Single-edge Precracked Beam Method Research Articles

Realizing near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia ceramics via current-ramp flash sintering

This study demonstrates the successful fabrication of near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia (1.5YSZ) ceramics using current-ramp flash (CRF) sintering technique. The process involved regulating Joule heating in the samples by fine-tuning the input power through an alternating current field (nominal current density: 50 mA·mm−2) within a 3-min timeframe at a furnace temperature of 1100°C. This approach effectively enhanced grain size uniformity, which is crucial for preventing sample cracking associated with spontaneous tetragonal-to-monoclinic (T→M) phase transformation, thereby promoting densification with average relative densities exceeding 99%. The highest average fracture toughness of the 1.5YSZ samples was measured at 9.2 MPa·m0.5 using the standardized single-edge pre-cracked beam method. This toughness is approximately double that of commonly used 3YSZ samples produced by CRF sintering, all of which exhibited comparable average grain sizes and relative densities. Additionally, the 1.5YSZ samples demonstrated nearly identical resistance to low-temperature degradation (LTD) compared to the 3YSZ samples after accelerated hydrothermal aging at 140°C for 15 h, roughly equivalent to 60 years at 37°C. The reduced yttria concentration in 1.5YSZ facilitates T→M phase transformation at lower stress thresholds, enhancing toughness through increased crack shielding from a larger volume fraction of transformed grains. Furthermore, the uniform yttria distribution in 1.5YSZ, revealed by scanning transmission electron microscopy, compensates for the reduced tetragonal phase stability and contributes to improved LTD resistance. Notably, these exceptional properties were achieved at a furnace temperature 300°C lower and with a sintering duration several hours shorter than those of conventionally-sintered counterparts.

On the robustness of the Single-Edge-Precracked-Beam (SEPB) method for fracture toughness testing of ceramic materials

The Single-Edge-Precracked-Beam (SEPB) method for the testing of fracture toughness (KIc) of ceramic materials is a variation of the Single-Edge-Cracked-Beam (SEPB) method for the test of metallic materials that creates a sharp precrack not by fatigue, but by a “bridge indentation” technique that induces a crack to pop-in under compression loading. This precrack is sharp and overcomes the issue of blunt notch-root radii resulting from the Single-Edge-V-Notched-Beam (SEVNB) method, which overestimates the KIc of ceramics with submicrometric microstructures. However, the difficulties in obtaining a valid precrack and measurement make of the SEPB a low-yield, time- and cost-intensive method. Here we evaluate the SEPB method using large datasets of two fine dental grade ceramics to assess the sensitivity of the validity criteria in ISO 15732 and ASTM C 1421 and the effect of their violations in the obtained KIc. We compile datasets including or excluding validity criteria and combine those of the two standards to establish their effect on the mean and scatter of the data. We discuss the appropriateness of the validity ranges and of some criteria as a whole, while arguing for tightening the criterion precrack frontline asymmetry and for the dropping of criteria regulating the extent of stable crack extension.

Environment dependence of KIc of glass

The role of temperature (T) and moisture on fracture toughness (KIc) of glasses, as measured by the single-edge precracked beam method (SEPB), is investigated. The brittle to ductile temperature, shown by an increase of KIc, is ~Tg. At T below 0.9Tg, the decrease of the bond strength with T results in the decrease of both the fracture surface energy and Young's modulus, so that KIc decreases with T. From above Tg to below 1.1Tg, the work of fracture increases because of dissipative processes, including confined viscoplasticity and blunting at the crack front. From 1.1Tg, viscous flow extends to the whole specimen and becomes more uniform, resulting in the macroscopic deformation of the specimen, eventually preventing against fracture. The results on humidity sensitive glasses show that both precracking and final fracture steps need to be performed in inert environment to meet the intrinsic KIc value, unless the test is fast enough.

