Biaxial Flexure Research Articles

Quantifying the Strength of a Resin-coated Dental Ceramic

Resin luting all-ceramic restorations increases clinical performance; however, the strengthening mechanisms are not fully understood. The authors have previously proposed the existence of a resin-ceramic hybrid layer, and the hypothesis tested was that ceramic strength enhancement was conferred by the characteristics of the resin-ceramic hybrid layer. Dentin porcelain discs were polished with a P4000-grade abrasive paper, and half were centrally indented at 9.8 N. Further discs were alumina-air-abraded. Groups of 30 specimens were coated with resin cement thicknesses varying from 0 to 250 +/- 20 microm before bi-axial flexure testing. Following investigation of residual stresses by annealing, regression analysis enabled us to calculate the magnitude of 'actual' strengthening for a theoretical 'zero' thickness of resin cement on each surface texture. Accounting for resin bulk strengthening, resin cement coating significantly increased the mean strength that was attributed to a resin-ceramic hybrid layer sensitive to surface texture.

Biaxial flexure strength and low temperature degradation of Ce‐TZP/Al2O3 nanocomposite and Y‐TZP as dental restoratives

The purpose of the present study was to evaluate the mechanical durability of a zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al(2)O(3) nanocomposite) in comparison to yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) and discuss its application on ceramic dental restorations. The disk-shaped specimens of both materials were stored in physiological saline solution at 80 degrees C for 30 days, in 4% acetic acid at 80 degrees C for 30 days, and in an autoclave at 121 degrees C for 10 days. Before and after storage, specimens were subjected to the biaxial flexure test and to the determination of the monoclinic zirconia content. After autoclaving, Y-TZP showed remarkable increasing of the content of monoclinic zirconia: 0.3 vol % before and 49.9 vol % after, and slight decreasing of biaxial flexure strength: 1046 MPa before and 892 MPa after; whereas Ce-TZP/Al(2)O(3) nanocomposite showed no significant difference in the monoclinic content (4.8-5.5 vol %) and the biaxial flexure strength (1371-1422 MPa) after storage in any conditions. It is concluded that, compared to Y-TZP, the Ce-TZP/Al(2)O(3) nanocomposite has a high biaxial flexure strength along with a satisfactory durability in terms of low-temperature aging degradation in above conditions. This study indicates that the Ce-TZP/Al(2)O(3) nanocomposite demonstrates excellent mechanical durability for dental restorations such as all-ceramic bridges.

Simple Expressions for the Evaluation of Stresses in Sphere-Loaded Disks Under Biaxial Flexure

Abstract In testing ceramic materials, sphere-loaded disk tests are of increasing importance. Three different tests including one, four, and six spheres are addressed. For the first two tests, the maximum stress in the disk center was already studied in several papers. In this note, simple analytical relations for the distribution of the tangential stress will be given. The stress distribution in the center region is, for instance, necessary for the determination of effective surface and volume as used for Weibull statistics. For special applications also, stresses across larger distances from the center are desirable. As an example, strength tests on coarse-grained alumina are mentioned briefly which need a fracture mechanics evaluation for interpretation.

Light-Cured Dimethacrylate-Based Resins and Their Composites: Comparative Study of Mechanical Strength, Water Sorption, and Ion Release

This study explored how resin type affects selected physicochemical properties of complex methacrylate copolymers and their amorphous calcium phosphate (ACP)-filled and glass-filled composites. Two series of photo-polymerizable resin matrices were formulated employing 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]propane (Bis-GMA) or an ethoxylated bisphenol A dimethacrylate (EBPADMA) as the base monomer, Unfilled copolymers and composites filled with a mass fraction with 40 %, 35 % and 30 %, respectively, of ACP or the un-silanized glass were assessed for biaxial flexure strength (BFS), water sorption (WS) and mineral ion release upon immersion in HEPES-buffered saline solution for up to six months. Substituting EBPADMA for Bis-GMA significantly reduced the WS while only marginally affected the BFS of both dry and wet copolymers. Independent of the filler level, both dry and wet ACP composites formulated with either BTHM or ETHM resins were mechanically weaker than the corresponding copolymers. The BFS of ACP composite specimens after 1 month in saline did not further decrease with further aqueous exposure. The BFS of glass-filled composites decreased with the increased level of the glass filler and the time of aqueous exposure. After 6 months of immersion, the BFS of glass-filled BTHM and ETHM composites, respectively, remained 58 % and 41 % higher than the BFS of the corresponding ACP composites. Ion release data indicated that a minimum mass fraction of 35 % ACP was required to attain the desired solution supersaturation with respect to hydroxyapatite for both the BTHM and ETHM derived composites.

