Significant Decrease In Flexural Strength Research Articles

Effect of different aging methods on the mechanical behavior of multi-layered ceramic structures

ObjectiveTo evaluate the effect of two aging methods (mechanical cycling and autoclave) on the mechanical behavior of veneer and framework ceramic specimens with different configurations (monolithic, two and three-layers). MethodsThree ceramics used as framework for fixed dental prostheses (YZ—Vita In-Ceram YZ; IZ—Vita In-Ceram Zirconia; AL—Vita In-Ceram AL) and two veneering porcelains (VM7 and VM9) were studied. Bar-shaped specimens were produced in three different designs: monolithic, two layers (porcelain–framework) and three layers (porcelain–framework–porcelain). Specimens were tested for three-point flexural strength at 1MPa/s in 37°C artificial saliva. Three different experimental conditions were evaluated (n=10): control; mechanical cycling (2Hz, 37°C artificial saliva); and autoclave aging (134°C, 2 bars, 5h). Bi-layered specimens were tested in both conditions: with porcelain or framework ceramic under tension. Fracture surfaces were analyzed using stereomicroscope and scanning electron microscopy. Results were statistically analyzed using Kruskal–Wallis and Student-Newman–Keuls tests. ResultsOnly for AL group, mechanical cycling and autoclave aging significantly decreased the flexural strength values in comparison to the control (p<0.01). YZ, AL, VM7 and VM9 monolithic groups showed no strength degradation. For multi-layered specimens, when the porcelain layer was tested in tension (bi and tri-layers), the aging methods evaluated also had no effect on strength (p≥0.05). Total and partial failure modes were identified. SignificanceMechanical cycling and autoclave aging protocols had no effect on the flexural strength values and failure behavior of YZ and IZ ceramic structures. Yet, AL monolithic structures showed a significant decrease in flexural strength with any of the aging methods.

Influence of aging on flexural strength of translucent zirconia for monolithic restorations

Statement of problemConcern has been raised with regard to the low-temperature degradation (LTD) of translucent yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) for monolithic zirconia restorations. PurposeThe purpose of this in vitro study was to assess the LTD behavior of 4 commercially available translucent Y-TZP materials by accelerated aging specimens in steam at 134°C, 0.2 MPa. Material and MethodsThin bars (22×3×0.2 mm) of Y-TZP, including Katana ML (Kuraray Noritake Dental Inc), Katana HT13 (Kuraray Noritake Dental Inc), Prettau (Zirkonzahn), and BruxZir (Glidewell Laboratories) (n=30 for each group), were machined from sintered blocks. Control specimens were assessed in the nonaged condition. Artificially ageing (n=5 per group at 5, 50, 100, 150, and 200 hours) was conducted in steam at 134°C at 0.2 MPa. The specimens were characterized, tested in 4-point flexure, and the fracture surfaces were analyzed. The monoclinic-to-tetragonal (m/t) peak intensity ratio measured by x-ray diffraction was used to calculate the monoclinic phase fraction and monitor LTD. Linear regression with heteroscedasticity-consistent robust standard errors was used to test for the effect of LTD (aging time) on (σf) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between σf and monoclinic phase fraction (α=.05). ResultsArtificial aging resulted in LTD as shown by an increase in the monoclinic phase fraction for all specimens. After aging for 200 hours, the mean ±SD monoclinic phase fraction increased from 2.90 ±0.34% to 76.1 ±0.64% for Prettau, 2.69 ±0.18% to 76.0 ±0.26% for BruxZir, 4.6 ±0.19% to 35.8 ±0.80% for Katana HT13, and 3.57 ±0.35% to 33.2 ±1.1% for Katana ML (all P<.001). Flexural strength changed from a mean ±SD of 1612 ±197 MPa to all fractured during aging for Prettau (P<.001); 1248 ±73.5 MPa to all fractured during aging for BruxZir (P<.001); 1052 ±84.2 to 1099 ±70 MPa ±130 for Katana HT13 (P=.45); and from 875 ±130 to 909 ±70 MPa (P=.82) for Katana ML. The mean flexural strength values of Prettau and BruxZir decreased with an increase in the monoclinic phase with Spearman rank correlation coefficients of −0.80 (P=.001) for Prettau and −0.63 (P=.022) for BruxZir. No significant changes in flexural strength were measured for Katana ML or Katana HT13 (P>.05). ConclusionsThe LTD of Y-TZP resulted in a significant decrease in flexural strength of Prettau and BruxZir, whereas Katana ML and Katana HT13 exhibited less LTD and no significant decrease in flexural strength.

