The fracture toughness (KIc) of of LCM-alumina (prepared at LITHOZ GmbH, Austria) was tested by using the Single-Edge-V-Notched Beam (SEVNB) method. Notches are made into prismatic bend-bars in all three direction X, Y and Z to recognize the value of fracture toughness of the material in all three directions. The microstructure was revealed with optical microscopy as well as Scanning Electron Microscopy (SEM). The results indicate that the fracture toughness in Y-direction has the highest value (3.10 MPam1/2) that is followed by the one in X-direction which is just a bit lower (2.90 MPam1/2). The Z-direction is found to have a similar fracture toughness (2.95 MPam1/2). This is supported by a homogeneous microstructure showing no hint of the layers used during production.

Testing in the micro regime is relevant to many technological issues, including, for example, evaluating the performance of established plant or new components through removal of small scale samples; assessing point to point variation in welded structures; characterising expensive advanced alloys that typically are only available in small quantities; and testing of sub-sized components. The use of miniature testpieces requires attention to the scale of the underlying microstructure relative to the overall testpiece dimensions, and presents challenges to the measurement of stress and strain in small volumes. The subject is extensive and the emphasis of the current paper is on a discussion of issues associated with the measurement of strength and strain, particularly with the latter assessed using non-contact optical techniques based on Digital Image Correlation. The current article thus concentrates on the testing situation at the intermediate scale (in the range 0.5-5 mm) where testpieces are small compared with the size of engineering components and in the range where representative mechanical behaviour of the underlying microstructure must be considered. Metrology issues are addressed through specific case studies concerning; micro uniaxial tests; small punch tests; sub-sized ceramic strength tests and the microindentation technique. Particular attention is paid to the uncertainties in measurement arising from the use of microscale tests to provide quantifiable levels of confidence in the ability of such tests to discriminate material behaviour.

Laminates with strong interfaces and having compressive residual stresses in the outer layer are designed to have high strength and high reliability. Bending strength tests on such laminate specimens were performed and compared with tests made on monolithic specimens made from the outer layer material. In both types of specimens fracture started from similar flaws, but the laminate specimens showed a higher strength and Weibull modulus. In the case of the monolithic material the toughness is constant but in the case of the laminates it increases – due to the action of the residual stresses – with crack extension (i. e. it is an apparent toughness). The influence of the apparent toughness curve on the fracture statistics is discussed. It is also shown that the statistics of the laminate can be determined using the statistics of the monolithic material, if the influence of the residual stresses on the toughness and strength is properly taken into account.

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.

Layered ceramics have been proposed as an alternative choice for the design of structural ceramics with improved fracture toughness, strength and reliability. The use of residual compressive stresses, either at the surface or in the internal layers, may improve the strength as well as the crack resistance of the material during crack growth. In this work, two alumina–zirconia laminates designed with external (ECS-laminates) and internal (ICS-laminates) compressive stresses have been investigated using a fracture mechanics weight function analysis. An optimal architecture that maximises material toughness and strength has been found for each design as a function of geometry. From a flaw tolerant viewpoint, ECS-laminates are suitable for ceramic components with small cracks or flaws which are embedded in or near the potential tensile surface of the piece. On the other hand, the existence of large cracks or defects suggests the use of ICS-laminates to attain a more reliable response.

