The effect of different ceramic materials on the fracture resistance of dentin-bonded crowns

Using stereomicroscopy, light microscopy, and scanning electron microscopy, we investigated the development of vasa vasorum in the proliferated neointima of the autovein graft and its anastomoses implanted in the canine femoral artery against a background of poor distal runoff. In the stereomicroscopic examination, a microfil silicone rubber compound (MF) was injected transluminally or via perivascular vasa, and the vascular specimen was prepared for clearing by immersion in a methyl-salicylate solution. Vessel interstices filled with MF were found adjacent to the suture materials within 5 days of grafting. Fourteen days after implantation, luminally originating vasa vasorum were often visible in the neointima along the suture line and distributed into the media and adventitia connecting to the original vasa vasorum. At 6 months or more after grafting, many orifices of luminally originating vasa vasorum were seen along the suture line of both proximal end-to-end and distal end-to-side anastomoses and distributed into the thickened neointima forming a vasa network when the neointima had proliferated to over 250 microm in depth. On the other hand, some clefts filled with MF were found in mural thrombi deposited on the vascular sinus of the graft within 5 days, and these appeared to be one of the sources of luminally originating vasa vasorum on the graft distant from the suture line. Moreover, the development of numerous vasa vasorum was constantly demonstrated in the neointima when it had proliferated to over 250 microm in depth.     

The effect of alloying elements and microstructure on the strength and fracture resistance of pearlitic steel

The processes of ductile and brittle fracture in fully pearlitic steel and their relation to both the scale of the microstructure and the presence of substitutional alloy elements have been investigated at room temperature using smooth tensile and over a range of temperatures using V-notched Charpy impact specimens. The results show that the early stages of cracking, revealed in both types of specimen, are largely the result of shear cracking of the pearlite lamellae. These cracks grow and can reach a size when they impinge upon the prior austenite boundary; afterward the character of fracture can be either microvoid coalescence or cleavage, depending on test conditions and metallurgical variables. Further, the carbide plates of the pearlite lamellae can act as barriers to the movement of dislocations as is the case normally with grain boundaries. For pearlite an optimum spacing of approximately 0.2 μm resulting from a balance between carbide plate thickness and interlamellar spacing was found to enhance toughness, although such changes are much smaller than corresponding changes due to varying alloy elements. Specific alloy elements used herein strengthened the lamellar ferrite in pearlite, inhibiting the movement of dislocations while also usually decreasing the lamellar cementite plate thickness for the same spacing. This dual behavior results in enhanced resistance to the initiation and propagation of microcracks leading to an improvement in strength, ductility, and toughness. The most effective alloy elements for the composition ranges studied in fully pearlitic steels are Si and Ni for strength improvement, and Ni and Mn for toughness.     

The effect of reaction products on the fracture resistance of a metal/ceramic interface

Solid state diffusion bonds between a Ta(Ti) alloy and Al₂O₃ have been used to study the effect of reaction products on the fracture resistance of a metal/ceramic interface. The reaction layer consists of brittle intermetallics, consistent with the TiTaAl ternary phase diagram. The interface fracture energy Γ_i was found to be dominated by periodic tunnel cracking of the reaction products, and was relatively insensitive to layer thickness, because of the self-similar nature of the periodic cracking. The contribution to Γ_i from plastic dissipation within the alloy was minimal because the intervening reaction layer inhibited yielding. A sequence of cracking observations was used to bound the unknown values of the fracture energy of the reaction products, the residual stress within the layer and the fracture energy of the reaction product/sapphire interface.     

The effect of matrix strength on the fracture resistance of an alpha-Beta titanium alloy,corona-5

This paper presents the results of a study of the effect of matrix yield strength, at constant Widmanstätten α microstructure, on the fracture resistance of an α-β Ti alloy, CORONA-5. Fracture initiation resistance,J _q, and the stable crack growth resistance,T, were evaluated by the single specimen, unloading compliance method for four different microstructures and three yield strengths. The microstructures involved coarse or fine Widmanstätten α particles in a heat treated β-matrix; the yield strength ranged from 765 to 1018 MPa. It was found thatJ _q/σ_Y, where σ_Y is the effective yield strength, decreased with increasingσ _Y.T/σ _Y also decreased with increasing σ_Y for fine structures. For the coarse α structures, however, T/σ_Y revealed intermediate maxima. Coarser structures, in general, revealed higher values ofJ _q/σ_Y andT/σ _Y. The cause was found primarily to be due to the effect of increased α particle thickness. The effect of grain size was secondary. J_q/σ_Y increased with increasing tensile strain hardening rate, obtained at the onset of void nucleation. T/σ_Y was found to decrease with increasing tensile void growth rate. In general, J_q/σ_Y and T/σ_Y revealed different relationships with microstructure. Fatigue precrack front- and the stable crack length-tortuosities did not yield any general relationship to fracture resistance at different yield strengths.     