Evaluation of polymethyl methacrylate resin mechanical properties with incorporated halloysite nanotubes.

PURPOSEThis study inspects the effect of incorporating halloysite nanotubes (HNTs) into polymethyl methacrylate (PMMA) resin on its flexural strength, hardness, and Young's modulus.MATERIALS AND METHODSFour groups of acrylic resin powder were prepared. One group without HNTs was used as a control group and the other three groups contained 0.3, 0.6 and 0.9 wt% HNTs. For each one, flexural strength, Young's modulus and hardness values were measured. One-way ANOVA and Tukey's test were used for comparison (P<.05).RESULTS At lower concentration (0.3 wt%) of HNT, there was a significant increase of hardness values but no significant increase in both flexural strength and Young's modulus values of PMMA resin. In contrast, at higher concentration (0.6 and 0.9 wt%), there was a significant decrease in hardness values but no significant decrease in flexural strength and Young's modulus values compared to those of the control group.CONCLUSIONAddition of lower concentration of halloysite nanotubes to denture base materials could improve some of their mechanical properties. Improving the mechanical properties of acrylic resin base material could increase the patient satisfaction.

Effect of thermo-oxidative aging on three-dimensional and four-directional braided carbon fiber/epoxy composite

An experimental study was performed to investigate the effect of high temperature exposure on flexural properties of the three-dimensional and four-directional braided carbon fiber/epoxy composite and the corresponding neat resin. For this purpose, samples were exposed to 90℃, 120℃, 150℃, and 180℃ (below and above the glass transition temperature of the matrix resin) for various periods of time up to 13 days. The flexural properties combining with Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), weight loss, scanning electron microscope (SEM), and optical microscope analyses were performed on the unaged and aged samples. The results revealed that the thermo-oxidative behavior of the braided composite was significantly different from the neat resin. The flexural strength of the neat resin decreased dramatically due to the oxidation degradation of the sample surface when the aging temperature was equal to its glass transition temperature. The extensive degradations of both the matrix and the fiber/matrix interface, especially for the interface, were responsible for the significant decrease in flexural strength of the braided composite. The flexural strength retention rates of the braided composite were higher than that of the neat resin after exposure to 90℃, 120℃ and 150℃ because the integral structure of the braided composite can make the fiber bear flexural force together although the matrix resin and the fiber/matrix interface had a certain degree of damage after longtime thermo-oxidative aging.

Effect of hydrothermal degradation on three types of zirconias for dental application

