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

Many bones within the axial and appendicular skeleton are subjected to repetitive loading during the course of ordinary daily activities. If this loading is of sufficient magnitude or duration, failure of the bone tissue may result. Until recently the structural analysis of these fractures has been limited to two-dimensional sections. Due to the inherent destructiveness of this method, dynamic assessment of fracture progression has not been possible. An image-guided technique to analyze structural failure has been developed utilizing step-wise micro-compression in combination with time-lapsed micro-computed tomographic imaging. This technique allows, for the first time, direct three-dimensional visualization and quantification of fracture initiation and progression on the microscopic level and relates the global failure properties of trabecular bone to those of the individual trabeculae. The goals of this project were first to design and fabricate a novel micro-mechanical testing system, composed of a micro-compression device and a material testing and data acquisition system; and second, to validate the testing system to perform step-wise testing of trabecular bone specimens based on image-guided failure analysis. Due to the rate dependant properties of bone, stress relaxation was a concerning factor with respect to the step-wise testing method. In order to address these concerns, the results of the step-wise testing method were compared to those obtained from a conventional continuous test (considered to be the gold standard for the step-wise compressive mechanical testing) over the same total strain range and testing conditions. This was performed using porous aluminum alloy samples with highly reproducible and homogenous structural properties as well as trabecular bone samples from a single whale vertebra. Five cylinders from aluminum foam and trabecular whale bone each were compressed and imaged in a step-wise fashion from 0% to 20% strain at intervals of 2%, 4%, 8%, 12%, 16% and 20%. Mechanical properties obtained from the continuous and step-wise methods were not significantly different for both aluminum foam and whale bone specimens (p>0.05). Both testing methods yielded very similar stress-strain graphs with almost identical elastic and plastic regions with overlaying standard error bars for both whale bone and aluminum foam specimens. This was further concurred by performing regression analyses between the stress data from both testing methods (r(2)=0.98 for whale bone and aluminum foam specimens). Animations of fracture initiation and progression revealed that failure always occurred in local bands with the remaining regions of the structure largely unaffected independent of structure type. In conclusion, we found step-wise micro-compression to be a valid approach for image-guided failure assessment (IGFA) with high precision and accuracy as compared to classical continuous testing. We expect findings from upcoming studies of IGFA of human vertebral bone to improve our understanding of the relative importance of densitometric, morphological, and loading factors in the etiology of spontaneous fractures of the spine. Eventually, this improved understanding may lead to more successful approaches to the prevention of age-related fatigue fractures.
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PMID:Time-lapsed microstructural imaging of bone failure behavior. 1467 68

Clinicians at the Shriner's Hospitals for Children have noticed unexpected catastrophic failures in the 2024-T3 aluminum prosthetic pylons of their juvenile patients. They postulated that small "dings", caused by sharp impacts with playground equipment or other obstacles, might be causing stress concentrations in the pylons. To investigate this, students at the United States Air Force Academy performed a series of tests to determine potential failure modes for the pylon. Initially, a technique for introducing small reproducible "dings" was established. These damaged specimens were then subjected to compressive axial tests to failure, axial fatigue tests (5-100 lbs at 5 Hz), and finally four point bending fatigue tests (370 in-lbs at 5 Hz). The maximum compressive load of the damaged specimen was virtually identical to that of the control specimen (14 kips), and the axial fatigue test was halted with no damage after one million cycles. Two bending fatigue tests resulted in cracks and eventual failure at the load application points of the fixture on the pylon. This has caused us to speculate that failure of the pylons is not due to the dings, but may be due to over-tightening of the pylon attachment points to the foot and the deficient limb. Further testing will be conducted to examine this possibility.
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PMID:Structural testing of a juvenile prosthetic foot pylon. 1513 37

Aluminium-based adjuvants have been used throughout the world since 1926, and their safety profile is such that they have long been the sole adjuvants registered for clinical use. Their safety has nevertheless been questioned in France over the last few years following the demonstration that aluminium could persist for prolonged periods at the injection site, within macrophages gathered around the muscular fibers and forming a microscopic histological lesion called "macrophagic myofasciitis (MMF)". This image has been observed in patients undergoing a deltoid muscular biopsy for diagnostic purposes of various symptoms essentially including muscular pain and fatigue, in association with a large panel of various symptoms and diseases, including those of an autoimmune nature. Studies of the clinical, biological and epidemiological characteristics undertaken to identify a possible association between the MMF histological image and a systemic disease have remained negative. As of today, available evidence indicates that although vaccine aluminium may persist at the site of injection for years ("vaccine tattoo"), this does not reflect the existence of a diffuse inflammatory muscular disease and is not associated with a specific clinical disease. The existence of sampling bias inherent to the complexity of the clinical and pathological diagnoses remains the most likely hypothesis.
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PMID:[Vaccine adjuvants and macrophagic myofasciitis]. 1514 82

