Surface Crack In Flexure Method Research Articles

Fracture toughness and fractal analysis of ceramic benchmark materials.

Previous studies have reported various methods of measuring the fracture toughness of brittle ceramics. The purpose of the present research was to use a new method of fractal dimension measurement on benchmark materials (silica glass, Viosil SX, Shin-Etsu, n = 13, and silicon nitride standard reference material, SRM2100, NIST, n = 10), to compare the fracture toughness calculated using different methods, and to study the effect of noise filtering on the fractal dimension and fracture surface roughness. Fracture toughness was determined using surface crack in flexure method according to ASTM C1421 and fractal analysis method. Fractal dimension was determined using the Minkowski Cover algorithm on atomic force microscope scans of epoxy replicas of fracture surfaces. The mean ± standard deviation of fracture toughness using surface crack in flexure method and fractals method were 0.97 ± 0.18 MPa·m1/2 and 1.03 ± 0.07 MPa·m1/2 for silica glass and 4.62 ± 0.14 MPa·m1/2 and 2.54 ± 0.07 MPa·m1/2 for silicon nitride, respectively. The mean ± standard deviation of fractal dimension was 2.17 ± 0.03 for silica glass and 2.13 ± 0.01 for silicon nitride. The mean ten-point roughness (Rz) before and after noise filtering was 89 ± 102 nm and 87 ± 101 nm for silica glass and 355 ± 132 nm and 357 ± 134 nm for silicon nitride, respectively. Noise filtering had no significance on the fracture surface roughness of the two materials. The newly developed fractal analysis method can be used to predict the baseline fracture toughness of specimens with unknown failure stress.

40 - Toughening Via Revitrification of Li2SiO3 Crystals in Obsidian

The aim of this study was to illustrate the microstructural and compositional evolution of a lithium silicate glass-ceramic and to draw conclusions about its mechanical behaviour. Blocks of Obsidian (Glidewell Laboratories, USA) were processed into slices measuring 12x12x1.5mm3 for special heat treatment. The heating ramp was interrupted at temperatures between 700 C and 820 C (dwell time at 820 C between 0 min and 10 min) in order to freeze the respective microstructural state. The crystallization peaks of the base and the pre-crystallized glass were detected by DSC. The coefficient of thermal expansion and Tg were derived from DTA. XRD was performed to quantify and characterize the crystal phase fraction, whose microstructural changes were visualised using FE-SEM. The ball on three balls surface crack in flexure method was used to record the development of fracture toughness. The microstructural evolution during crystallization firing revealed the progressive revitrification of the original Li 2 SiO 3 crystals at the boundaries of nanometric single coherent scattering domains (CSD) starting at 740 C and extending throughout the isothermal stage. The crystal fraction decreased monotonically from 41vol.% of 5 m-sized poly-crystals to 35vol.% 0.5m-sized single-crystals, in opposite trend to the CSD sizes. The KIc accompanied the reverse trend of crystallinity, departing from 1.63±0.02 MPa ↔ m at the pre-crystallized stage to 1.84±0.06 MPa↔m after 10 min at 820 C in a sigmoidal shape. Toughening appeared counter-intuitive in view of the decreasing crystal fraction and size, to rather relate to the relaxation of the residual stresses in the interstitial glass, as let suggest from the disappearance of the extensive microcracking. During heat-treatment, the CTE decreased from 13.7×10-6 K-1 to 12.8×10-6 K-1. Toughening of Obsidian occurs via revitrification and isotropization of Li 2 SiO 3 crystals and consequent relaxation of anisotropic mismatch in coefficient of thermal expansion between crystal and glass, despite decrease in crystallinity and crystal size.

Toughening by revitrification of Li2SiO3 crystals in Obsidian® dental glass-ceramic