Concern has been expressed with regard to hydrothermal aging of yttria-stabilized tetragonal zirconia polycrystalline. The purpose of this study was to assess the accelerated aging characteristics of a new yttria-stabilized tetragonal zirconia polycrystalline material and 2 commercially available yttria-stabilized tetragonal zirconia polycrystalline materials by exposing specimens to hydrothermal treatments in steam at 134°C, 0.2 MPa, and at 180°C, 1.0 MPa. Thin bars of zirconia: Prettau, Zirprime, and a new zirconia, ZirTough, n = 55 for each brand (22 × 3 × 0.2 mm) were cut and ground from blocks sintered according to the manufacturer's specifications. The control specimens for each group were evaluated in the nonaged condition, and their chemical composition was measured with energy dispersive spectroscopy. The experimental specimens were artificially aged under standard autoclave sterilization conditions, 134°C at 0.2 MPa (n = 5 per group at 5, 50, 100, 150, and 200 hours), and under standard industrial ceramic aging conditions, 180°C at 1.0 MPa (n = 5 per group at 8, 16, 24, and 48 hours). The tetragonal to monoclinic transformation was measured by using x-ray diffraction for all groups. Flexural strength was measured with a 4-point bend test (ASTM 1161-B) for all the groups, and the fracture surfaces were examined with scanning electron microscopy. The data were analyzed as a function of aging time. To test for an aging effect on the flexural strength and the monoclinic-tetragonal ratio, a 1-way ANOVA (with heteroscedasticity-consistent standard errors) was used to test for a general time effect. For the analyses of the monoclinic-tetragonal ratio, the same specimens were used at 0 hours and after aging, and the data were analyzed with an ANOVA for an incomplete block design. The relationship between flexural strength and monoclinic-tetragonal ratio was assessed with the Spearman rank correlation coefficient based on the average value at each aging. After 200 hours at 134°C and 0.2 MPa, the flexural strength decreased from a mean (standard deviation) of 1328 ± 89.9 MPa to all fractured during aging for Prettau (P < .001); 1041 ± 130 to 779 ± 137 MPa for Zirprime (P = .<.001) and 1436 ± 136 to 1243 ± 101 MPa for ZirTough (P = .017). After 200 hours at 134°C and 0.2 MPa, a portion of the tetragonal crystals transformed to the monoclinic phase in all specimens. The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 78.8% ± 2.0% for Prettau, 1.95% ± 0.48% to 74.8% ± 0.52% for Zirprime, and 12.4% ± 0.60% to 31.4% ± 4.4% for ZirTough (all P < .001). After 16 hours at 180°C and 1.0 MPa, all Prettau specimens had spontaneously fractured during aging. The Zirprime and ZirTough specimens were intact after 48 hours at 180°C and 1.0 MPa, and the mean (standard deviation) flexural strength had decreased from 1041 ± 130 MPa to 595 ± 88.4 MPa for Zirprime and 1436 ± 136 MPa to 1068 ± 76.8 MPa for ZirTough (all P < .001). The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 79.0% ± 0.13% for Prettau, from 1.95% ± 0.48% to 68.1% ± 4.4%, for Zirprime, and from 12.4% ± 0.60% to 39.5% ± 5.56% for ZirTough (all P<.001). The flexural strength decreased with an increase in the monoclinic phase for all the groups (Spearman rank correlation coefficients, -0.71 to -1.0). Scanning electron microscope micrographs revealed a transformed layer on the fracture surfaces. The decrease in flexural strength was related to the increase in monoclinic phase from long-term degradation. Hydrothermal aging of zirconia caused a statistically significant decrease in flexural strength of thin bars of zirconia, which was the result of the transformation from tetragonal to monoclinic crystal structure. ZirTough exhibited the least decrease in strength and smallest amount of monoclinic phase after aging.

A Comparative Evaluation of Flexural Strength of Commercially Available Acrylic and Modified Polymethylmethacrylate: An in vitro Study

The purpose of this study was to evaluate and compare the flexural strength of commercially available acrylic (trevalone) and modified polymethylmethacrylate (PMMA). Four groups were tested; Group 1- control group regular MMA, group 2-2% methacrylic acid, 88% MMA, group 3-16% methacrylic acid, 84% MMA group 4- 20% methacrylic acid, 80% MMA 15 resin specimens of each group were polymerized. After processing, the specimens were subjected for flexural strength testing using three point bending test in a Universal Testing Machine. All data was statistically analyzed with one-way ANOVA, differences within the groups were analyzed by Scheffe's analysis. As the ratio of incorporated methacrylic acid to PMMA increased, the flexural strength decreased. Analysis of data revealed a significant decrease in flexural strength of specimens (p < 0.000) after incorporation of 12%, 16%, 20% methacrylic acid to heat polymerized acrylic resin, when compared with the control group. Lowest flexural strength was observed with specimens containing 20% methacrylic acid and highest flexural strength was observed with specimens containing conventional monomer without methacrylic acid. It was observed that as the concentration of methacrylic acid in heat polymerized acrylic resin increases, the flexural strength decreases. Lowest flexural strength was observed with specimens containing 20% methacrylic acid and highest flexural strength was observed with specimens containing conventional monomer without methacrylic acid. The major advantages of addition of methacrylic acid to polymethylmethacrylate could be for the elderly people with restricted manual dexterity or cognitive disturbances, especially for patients who do not follow an adequate denture cleansing protocol and diabetic patients who are more susceptible for denture stomatitis.