Translucency and biaxial flexural strength of four ceramic core materials

Objectives To assess the relative translucencies and flexural strengths of four dental restorative ceramic core materials. Methods Eight disk specimens (14 mm in diameter × 0.5 ± 0.05 mm in thickness) were prepared for each group of four ceramic core materials (IPS Empress 2 dentin, VITA In-Ceram Alumina, VITA In-Ceram Zirconia, Cercon Base Zirconia), according to the manufacturers’ instructions. A color meter was used to measure the relative translucencies of the specimens. The biaxial flexure test (ISO 6872) was then used to measure their flexural strengths. Data for relative translucency (0.0–1.0), fracture load (N) and biaxial flexural strength (MPa) were analyzed by one-way ANOVA, followed by Tukey's multiple comparison test for significant findings ( α = 0.05). Results For relative translucency: IPS Empress 2 (0.78 ± 0.03), VITA In-Ceram Alumina (0.94 ± 0.01), VITA In-Ceram Zirconia (1.00 ± 0.01), Cercon Base Zirconia (1.00 ± 0.01), P < 0.0001. For biaxial flexural strength: IPS Empress 2 (355.1 ± 25.7), VITA In-Ceram Alumina (514.0 ± 49.5), VITA In-Ceram Zirconia (592.4 ± 84.7), Cercon Base Zirconia (910.5 ± 95.3), P < 0.0001. Significance IPS Empress 2 and VITA In-Ceram Alumina were significantly more translucent than the two opaque zirconia-containing core materials. IPS Empress 2 was significantly weaker, and Cercon Base Zirconia was significantly stronger, than the other two ceramic core materials.

A novel indirect tensile test method to measure the biaxial tensile strength of concretes and other quasibrittle materials

A novel indirect tensile test method, the biaxial flexure test (BFT) method, has been developed to measure the biaxial tensile strength of concretes. The classical modulus of rupture (MOR) test has been generalized to three dimensions. In this method, we use a circular plate as the new test specimen. This plate is supported by an annular ring. We apply an external load to this specimen through a circular edge. The centers of the specimen, the loading device and the support are identical. The biaxial tensile strength measured by this new method is about 19% greater than the uniaxial tensile strength obtained from the classical modulus of rupture test as reported by other researchers. However, at the same time, we also found that the stochastic deviation of the biaxial tensile strength is about 63% greater than the uniaxial strength.

Application of analytical stress solutions to bi-axially loaded dental ceramic–dental cement bilayers

Objective Increased consideration of dental ceramics and dental cements as a singular structural unit for in-vitro mechanical testing has resulted in the reporting of a wide range of analytical methods to calculate the failure stresses. Therefore a comparison of observations between studies is complicated by the use of dissimilar stress solutions despite the employment of a similar testing methodology and specimen geometry. Materials and methods Three analytical solutions to calculate failure stresses in bi-axially loaded dental ceramic–dental cement bilayers were appraised for a commonly utilized testing geometry (ball-on-ring). Clinically relevant datasets were generated from the bi-axial flexure testing of uncoated and dental cement coated aluminous core ceramic exposed to differing ceramic surface preparations. A Weibull statistical approach was utilized in order to provide insight into the impact of the analytical method on both the scale ( σ 0) and distribution ( m) of the failure stress data. Results Calculation of the bi-axial flexure stress utilizing Timoshenko's analysis resulted in an increase in σ 0 for the uncoated (6%) and dental cement coated (11–12%) aluminous core ceramic, when compared with the bilayered solutions reported by Rosenstiel and Hsueh. However, the shape of the failure distributions illustrated by the consistency of m and associated 95% confidence intervals was not influenced by the analytical stress solution employed. Significance The choice of the analytical method chosen to calculate failure stresses in bi-axially loaded dental ceramic–dental cement bilayers will impact on the magnitude of the reported strength. Comparison between the failure stresses of uncoated and cement coated dental ceramics is more accurately represented by bilayer solutions, which account for the mismatch between the elastic constants of dissimilar materials. However, within the context of dental cement coated dental ceramics of clinically relevant dimensions, the choice of solution is unlikely to impact on the interpretation of the observations previously reported in the dental literature.

