Higher Weibull Modulus Research Articles

Development of experimental HEMA-free three-step adhesive system

Objective To evaluate the influence of Bis-EMA 30 on the resin-to-dentin microtensile bond strength (μTBS) and structural reliability of the experimental three-step etch-and-rinse adhesive systems. Methods Five experimental primers containing different dimethacrylate monomer concentrations (0, 10, 20, 40, 60 wt% of the Bis-EMA 30, P0.P60) added to acid monomer and solvents (ethanol/water), and a resin bond (Bis-GMA/TEGDMA, 50/50 wt%) were formulated. The adhesive system Scotchbond™ MultiPurpose (SBMP, 3M ESPE) was tested as commercial reference. Sixty bovine incisors were randomly separated into six groups, and their superficial coronal dentin was exposed. After acid etching and rinsing, the excess water was removed from the surface with absorbent paper. Each experimental primer was actively applied (30 s), followed by a mild air stream (10 s). The experimental adhesive resin was applied and light activated for 20 s. Resin composite restorations were incrementally built up. The restored teeth were stored in distilled water at 37 °C for 24 h, and then sectioned to obtain sticks with a cross-sectional area of approximately 0.5 mm 2, after which 24 specimens per group were subjected to the μTBS test. Data (MPa) were analyzed by One-way ANOVA, Tukey test ( α = 0.05) and Weibull analysis. Results The P40 group showed μTBS means similar to those of the control (SBMP), whereas both had statistically higher values when compared with the other groups ( p < 0.001). Moreover, P40 showed higher structural reliability, represented by the high Weibull modulus and characteristic strength values. The lowest μTBS was observed in the P0, P10 and P20 groups, which also had low structural reliability. Significance Bis-EMA 30 is a promising monomer to be considered as a substitute for HEMA in adhesive system compositions.

Influence of surface treatments on surface roughness, phase transformation, and biaxial flexural strength of Y-TZP ceramics.

The aim of this study was to evaluate the influence of surface grinding and sandblasting on surface roughness, phase chances, and biaxial flexural strength of yttria-stabilized tetragonal zirconia (Y-TZP) materials. Thirty disk specimens of Cercon (C), DentaCAD (DC), Zirkonzahn (ZZ) were fabricated. The specimens were divided into three groups according to surface treatment (control, ground, and sandblasted). Surface roughness was measured, and X-ray diffraction analysis was performed. Finally, biaxial flexural strength was determined. The data was analyzed by two-way ANOVA. Weibull statistics was used to analyze the variability of strength. The effects of surface treatments on surface roughness values were different for each material. X-ray diffraction analysis revealed that control groups of C and ZZ were composed of tetragonal zirconia. Relative amount of monoclinic zirconia (SD) was 7.366 (0.716)% in the DC control group. In all materials, transformation occurred after treatments. Grinding decreased and sandblasting increased the strength of control groups in all materials. Ground C and DC specimens had higher Weibull modulus than control groups while lower m was found for ground ZZ. Sandblasting, resulted in lower m compared with grinding for all materials although increased strength. The roughness and crystalline phase of Y-TZP materials were influenced by surface treatments. Biaxial flexural strength of materials decreased after grinding and increased after sandblasting. The low m of sandblasted groups may indicate further weakening of the materials, resulting in unexpected failures.

The Effect of Particle Size Distribution on the Microstructure and the Mechanical Properties of Boron Carbide‐Based Reaction‐Bonded Composites

The presence of unreacted, free silicon lowers the mechanical properties of reaction‐bonded boron carbide. The fraction of free silicon can be reduced by increasing the green density of the initial boron carbide performs. The use of multimodal boron carbide mixtures allows attaining 75% green density. After reaction bonding with molten silicon, the composites consist of four phases, namely the original B4C particles, the B12(B,C,Si)3 phase, product of the dissolution–precipitation process, β‐SiC, and residual Si. The volume fraction of residual Si in the composites is in the 8–10% range. The infiltrated composites display elevated values of the mechanical properties with a high Weibull modulus.

Tensile properties of glassy MgZnCa wires and reliability analysis using Weibull statistics

Mg-based metallic glass wires were produced via the melt-extraction technique. Their mechanical properties were evaluated by carrying out tensile tests on electrochemically polished dogbone-shaped wire samples, and their reliability was estimated using Weibull analysis. The wires exhibit a tensile strength of 675–894 MPa with a characteristic strength of 817 MPa and a Weibull modulus of 20.6. The high Weibull modulus is explained by the plastic behavior and necking of the tensile samples. The tensile tests also reveal an extended amount of homogeneous plastic deformation, which can be assigned to the circular geometry and flawless surface quality of the specimens.