Fracture toughness of a metal\u2013organic framework glass

Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm3) from zeolitic imidazolate frameworks. Here, fracture toughness (KIc) of a representative glass, namely ZIF-62 glass (Zn(C3H3N2)1.75(C7H5N2)0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. KIc is determined to be ~0.1 MPa m0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson’s ratio to that of many ductile metallic and organic glasses.

Mechanical properties of partially stabilized zirconia for dental applications

ABSTRACTIn esthetic dental care, porcelain-fused-to-metal crowns have long been used for dental restoration. Since a dental prosthesis is placed in the harsh environment of the oral cavity over long periods, metal allergy due to eluate from the metal frame has proved to be problem, prompting the development of new methods for tooth repair. Zirconia ceramics are used in dental prostheses for tooth restoration because of their excellent strength and high biocompatibility. They are widely used clinically for cosmetic dentistry. This study evaluates the suitability of yttria-based zirconia and ceria-based nano-zirconia for use in dentistry. The mechanical properties of zirconia powders with different compositions are evaluated in terms of sintering density, crystalline phases, structure, translucency, bending strength, fracture toughness, and impact toughness. In addition, a fracture strength test was carried out on sintered bodies that imitate a single crown coping assuming a molar tooth. Yttria-based zirconia was found to have a natural tooth tone and excellent light transmission. Ceria-based nano-zirconia was found to have more than double the fracture toughness (single-edge pre-cracked beam method) and bending strength compared to yttria-based zirconia and excellent strength for dental prostheses.

Accurate evaluation of critical flaw size in structural ceramics via femtosecond laser

Critical flaw size of structural ceramics is generally less than 100 μm and is therefore too small to be reliably evaluated. In this study, micro-sized flaws (as small as 10 µm) were introduced via femtosecond laser for the first time, and the critical flaw sizes of ZrB2 and ZrB2-SiC-Graphite ceramics (12.9 µm and 36.1 µm, respectively) were accurately evaluated by linear extrapolation in double logarithmic coordinates. The critical flaw sizes calculated from the indentation fracture toughness (14.3 µm and 34.3 µm for ZrB2 and ZrB2-SiC-Graphite ceramics, respectively) agreed well with the experimental results, while significant overestimation would occur by using fracture toughness values measured by means of single-edge notched beam, single-edge V-notched beam and single-edge precracked beam methods, which was mainly due to the fact that the notches in these three tests had the depth scale nearly two orders of magnitude higher than those of the inherent flaws.

Fracture strength and toughness of chemical-vapor-deposited polycrystalline diamond films

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25 °C, using a single-edge pre-cracked beam method.

Interaction between Deformation and Crack Initiation under Vickers Indentation in Na2O–TiO2–SiO2 Glasses

An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in Na2O-TiO2-SiO2 glasses was investigated. Vickers indentation was used to examine hardness and average cracking length, fracture toughness was measured through the single-edge pre-cracked beam method (SEPB), and volumes of densification and shear flow around indents were measured using atomic force microscopy (AFM). Relations between the critical crack initiation load and hardness, average crack length, fracture toughness, and the volume fractions of densification and shear flow were studied. No correlations were observed between hardness, average crack length, or fracture toughness with the critical crack initiation load. A link between the minimum in crack initiation load and a change in deformation mechanisms (densification vs. shear flow) was observed.

Application of ASTM C1421 to WC-Co fracture toughness measurement

The accurate measurement of fracture toughness of cemented carbides is essential for material development, characterization, and application; however, varying methodologies are used to characterize this critical material property. In this study, the surface crack in flexure and single-edge precracked beam methods in ASTM C1421, along with the Palmqvist indentation/crack length method outlined in ISO 28079, were used to determine the fracture toughness of tungsten carbide (WC) materials containing between 10 and 25wt.% cobalt (Co). The surface crack in flexure method was not successful because an appropriate precrack did not form underneath the Knoop indentation in any of these compositions. Cracks also did not form at the tips of the Vickers indentations in WC with 15% and higher Co content, rendering the Palmqvist method useless for these materials. The single-edge precracked beam method was the only method that yielded consistent and repeatable toughness values for all compositions.