Prestresses in bilayered all‐ceramic restorations

A general trend in all ceramic systems is to use veneering ceramics of slightly lower thermal expansion coefficients compared with that of the framework resulting in a positive mismatch in thermal expansion coefficient (+DeltaTEC). The concept behind this TEC mismatch is to generate compressive stresses in the weaker veneering ceramic and thus enhance the overall strength of the restoration. This technique had excellent results with porcelain fused to metal restorations (PFM). However, there are concerns to apply this concept to all-ceramic restorations. The aim of this research was to determine the stresses in bilayered all-ceramic restorations due to the mismatch in TEC. Two commercial veneering ceramics with a TEC lower than that of zirconia (+DeltaTEC); NobelRondo zirconiatrade mark and Lava Ceramtrade mark, plus one experimental veneering ceramic with an identical TEC that matches that of zirconia (DeltaTEC = 0) were used to veneer zirconia discs. The specimens were loaded in biaxial flexure test setup with the veneer ceramic in tension. The stresses due to load application and TEC mismatch were calculated using fractography, engineering mathematics, and finite element analysis (FEA). In this study, the highest load at failure (64 N) was obtained with the experimental veneer where the thermal mismatch between zirconia and veneering ceramic was minimal. For the two commercial veneer ceramics the magnitude of the thermal mismatch localized at the zirconia veneer interface (42 MPa) exceeded the bond strength between the two materials and resulted in delamination failure during testing (ca. 50 MPa). For all-ceramic zirconia veneered restorations it is recommended to minimize the thermal mismatch as much as possible.

Crack propagation in directionally solidified eutectic ceramics

Improving the strength and toughness of directionally solidified eutectic ceramics is essential for practical applications. In this respect, a biaxial disc flexure testing device has been designed. This system, which presents the ring supported/ring loaded test geometry, is essentially used as a means for the nucleation and early propagation of cracks, with the flexure test being stopped as soon as a load drop is detected in the loading curve. The composites under investigation are either binary (Al 2O 3/Y 3Al 5O 12, Al 2O 3/Er 3Al 5O 12 and Al 2O 3/GdAlO 3) or ternary (Al 2O 3/Y 3Al 5O 12/ZrO 2, Al 2O 3/Er 3Al 5O 12/ZrO 2 and Al 2O 3/GdAlO 3/ZrO 2) eutectics. Examination of cracks is performed in field emission gun-scanning electron microscope on the polished surface of the biaxial bending test specimens, with attention being paid to the possibility of crack deflection in the various phases and in the phase boundaries, a phenomenon which may markedly improve the toughness of these eutectic ceramics. These observations are correlated to internal stress calculations, such as normal stress acting on the phase boundaries.

Thermal stress analysis of cordierite materials subjected to thermal shock

Cordierite disks were obtained by slip casting and sintering at 1,450 °C for 2 h. Sintered disks were indented at the center of one flat surface. The fracture strength of indented disks was measured in biaxial flexure. The critical temperature differential was determined by sudden cooling of the center of the indented surface at high temperature, using a high-velocity air jet at room temperature. The initial temperature was successively increased by 10 °C until crack propagation was detected with the naked eye. Temperature and stress distributions during air impinging were calculated by a finite element analysis. The heat transfer coefficient was estimated by fitting experimental temperature profiles. A radial variation of the coefficient that considers its increment beyond the stagnation region and a diminution toward the disk periphery was assumed. The calculated cordierite thermal stresses were analysed in relation to experimental data. Also, the comparison with alumina tested in similar conditions was included.