In vitro comparison of flexural strength and elastic modulus of three provisional crown materials used in fixed prosthodontics

Objectives: To evaluate and compare the flexural strength and the elastic moduli of three provisional crown materials (methyl methacrylate based autopolymerized resin, bis acryl composite based autopolymerized resin and urethane dimethacrylate based light polymerized resin) after storing in artificial saliva and testing at intervals of 24 hours and 7 days. Study design: A metal master mould with four slots of dimensions 25x2x2 mm was fabricated to obtain samples of standard dimensions. A total of 135 specimens were thus obtained with 45 each of three provisional materials. Further 15 samples of each group were tested after storing for one hour at room temperature and again at intervals of 24 hours and 7 days after storing in artificial saliva. Three point flexural tests were carried out in the universal testing machine to calculate the flexural strength and the elastic modulus. The changes were calculated and data was analyzed with Fisher’s test and ANOVA. Results: The flexural strength of the methyl methacrylate resin reduced significantly while bis-acrylic composite resin showed a significant increase in its flexural strength after storing in artificial saliva for 24 hours and the values of both remained constant thereafter. Contrary to these findings, light polymerized resin showed a significant decrease in flexural strength after storing in artificial saliva for 24 hours and then significantly increased in flexural strength after 7 days. However the changes in the values for elastic modulus of respective materials were statistically insignificant. Conclusion: Methacrylate based autopolymerizing resin showed the highest flexural strength and elastic moduli after fabrication and after storing in artificial saliva and for 24 hours and 7 days. Bis-acrylic composite resin showed the least flexural strength and elastic moduli. Key words:Provisional restorations, interim restorations, Methyl Methacrylate, composite restoration, flexural strength, elastic moduli.

Effect of Airborne‐Particle Abrasion and Aqueous Storage on Flexural Properties of Fiber‐Reinforced Dowels

A great range of clinical failures have been observed with fiber-reinforced dowels, often attributed to fracture or bending of the dowels. This study investigated flexural properties of fiber-reinforced dowels, with and without airborne-particle abrasion, after storage in aqueous environments over time. Scanning electron microscopy (SEM) was used to analyze the mode of failure of dowels. Two dowel systems (ParaPost Fiber Lux and FibreKor) were evaluated. Ten dowels of each system were randomly assigned to one of six experimental groups: 1--control, dry condition; 2--dowels airborne-particle abraded and then stored dry; 3--dowels stored for 24 hours in aqueous solution at 37°C; 4--dowels airborne-particle abraded followed by 24-hour aqueous storage at 37°C; 5--dowels stored for 30 days in aqueous solution at 37°C; 6--dowels airborne-particle abraded followed by 30-day aqueous storage at 37°C. Flexural strength and flexural modulus were tested for all groups according to American Society of Testing and Materials (ASTM) standard D4476. One failed dowel from each group was randomly selected to be evaluated with SEM equipped with energy dispersive spectroscopy (EDS) to characterize the failure pattern. One intact dowel of each system was also analyzed with SEM and EDS for baseline information. Mean flexural modulus and strength of ParaPost Fiber Lux dowels across all conditions were 29.59 ± 2.89 GPa and 789.11 ± 89.88 MPa, respectively. Mean flexural modulus and strength of FibreKor dowels across all conditions were 25.58 ± 1.48 GPa and 742.68 ± 89.81 MPa, respectively. One-way ANOVA and a post hoc Dunnett's t-test showed a statistically significant decrease in flexural strength as compared to the dry control group for all experimental groups stored in water, for both dowel systems (p < 0.05). Flexural modulus for both dowel systems showed a statistically significant decrease only for dowels stored in aqueous solutions for 30 days (p < 0.05). Airborne-particle abrasion did not have an effect on flexural properties for either dowel system (p > 0.05). SEM and EDS analyses revealed differences in composition and failure mode of the two dowel systems. Failed dowels of each system revealed similar failure patterns, irrespective of the experimental group. Aqueous storage had a negative effect on flexural properties of fiber-reinforced dowels, and this negative effect appeared to increase with longer storage times. The fiber/resin matrix interface was the weak structure for the dowel systems tested.