Kurzfassung Der 4-Kugelversuch ist ein neuer Festigkeitstest zur Ermittlung der Zugfestigkeit spröder Werkstoffe unter bi-axialer Beanspruchung. Eine Scheibe wird auf drei Kugeln gelagert und auf der gegenüberliegenden Seite mit einer vierten Kugel zentrisch bis zum Bruch beansprucht. Wegen der gut definierten und stabilen Lasteinleitung eignet sich dieser Test auch zur Prüfung von Proben mit leicht gekrümmten Oberflächen. Die Prüfung gesinterter Proben ohne aufwendige und kostenintensive Hartbearbeitung ist somit möglich. Beim Kontakt von Kugeln und Platten entstehen in den Platten Zugspannungen, die in spröden Werkstoffen zur Entstehung von Rissen führen können (Hertz Risse). In dieser Arbeit wird der Frage nachgegangen, ob bei der Lasteinleitung solche Schäden entstehen und ob Kontaktrisse zu einer Verringerung der Probenfestigkeit führen. Die hier vorgestellten Arbeiten an Proben mit künstlich eingebrachten Rissen und an Proben ohne Vorschäden zeigen, dass Risse in der Probe im Bereich der Krafteinleitung (Kontakt zwischen Kugeln und Probe) keinen Einfluss auf die Festigkeit haben.

Ceramic multilayer composites have been developed in recent years to enhance toughness and reliability of ceramics. It has been demonstrated by theoretical as well as experimental means, that surface compressive stresses protect the composite against the negative action of surface flaws. The behaviour of an alumina–alumina/zirconia laminate having significant compressive residual stresses at its alumina surface is investigated. Compared to alumina specimens its strength is increased by the amplitude of the residual compressive surface stress, which is also a lower threshold value for strength. The consequences of that behaviour for the fracture statistics and reliability are discussed.

It is claimed in almost every experimental work on ceramics that the strength is Weibull distributed. The literature demonstrates that this is not valid in any case, but it is up to now the backbone in the design of brittle components. An overview on situations that deviate from Weibulls statistics is presented (multi-modal flaw distribution, R-curves, etc.). It is also shown that testing specimens with different volumes may help to understand the real strength distribution. Inaccuracies that arise from using the Weibull’s theory are presented. Monte Carlo simulations on the basis of the standardised testing procedure (30 specimens) clearly reveal that these deviations can be hardly detected on the basis of small samples.

In asymmetrical 3-layer laminates with constant overall and inner layer thickness, the residual compressive stresses in the two outer layers are not longer the same: compression is higher in the thinner layer.Therefore, it can be expected that the strength also depends on the outer layer thickness. Experimental evidence of this behavior was found by measuring the bending strength of asymmetrical tri-layers. A value of the thermal expansion mismatch was determined by fitting the theoretical expression to the experimental data.

In this work, the geometry effect on the thermal shock behaviour of a nine layered Al2O3- 5%tZrO2/Al2O3-30%mZrO2 ceramic fabricated by slip casting has been studied. A finite element model has been used to estimate the magnitude and location of the maximum thermal stresses in the layered material as well as the influence of the variation of this layered architectural design in the thermal shock crack initiation and extend throughout the specimens of study. Experimental tests on various samples have been carried out to validate the model. The residual stress distribution profile in the laminate, due to the elastic mismatch of the different layers along with the zirconia phase transformation on the Al2O3-30%mZrO2 layers, conditions the thermal shock response of the material. It is demonstrated how the variation of the outer most layer thickness in the laminates modifies the stress state in the surface, affecting the thermal shock crack initiation.

Silicon nitrides with sufficient strength for structural applications have been developed 20 years ago. A break-through in the use of these ceramics for structural applications did not yet take place. Most probably, the reason for this is a significant lack of design relevant data. TC 6 “Ceramics” of the European Structural Integrity Society (ESIS) has established a research program in order to determine a complete set of material properties and data indispensable for design for a commercially available silicon nitride ceramic. The material chosen as the ESIS Silicon Nitride Reference Material is a gas pressure sintered silicon nitride containing ∼3 wt.% Al2O3 and ∼3 wt.% Y2O3. The results available to the present indicate that this specific material has a good mechanical performance at room temperature and up to ∼800 °C. Above this temperature the large amount of amorphous grain boundary phase causes a detrimental influence on the environmental assisted crack growth properties and the creep performance.

