Increase In Fracture Strength Research Articles

Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement

ObjectivesTo determine, by means of static fracture testing the effect of the tooth preparation design and the elastic modulus of the cement on the structural integrity of the cemented machined ceramic crown-tooth complex. MethodsHuman maxillary extracted premolar teeth were prepared for all-ceramic crowns using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita MK II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3). The machined all-ceramic crowns were resin bonded to the tooth structure using one of three cements with different elastic moduli: Super-Bond C&B, Rely X Unicem and Panavia F 2.0. The specimens were subjected to compressive force through a 4mm diameter steel ball at a crosshead speed of 1mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the fracture point was recorded for each specimen in Newtons (N). These values were compared to a control group of unprepared/unrestored teeth. ResultsThere was a significant difference between the control group, with higher fracture strength, and the cemented samples regardless of the occlusal design and the type of resin cement. There was no significant difference in mean fracture load between the two designs of occlusal preparation using Super-Bond C&B. For the Rely X Unicem and Panavia F 2.0 cements, the proposed preparation design with a flat occlusal morphology provides a system with increased fracture strength. SignificanceThe proposed novel flat design showed less dependency on the resin cement selection in relation to the fracture strength of the restored tooth. The choice of the cement resin, with respect to its modulus of elasticity, is more important in the anatomic design than in the flat design.

Effect of Carbon Nanotube Dispersion on Mechanical Properties of Aluminum-Silicon Alloy Matrix Composites

This study has been carried out to reinforce the commonly believed fact that the dispersion of carbon nanotubes in a composite has a profound effect on the properties of the composite. In this study, ball milling was carried out using two different parameters to obtain distinctly different degrees of dispersion of carbon nanotubes (4 wt.%) in Al-9 wt.% Si powders. Composite disks, 80 mm in diameter, having good and bad dispersions of carbon nanotubes were obtained by hot pressing. Optical micrographs and Raman spectroscopy images showed the presence of larger carbon nanotube clusters in the bad dispersion sample. Transmission electron microscopy images confirmed the presence of large clusters in the bad dispersion sample, while the good dispersion sample showed individual carbon nanotubes in the Al matrix. Nanoindentation results indicated a 41% increase in the hardness and a 27% increase in the elastic-to-plastic work ratio, while compression tests indicated a 185% increase in compression yield strength and a 109% increase in fracture strength with improvement in carbon nanotube’s dispersion.

Controlling Mechanical Properties of Cell‐Laden Hydrogels by Covalent Incorporation of Graphene Oxide

Graphene-based materials are useful reinforcing agents to modify the mechanical properties of hydrogels. Here, an approach is presented to covalently incorporate graphene oxide (GO) into hydrogels via radical copolymerization to enhance the dispersion and conjugation of GO sheets within the hydrogels. GO is chemically modified to present surface-grafted methacrylate groups (MeGO). In comparison to GO, higher concentrations of MeGO can be stably dispersed in a pre-gel solution containing methacrylated gelatin (GelMA) without aggregation or significant increase in viscosity. In addition, the resulting MeGO-GelMA hydrogels demonstrate a significant increase in fracture strength with increasing MeGO concentration. Interestingly, the rigidity of the hydrogels is not significantly affected by the covalently incorporated GO. Therefore, this approach can be used to enhance the structural integrity and resistance to fracture of the hydrogels without inadvertently affecting their rigidity, which is known to affect the behavior of encapsulated cells. The biocompatibility of MeGO-GelMA hydrogels is confirmed by measuring the viability and proliferation of the encapsulated fibroblasts. Overall, this study highlights the advantage of covalently incorporating GO into a hydrogel system, and improves the quality of cell-laden hydrogels.