Studies on Bamboo/Glass Fiber Reinforced USP and VE Resin

The effects of weight fraction and length of short bamboo and glass fibers on flexural strength and inter-laminar shear strength (ILSS) of vinyl ester resin and unsaturated polyester (USP) resins were investigated. The bamboo fibers were isolated from the bamboo plant by treating them with dilute solution of sodium hydroxide (0.1 and 1 N NaOH) for 72 h and then boiling them in a pressure cooker for 30 min. The fibers were separated, dried, and cut into 5 or 10 mm lengths. Composites were fabricated using vinyl ester resin or USP resin as binders. The flexural properties of glass/bamboo fiber reinforced vinyl ester resins were higher than those based on USP resins. This may be due to better wetting of the fibers by VE resins due to their low viscosity and possibility of hydrogen bond formation with hydroxyl group on bamboo fibers. Hybrid composites were fabricated using 50 wt% of fibers. The wt% of bamboo fibers in these hybrid composites was varied as 25, 50, and 75 wt% of chopped glass fibers (5 mm). Replacement of 25 wt% of glass fibers did not affect the flexural modulus and a marginal increase in ILSS was observed. However, replacement of 75% glass fibers by bamboo fibers resulted in a significant decrease in flexural strength, modulus and ILSS.

Invitro evaluation of flexural strength and flexural modulus of elasticity of different composite restoratives

Background & Objectives : The aim of the study was to evaluate the flexural strength and flexural modulus o f five commercially available composites namely Fetric Ceram (Hybrid), Filtek P-60 (Packable), Dyract (Compomer), Filtek Flow (Flowable) and Admira (Ormocer). after aging in water. Methodology: 20 specimens were made using each of the 5 composite materials . and randomly divided into two subgroups containing 10 specimens each. Specimens were stored in distilled water (Subgroup A: 7 days & Subgroup 13: 30 days). Thermocycling of all the specimens was done for 5000 cycles, to simulate the oral conditions. Flexural strength and modulus of the specimens were assessed in a Universal Testing Machine. Data was analyzed using one way ANOVA /Students t-test at a significance level of 0.05. Results: Results have shown that highest flexural strength and modulus was seen for Group I1 (Filtek P-­60) and Group I (Tetric Ceram), followed by Group V (Admira). Group Ill (Dyract) showed lowest flexural strength and Group IV (Filtek flow) showed lowest flexural modulus. After aging in water all the groups showed decreased flexural strength, except for Group IV (Filtek Flow), for which the strength increased. Flexural modulus of all the composites tested increased slightly after aging in water, but was not significant. Interpretation & Conclusion: The effect of aging in water on flexural strength and modulus was material dependent. A significant decrease in flexural strength was observed for all the composites, except for Group IV (Filtek flow), after aging in water. Aging had no significant effect on the flexural modulus of any of the composites tested.