Water uptake and strength characteristics of a nanofilled resin-based composite

Objectives To investigate the influence of short- and medium-term immersion on water uptake and mechanical properties of a so-called ‘nanofilled’ compared with a conventional resin-based composite (RBC). Method For each material (a microhybrid, Filtek Z250, FZ and ‘nanofilled’ RBC, Filtek Supreme in body and translucent shades, FSB and FST; 3M ESPE, St. Paul, USA) five specimen groups ( n = 20) were tested. Mean bi-axial flexure strength (BFS) of each group was determined following 24 h ‘dry’ and 24 h, 3, 6 and 12 months in a water-bath maintained at 37 ± 1 °C prior to testing. The extent of water uptake following each storage regime was determined using a near-infrared spectroscopy (NIRS) technique. Results No significant difference in BFS was identified for each material stored dry or wet for 24 h ( F = 2.7; P = 0.07) whilst BFS decreased following 3, 6 and 12 months ( F = 6.6; P < 0.001). A significant decrease in BFS of FZ was observed following 3 and 6 months (147 ± 34 and 102 ± 30 MPa; P < 0.001) with no further reduction following 12 months (94 ± 13 MPa; P > 0.05). In contrast, no significant decrease in BFS of FSB (97 ± 34 MPa) and FST (112 ± 28 MPa) was recorded following 6 compared with 3 months immersion ( P > 0.05). Storage for 12 months resulted in a further significant strength reduction of FSB and FST (56 ± 11, 82 ± 12 MPa; P = 0.004, P < 0.001, respectively). Water uptake of FZ and FSB increased up to 3 months before equilibrating, whereas water content of FST increased following all storage periods. Conclusions Strength degradation occurred at different rates between material types. Water uptake and mechanical properties of test materials were influenced by the size and morphology of the reinforcing particulate phase. The use of nanoparticles and associated agglomerates in modern RBCs exhibit distinct mechanical and physical properties compared with a conventional RBC type.

Comparison of an experimental bone cement with surgical Simplex® P, Spineplex® and Cortoss®

Conventional polymethylmethacrylate (PMMA) cements and more recently Bisphenol-a-glycidyl dimethacrylate (BIS-GMA) composite cements are employed in procedures such as vertebroplasty. Unfortunately, such materials have inherent drawbacks including, a high curing exotherm, the incorporation of toxic components in their formulations, and critically, exhibit a modulus mismatch between cement and bone. The literature suggests that aluminium free, zinc based glass polyalkenoate cements (Zn-GPC) may be suitable alternative materials for consideration in such applications as vertebroplasty. This paper, examines one formulation of Zn-GPC and compares its strengths, modulus, and biocompatibility with three commercially available bone cements, Spineplex, Simplex P and Cortoss. The setting times indicate that the current formulation of Zn-GPC sets in a time unsuitable for clinical deployment. However during setting, the peak exotherm was recorded to be 33 degrees C, the lowest of all cements examined, and well below the threshold level for tissue necrosis to occur. The data obtained from mechanical testing shows the Zn-GPC has strengths of 63 MPa in compression and 30 MPa in biaxial flexure. Importantly these strengths remain stable with maturation; similar long term stability was exhibited by both Spineplex and Simplex P. Conversely, the strengths of Cortoss were observed to rapidly diminish with time, a cause for clinical concern. In addition to strengths, the modulus of each material was determined. Only the Zn-GPC exhibited a modulus similar to vertebral trabecular bone, with all commercial materials exhibiting excessively high moduli. Such data indicates that the use of Zn-GPC may reduce adjacent fractures. The final investigation used the well established simulated body fluid (SBF) method to examine the ability of each material to bond with bone. The results indicate that the Zn-GPC is capable of producing a bone like apatite layer at its surface within 24 h which increased in coverage and density up to 7 days. Conversely, Spineplex, and Simplex P exhibit no apatite layer formation, while Cortoss exhibits only minimal formation of an apatite layer after 7 days incubation in SBF. This paper shows that Zn-GPC, with optimised setting times, are suitable candidate materials for further development as bone cements.