The fracture resistance of dental materials

A novel test procedure was developed that provides the basis for a standard plain strain fracture toughness test to evaluate fracture resistance of resin-based materials. This procedure utilizes a notched disc specimen. Using this technique, a new concept of Torque to initiate fracture (T)' was suggested. Also, fracture resistance can be assessed by means of critical stress intensity factor (K[IC]). Using this method, the fracture resistance of direct restorative materials, resin-based inlay/onlays and luting cements can be evaluated.     

The fracture resistance of metal-ceramic interfaces

The effects of interface structure and microstructure on the fracture energy, Γ_i, of metal-ceramic interfaces are reviewed. Some systems exhibit a ductile fracture mechanism and others fail by brittle mechanisms. In the absence of either interphases or reaction products, Γ_i is dominated by plastic dissipation (for both fracture mechanisms), leading to important effects of metal thickness, h, and yield strength, σ₀. Additionally, Γ_i is larger when fracture occurs by ductile void growth (for the same h and σ₀). A fundamental understanding now exists for the ductile fracture mechanism. However, some basic issues remain to be understood when fracture occurs by brittle bond rupture, particularly with regard to the role of the work of adhesion, W_ad. Interphases and reaction products have been shown to have an important effect on Γ_i. A general trend found by experiment is that Γ_i scales with the fracture energy of the interphase itself, wherein Γ_i tends to increase for the interphase sequence: amorphous oxides > crystalline oxides > intermetallics. However, there also appear to be important effects of the residual stresses in the interface (which influence the fracture mechanism and the layer thickness.     

The effect of dentin primer on the tensile bond strength to human enamel

Four third-generation dentin bonding products (Scotchbond Multi-Purpose, Optibond, All-Bond 2, and Prisma Universal Bond 3) were tested to evaluate their tensile bond strength to enamel. Test enamel specimens were etched, primed, and polymerized according to each manufacturer's directions. Control specimens were treated identically except the primer application was eliminated. The results demonstrated that the dentin primer significantly increased the tensile bond strength of All-Bond 2, significantly decreased the tensile bond strength of Scotchbond Multi-Purpose and Optibond, and had no significant effect for Prisma Universal Bond 3. A one-way analysis of variance was run between the eight groups tested, and three significant subsets were found (P < .05). The subset with the highest mean tensile bond strengths consisted of Prisma Universal Bond 3 primed and nonprimed, All-Bond 2 primed, and Optibond nonprimed.     

The effect of constraint on elastic-plastic fracture

The influence of the local constraint in the near field zone of cracks in terms of the parameter σ_m/σ_v = f (r, θ, s) on fracture initiation and fracture resistance is investigated for several crack configurations in specimens and components. Experimental results from various authors are compared with corresponding numerical findings. Methods to transfer test results obtained for specimens to more complex situations in components are reported.     

Laser-induced photoacoustic effects in the dentin]

To study photoacoustic effects in dentin caused by UV-laser ablation, laser-induced shockwaves were measured using piezoelectric PVDF films. Above the tissue-specific energy threshold for photoablation the amplitude of the acoustic shock waves is proportional to the applied laser energy density. Laser energy densities of 2 J/cm2 cause pressure amplitudes of 50 bar, densities of 20 J/cm2 cause pressure amplitudes of 1000 bar. To avoid microcracks in dentine the maximum laser energy density to prepare dentin should be limited to approximately 20 J/cm2.     

Influence of interface impurities on the fracture energy of UHV bonded niobium-sapphire bicrystals

Defined interface doping is used to study the influence of segregants on the fracture energy of metal-ceramic interfaces. In a doping chamber connected to a UHV diffusion boding facility, sputter cleaned surfaces were covered with thin films of dopant in the range between submonolayers and nanometers. A quartz oscillator monitors the film thickness. After surface doping of one of the two surfaces to be bonded, the interface is produced by pressure bonding. Fracture measurements and AES measurements on in situ fractured samples are used to obtain a relationship between the fracture energy and the interface concentration of the impurity. Niobium-sapphire bicrystals were chosen for the first experiments because of their similar thermal expansion behaviour. Bicrystals with three different crystallographic orientations were studied. The segregating elements were silver and sulphur. Both elements are insoluble in niobium and sapphire. With increasing layer thickness and increasing interface concentration of silver the fracture energy decreased. Depth profiling of UHV fractured samples showed that the concentration profiles are similar to those of natural segregation. Silver reduces the fracture energy according to the models of Nicholas and Seah and Hondros. Sulphur was present in the bulk of the niobium and was enriched at the metal surface by segregation anneals prior to bonding. It shows a stronger tendency to segregate to free niobium (100) surfaces than to niobium-sapphire interfaces. Sulphur behaves as proposed for cohesion-reducing impurities in the model of Rice and Wang.     