Dynamic fatigue of cracked piezoelectric ceramics under electromechanical loading: three-point bending test and finite element analysis

This paper studies the dynamic fatigue or slow crack growth in piezoelectric ceramics under electromechanical loading by a combined numerical-experimental approach. Constant load-rate testing was conducted in three-point flexure using the single-edge precracked-beam specimens under zero and positive electric fields, and the effects of electric field and loading-rate on the fracture load and crack propagation were examined. A finite element analysis was also employed to calculate the energy release rate for the permeable, impermeable and open crack models, and the effect of electric field on the energy release rate was discussed. Crack propagation velocity versus energy release rate curves at various loading-rate were then estimated based on the finite element analysis using measured data. Piezoelectric ceramics of the lead zirconate titanate (PZT) class have been used for a number of years as sensors and actuators. The high mechanical stresses and intense electric fields in PZT ceramics can induce cracking that can lead to premature failure of the piezoelectric devices. The properties of PZT ceramics are also susceptible to degradation under electromechanical loading. Therefore, an understanding of piezoelectric fracture [Shindo et al. 2003; 2005] and fatigue [Cao and Evans 1994; Lynch et al. 1995] is a key issue for the efficient and reliable design of the piezoelectric devices. Shindo et al. [2007] conducted an experimental and analytical study of the static fatigue behavior of PZT ceramics under electromechanical loading. Narita et al. [2007] also reported experimental and numerical examination of the fatigue crack growth in PZT ceramics under a cyclic mechanical load and a constant electric field. Certain environments may affect formation and extension of cracks over time and at stress levels well below that which causes immediate failure to occur. This process is called dynamic fatigue or slow crack growth. Slow crack growth parameters have been estimated for only a few commercial ceramics and glass, and no one has investigated the resistance of piezoelectric ceramics to slow crack growth and the influence of electric field on the dynamic fatigue behavior. In this study, we report numerical and experimental examination of the dynamic fatigue or slow crack growth in piezoelectric ceramics under electromechanical loading. A crack was created normal to the poling direction. Constant load-rate testing was conducted in three-point flexure under zero and positive electric fields using single-edge precracked-beam method. A finite element analysis was also used to evaluate the energy release rate for the permeable, impermeable and open crack models, and the effect

Electric Delayed Fracture and Localized Polarization Switching of Cracked Piezoelectric Ceramics in Three-point Bending

This article discusses the delayed fracture and localized polarization switching near a crack tip in three-point bending piezoceramics under electromechanical loading. We have used finite element analysis to study delayed fracture experiments with single-edge precracked-beam method. A nonlinear finite element analysis was employed to calculate the fracture mechanics parameters such as energy release rate for the permeable, impermeable, and open crack face boundary conditions. Recent proposed energetically consistent boundary conditions were also considered. The effects of applied electric field and localized polarization switching on the fracture mechanics parameters were then examined.

Correlation of wear behavior and indentation fracture resistance in silicon nitride ceramics hot-pressed with alumina and yttria

Nine kinds of silicon nitrides with different microstructures were fabricated by controlling both sintering conditions and amounts of sintering additives, Al 2O 3 and Y 2O 3. The wear behavior of the various Si 3N 4 ceramics was investigated in sliding contact test without lubricant. The specific wear rate varied notably from 6 × 10 −6 to 4 × 10 −4 mm 3 N −1 m −1 depending on the microstructures, whose ranking was difficult to predict directly from the hardness or fracture resistance obtained by the indentation fracture (IF) technique as well as the single-edge-precracked beam (SEPB) method. A good correlation was obtained between the specific wear rate and both mechanical properties when a lateral-crack chipping model was applied as the material removal process. However, the correlation was lost when the fracture toughness obtained by the SEPB method was employed, indicating that the conventional long-crack toughness is inappropriate for analyzing the wear behavior of Si 3N 4 exhibiting a rising R-curve behavior.