Analysis of macroscopic crack branching patterns in chemically strengthened glass

Residual stress profiles were introduced in sodium aluminosilicate glass disks using an ion-exchange process. They were fractured in two loading conditions: indentation and biaxial flexure. The fractal dimension of the macroscopic crack branching pattern called the crack branching coefficient (CBC), as well as the number of fragments (NOF) were used to quantify the crack patterns. The fracture surfaces were analyzed to determine the stresses responsible for the crack branching patterns. The total strain energy in the body was calculated. The CBC was a good measure of the NOF. They are directly related to the tensile strain energy due to the residual stress profile for fractures due to indentation loading. However, in general for materials with residual stresses, CBC (or NOF) is not related to the strength or the stress at fracture, or even to the total stored tensile strain energy. Instead, the CBC appears to be related, in a complex manner, to the distribution of stresses in the body. Therefore, in general, the characterization of the CBC of fractured materials cannot be used to ascertain the prior stress distribution.

Mechanical Properties of Dental Zirconia Ceramics Changed with Sandblasting and Heat Treatment

Two types of tetragonal zirconia polycrystals (TZP), a ceria-stabilized TZP/Al2O3 nanocomposite (CZA) and a conventional yttria-stabilized TZP (Y-TZP), were sandblasted with 70-microm alumina and 125-microm SiC powders, then partially annealed at 500-1200 degrees C for five minutes. Monoclinic ZrO2 content was determined by X-ray diffractometry and Raman spectroscopy. Biaxial flexure test was conducted on the specimens before and after the treatments. Monoclinic ZrO2 content and biaxial flexure strength increased after sandblasting, but decreased after heat treatment. However, in both cases, the strength of CZA was higher than that of Y-TZP. Raman spectroscopy showed that a compressive stress field was introduced on the sample surface after sandblasting. It was concluded that sandblasting induced tetragonal-to-monoclinic phase transformation and that the volume expansion associated with such a phase transformation gave rise to an increase in compressive stress on the surface of CZA. With the occurrence of such a strengthening mechanism in the microstructure, it was concluded that CZA was more susceptible to stress-induced transformation than Y-TZP.

Relation between biaxial flexure strength and phase transformation of zirconia with surface treatments

In this study, two types of tetragonal zirconia polycrystals (TZP) were used: a conventional yttria stabilized TZP (3Y-TZP) and a ceria stabilized TZP/alumina nanocomposite (CZA). All specimens were sandblasted with 70 μm-alumina and 125 μm-SiC. Heat treatment was performed at 1100°C for 10 min. XRD patterns of the specimens with grinding, sandblasting and heat treatment were determined, and the content of monoclinic zirconia was calculated using the XRD peak intensity ratios. The biaxial flexure strength of both zirconia increased with the sandblasting and decreased with the heat treatment. The biaxial flexure strength of CZA was higher than that of 3Y-TZP in each treatment. After sandblasting, the content of the monoclinic zirconia of CZA was much higher than that of 3Y-TZP. Although the monoclinic content with SiC-sandblasting was larger than alumina-sandblasting, the biaxial flexure strength with SiC-sandblasting was smaller than that with alumina-sandblasting. These results indicated that the volume expansion due to the phase transformation from tetragonal to monoclinic zirconia induced the compressive stress in the surface area. Furthermore, it suggests that CZA is more susceptible to stress-induced phase transformation by sandblasting than 3Y-TZP.

Effect of Sintering Condition, Sandblasting and Heat Treatment on Biaxial Flexure Strength of Zirconia

The effect of sintering condition, sandblasting and heat treatment on biaxial flexure strengths of the zirconia/ alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite, referred to NANOZR) was evaluated in comparison to that of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The disc-shaped specimens of NANOZR and Y-TZP were cut from the full-sintered or middle-sintered ones. The discs cut from the middle-sintered ones were finally sintered at the same temperature for the full-sintered one. These four kinds of disc were treated in various conditions combined with the sandblasting, the heat treatment, and the storage. The biaxial flexure strength of both middle- and full-sintered Y-TZP decreased with the autoclaving, whereas those of both NANOZR did not change with it. The monoclinic content of both the materials increased with the sandblasting and decreased with the heat treatment. Regardless of the sintering condition, the monoclinic content of the Y-TZP dramatically increased with the autoclaving and those of NANOZR remarkably increased with the sandblasting. Regardless of the different surface roughness, the biaxial flexure strengths of both materials strongly depended on the content of monoclinic ZrO2 on the surface.