Effects of phosphorus doping on the bending strength of a single crystal silicon microbeam

The effects of phosphorus doping on the bending strength of a single crystal silicon (SCS) microbeam are reported. First, a specially designed bending microstructure test and a simple bending test device are presented. Secondly, the results of bending strength tests for six groups of specimens with different phosphorus doping concentrations are presented. The test results are comparatively analysed using a Weibull distribution, revealing that the bending strength has a monotonically increasing relationship with the phosphorus concentration. The effect of surface roughness on the bending strength is also investigated. Finally, the mechanism underlying this phenomenon is demonstrated. The results indicate that the bond strength of a phosphorus–silicon pair is larger than that of a silicon–silicon pair, and the phosphorus atoms increase the energy necessary to create new surfaces. Therefore more energy is required to fracture the microbeam, and the fracture strength increases. Research on this topic therefore shows some practical significance for improving the bending strength of a SCS microstructure.

Enhancing graphene reinforcing potential in composites by hydrogen passivation induced dispersion

To take full advantages of the structural uniqueness and exceptional properties of graphene as reinforcement in composites, harvesting well-dispersed graphene is essential. On the other hand, it is challenging to achieve simultaneously high stiffness, strength and toughness in engineered materials because of the trade-off relations between these properties. Here we demonstrate that the graphene reinforcing potential can be significantly enhanced through the excellent dispersion of graphene sheets in the matrix material and the strong graphene-matrix bonding by the coupled hydrogen passivation and ultrasonication technique. The fabricated graphene/epoxy composites exhibit simultaneously remarkable increase in elastic modulus, fracture strength, and fracture energy. We found that the inlet hydrogen atoms in the hydrogen passivation serve as a source of the second atoms to terminate the C dangling bonds and form more stable C-H bonds, separating graphene flakes and promoting the binding with the matrix material.

Effect of two different types of fibers on the fracture resistance of endodontically treated molars restored with composite resin.

To compare the in vitro fracture resistance of endodontically treated molars with mesio-occluso-distal (MOD) cavities restored with two different types of fibers. Sixty extracted human mandibular first molars were selected for the study and randomly assigned to six groups (n = 10). Group 1 served as the control. In groups 2 through 6, endodontic access and standard MOD cavities were prepared. Following root canal treatment, group 2 was left unrestored. In group 3, the teeth were restored with composite resin (Venus, Heraeus Kulzer). In group 4, flowable composite resin (Venus, Heraeus Kulzer) was used before restoring the teeth with composite resin. In group 5, leno-woven ultrahigh molecular weight polyethylene ribbon fiber (Ribbond) was inserted in the flowable resin in a buccal to lingual direction, and the teeth were then restored with composite resin. In group 6, translucent glass fiber (Vectris, Ivoclar) was adapted over the flowable resin in the bucco-lingual direction and restored with composite resin. The specimens were stored in 100% humidity at 37°C for 1 day. Compressive loading of the teeth was performed using a universal testing machine at a crosshead speed of 0.5 mm/min. The mean load necessary to fracture the samples was recorded in Newtons (N). Data were subjected to analysis of variance (ANOVA) and Duncan's post-hoc test, where significance was set at p < 0.001. The highest and the lowest mean fracture strengths were found in sound teeth (1598.8 N) and unrestored teeth (393.7 N), respectively. The mean load necessary to fracture the samples was 958.6 N in the polyethylene ribbon group (group 5), 913.2 N in the glass-fiber group (group 6), 699.7 N in teeth restored with flowable resin and composite (group 4), and 729.3 N in group 3 with composite resin alone. Statistical analysis showed significantly higher fracture resistance of both the fiber groups compared to composite resin alone (p < 0.001). Both polyethylene ribbon and glass fiber under MOD composite restorations significantly increased fracture strength with no statistical difference between the two groups. Therefore, both polyethylene- and glassfiber- reinforced composites can be used for access cavity restorations in teeth with weakened cusps.

Effects of Surface Chemical Structure on the Mechanical Properties of Si1–xGex Nanowires

The Young's modulus and fracture strength of Si(1-x)Ge(x) nanowires (NWs) as a function of Ge concentration were measured from tensile stress measurements. The Young's modulus of the NWs decreased linearly with increasing Ge content. No evidence was found for a linear relationship between the fracture strength of the NWs and Ge content, which is closely related to the quantity of interstitial Ge atoms contained in the wire. However, by removing some of the interstitial Ge atoms through rapid thermal annealing, a linear relationship could be produced. The discrepancy in the reported strength of Si and Ge NWs between calculated and experimented results could be related to SiO(2-x)/Si interfacial defects that are found in Si(1-x)Ge(x) NWs. It was also possible to significantly decrease the number of interfacial defects in the NWs by incorporating a surface passivated Al2O3 layer, which resulted in a substantial increase in fracture strength.