Effect of the sulfidation process on the mechanical properties of a CoMoP/Al 2O 3 hydrotreating catalyst

Mathematical models for the mechanical properties of a CoMoP/Al 2O 3 hydrotreating catalyst in the sulfidation process are developed with response surface methodology. A Doehlert design is performed to collectively study the effects of sulfidation temperature, time and heating rate on the mean strength, Weibull modulus and pellet density. Analysis of variance reveals that the models developed for the mean strength and Weibull modulus are adequate. The validity of the models is also verified by experimental data. Analyses of response surfaces show that in the great part of the experimental domain examined, as the sulfidation temperature increases the mean strength decreases, while the Weibull modulus increases at first and then decreases. The middle level of the sulfidation time results in smaller mean strength and higher Weibull modulus. The Weibull modulus decreases with increasing heating rate; however, the effect of the heating rate on the mean strength is statistically less pronounced. It is concluded that there is a great potential for improving the catalyst mechanical reliability in the sulfidation process, while the pellet density does not vary significantly.

Reliability Evaluation of Hydrothermal Crystallization on Plasma-Sprayed HA Coatings by the Weibull Model

Plasma-sprayed HA coatings (HACs) on Ti-6Al-4V substrates with post-heat treatments were employed to improve the microstructural homogeneity, bonding strength and reliability of the HACs. A defect-healing effect can be recognized to diminish coating defects with the hydrothermal treatment, and 150°C hydrothermally-treated HACs shows a significant improved bonding strength than 600°C vacuum heating HACs because of its dense structure from the defect-healing effect. Low-temperature hydrothermal treatment demonstrates a more superior crystallization effect than vacuum heat treatment. Based on the statistical calculation by the Weibull distribution function, hydrothermally-treated HACs show a wear-out failure with a higher Weibull modulus than vacuum heating HACs. Hydrothermally-treated HACs possessed better reliability can be attributed to the suppression of defect content (about 2.6-3.2 volume %) and its smaller strength data fluctuation.

An estimation of ceramic fracture at singularities by a statistical approach

A statistical analysis based on weakest link theory is employed to describe the brittle fracture induced at singularities in ceramic materials. Relationships are stated between the Weibull probability and the notch geometry. For low Weibull's modulus, survival probabilities only depend on the generalised stress intensity factor, whereas for high Weibull's modulus, probabilities also depend on the notch tip radius. The effect of the tip radius on the failure probabilities is equivalent to the exclusion of a small volume surrounding the notch tip. From these results, a numerical methodology based on the finite element analysis is proposed to state if singularity is harmful for a ceramic structure. Elsewhere, for a notch with high stress singularity order and symmetrically loaded, the Batdorf's theory gives the same results than the Weibull one. In the case of a low stress singularity order, the prediction of failure can strongly depend on the multi-axial criterion.

Fabrication and Mechanical Properties of In Situ Monazite-Coated Alumina Fiber-Reinforced Alumina/YAG Composites

Monazite(LaPO4)-coated alumina-fiber/alumina-YAG (Y3Al5O12) matrix composites were fabricated by in-situ coating of monazite followed by hot-pressing, and the effects of coating and sintering condition on mechanical properties of the composite were examined. Alumina powder and YAG powder (weight ratio, 95:5) were used as raw materials for green sheets, which was fabricated by tape casting technique. Monazite was synthesized by the in-situ reaction of La(NO3) solution with H3PO4 on the surface of fibers. After slurry infiltration into the coated fiber bundles, the fiber cloths were laminated with the green sheets alternately, then they were heat-treated, finally sintered by hot-pressing at various temperatures. The mechanical properties of the composites were changed by the fabrication conditions. Non-brittleness of the composites reduced with the increase of sintering temperature. The composites sintered at 1200oC showed the highest Weibull modulus and pseudo-ductility.

