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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The hypothesis under examination in this paper is that the lifetimes of dental restorations are limited by the accumulation of contact damage during oral function; and, moreover, that strengths of dental ceramics are significantly lower after multi-cycle loading than after single-cycle loading. Accordingly, indentation damage and associated strength degradation from multi-cycle contacts with spherical indenters in water are evaluated in four dental ceramics: "aesthetic" ceramics-porcelain and micaceous glass-ceramic (MGC), and "structural" ceramics-glass-infiltrated alumina and yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). At large numbers of contact cycles, all materials show an abrupt transition in damage mode, consisting of strongly enhanced damage inside the contact area and attendant initiation of radial cracks outside. This transition in damage mode is not observed in comparative static loading tests, attesting to a strong mechanical component in the fatigue mechanism. Radial cracks, once formed, lead to rapid degradation in strength properties, signaling the end of the useful lifetime of the material. Strength degradation from multi-cycle contacts is examined in the test materials, after indentation at loads from 200 to 3000 N up to 10(6) cycles. Degradation occurs in the porcelain and MGC after approximately 10(4) cycles at loads as low as 200 N; comparable degradation in the alumina and Y-TZP requires loads higher than 500 N, well above the clinically significant range.
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PMID:Lifetime-limiting strength degradation from contact fatigue in dental ceramics. 1072 73

The fatigue properties of fine-grain Y-TZP in cyclic flexural testing are studied. Comparative tests on a coarser-grain alumina provide a baseline control. A bilayer configuration with ceramic plates bonded to a compliant polymeric substrate and loaded with concentrated forces at the top surfaces, simulating basic layer structures in dental crowns and hip replacement prostheses, is used as a basic test specimen. Critical times to initiate radial crack failure at the ceramic undersurfaces at prescribed maximum surface loads are measured for Y-TZP with as-polished surfaces, mechanically predamaged undersurfaces, and after a thermal aging treatment. No differences in critical failure conditions are observed between monotonic and cyclic loading on as-polished surfaces, or between as-polished and mechanically damaged surfaces in monotonic loading, consistent with fatigue controlled by slow crack growth. However, the data for mechanically damaged and aged specimens show substantial declines in sustainable stresses and times to failure in cyclic loading, indicating an augmenting role of mechanical and thermal processes in certain instances. In all cases, however, the sustainable stresses in the Y-TZP remain higher than that of the alumina, suggesting that with proper measures to avoid inherent structural instabilities, Y-TZP could provide superior performance in biomechanical applications.
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PMID:Fatigue and damage tolerance of Y-TZP ceramics in layered biomechanical systems. 1536 41

A study has been made of the effects of sandblasting on the strength of Y-TZP and alumina ceramic layers joined to polymeric substrates and loaded at the top surfaces by a spherical indenter, in simulation of occlusal contact in ceramic crowns on tooth dentin. The sandblast treatment is applied to the ceramic bottom surface before bonding to the substrate, as in common dental practice. Specimens with polished surfaces are used as a control. Tests are conducted with monotonically increasing (dynamic) and sinusoidal (cyclic) loading on the spherical indenter, up to the point of initiation of a radial fracture at the ceramic bottom surface immediately below the contact. For the polished specimens, data from the dynamic and cyclic tests overlap, consistent with a dominant slow crack growth mode of fatigue. Strengths of sandblasted specimens show significant reductions in both dynamic and cyclic tests, indicative of larger starting flaws. However, the shift is considerably greater in the cyclic data, suggesting some mechanically assisted growth of the sandblast flaws. These results have implications in the context of lifetimes of dental crowns.
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PMID:Effect of sandblasting on the long-term performance of dental ceramics. 1538 95