As a predominantly lithium-metasilicate-containing glass-ceramic, Obsidian® (Glidewell Laboratories, USA) has a peculiar composition and microstructure among other dental lithium silicates, warranting an evaluation of the crystallization process to establish relationships between microstructural evolution and mechanical properties. Blocks of the pre-crystallized material were processed into slices measuring 12 × 12 × 1.5 mm3 and subjected to the mandatory crystallization firing by interruption the heating ramp at temperatures between 700 °C and 820 °C (dwell time between 0 min and 10 min). The crystallization peaks of the base and the pre-crystallized glass were obtained by differential scanning calorimetry (DSC). The coefficient of thermal expansion and the glass transition temperature were derived from differential thermal analysis (DTA). X-ray diffraction (XRD) was performed to quantify and characterize the crystal phase fraction, whose microstructural changes were visualised using FE-SEM. The ball-on-three-balls surface crack in flexure method was used to track the evolution of fracture toughness. The microstructural evolution during crystallization firing was characterized by two regimes of growth: (i) the progressive revitrification (dissolution) of the 5 μm-sized Li2SiO3 polycrystals manifested at the boundaries of nanometric single coherent scattering domains (CSDs); (ii) the non-isothermal period is marked by an Ostwald ripening process characterized by the growth of the single crystalline structures into 0.5 μm polycrystals. The decrease in the crystal fraction of Li2SiO3 crystals from 41 vol.% to 37 vol.% is accompanied by the formation of a small amount of Li3PO4 (6 vol.%), maintaining the total crystal phase fraction mostly constant. The KIc accompanied the reverse trend of crystallinity, departing from 1.63 ± 0.02 MPa√m at the pre-crystallized stage to 1.84 ± 0.06 MPa√m after 10 min at 820 °C in a linear trend. Toughening appeared counter-intuitive in view of the decreasing crystal fraction and size, to rather relate to the relaxation of the residual stresses in the interstitial glass due to the spheroidization of the initially anisotropic, elongated Li2SiO3 crystals into round, nearly equiaxed particles, as let suggest from the disappearance of the extensive microcracking.

Concurrent kinetics of crystallization and toughening in multicomponent biomedical SiO2-Li2O-P2O5-ZrO2 glass-ceramics

We investigate here the crystallization kinetics of two pre-crystallized multicomponent dental glass-ceramics (IPS e.max® CAD, Ivoclar-Vivadent; Suprinity® PC, Vita Zahnfabrik) by interrupting the heat treatment at different periods of the non-isothermal and isothermal stage at 840°C. Differential scanning calorimetry (DSC) was used to characterize the crystallization peaks of the pre-crystallized and the base glass, and dilatometry was used to determine the coefficient of thermal expansion and Tg after different times at the maximum temperature. The evolution of crystallinity was tracked by XRD, while the evolution of fracture toughness was obtained by the ball-on-three-balls surface crack in flexure method. The change in microstructural features was made visible using FE-SEM. We show that the isothermal stage is dominated by the transformation of Li2SiO3 in Li2Si2O5, inducing the impingement of nanocrystals into clusters. The residual glass became thereby less polymerized. The kinetics of toughening followed closely the evolution of crystallinity in both systems.

Comparison of fracture toughness testing by the single edge v-notch beam and the surface crack in flexure method on silicon nitride

This work is intended to compare the SEVNB and the SCF methods for fracture toughness testing. Both of the test methods are applied to a silicon nitride. Both methods are standardized and both have some minor shortcomings with respect to practicability and match of the theoretical description of the test situation with the actual test situation. SEVNB is easy to perform, but uses a notch of known depth (+ small micro-cracks of unknown size in front of a notch) instead of a crack, SCF uses a presumably residual stress-free indentation crack which is sometimes difficult to measure, not always semi-elliptical, as assumed to apply the evaluation formula, and may be accompanied by lateral cracks. Both methods will be analysed with respect to possible experimental errors. Variations of the SCF method are applied in order to examine how pre-cracks can be introduced which can be detected on the fracture surface. The geometry of the SCF-pre-crack is varied by applying different loads and by removing different amounts of material. The geometry of the Knoop pre-cracks system is investigated. The results help to identify conditions under which the SCF method can be used successfully to measure the fracture toughness of silicon nitride ceramics.

Nanoceramic composites with duplex microstructure break the strength-toughness tradeoff

There is a strength and fracture toughness tradeoff in nanoceramic composite. The strength varies reciprocally with the grain size whereas the toughness contributed by compressive residual stress increases with the dimension of the second phase. In this work, a novel duplex microstructure with reinforced clusters composing of nanosized grains was proposed and validated using a model system of B4C-TiB2 ceramics densified by carbide boronizing. As-obtained ceramics exhibit excellent combined mechanical properties at room temperature, including Vickers hardness, Young’s modulus, flexural strength and fracture toughness (by surface crack in flexure method) of 32.1 ± 2.7 GPa, 506.9 ± 2.0 GPa,1175 ± 71 MPa and 5.1 ± 0.4 MPa m0.5, respectively. Both strength and toughness are at least ∼30 % higher than the counterparts with similar composition but homogenously distributed TiB2 grains. Graphite onion was confirmed to be an intermediate product during reactive sintering, it facilitated the grain pullout during fracture and retained the nanometric TiB2 grain in the cluster, both of which also contribute the toughening and strengthening mechanisms in the B4C-TiB2 ceramics.

Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

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 Effect of Indentation Load, Tilted Angle, and the Amount of Surface Removal to the Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

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 precrack. 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 precrack shape. In this work the influence of the variation of several experimental parameters is investigated. 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 were indentation load, orientation of the indentation crack with respect to the bending axis and the amount of material removed from the surface. Additional investigations were performed to determine the crack geometry for various indentation loads. A procedure was developed to facilitate the location and measurement of the precrack size on the fracture surfaces. The effect of measurement accuracy of the results is evaluated. 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. Observations from serial sectioning revealed that precracks obtained from HK20 and HK30 have an irregular precrack shape with large lateral cracks. It is suggested to use HK5 and HK10 to produce semi-elliptical surface cracks.

Short crack fracture toughness in (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 relaxor ferroelectrics

AbstractThe fracture toughness in the lead‐free relaxor ferroelectric (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 was investigated utilizing the surface crack in flexure method. To allow a comprehensive assessment, unpoled, and poled samples from the rhombohedral, the tetragonal, and the morphotropic phase regime were considered. It was found that the fracture toughness is up to 23% higher for the poled state. In order to cover the transition from ferroelectric to relaxor phase, the temperature dependence of 0.97(Na1/2Bi1/2)TiO3–0.03BaTiO3 was studied as well. Fracture toughness values of up to 2 MPam1/2 were determined, which are considerably above data for lead zirconate titanate materials. The results are rationalized using a simple transformation toughening‐type model in conjunction with investigations into the ferroelastic behavior. The presented model can be applied without fitting parameters but utilizes measurements of the coercive stress and remanent strain as well as the elastic modulus.

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.

International round-robin test on an improved indentation fracture (IF) method performed through high-magnification microscopy with a traveling stage

An international round-robin test on an improved indentation fracture (IF) method was conducted successfully with seven laboratories using solid-phase sintered silicon carbide, silicon nitride, and alumina. An objective lens with a high magnification of either 40× or 50× and a calibrated traveling stage were employed to enable both clear identification of the crack tips in high resolution and precise measurement of the crack length. Misreading of the crack length by each laboratory was reduced compared with the conventional IF method, resulting in a moderate scatter in the indentation fracture resistance (KIFR) data collected by the participating laboratories. The coefficient of variance for the three samples was less than 8%, which is comparable to those typical of standard fracture toughness tests such as the surface crack in flexure method and the single-edge V-notch method. Additionally, the results were consistent with those of our previous domestic round robin test, thus confirming the reliability of the improved IF method both internationally and domestically.

Grain size dependence of fracture toughness for fine grained alumina

► Fully dense fine grained aluminas with grain size from 290 nm to 3.3 μm were obtained. ► Fracture toughness values with different grain sizes were statistically analyzed. ► Fracture toughness in fine grained alumina is independent of grain size. ► Characteristic fracture toughness in fine grained alumina is 3.37 MPa m 1/2 . The fracture toughness of a series of fully dense polycrystalline alumina samples prepared by electrical field assisted sintering with grain sizes varying from ∼290 nm to ∼3.3 μm was measured using the surface crack in flexure method. Fracture toughness values for different grain sizes were statistically analyzed based on the Weibull distribution. It was found that the fracture toughness in fine grained alumina is independent of grain size. The characteristic fracture toughness calculated from the Weibull distribution is 3.37 MPa m 1/2 .

Fractography and Fracture Toughness Measurement

Using a variety of advanced ceramic materials, a comparison has been conducted of fracture toughness test methods using the single edge vee-notch beam method and the surface crack in flexure method, the latter restricted to optical fractography. Good agreement has been found between the two methods on materials which were amenable to the SCF method. It has further been shown that the SEVNB method can produce reliable results on materials to which the SCF method is not readily applicable.

Relationship between fracture toughness and flexural strength in dental porcelains

The aim of this study was to investigate the correlation between fracture toughness (K(Ic)) and flexural strength (FS) in dental porcelains. Porcelains with different leucite contents and clinical indications were used (A, B, C, D, and E). K(Ic) was determined by surface crack in flexure method (SCF) and FS was determined by four-point-bending test. Microstructural characterization was also carried out. The leucite contents of porcelains A, B, C, D, and E were, respectively, 22, 22, 6, 15, and 0%. Materials with higher leucite content (A and B) presented significantly higher K(Ic) values compared to materials with lower leucite content (C and E). The Weibull moduli (m) of porcelains A and B were statistically higher than those of the other three materials. Regarding characteristic strength (sigma(0)), porcelains D and E showed similar values and statistically higher than those of the other materials which were statistically different from each other. According to the regression analysis, sigma(0) increased with the increase of K(Ic) until approximately 0.75 MPa m(1/2). After that, the increase in K(Ic) was accompanied by a decrease in sigma(0). However, the Weibull modulus increased with the increase in K(Ic), especially for values greater than 0.80 MPa m(1/2).