Effects of the Loading Rate and Humidity in the Fracture Toughness Testing of Alumina

To test the fracture toughness of alumina, a Surface-Crack-in-Flexure (SCF) method, a Single-Edge-Precracked-Beam (SEPB) method and a Single-Edge-V-Notched-Beam (SEVNB) method were used at crosshead rates ranging from 0.005 ㎜/min to 2 ㎜/min and relative humidity ranging from 15% to 80%. The results show that the fracture toughness tested by the SCF method increases with either an increasing loading rate or decreasing relative humidity; in contrast, the toughness by the SEPB method and the SEVNB method does not depend on the loading rate or the relative humidity. Theoretical analysis of the way slow crack growth affects the apparent fracture toughness indicates that the three testing methods have different effects with respect to the loading rate and the relative humidity; moreover, these differences are attributable to differences in the size of the cracks or notches.

Fracture Toughness of Dental Porcelains Evaluated by IF, SCF, and SEPB Methods

Fracture toughness of six dental porcelains with leucite content ranging from 0 to 22 vol% was evaluated by indentation fracture (IF), surface crack in flexure (SCF), and single edge pre‐cracked beam (SEPB) methods. The results of the IF method were similar to those of the SCF method for all the porcelains investigated. The results of the SEPB were similar to those of the other two methods only for the glassy porcelains, but for leucite‐based porcelains this method resulted in higher values of KIc. Based on microstructure, fractographic analysis, and an additional single edge V‐notched beam test, it was concluded that the pre‐crack size influences the value of KIc for porcelains reinforced by leucite. For design and failure analysis purposes, the KIc determined by SCF method should be preferred, since fracture of dental restorations usually starts from small surface cracks.

Fracture Toughness Measurement of Dental Ceramics Using the Indentation Fracture Method with Different Formulas

This study examined fracture toughness (KIC) measurements obtained using the indentation fracture (IF) method with a view to improving their reliability. The KIC values of five dental ceramics were measured using the IF method with five different formulas, and the single-edge precracked beam (SEPB) method was used as a control. The elastic moduli of the dental ceramics were evaluated by dynamic hardness test. Load conditions of the dental ceramics that produced a median/radial crack for the IF method formulas were investigated. Based on the resultant c/a and P/c1.5 values, the indentation load (P) required for median/radial crack occurrence varied greatly from 29.4 to 196 N depending on the ceramic used. Among the five formulas, none of the KIC values obtained by the IF method with Miyoshi's formula differed significantly (p > 0.05) from the values obtained using SEPB method. These results suggested that, after an appropriate indentation load is determined, reliable KIC values for small dental ceramic specimens can be easily obtained using the IF method if Miyoshi's formula is used in combination with the dynamic hardness test.

Evaluation of Fracture Toughness of Porous Ceramics

The indentation fracture (IF) method, the single-edge precracked beam (SEPB) method, and the single-edge V-notched beam (SEVNB) method were applied to evaluate the fracture toughness of four kinds of porous ceramics of SiC, Al2O3 and Mg2Al4Si5O18 with porosity ranging from 37 to 43%. The microstructures of these materials were composed of ceramics grains, glassy grain boundaries and pores. Each grain was joined together with the glassy grain boundary phase. The IF and SEPB methods were not applicable because both precracks and indenter traces were not visible. On the other hand, the SEVNB method was applicable because the V-notch could be easily machined by grinding. In the case of the SEVNB method, the applied load versus back-face strain plots under four-point bending showed nonlinearity prior to the maximum load. The R-curve behavior was estimated from the compliance change of specimens. The fracture toughness of porous ceramics was smaller than that of dense ceramics, and increased with increasing crack extension. Since the stable crack predominantly propagated along glassy grain boundaries, the R-curve behavior depended on the loading rate and matrix grain size. The increment of the R-curve by grain bridging became larger for coarser-grain sized ceramics.

Evaluation of Fracture Toughness of Porous Ceramics.