The Effect of Fe-Rich Intermetallics on the Weibull Distribution of Tensile Properties in a Cast Al-5 Pct Si-3 Pct Cu-1 Pct Fe-0.3 Pct Mg Alloy

A comparison was made between the tensile properties of an Al-5 pct Si-3 pct Cu-1 pct Fe-0.3 pct Mg (319-type) alloy in the as-cast condition. The alloy was either unmodified or modified with Mn or Sr. The additions of 0.6 wt pct Mn, or 0.9 wt pct Mn, or 300 ppm Sr were made. All modifying additions increased the average and decreased the scatter in tensile strength, and increased elongation, with the Sr addition being most effective among the three. Fractographic examinations showed that the β phase, the polyhedral α phase, and porosity appear to be the main defects responsible for failure. The distribution of tensile strength has been quantitatively assessed using both two- and three-parameter Weibull distribution functions. In the two-parameter Weibull analysis, the high Weibull modulus (46) achieved by Sr modification is due to its binary effect on eutectic silicon and β platelets. Reservations about potentially misleading implications of the two-parameter approach are explored. The three-parameter Weibull analysis provides new information. In particular, minimum values of strength below which the material is extremely unlikely to fail are found. These values again show the clear benefit of Sr addition. The results are explained by the effects of modifying agents on the microstructure.

Investigation on reliability of nanolayer-grained Ti3SiC2 via Weibull statistics

Weibull modulus of bending strength of nanolayer-grained ceramic Ti3SiC2 was estimated with over 50 specimens, using the least square method, the moment method and the maximum likelihood technique, respectively. The result demonstrated that the m-value of this layered ceramic ranged from 25 to 29, which is much higher than that of traditional brittle ceramics. The reason of high Weibull modulus was due to high damage tolerance of this material. Under stress, delamination and kinking of grains and shear slipping at interfaces give this material high capacity of local energy dissipation and easy local stress relaxation, leading to the excellent damage tolerance of Ti3SiC2. The effect of amounts of specimens on the reliability of the estimated m-values was also investigated. It was confirmed that the stability of the estimated m-value increased with increasing numbers of specimens. The parameter obtained using the maximum likelihood technique showed the highest reliability than other methods. The ranges of failure probability were determined using the Weibull estimates calculated from the maximum likelihood technique.

Distribution of the Mechanical Strength of Solid Catalysts

The mechanical strength of solid catalysts is distributed over a wide range of values. The suitability of the normal, lognormal and Weibull distributions to model the catalyst strength variation was judged by three goodness-of-fit criteria: the coefficient of determination, Akaike information criterion and Kolmogorov–Smirnov test. It is concluded that the Weibull distribution is most universal to represent the catalyst strength data, though sometimes it may not be the optimal candidate. It is elucidated that the catalyst strength variation results from the brittle fracture nature of the mechanical failure of solid catalyst materials. The low-strength/probability part of the catalyst strength distribution is the key domain for the mechanical reliability, while the mean strength that has traditionally been taken as a measure of the catalyst strength is of less importance. It is also pointed out that a mechanical strength distributed in a narrow window, i.e., with a high Weibull modulus, is beneficial to industrial applications of solid catalysts.

Investigation of Shear-Peel Bond Strength of Orthodontic Brackets on Enamel after Using Pro Seal™

The aim of this investigation was to ascertain whether the use of a fluoride-releasing, light-curing sealant (Pro Seal, Reliance Orthodontic Products, Itasca, IL, USA) results in differences in shear-peel bond strength when teeth are etched via the conventional acid-etching method and with self-etching primers (SEP), respectively. We also examined whether Pro Seal functions as a substitute for the bonding agents recommended by the manufacturers. To carry out this study, we randomly allocated 300 extracted molars (ISO 11405) into twelve groups (n=25). The brackets used (Victory Series, Twin UBi 0T/0A .022, 3M Unitek, Seefeld, Germany) were bonded using the same layer thickness of adhesive. The teeth were then stored for 24 h in de-ionized water (37 degrees C) and subsequently thermocycled. The brackets were debonded using a universal testing machine with a crosshead speed of 0.5 mm/min. For statistical evaluation we used the one-way analysis of variance (ANOVA [alpha = 0.05]) and the post hoc Tukey test and Weibull analysis. We could establish that the use of Pro Seal does not have a negative influence on shear-peel bond strength. When the compomer Assure was used, Pro Seal even led to an increase in shear-peel bond strength, especially when it was applied using the bonding agent recommended by the manufacturer. The highest Weibull modulus (m) was found for the self-etching primer IDEAL 1 without using Pro Seal (m=6.5) and the lowest for First Step SEP when using Pro Seal (m=2.2). There were also clear differences in characteristic shear-peel bond strength (sigma(0)) within some of the groups investigated. Although the statistical evaluation showed no significant negative influence on the shear-peel bond strength either when Pro Seal was used in addition, or when that fluoride-releasing, light-curing sealant was substituted for the bonding agent recommended by the manufacturer, the Weibull analysis showed that the use of Pro Seal in combination with SEP resulted in bond strength values that were less reliably reproducible.