Thermo-hydrolytic stability of core foundation and restorative composites

Statement of problem The use of weak and less durable materials in restoring teeth may result in weak restorations unable to withstand intraoral conditions. Purpose The purposes of this study were to evaluate the effect of thermo-hydrolytic stress on the flexural strength and flexural modulus of core foundation composites with direct restorative composites and determine mass percentage of filler content. Material and methods A total of 216 specimens, from 9 brands of commercially available composites (Coreflo, DC Core, Photocore, APX, Litefil II A, Surefil, TPH Spectrum, Z100, and Z250) were fabricated following ISO Standard 4049. Flexural strength (MPa) and flexural modulus (GPa) were determined on bar-shaped specimens (25 × 2 × 2 mm) before and after storage in boiling water for 24 hours (n=12). The filler content in composite was determined by incineration using a thermogravimetric analyzer. The data were analyzed using 2-way analysis of variance and the Student t test (α=.05). Results Filler content of the tested composites was 66.6 to 81.8 mass %. Significant differences in both flexural strength and flexural modulus existed among materials, the effect of boiling and interaction ( P<.05). Coreflo, DC Core, Z100, and Z250 demonstrated a significant decrease in flexural strength after boiling ( P<.05). Z250 showed a significant decrease in flexural modulus after boiling ( P=.001), while Surefil showed a significant increase in flexural modulus ( P=.007). Conclusion Within the limitations of this study, it can be concluded that composites were affected differently by moist heat stress. Some composites showed a degradation of flexural properties while some retained flexural properties. Stability of the composites varied among brands.

Effect of simulated resin-bonded fixed partial denture clinical conditions on resin cement mechanical properties.

The purpose of this study was to determine changes in flexural properties of resin cement under simulated resin-bonded fixed partial denture (RBFPD) clinical conditions using aqueous ageing and cyclic loading. Panavia F flexural modulus and strength were measured by static loading to failure after 48-h and 60-day aqueous ageing at 37 degrees C with and without simulated cyclic occlusal loading. Panavia F sorption and solubility were also measured. Scanning electron microscopy (SEM) was used to characterize the morphology of the fractured surfaces. A two-factor anova (P </= 0.05) indicated that cyclic loading produced a significant increase in the flexural modulus with no significant effect on the flexural strength. Conversely, aqueous ageing time produced a significant decrease in flexural strength with no effect on the flexural modulus. The SEM fracture analysis indicated that resin matrix fracture occurred in static-aqueous specimens; while in the aqueous-cycled specimens, resin matrix fracture occurred in addition to an increasing proportion of filler/resin interface fracture. Collectively, these outcomes suggest that initial degradation under simulated resin cement clinical function may be related to breakdown of the filler/resin interface bond, which could contribute to in vivo RBFPD resin cement cohesive failure.

Mechanical property characterization of resin cement after aqueous aging with and without cyclic loading

Objective. The purpose of this study was to determine changes in flexural properties of resin cement under cyclic loading and aqueous aging. Methods. Panavia F flexural modulus and strength were measured by static loading to failure after 48-h, 31-d, and 60-d aqueous aging at 37 °C with and without cyclic loading at clinical stress levels. Six specimens were used for each experimental condition. Scanning electron microscopy (SEM) was also used to characterize the morphology of the fractured surfaces to potentially identify resin cement components particularly susceptible to degradation under simulated clinical function. Results. A two-factor ANOVA ( p≤0.05) and Least Significant Difference post hoc test indicated that cyclic loading produced a significant increase in the flexural modulus with no significant effect on the flexural strength. In contrast, aqueous aging time produced a significant decrease in flexural strength with no effect on the flexural modulus. The SEM fracture analysis indicated that resin matrix fracture occurred in static-aqueous specimens; while in the aqueous-cycled specimens, resin matrix fracture occurred in addition to an increasing proportion of filler/resin interface fracture as a result of both increased aqueous aging and increased flexural cycles. Significance. Evidence suggests that after aqueous aging with cyclic loading to simulate resin cement clinical function, initial degradation may be related to breakdown of the filler/resin interface bond. Such breakdown is potentially reflective of slow crack propagation that may contribute to in vivo resin cement cohesive failure.

Thermal Shock Behavior of an Alumina‐SiC Whisker Composite

Thermal shock testing of an alumina‐20 vol% SiC whisker composite showed no decrease in flexural strength with temperature differences up to 900°C. Alumina, on the other hand, normally shows a significant decrease inflexural strength with a temperature change of &gt;400°C. The improvement in the thermal shock resistance of the composite is believed attributable to the increased fracture toughness of this material.