The indentation fracture (IF) method, the single-edge precracked beam (SEPB) method, and the single-edge V-notched beam (SEVNB) method were applied to evaluate the fracture toughness of four kind of porous ceramics of SiC, Al2O3 and Mg2Al4Si5O18 with porosity ranging from 37 to 43%. The microstructures of these materials were composed of ceramics grains, glassy grain boundaries and pores. Each grain was joined together with the glassy grain boundary phase. The IF and SEPB methods were not applicable because both precracks and indenter traces were not visible. On the other hand, the SEVNB method was applicable because the V-notch could be easily machined by grinding. In the case of the SEVNB method, the applied load versus back-face strain plots under four point bending showed nonlinearity prior to the maximum load. The R-curve behavior was estimated from the compliance change of specimens. The fracture toughness of porous ceramics was smaller than that of dense ceramics, and increased with increasing crack extension. Since the stable crack predominantly propagated along glassy grain boundaries, the R-curve behavior depended on the loading rate and matrix grain size. The increment of R-curve by grain briding got bigger for coarser-grain sized ceramics.

Effect of grain growth and measurement on fracture toughness of silicon nitride ceramics

Two kinds of α-Si3N4 powders with different properties (high purity and low purity) were pressureless sintered using MgAl2O4-ZrO2 as sintering additives. The grain growth was controlled by sintering time and temperature. The fracture toughness was determined using indentation microfracture (IM) and single-edge-precracked-beam (SEPB) methods. A discrepancy of fracture toughness was found between the values obtained by these two methods, and the SEPB method provided higher fracture toughness than the IM method. Materials with more large elongated Si3N4 grains gave higher fracture toughness, R-curve behavior and larger discrepancy. This is contributed to the effects of grain bridging, pull-out and deflection by the large elongated Si3N4 grains. Comparing the specimens from two kinds of α-Si3N4 powders with different purity level, both gave high fracture toughness of over 9 MPa · m1/2 by the SEPB method, while the E10 samples have a little higher flexural strength.

Mechanical properties Of Si3N4 cerarnics reinforced with SiC whiskers and SiC platelets

Silicon nitride ceramics reinforced with SiC whiskers and SiC platelets were fabricated by hot pressing and their mechanical properties were studied. They showed higher fracture energy than conventional composites, particularly when they were consolidated by gas-pressure hot pressing at high temperature. A high fracture toughness (10.7 MPa m1/2) which was measured by the single-edge pre-cracked beam method was achieved. Furthermore, a unique method to observe the crack propagation behaviours directly in a scanning electron microscope with loading devices was developed. As a result, much bridging and pull-out of the whiskers and the elongated Si3N4 grains, and crack deflection along the platelets, were observed behind the crack tip. This means that these grains are effective in enhancing the fracture resistance during crack propagation.

VAMAS Round Robin on Fracture Toughness of Silicon Nitride

Eight laboratories in Germany, Japan, U.K., and U.S. participated in the VAMAS round robin. The fracture toughness of silicon nitride at room temperature and at 1200deg;C was measured by three methods: the single‐edge V‐notched beam (SEVNB), single‐edge precracked beam (SEPB), and chevron notched beam (CNB). The obtained values show hardly any crosshead speed dependence, irrespective of test temperature and atmosphere. Results may have been influenced by a small amount of slow crack growth, but distinct R‐curve behavior could not be detected within the scope of the tests. The values at 1200deg;C in N2 can be measured by the SEVNB and SEPB methods with small scatters. The oxidation of silicon nitride, caused by heating in air, increases the SEVNB and SEPB values. The CNB values are free from the effects of test temperature and atmosphere, but they show a large scatter between laboratories. However, the chevron V‐notched beam (CVNB) method, which is an improved CNB method, shows values with a small scatter, irrespective of the measurement conditions. The SEVNB and SEPB measurements in N2 and the CVNB measurement under any conditions are recommended for the measurement of high‐temperature fracture toughness.