The strength degrading effects of sharp-contact damage are examined for Y-TZP ceramic plates bonded to a plastic substrate. Contacts are made with Vickers and Berkovich diamond indenters at low loads (0.1 N to 100 N) in the ceramic lower surfaces prior to bonding to the substrates. The indentations remain in the subthreshold region, that is, without visible corner radial cracks, over the lower region (< 10 N) of the load range. A concentrated load is then applied sinusoidally to the ceramic upper surface, with the loading axis centered on the subsurface indentation flaw, thereby subjecting the flaw to cyclic tension. Relative to polished surfaces, the indentations diminish the single-cycle strength by an amount that increases with increasing indentation load. The critical number of cycles required to cause failure from the indentation flaws is then measured at specified maximum lower surface tensile stresses. At each indentation load, the strength of the ceramic plates diminishes with increased cycling. The scale of degradation is compared with that from previous studies on Y-TZP surfaces containing larger-scale surface damage: sandblast damage, as used in dental crown preparation; and blunt-contact trauma from a spherical indenter at 3000 N. These other damage modes are shown to be equivalent in their strength-degrading capacity to diamond pyramid indentations in the microscale load range 0.1 N to 1 N, attesting to the highly deleterious nature of sharp particulate contacts. The mechanism of fatigue is considered in terms of microcrack evolution within the indentation damage zone.
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PMID:Fatigue sensitivity of Y-TZP to microscale sharp-contact flaws. 1555 Dec 60

The purpose of this study was to evaluate microstructural and crystallographic phase changes after grinding 3Y-TZP dental ceramics. Ceramic blanks were sintered according to manufacturer's recommendations and divided into four groups: (A) as-sintered control, (B) diamond-ground manually under water, (C) ground and polished, and (D) ground and annealed at 1000 degrees C for 1 h. Bulk specimens were analyzed by X-ray diffraction to characterize the crystalline phases. The microstructure was investigated by SEM. XRD analyses showed that the control group and the group that was ground and annealed contained only tetragonal zirconia. However, after grinding or after grinding followed by polishing, rhombohedral zirconia and strained tetragonal zirconia were present, without any detectable amount of monoclinic zirconia. Annealing led to the disappearance of both residual lattice strain and the rhombohedral phase. The microstructure of the ground and polished specimens was characterized by significant residual surface damage associated with grain pullout to a depth of about 20 microm. This type of damage could have an impact on the long-term fatigue behavior of 3Y-TZP.
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PMID:Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. 1618 29

All-ceramic dental restorations exhibit enhanced esthetics and biocompatibility as compared to traditional metal-based prosthesis. However, long-term fatigue and subcritical crack growth in the presence of water and cyclic loading can decrease the strength of ceramic components over time. We investigated the cyclic fatigue in water of three dental materials currently used as frameworks in all-ceramic restorations: a 3 mol%-yttria partially stabilized zirconia (3Y-TZP, Cercon, Degudent GmbH), a Al(2)O(3)-ZrO(2)-Glass composite (Inceram-Zirconia, Vita Zahnfabrik GmbH) and a Li(2)O.2SiO(2) glass ceramic (Empress 2, Ivoclar Vivadent AG). Fatigue and fast fracture tests were performed to determine the Weibull distribution of lifetime and initial mechanical strength for each framework component. In spite of its noticeable susceptibility to fatigue in water, the 3Y-TZP material was found to be particularly suitable for the preparation of posterior all-ceramic bridges due to its high initial mechanical strength. Guidelines are provided for the selection of materials and the design of all-ceramic posterior bridges exhibiting lifetime longer than 20 years under severe wet and cyclic loading conditions.
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PMID:In vitro lifetime of dental ceramics under cyclic loading in water. 1733 47

