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

The high cycle fatigue strength of porous coated Ti-6A1-4V is approximately 75% less than the fatigue strength of uncoated Ti-6A1-4V. This study separates the effects of three parameters thought to be responsible for this reduction: interfacial geometry, microstructure, and surface alterations brought about by sintering. To achieve the goal of one parameter variations, hydrogen-alloying treatments, which refined the lamellar microstructure of beta-annealed and porous coated Ti-6A1-4V, were formulated. The fatigue strength of smooth-surfaced Ti-6A1-4V subjected to hydrogen-alloying treatments is 643-669 MPa, significantly greater than the fatigue strength of beta-annealed Ti-6A1-4V (497 MPa) and also greater than the fatigue strength of pre-annealed, equiaxed Ti-6A1-4V (590 MPa). The fatigue strength of porous coated Ti-6A1-4V, however, is independent of microstructure. This leads to the conclusion that the notch effect of the surface porosity does not allow the material to take advantage of the superior fatigue crack initiation resistance of a refined alpha-grain size. Thus, sinternecks acts as initiated microcracks and fatigue of porous coated Ti-6A1-4V is propagation controlled.
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PMID:A parametric study of the factors affecting the fatigue strength of porous coated Ti-6A1-4V implant alloy. 227 82

Previous studies have shown that the application of a porous Ti-6A1-4V alloy coating results in a substantial decrease in the substrate material fatigue properties. This phenomenon is due to both a microstructural change from the equiaxed alpha-beta microstructure to a lamellar structure and a notch effect created by the porous coating. The lamellar microstructure has been shown to exhibit the worst fatigue properties of the most common structures obtained in Ti-6A1-4V alloy. This study examined various post-sintering heat treatments which would alter the material microstructure and possibly result in improved fatigue performance. The heat treatments examined produced alternate microstructures to the lamellar structure and some approached the totally transformed acicular alpha structure. The acicular alpha structure has been shown to exhibit the best fatigue properties for Ti-6A1-4V alloy in the notched condition.
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PMID:Post-sintering heat treatments for porous coated Ti-6A1-4V alloy. 408 50

Grit blasting is a common procedure of roughening surfaces to promote physical attachment of porous coatings, but it has been shown to reduce fatigue strength. Shot peening is known to increase fatigue strength by inducing compressive surface stresses; however, it is not known how subsequent grit blasting affects these benefits. This study examines the endurance limits, Se, of ELI grade Ti-6A1-4V specimens under rotating cyclic bending, including polished (control); belted and beaded; belted, beaded, and grit blasted; and belted, beaded, shot peened, and grit blasted. Belting and beading resulted in a slight increase in Se, grit blasting caused a 15% reduction in Se from polished. Fifty percent of this reduction was recovered when shot peening preceded grit blasting, suggesting that residual compressive surface stresses, induced by peening, were not eliminated by the blast process. Roughness averages and RMS values did not correlate with Se trends. SEM results showed classical fatigue fractures, consistent with surface crack initiation.
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PMID:Effects of precoating surface treatments on fatigue of Ti-6A1-4V. 749 7

Porous-coated Ti-6A1-4V has a fatigue strength approximately one-third that of the uncoated alloy. The interfacial geometry between the porous coating and the implant substrate is notchlike, leading to stress concentrations that have been shown to be the main cause for the reduction in fatigue strength. In this study, the effect of interfacial geometry on fatigue strength of porous-coated Ti-6A1-4V is quantified. The interface between porous coating and implant is modeled using linear elastic, plane strain finite element analysis. Integrated with the numerical analysis is an experimental verification of enhanced fatigue behavior. Changes in interfacial geometry are conceived, and their effectiveness in reducing stress concentrations are determined. A doubling of fatigue strength can be achieved for newly conceived geometries over conventional porous coating geometries.
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PMID:Effect of a change in interfacial geometry on the fatigue strength of porous-coated Ti-6A1-4V. 800 46

The aim of this study was to determine the fatigue resistance of the cast clasps of removable partial dentures. The different commercial types of metals used included five cobalt-chromium alloys, pure titanium, one titanium alloy (Ti-6A1-4V) and one gold alloy (type IV) that was either unhardened or age-hardened (n = 5 per group). The test method used was a constant-deflection fatigue test in which the force required to deflect the clasp for 0.6 mm and the number of loading cycles required to fracture the clasp were determined. The fatigue fracture surface of the clasps was examined with a scanning electron microscope. The results revealed that a fatigue fracture occurred in the cobalt-chromium clasp after approximately 25,000 loading cycles, in the pure titanium clasps after 4500 loading cycles, in the titanium alloy clasp after 20,000 loading cycles, and in the gold alloy clasp after 21,000 loading cycles. The means differed significantly (p < 0.001). Activation of the clasp by bending it 0.5 mm increased the fatigue resistance of the cobalt-chromium alloy and gold alloy clasps but decreased the fatigue resistance of both pure titanium and titanium alloy clasps (p < 0.005). The results of this study suggest that significant differences exist in the fatigue resistance of removable denture clasps made from different commercial cast metals, which may cause loss of retention of the removable partial denture and clasp failures.
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PMID:Deflection fatigue of cobalt-chromium, titanium, and gold alloy cast denture clasp. 853 Nov 62