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 and Flexural Strength of Chemically Vapor-Deposited Silicon Carbide As Determined Using Chevron-Notched and Surface Crack in Flexure Specimens

Two different techniques used to measure the fracture toughness of silicon carbide (SiC) produced by chemical vapor deposition (CVD) are compared, and the influence of the material growth direction on toughness and flexural strength are evaluated. Fracture toughness values for monolithic CVD SiC are found to be independent of the CVD growth direction and test temperature from ambient to 1100°C with values ranging from 2.8 to 5.5 MPa·m1/2. This isotropic nature is notably different from hot-pressed α-SiC and is likely a result of the cubic β-SiC structure produced by the CVD process and the transgranular fracture mode. The range of fracture toughness results obtained using a chevron-notched specimen (2.8–4.0 MPa·m1/2) was less than values obtained using a surface crack in flexure method (3.7–5.5 MPa·m1/2), and these differences are statistically significant. These differences can be attributed to difficulties in resolving the true precrack size for the surface crack in the flexure specimen. Flexural strength values for monolithic CVD SiC are found to be isotropic with respect to the CVD growth direction from room temperature to 1090°C, as observed for the fracture toughness values. Flexural strength values measured at 1090°C are significantly higher (69% to 10% higher) than those measured at room temperature, while the modulus measured at 1090°C is less (−25% to −10% lower) than that measured at room temperature. Since the fracture toughness values are independent of temperature between room temperature and 1090°C, the increased flexural strength at elevated temperature is difficult to explain on the basis of classical fracture mechanics and must be influenced by other factors. Surface machining flaws on flexure specimens are shown to produce a 35% to 50% decrease in the flexure strength of monolithic CVD SiC that was quantified using the fracture toughness and flaw-size data.

Effect of Lateral Cracks on Fracture Toughness Determined by the Surface‐Crack‐in‐Flexure Method

The surface‐crack‐in‐flexure (SCF) method uses a Knoop indenter to create small, semielliptical surface precracks in beam specimens. Lateral cracks may interfere with the primary median crack and cause errors of up to 10% in determination of fracture toughness, particularly for materials for which the fracture toughness is ∼3 MPa·m1/2 or less. Although the residual‐stress‐damage zone is ground or polished away by hand by removing 4.5–5 times the indentation depth, this amount may not be sufficient to completely remove the lateral cracks in low‐fracture‐toughness materials. A series of tests were conducted on sintered alpha silicon carbide with different amounts of material removed after indentation. Once the lateral cracks were fully removed, the SCF results concurred with single‐edged‐precracked‐beam and chevron‐notched‐beam data collected in accordance with ASTM Designation C1421. A simple remedy for the SCF method is to examine the outer ground surface for remnants of lateral cracks before fracture and to remove more material if necessary.

Refinements to the Surface Crack in Flexure Method for Fracture Toughness of Ceramics

Refinements to the Surface Crack in Flexure Method for Fracture Toughness of Ceramics

Effect of crosshead speed on the fracture toughness of soda-lime glass, Al2O3 and Si3N4 ceramics determined by the surface crack in flexure (SCF) method

The fracture toughness of soda-lime glass, Al2O3 and Si3N4 specimens was measured by the surface crack in flexure method. For the soda-lime glass specimens, the fracture toughness was calculated from the initial crack size and flexure strength, and the value increased with increasing crosshead speed. This trend seems to be related to the difficulty in determining the critical crack size at fracture, since slow crack growth occurs during bending test. For the Al2O3 specimens, a halo region (stable crack growth region) was formed around the initial precrack during bending test. The halo size increased and the resultant flexure strength decreased with decreasing in the crosshead speed. The halo region, however, could not be observed in the Si3N4 specimens. Despite of the difference in the appearance of halo region, the fracture toughness of the Al2O3 and Si3N4 specimens was constant irrespective of the crosshead speed when the values were calculated with the critical crack sizes at fracture (halo incorporated crack sizes). The constant fracture toughness with the crosshead speed could be explained by the relation between the changes of halo size (thus critical crack size at fracture) and resultant flexure strength.