種結晶を用いた窒化ケイ素セラミックスの微構造制御と機能発現

It has been reported that the size and distribution of β-Si3N4 particles existing in silicon nitride raw powder are crucial factors governing the microstructure of silicon nitride ceramics. Considering such the grain growth behavior, a processing strategy for controlling the microstructure of silicon nitride ceramics has been developed. The key point of this process is to control the size, distribution and orientation of large elongated β-Si3N4 grains via seeding of morphologically regulated β-Si3N4 single crystal particles. Silicon nitrides with textured microstructure where elongated β-Si3N4 grains were unidirectionally or two-dimensionally orientated could be fabricated by combining the seeding with forming methods of extrusion or tape casting. The textured silicon nitride exhibited superior mechanical properties such as high strength, high fracture toughness, high creep resistance, and had a high Weibull modulus of about 50. Moreover it has been revealed that the material exhibits unique tribological and thermal properties.

Multilayer coatings with improved reliability produced by aqueous electrophoretic deposition

Laminates in the system Al 2O 3/Y-TZP have been produced by an aqueous electrophoretic deposition (EPD) process combined with the use of thermogelling polysaccharides. Mixtures with relative volume ratios of Al 2O 3 to Y-TZP of 95/5, 60/40 and 0/100 have been studied in terms of colloidal stability, gelation behaviour and EPD kinetics. A graded symmetrical architecture, with external layers of A95Z5, and intermediate layers of A60Z40 sandwiching a layer of A0Z100 has been designed by controlling the sequential EPD kinetics of each suspension. The fracture strength of monoliths and laminates has been studied by three point bending and Weibull analysis. Even though the multilayer does not present strength values significantly higher than those of the A95Z5 monolith it presents enhanced reliability, as demonstrated by a higher Weibull modulus. Reliability of the laminate, under critical fracture conditions such as those involved in strength testing, is mostly due to a strong effect of the reduced thickness of the A95Z5 layers as compared to total thickness of the A95Z5 monolith.

Effects of the entrained surface film on the reliability of castings

The tilt pouring and gravity top pouring of an Al-4.5 pct Cu alloy have been studied. A computercontrolled rollover casting wheel was used to perform the tilt pouring. Filling sequences with tranquil or turbulent flow patterns have been visualized using real-time X-ray video radiography and modeled using a computational fluid dynamics (CFD) code. The area of the free surface film entrained into the bulk of liquid metal in different filling conditions has been calculated using a filling sequence free from surface turbulence as a baseline. The tensile properties of the castings have been quantitatively assessed for reliability using a two-parameter Weibull distribution function. The study reveals that the liquid metal flow in the mold filling process can be accurately simulated using a CFD code. In addition, the computed total surface area of the entrained surface film can be used as a criterion to judge the deterioration of reliability. The high Weibull modulus achieved by filling a mold without surface turbulence was reduced by a factor of 2.5 of its original value by entrained surface films. Entrainment of bubbles required surface turbulence, but folded films could be entrained simply by contraction of the free surface, creating excess surface film that necessarily folds inward.

Strength and Reliability of Porous Ceramics Measured by Sphere Indentation on Bilayer Structure

The importance of porous ceramics has been increasingly recognized and adequate strength of porous ceramics is now required for structural applications. Porosities of porous ceramics act as flaws in inner volume and outer surface which result in severe strength degradation. The effect of pore structure, however, on strength and reliability of porous ceramics has not been clearly understood. We investigate the relationship between pore structure and mechanical properties using a sphere indentation on bilayer structure, porous ceramic top layer with soft polymer substrate. Porous alumina and silica were prepared to characterize the isolated pore structure and interconnected pore structure, respectively. The porous ceramic with 1mm thickness were bonded to soft polycarbonate substrate and then fracture strengths were estimated from critical loads for radial cracking of porous ceramics during sphere indentation from top surface. This simple and reproducible technique provides Weibull modulus of strength of porous ceramics with different pore structure. It shows that the porous ceramics with isolated pore structure have higher strength and higher Weibull modulus as well, than those with interconnected pore structure even with the same porosity.