Toughened carbon nanotube–iron–mullite composites prepared by spark plasma sintering

Carbon nanotube–iron–mullite nanocomposite powders were prepared by a direct method involving a reduction in H 2 –CH 4 and without any mechanical mixing step. The carbon nanotubes are mostly double- and few-walled (3–6 walls). Some carbon nanofibers are also observed. The materials were consolidated by spark plasma sintering. Their electrical conductivity is 2.4 S/cm whereas pure mullite is insulating. There is no increase in fracture strength, but the SENB toughness is twice than the one for unreinforced mullite (3.3 vs. 1.6 MPa m 1/2 ). The mechanisms of carbon nanotube bundle pullout and large-scale crack-bridging have been evidenced.

Factors Affecting the Fracture Strength of Freestanding Diamond Films

As an emerging brand new type of engineering material for a variety of important high technology applications, the deep understanding of the mechanical behavior of freestanding diamond films has become an emergent task of vital importance. Unfortunately the mechanical behavior of this brand new material is not fully understood. Effects that affect the fracture strength are still not very clear except that the fracture strength of freestanding diamond films is only depended on the grain size (film thickness), and is insensitive to the microscopic defects, and the strength is considerably higher when the nucleation side is in tension than that when the growth side is in tension. However, this is not the full story. Based on the experimental date accumulated in USTB (University of Science and Technology Beijing) for high power dc arcjet diamond films, other factors that may affect the fracture strength of freestanding diamond films are discussed in detail. Effects of the quality of the diamond films, impurities (nitrogen, hydrogen, non-diamond carbon etc.), film morphology and texture, on fracture strength are discussed. Effects of the deposition parameters are explained. Advantages for small amount of nitrogen addition, and for the use of higher substrate temperatures in the increase in fracture strength are demonstrated, which have already been applied in the mass production of tool grade freestanding diamond films. It is hoped that the present paper will be helpful for those who wish to understand and use this brand new type of engineering material

Волокнистая ультрамелкозернистая структура и свойства прокатанной низкоуглеродистой стали 12ГБА

It is shown that the intensive warm rolling leads to the formation of ultrafinegrained (UFG) structure with an average grain size 0.5 in transverse and to two times increase in strength by comparison to the initial coarse-grained (CG) state. Impact tests have shown increase of fracture strength in fibrous UFG structure at negative temperatures. It was also established that the type of fracture in UFG state remains ductile until -80 deg. whereas in CG material a brittle rupture is observed at this temperature. <br />

Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior

This paper deals with the uniaxial compression behavior of porous ceramics within a wide range of porosity, varying from 30 to 75 vol%. The load–displacement curves recorded on porous alumina samples showed a transition between a typical brittle behavior at porosity fractions below 60 vol% and a damageable, cellular-like behavior, at higher porosity fractions. This transition in fracture mode was confirmed by in situ compression tests in an X-ray tomograph. Based on a simple model taking into account the competition between the crack length initiating from spherical pores and the mean distance between pores, the porosity at which the transition took place was estimated. The influence of the pore size also depended on the volume fraction of pores: no size effect was noted at the lowest porosity whereas a statistical effect on the size of the solid walls was observed at higher porosity, with an increase in fracture strength with small pores.