The ball on three balls (B3B) test is a new method for biaxial strength testing of brittle materials. A detailed analysis of the stress field in the specimens and of possible measuring errors has been made recently. The B3B-testing method has several advantages compared to common three- or four-point bending tests: for example, the edges of the specimen have no influence on the testing results, small geometrical inaccuracies of the specimens or of the test jig have only a little effect on the maximum tensile stress and friction is of minor significance. Therefore the B3B-test seems to be suitable for miniaturisation. In this paper the practical applicability of the B3B-test is investigated by measuring the strength of three ceramic materials having a low (electro ceramics), an intermediate (alumina) and a high (silicon nitride) strength. Specimens of different size (the smallest specimens had a volume of less than 1 mm 3 ) with polished or with as-sintered surface were tested. The results are compared with bending test results. In total more than 600 strength tests were performed. A pronounced size effect on strength could be observed, which is discussed in the framework of fracture statistics (Weibull theory). In summary the B3B-test has been proven to be a cheap and easy new testing method, which can be used to determine the biaxial strength of brittle materials and which is particularly well suited for the testing of very small specimen.

Because of their high yield strength and hardness as well as their brittle fracture characteristics the behavior of cracks in ceramics can be described within the framework of linear elastic fracture mechanics. For fracture toughness (K Ic) measurements the test techniques which were developed for metallic materials are unfavorable, as an economical preparation is impossible in the case of ceramic materials. Therefore simple geometries e.g. bend bars became a preferred specimen shape for K Ic measurements. A major difficulty arises when sharp and well-defined pre-cracks for crack propagation studies have to be created. Several methods to overcome this problem are introduced. Additionally, methods to investigate small amounts of material are discussed.

In this work the strength of ceramic laminates is analyzed. Two different architectures with the same surface are studied. One of them, a Al2O3 / Zr(3Y)O2 laminate, contains compressive residual stresses, the other one, a monolithic Al2O3 laminate, has no residual stresses. Residual stresses are estimated analytically on the basis of the materials properties and by an indentation technique. The influence of the residual stress on the strength distribution is investigated. Strength distributions for laminates with compressive stresses at the surface follow a Weibull distribution only under certain conditions.

In this work, the thermal shock behavior of an Al2O3-5%tZrO2/Al2O3-30%mZrO2 multilayer ceramic is studied. On these materials, a tetragonal to monoclinic phase transformation within the Al2O3-30%mZrO2 layers takes place when cooling down from sintering. The latter induces an increase in volume and therefore compressive residual stresses arise in these layers. The residual stress distribution profile in the laminate influences the thermal shock response of the material. A finite element model has been developed to estimate both the thermal strain effects during the sintering process as well as the temperature distribution and stress profile within the laminate during thermal shock testing. Experimental tests on the monoliths and laminates were carried out and compared to the model. It is observed that the presence of the compressive layers within the laminate inhibits the penetration of thermal shock cracks into the body at even more severe conditions than in the monolithic material.

In order to better understand the fracture behavior of Al2O3-Al2O3/ZrO2 laminates, controlled crack growth experiments were carried out. The method proposed consists of a threepoint bending test of a sandwich beam in which a notched specimen is located in between two metallic bars. Under certain conditions that will be commented, stable crack growth can be obtained. The fracture pattern is related to the residual stresses present in the laminate. R-curves for the laminate were also calculated using the weight function method, including the influence of the free surface. Toughness values are presented in the R-curve after friction considerations.

Observation of the stable growth of indentation cracks in controlled bending tests is an attractive tool to study the R-curve behaviour of ceramic materials. In some crack growth experiments deviations from the ideal behaviour were observed––i.e. the cracks did not grow immediately upon application of the external load, but only after a certain applied stress is exceeded. The existence of such a threshold stress intensity for the onset of stable crack growth can be described by a disbalance of the residual stress intensity at the indentation crack tip and the fracture toughness after completion of the indentation cycle. A fracture mechanical description of this phenomenon is presented. The consequences which arise for the evaluation of crack growth data are explained by means of measured crack extension curves of Knoop indentation cracks in silicon nitride.