Fabrication and flexural properties of Tyranno-SA/SiC composites with carbon interlayer by CVI

Strength reliability and thermal stability of SiC/SiC composites are important as structural candidates for nuclear fusion reactor. Several CVI-Tyranno-SA (plain-woven)/SiC composites were fabricated and the flexural strength and the effects of the high temperature heat treatment in vacuum were investigated. The flexural strength and statistical properties of the as-fabricated composites showed a dependence on composite density. The highest average strength, 640 ± 69 MPa, with the highest Weibull modulus, m=12.2, was obtained by the composite with the highest density among the several composites. No significant change of the strength of the composite was noticed against heat treatment at temperature up to 1973 K in vacuum of ∼3 × 10 −4 Pa for 1 h, beyond which the strength decreased with the increasing of the heat treatment temperature.

Dynamic crack propagation with cohesive elements: a methodology to address mesh dependency

AbstractIn this paper, two brittle fracture problems are numerically simulated: the failure of a ceramic ring under centrifugal loading and crack branching in a PMMA strip. A three‐dimensional finite element package in which cohesive elements are dynamically inserted has been developed. The cohesive elements' strength is chosen to follow a modified weakest link Weibull distribution. The probability of introducing a weak cohesive element is set to increase with the cohesive element size. This reflects the physically based effect according to which larger elements are more likely to contain defects. The calculations illustrate how the area dependence of the Weibull model can be used to effectively address mesh dependency. On the other hand, regular Weibull distributions have failed to reduce mesh dependency for the examples shown in this paper.The ceramic ring calculations revealed that two distinct phenomena appear depending on the magnitude of the Weibull modulus. For low Weibull modulus, the fragmentation of the ring is dominated by heterogeneities. Whereas many cracks were generated, few of them could propagate to the outer surface. Monte Carlo simulations revealed that for highly heterogeneous rings, the number of small fragments was large and that few large fragments were generated.For high Weibull modulus, signifying that the ring is close to being homogeneous, the fragmentation process was very different. Monte Carlo simulations highlighted that a larger number of large fragments are generated due to crack branching. Copyright © 2003 John Wiley &amp; Sons, Ltd.

High Weibull Modulus HVOF Titania Coatings

The mechanical behavior of high-velocity oxyfuel (HVOF) sprayed titania (TiO2) coatings was evaluated using Vickers hardness measurements on the cross section and top surface. The distribution of hardness values for the cross-section and top surface under 25, 50, 100, 300, 500, and 1000 g loads was analyzed via Weibull statistics. The coating microstructure was evaluated using scanning electron microscopy (SEM). It was observed that the microstructural features were similar in the top surface and cross-section, different from the lamellar structure commonly found in thermal spray coatings. X-ray diffraction (XRD) analysis identified rutile as the major coating phase. The in-flight sprayed particle parameters such as temperature and velocity were determined using a commercial diagnostic system based on pyrometry and time-of-flight measurements. The uniformity of the microstructure resulted in a near isotropic behavior of the mechanical properties, such as hardness, in the coating cross-section and top surface. High Weibull modulus values were observed when compared with results of other thermal spray coatings available in the literature. These initial results should contribute to a more general understanding of the conditions necessary to achieve coatings with high uniformity and assist in the engineering of coating microstructures for specific applications.

Effects of the calcination conditions on the mechanical properties of a PCoMo/Al 2O 3 hydrotreating catalyst

Mathematical models for the effects of the calcination process conditions on the mechanical properties of a PCoMo/Al 2O 3 hydrotreating catalyst are developed using a response surface methodology. A central composite design is performed to study collectively the effects of calcination temperature, calcination time and heating rate on the mean strength and Weibull's modulus. A model is obtained for each response with multiple regression analysis and then is refined. Analysis of variance reveals that the models developed are adequate. The validity of the models is also verified by experimental data. Statistics reveals that there is a great potential for increasing the mechanical reliability in the calcination process. Analysis of response surface show that the mean strength and Weibull's modulus increase with the increase of calcination temperature. The middle level of heating rate results in smaller mean strength and higher Weibull's modulus. The mean strength increases as calcination time increases. However, calcination time has no significant effect on Weibull's modulus in the experimental domain examined.