Yacon Flour andBifidobacterium longumModulate Bone Health in Rats

Yacon flour has been considered a food with prebiotic potential because of the high levels of fructooligosaccharides, which allows for its use in formulating synbiotic foods. The purpose of this study was to evaluate the effect of yacon flour and probiotic (Bifidobacterium longum) on the modulation of variables related to bone health. Thirty-two Wistar rats were divided into 4 groups: control, yacon flour, diet+B. longum, and yacon flour+B. longum. After euthanasia, the bones were removed for analysis of biomechanical properties (thickness, length, and strength of fracture) and mineral content (Ca, Mg, and P); the cecum was removed for analysis of the microbiota and short-chain fatty acids. Tibia Ca, P, and Mg content was significantly (P<.05) higher in groups fed diet+B. longum, yacon flour+B. longum than in the control group. An increase in fracture strength was observed in the yacon flour (8.1%), diet+B. longum (8.6%), and yacon flour+B. longum (14.6%) in comparison to the control group. Total anaerobe and weight of the cecum were higher (P<.05) in rats consuming the yacon flour diet compared with the other groups. Cecal concentration of propionate was higher in all experimental groups compared with the control (P<.05). Yacon flour in combination with B. longum helped increase the concentration of minerals in bones, an important factor in the prevention of diseases such as osteoporosis.

In vitro evaluation of the effect of post system and length on the fracture resistance of endodontically treated human anterior teeth

The aim of this study was to evaluate the effect of post system and length on the fracture resistance of endodontically treated human anterior teeth. Seventy-five extracted human incisors were endodontically treated, out of which 60 were decoronated 2mm above the cementoenamel junction and divided into two experimental groups based on the type of post system to be used: glass fiber post (GFP) and Ribbond fiber post groups (RFP). Endodontically treated human anterior teeth in which no post was placed served as control group. Each group was divided into two subgroups according to the length of post space: 5 and 10mm and all the samples were restored with metal crowns. The fracture resistance was measured by applying loads at an angle of 130° to the long axis of teeth in an Instron universal testing machine. The results revealed that GFP group at 10-mm post space length showed the significantly highest fracture resistance (740.2133N) among all groups and subgroups. Decrease in post length resulted in the decrease in fracture resistance in GFP group (425.1867N), whereas in group RFP 5-mm subgroup (299.6200N) showed significantly higher fracture resistance than 10-mm subgroup (216.9300N) but lesser than the control (437.8733N) in both the subgroups. Glass fiber posts efficiently increase the fracture resistance of an endodontically treated tooth but the determination of optimal post length is also essential. The present investigation highlights the significance of using glass fiber posts in the restoration of endodontically treated teeth. Endodontically treated teeth restored with glass fiber posts showed increased fracture strength and favorable mode of fracture, and are therefore highly recommended to achieve better clinical outcomes.

Structure and Mechanism of Strength Enhancement in Interpenetrating Polymer Network Hydrogels

Hydrogels with high modulus and fracture strength are obtained by interpenetrating a tightly cross-linked poly(ethylene glycol) (PEG) network with a loosely cross-linked poly(acrylic acid) (PAA) network. Small-angle X-ray and neutron scattering (SAXS/SANS) are used in conjunction with swelling measurements to determine the structure of PEG/PAA interpenetrating polymer networks (IPNs) and to measure the average PEG chain extension within the IPN. At pH 7.4, the PEG chains within the IPN are extended to 45–70% of their maximum achievable length as a result of expansion of the ionized PAA network within the IPN. Near these high extension ratios, the force required to further strain the PEG chains is increased due to the entropic effects of finite chain extensibility. This leads to PEG/PAA IPN hydrogels with a 3-fold increase in both compressive modulus and fracture strength compared to PEG single networks with the same polymer volume fraction. The structure, mechanical properties, and mechanisms of strength enhancement for PEG/PAA IPN hydrogels are notably different than for the high toughness double-network hydrogels previously described by Gong et al.

Molecular dynamics study on diamond nanowires mechanical properties: Strain rate, temperature and size dependent effects

Using molecular dynamics simulations, strain rate, temperature and size dependent mechanical properties of < 001 > orientation diamond nanowires are investigated. It is found that, for the same cross-sectional areas, strain rates have almost no effect on yield strength and Young's modulus, provided strain rates are within the range from 0.001 to 0.025 ps − 1 . Our calculated results have also indicated that, at the temperature ranging from 100 to 500 K, diamond nanowires' yield strength, Young's modulus, fracture strength and fracture strain are all decreasing with increasing temperature. Furthermore, at the temperature of 300 K, yield strength, Young's modulus, fracture strength and fracture strain increase dramatically with increasing cross sectional area. Finally, orientation dependent diamond nanowires mechanical properties are studied.