The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

The effect of microetching on the bond strength of metal brackets when bonded to previously bonded teeth: an in vitro study

The purpose of this in vitro study was to answer the following questions for three different metal brackets: (1) when rebonding a previously bonded tooth, how do shear bond strengths compare for new brackets, new microetched brackets, and debonded microetcher cleaned brackets? and (2) how do the different bracket types compare with respect to the time required to remove composite resin from their bonding pads with a microetcher? Ninety human premolars and canines previously debonded of metal brackets were randomly assigned to 9 groups of 10 teeth each. New, new etched, and debonded etched cleaned brackets of each type were bonded with composite resin onto teeth, and the bonds tested to failure for shear bond strength. An analysis of variance and Duncan's multiple range test were used to compare bracket/enamel bond strength. Within each bracket type no significant differences were found between mean bond strengths for new, new etched, and debonded etched conditions, a finding that supports the use of microetching to clean accidentally debonded brackets. Mean times for removal of resin from bonding pads with a microetcher varied from 9.3 seconds to 11.9 seconds, with bracket (M) requiring significantly less time for removal of resin.     

The effect of dentin disinfectants on shear bond strength of resin-modified glass-ionomer materials

The purpose of this study was to investigate the effect of dentinal disinfection with a 2% chlorhexidine or a 0.11% I2-KI/CuSO4 solution on the shear bond strength of three resin-modified glass-ionomer cements: Fuji II LC, Photac-Fil, and Vitremer. The occlusal surfaces of extracted human teeth were flattened to dentin. Specimens were randomly assigned to one of nine treatment groups (n = 12). For each glass-ionomer material, there was a control group and two treatment groups in which the dentin was treated with either a 2% chlorhexidine or a 0.11% I2-KI/CuSO4 solution before the dentin was treated with the recommended dentin conditioner prior to glass-ionomer bonding. Specimens were stored for 1 day in water, thermocycled, and tested in shear until failure. The chlorhexidine solution did not significantly affect the shear bond strengths of any of the cements, but the I2-KI/CuSO4 solution significantly lowered the bond strengths of Vitremer and Fuji II LC compared to the controls.     

The effect of hydrogen on the fracture of alloy x-750

The effect of hydrogen on the fracture of a nickel-base superalloy, alloy X-750, was investigated in the HTH condition. The effect of hydrogen was examined through tensile testing incorporating observations from scanning electron microscopy and light microscopy. The ductility at 25 °C, as measured by elongation to failure for tensile specimens, was reduced from 21 pct for noncharged specimens to 7.3 pct for 5.7 ppm hydrogen and to 3.5 pct for 65 ppm hydrogen. The elongation to failure was a function of the strain rate and test temperature. For hydrogen-charged specimens, the elongation decreased as the strain rate decreased at a constant temperature, while for a constant strain rate and varying temperature, there was a maximum in embrittlement near 25 °C and no embrittlement at -196 °C. For the noncharged specimens, the elongation monotonically increased as temperature increased, while there was no noticeable effect of strain rate. Prestraining prior to charging dramatically decreased elongation after hydrogen charging. When the strain rate was increased on the prestrained specimens, more plastic deformation was observed prior to failure. Failure did not occur until the flow stress was reached, supporting the proposition that plasticity is required for failure. The intergranular failure mechanism in alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. The void initiation strain, as determined from tensile data and from sectioning unfractured specimens, was observed to be much lower in the hydrogen-charged specimens as compared to noncharged specimens. The reduced ductility may be explained by either a reduction of the interfacial strength of the carbide-matrix interface or a local hydrogen pressure at the carbide-matrix interface.     

The fracture resistance of a binary TiAl alloy

The fracture resistance of a binary Ti-47Al (in at. pct) alloy has been investigated. The binary alloy was cast, forged, and heat treated to a fully lamellar microstructure with a colony size of either 640 or 1425 μm. Fracture toughness tests were performed in a scanning electron microscope (SEM) equipped with a loading stage. Direct observations of the fracture process indicated that crack extension commenced at a stress intensity level of 1.2 to 4 MPa√m. The crack path was primarily interlamellar and crack extension across an individual colony or across similarly oriented colonies was relatively easy. In contrast, crack arrest was prevalent when the crack encountered the boundaries of unfavorably oriented colonies. To extend into an unfavorably oriented neighboring colony, the K level of the approaching crack had to be increased significantly to renucleate a microcrack at a location away from the crack tip, resulting in the formation of an interconnecting ligament that must be fractured to further crack growth. This interaction between the crack and the microstructure led to a large variation in the slope of the K _R curves. Comparison of the K _R curves for the binary Ti-47Al alloy against published data for quinary Ti-47Al-xNb-yCr-zV alloys indicates that the initiation toughness of the quinary alloys is higher by a factor of 5 to 10, implying the existence of a significant beneficial effect of alloying additions on the initiation toughness.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 . Charging with hydrogen decreased the fracture toughness, K _lc, to 52 at a rapid loading rate and further decreased the toughness to 42 for a slow loading rate. This is consistent with the rate-limiting step forthe embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.