Nonlinear finite element analysis of four-pile caps supporting columns subjected to generic loading

The paper presents the development of an adaptable strut-and-tie model that can be applied to the design or analysis of four-pile caps that support axial compression and biaxial flexure from a supported rectangular column. Due to an absence of relevant test data, the model is validated using nonlinear finite element analyses (NLFEA). The results indicate that the use of the proposed model would lead to safe and economical designs. The proposed model can be easily extended to any number of piles, providing a rational procedure for the design of wide range of pile caps.

Bi-Axial Flexure Strength, Weibull Modulus and Fracture Mode of Alumina Glass-Infiltrated Core/Veneer Ceramic Composites

The purpose of this study is to determine the bi-axial flexural strength, weibull modulus and fracture mode of bilayered alumina glass-infiltrated core and the veneering porcelain. Forty disk specimens were fabricated from alumina glass-infiltrated core (HSDC-A) and veneer porcelain (Vintage AL). The specimens were equally divided into four groups as: MV, monolithic specimens of veneer material; MC, monolithic specimens of core material; BV, bilayered specimens with the veneer in tension; BC, bilayered specimens with core material in tension. Mean flexure strength, standard deviation and associated Weibull modulus were determined using bi-axial flexure (ball-on-ring) for each group. Both optical and scanning electron microscopy were employed for identification of the fracture mode and origin. The surface loaded in tension influenced the bi-axial flexural strength and reliability of the composites. The frequency of specimen delamination, Hertzian cone formation and sub-critical radial cracking in the bilayered discs are also dependent on the surface loaded in tension.

The potential of a resin-composite to be cured to a 4 mm depth

Objectives The potential of a recently marketed resin-based composite (RBC), namely X-tra fil (shade A3), which claims to be amenable to curing to a depth of 4 mm was investigated. Methods Disc-shaped specimens (11 mm diameter, 2 mm thickness) of Filtek™ Z250, Admira and X-tra fil were tested in bi-axial flexure to determine the strength. Water sorption, water solubility and Vickers hardness measurements were determined following short-term (0.1, 0.5, 1, 4, 24 and 48 h) and medium-term (1, 4, 12 and 26 weeks) water immersion on disc-shaped specimens (11 mm diameter, 1 mm thickness). The top (t) 0–1 mm depth and bottom (b) 3–4 mm depth of X-tra fil were tested for water sorption, water solubility and Vickers hardness measurements. In addition an analysis of pulpal cell cytotoxicity at 1, 2, 3, 7 and 14 days was also performed on the RBCs. Results No significant differences in the bi-axial flexure strength or top to bottom hardness ratios were evident between the materials examined. However, the water sorption and water solubility values obtained for Filtek™ Z250 (12.3 ± 1.8 and 2.7 ± 1.6 μg/mm 3, respectively) and Admira (16.0 ± 1.5 and 4.3 ± 0.2 μg/mm 3, respectively) were increased compared with X-tra fil (t) (5.4 ± 0.7 and 0.8 ± 0.2 μg/mm 3, respectively) and X-tra fil (b) (6.8 ± 0.6 and 2.4 ± 1.1 μg/mm 3, respectively) but within the ISO specification standard of ≤40 and ≤7.5 μg/mm 3, respectively. No statistically significant differences were identified on cell viability between the RBCs used in the current study. Significance The manufacturer claims that the recently marketed material X-tra fil could be cured to a depth of 4 mm appear to be vindicated and the performance in terms of flexure strength, water uptake and biocompatibility are comparable with conventional RBCs.

Resin Elasticity and the Strengthening of All-ceramic Restorations

Resin luting of all-ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three-point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain discs were air-abraded. One group acted as a control, and 3 additional groups were coated with 120 +/- 20 microm of each resin prior to bi-axial flexure testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R(2) = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.