Characterisation of interface of steel/magnesium FSW

Mild steel with/without zinc coating and magnesium alloy AZ31B were lap joined by friction stir welding during the tool plunging through the magnesium into the steel. With increasing welding speed, the fracture strength increases. At the interface of zinc coated steel and magnesium, a liquid eutectic layer was detected for higher but not for lower welding speeds. At the interface between the steel without zinc coating and magnesium, no melting could be detected.

Correlation of Thermal Conduction Properties With Mechanical Deformation Characteristics of a Set of SiC–Si3N4 Nanocomposites

New developments in high temperature ceramic materials technology have focused on obtaining nanocomposite materials with nanoscale features for an optimal control of thermal and mechanical properties. One example is the silicon carbide (SiC)–silicon nitride (Si3N4) nanocomposites with nanosized SiC particles placed either in microsized Si3N4 grains or along Si3N4 grain boundaries (GBs). This work focuses on analyzing the influence of GBs, interfaces, and impurities on thermal and mechanical properties of a set of SiC–Si3N4 nanocomposites at three different temperatures (300 K, 900 K, and 1500 K). Nanocomposite thermal conductivity values predicted in this study are smaller in comparison to the bulk Si3N4 values (∼30 W/m K). Even with the volume fraction of SiC phase being limited to maximum 40%, it is shown that the thermal conductivity values could be reduced to less than those of the bulk SiC phase (∼3 W/m K) by microstructural feature arrangement. Nanocomposite phonon spectral density values show a short rage structural order indicating a high degree of diffused phonon reflection. Visual analyses of the atomistic arrangements did not reveal any loss of crystallinity in the nanocomposites at high temperatures. This indicates that structural arrangement, not the phase change, is a factor controlling thermal conduction as a function of temperature. The nanocomposite deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si3N4–Si3N4 GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. GBs also contribute to reduction in thermal conductivity as well as increase in fracture strength. Replacement of sharp GBs by diffused GBs having C/N impurities, lowered thermal conductivity, and increased fracture strength. Decrease in SiC–Si3N4 interfaces by removal of SiC particles tends to favor an increase in thermal conductivity as well as fracture resistance. Overall, it is shown that for high temperature mechanical strength improvement, judicious placement of SiC particles and optimal control of GB atomic volume fraction are the main controlling factors.

Effect of thickness reduction on mechanical property and microstructure of Zr-based bulk metallic glass during warm-rolling in the supercooled liquid region

Warm rolling of Zr-based bulk metallic glass (BMG) alloy was carried out in the supercooled liquid region (SLR) to produce thin strips of BMG alloy. Warm-rolling caused a change in the mechanical properties of the BMG alloy. Rolling deformation up to a 50% thickness reduction led to an increase in fracture strength of the BMG alloy at room temperature. This increase in strength of the warm-rolled samples was attributed to a decrease in the free volume with deformation.

Effect of grain refinement on phase transformation behavior and mechanical properties of Cu-based alloy

A small amount of misch metal was added to Cu-Zn-Al alloy in order to study its effect on grain refinement, mechanical properties, phase transformation behavior and stabilization of martensite. It is found that the addition of misch metal is very effective for reducing the grain size. The coarse grains over 1 000 μm are refined to the size of 30 μm by the addition of 0.43% (mass fraction) misch metal. The grain size of thermo-mechanically treated alloys is barely affected by cold working. The fracture strength and ductility increase significantly with the increase of misch metal content when tensile test is carried out below M f temperature. Also, the fracture strength is larger in the case of post-quench ageing treatment than that in the case of direct quench ageing treatment. The fracture mode is changed from transgranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal.

Reverse size effect in the fracture strength of brittle thin films

The Weibull size effect, consisting of an increase in fracture strength with decreasing specimen size, constitutes the basis of the probabilistic design of brittle structures. Here we present evidence of a reverse size effect in submicron amorphous carbon films. Underlying this effect is the presence of critical defects of comparable size in all films. This leads to higher stress intensity factors, and concomitant decrease in strength, with decreasing thickness. The results have implications concerning the mechanical reliability of submicron scale devices.