The stability of radiofrequency (RF) magnetron sputtered calcium phosphate was studied under cyclically loaded conditions. The coatings were deposited on titanium bars and tested in either dry or wet conditions X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) spectroscopy were used to characterize the as-sputtered and tested coatings. XRD demonstrated that the amorphous structure after annealing at 650 C changed into a crystalline apatite structure. The residual stresses were determined by the XRD cos 2 i/i method. These residual film stresses were influenced by the coating conditions and the crystalline sputtered coating showed the presence of compressive stresses. SEM demonstrated that, after cyclic loading conditions in air, the crystalline sputter-coated Ti-6A1-4V bars showed a partial coating loss. Furthermore, in wet conditions (simulated body fluid) only the heat-treated sputter-coated bars appeared to be stable. On the other hand, the amorphous coating only showed signs of delamination in the more highly stressed regions, while in the less stressed regions a Ca-P precipitate was formed. On the basis of these results we conclude that calcium phosphate coatings subjected to cyclic loading conditions show an important difference in fatigue behaviour when tested in either dry or wet conditions.
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PMID:Stability of radiofrequency magnetron sputtered calcium phosphate coatings under cyclically loaded conditions. 911 52

The fatigue failure of hip joint prostheses will be expected to assume more importance in second generation implants aimed at younger, more active patients. Furthermore, new designs and material combinations including coatings (e.g. hydroxyapatite) may introduce fatigue problems that as yet have not been considered. The current research makes an initial attempt to develop accelerated fatigue testing procedures to enhance the methodology of hip implant lifetime prediction. Tests conducted on a 'model' four point bendbar testpiece (mill-annealed Ti-6A1-4V) highlighted that the accelerated test must be conducted in a physiological solution such as Ringer's at 37 degrees C. The introduction of superimposed block overloads (50 cycles) to signify stair ascent/descent or fast walking and single overloads to signify sit/stand movements or stumbling were found to reduce fatigue life by > 50%. The findings of this fatigue study were combined with biomechanics studies to construct a variable amplitude 'in-service' load spectrum for testing hip implants. Using ambulatory trial data, a simple load sequence was designed containing 4 single (sit/stand movements) and 3 block (stair ascent/descent) overloads that repeated ten times gave one days activity; single overloads repeated every 110 base cycles (normal walking) and block overloads 80, 110 (morning/evening) and 250 (daytime) base cycles.
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PMID:Development of fatigue lifetime predictive test methods for hip implants: part I. Test methodology. 969 4

Increased use of titanium alloys as biomaterials is occurring due to their lower modulus, superior biocompatibility and enhanced corrosion resistance when compared to more conventional stainless steels and cobalt-based alloys. These attractive properties were a driving force for the early introduction of alpha (cpTi) and alpha + beta (Ti-6A1-4V) alloys as well as for the more recent development of new Ti-alloy compositions and orthopaedic metastable beta titanium alloys. The later possess enhanced biocompatibility, reduced elastic modulus, and superior strain-controlled and notch fatigue resistance. However, the poor shear strength and wear resistance of titanium alloys have nevertheless limited their biomedical use. Although the wear resistance of beta-Ti alloys has shown some improvement when compared to alpha + beta alloys, the ultimate utility of orthopaedic titanium alloys as wear components will require a more complete fundamental understanding of the wear mechanisms involved. This review examines current information on the physical and mechanical characteristics of titanium alloys used in artifical joint replacement prostheses, with a special focus on those issues associated with the long-term prosthetic requirements, e.g., fatigue and wear.
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PMID:Titanium alloys in total joint replacement--a materials science perspective. 983 98

Cytochrome c oxidase (COX), the terminal enzyme of the electron transport chain, is a bigenomic enzyme with 13 subunits. The mechanism coordinating the transcription of these subunits is poorly understood. We investigated the role of nuclear respiratory factor-2 (NRF-2) in intragenomic regulation of nuclear COX genes. Vector-mediated short-hairpin RNA interference against NRF-2alpha reduced all 10 COX nuclear subunit mRNAs and mRNAs of other genes involved in mitochondrial function/biogenesis. NRF-2 binding site was necessary for the rat COX 4i1 promoter to down-regulate in response to decreased energy demands in primary neurons. Over-expression of NRF-2 protein prevented the down-regulation of transcriptional activity by TTX. Finally, NRF-2 binding sites in isolation were sufficient for modulating COX subunit 4i1 and 6A1 promoters' activity in response to decreased energy demand. These results indicate that NRF-2 is a vital part of a molecular mechanism that senses upstream energy signals and modulates COX transcriptional levels in mammalian cells.
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PMID:Nuclear respiratory factor 2 senses changing cellular energy demands and its silencing down-regulates cytochrome oxidase and other target gene mRNAs. 1651 9