There is a rapidly growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. It was hypothesized that a mismatch in coefficient of thermal expansion between the veneering porcelain and the Y-TZP zirconia core of these prostheses causes transformation of the tetragonal to the monoclinic structure in Y-TZP zirconia at the interface boundary when exposed to fatigue loading, resulting in fracture at the interface boundary. Y-TZP zirconia discs were veneered with three porcelains differing in coefficient of thermal expansion. Finite element analysis was used to investigate the stress distribution in the bi-layered discs because of the mismatch in thermal expansion. Two of these three groups were fatigued with the veneering ceramic in tension. X-ray diffraction was used to measure the intensity of monoclinic and tetragonal zirconia phase present at the zirconia core surface after sintering, airborne abrasion, veneering, and fatigue loading. It was found that the sintered tetragonal structure was converted to monoclinic up to a depth of 27 mum after airborne abrasion, and reversed back to tetragonal after porcelain veneering with porcelain. Fatigue loading of veneered discs did not, even with the highest possible thermal mismatch stress, cause any conversion from tetragonal to monoclinic phase.
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PMID:Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs. 1791 51

Mechanical properties and slow crack growth (SCG) behavior of a 10Ce-TZP/Al2O3 nanocomposite currently developed as a biomaterial are considered. Fracture toughness is determined for sharp, long (double torsion) and short (indentation) cracks and a good agreement is found between the two types of cracks. The main toughening mechanism in the nanocomposite is the tetragonal to monoclinic phase transformation of the ceria-stabilized zirconia (Ce-TZP) phase. Transformation at the surface of ground specimens leads to surface compressive induced stresses and an increase in strength. Crack velocity curves (V-K(I) curves) are obtained under static and cyclic fatigue using the double torsion method. The static V-K(I) curve in air reveals the three stages characteristic of stress corrosion with a threshold K(I0) approximately 4.5 MPa m(1/2) and a fracture toughness of 8.8 MPa m(1/2) significantly higher than those of currently used inert bioceramics (i.e., alumina and Y-TZP). A crack growth accelerating effect is shown under cyclic loading, correlated with a decrease in the threshold. However, the cyclic fatigue threshold (4 MPa m(1/2)) still stands above that of current biomedical grade alumina and zirconia.
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PMID:Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia-alumina nanocomposite for medical applications. 1857 16

Despite the increasing utilization of all-ceramic crown systems, their mechanical performance relative to that of metal ceramic restorations (MCR) has yet to be determined. This investigation tested the hypothesis that MCR present higher reliability over two Y-TZP all-ceramic crown systems under mouth-motion fatigue conditions. A CAD-based tooth preparation with the average dimensions of a mandibular first molar was used as a master die to fabricate all restorations. One 0.5-mm Pd-Ag and two Y-TZP system cores were veneered with 1.5 mm porcelain. Crowns were cemented onto aged (60 days in water) composite (Z100, 3M/ESPE) reproductions of the die. Mouth-motion fatigue was performed, and use level probability Weibull curves were determined. Failure modes of all systems included chipping or fracture of the porcelain veneer initiating at the indentation site. Fatigue was an acceleration factor for all-ceramic systems, but not for the MCR system. The latter presented significantly higher reliability under mouth-motion cyclic mechanical testing.
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PMID:Reliability of metalloceramic and zirconia-based ceramic crowns. 2066 Jul 96

Veneer chipping and fracture are common failure modes for porcelain-veneered zirconia dental restorations. We hypothesized that the graded glass/zirconia/glass with external esthetic glass (e-GZG) can increase the lifetime and improve resistance to veneer chipping and fracture relative to porcelain-veneered zirconia, while providing necessary esthetics. Previously, we have demonstrated that a graded glass-zirconia surface possesses excellent resistance to occlusal-like sliding contact fatigue. Here, we investigated the sliding contact fatigue response of this graded glass-zirconia surface with external esthetic glass. This external glass is essential for shade options, for preventing excessive wear of opposing dentition, and for protecting Y-TZP from hydrothermal degradation. e-GZG plates were bonded to composite blocks and subjected to prolonged sliding contact up to 10 million cycles at 200 N in water. The resistance to sliding contact fatigue of e-GZG matches that of monolithic Y-TZP, and both of these materials demonstrated lifetimes that were orders of magnitude longer than that of porcelain-veneered zirconia. Graded e-GZG is a promising restorative material.
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PMID:Sliding contact fatigue of graded zirconia with external esthetic glass. 2166 5


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