This study evaluated the corrosion-fatigue life of laser-repaired specimens fabricated from commercially pure titanium (CP Ti) and Ti-6Al-4V alloy, tested under different storage conditions. For each metal, 30 dumbbell rods with a central 2.3 mm diameter were prepared by lost-wax casting with the Rematitan System. Simulating the failure after service, corrosion-fatigue life in different media at room temperature (air, synthetic saliva and fluoride synthetic saliva) was determined at a testing frequency of 10 Hz for intact specimens and after laser repairing, using a square waveform with equal maximum tensile and compressive stress that was 30% lower than the 0.2% offset yield strength. For laser welding, the fractured specimens were rejoined using a jig to align the sections invested in type-IV dental stone. The adjacent areas of the gap was air-abraded with 100 microm aluminum oxide, laser welded and retested under the same conditions as the initial intact specimens. The number of cycles at failure was recorded, and the fracture surface was examined with a scanning electron microscope (SEM). The number of cycles for failure of the welded and intact specimens was compared by anova and the Tukey test at a 5% probability level. Within the limitations of this study, the number of cycles required for fracture decreased in wet environments and the laser repairing process adversely affected the life of both metals under the corrosion-fatigue conditions.
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PMID:Corrosion-fatigue of laser-repaired commercially pure titanium and Ti-6Al-4V alloy under different test environments. 1538 45

Aluminium-based adjuvants have been used throughout the world since 1926, and their safety profile is such that they have long been the sole adjuvants registered for clinical use. Their safety has nevertheless been questioned in France over the last few years following the demonstration that aluminium could persist for prolonged periods at the injection site, within macrophages gathered around the muscular fibres and forming a microscopic histological lesion called "macrophagic myofasciitis (MMF)". This image has been observed in patients undergoing a deltoid muscular biopsy for diagnostic purposes of various symptoms essentially including muscular pain and fatigue, in association with a large panel of various symptoms and diseases, including those of an autoimmune nature. Studies of the clinical, biological and epidemiological characteristics undertaken to identify a possible association between the MMF histological image and a systematic disease have remained negative. As of today, available evidence indicates that although vaccine aluminium may persist at the site of injection for years ("vaccine tattoo"), this does not reflect the existence of a diffuse inflammatory muscular disease and is not associated with a specific clinical disease. The existence of sampling bias inherent to the complexity of the clinical and pathological diagnoses remains the most likely hypothesis.
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PMID:[Vaccine adjuvants and macrophagic myofasciitis]. 1565 65

The results of an extensive test program to characterize the behavior of typical aircraft structures under acoustic loading and to establish their fatigue endurance are presented. The structures tested were the three flap-like box-type of structures. Each structure consisted of one flat (bottom) and one curved (top) stiffener stiffened skin panel, front, and rear spars, and ribs that divided the structures into three bays. The three structures, constructed from three different materials (aircraft standard aluminum alloy, Carbon Fibre Reinforced Plastic, and a Glass Fibre Metal Laminate, i.e., GLARE) had the same size and configuration, with only minor differences due to the use of different materials. A first set of acoustic tests with excitations of intensity ranging from 140 to 160 dB were carried out to obtain detailed data on the dynamic response of the three structures. The FE analysis of the structures is also briefly described and the results compared with the experimental data. The fatigue endurance of the structures was then determined using random acoustic excitation with an overall sound pressure level of 161 dB, and details of crack propagation are reported.
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PMID:Comparison of structural response and fatigue endurance of aircraft flap-like box structures subjected to acoustic loading. 1595 53

Boron removal from wastewaters by electrocoagulation using aluminum electrode material was investigated in this paper. Several working parameters, such as pH, current density, boron concentration and type and concentration of supporting electrolyte were studied in an attempt to achieve a higher removal capacity. The experiments were carried out by keeping the pH of solution constant and optimum pH of solution was determined 8.0 for the aluminum electrode. Although energy consumption increased with decreasing boron concentration, which conductivity of these solutions were low, boron removal efficiency was higher at 100 mg/L than that of 1000 mg/L. Current density was an important parameter affecting removal efficiency. Boron removal efficiency and energy consumption increased with increasing current density from 1.2 to 6.0 mA/cm2. The types of different supporting electrolyte were experimented in order to investigate to this parameter effect on boron removal. The highest boron removal efficiency, 97%, was found by CaCl2. Added CaCl2 increased more the conductivity of solution according to other supporting electrolytes, but decreased energy consumption. The results showed to have a high effectiveness of the electrocoagulation method in removing boron from aqueous solutions.
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PMID:The investigation of parameters affecting boron removal by electrocoagulation method. 1598 28

Attached ultrasonic sensors can detect changes caused by crack initiation and growth if the wave path is directed through the area of critical crack formation. Dynamics of cracks opening and closing under load cause nonlinear modulation of received ultrasonic signals, enabling small cracks to be detected by stationary sensors. A methodology is presented based upon the behavior of ultrasonic signals versus applied load to detect and monitor formation and growth of cracks originating from fastener holes. Shear wave angle beam transducers operating in through transmission mode are mounted on either side of the hole such that the transmitted wave travels through the area of expected cracking. Time shift is linear with respect to load, and is well explained by path changes due to strain combined with wave speed changes due to acoustoelasticity. During subsequent in situ monitoring with unknown loads, the measured time of flight is used to estimate the load, and behavior of the received energy as a function of load is the basis for crack detection. Results are presented from low cycle fatigue tests of several aluminum specimens and illustrate the efficacy of the method in both determining the applied load and monitoring crack initiation and growth.
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PMID:An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth. 1645 66

The fatigue limits and fracture characteristics for three Pd-Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91, Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress amplitude (R-ratio) of -1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits for these alloys are attributed to their complex microstructures and perhaps to casting defects.
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PMID:Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd-Ag dental casting alloys. 1720 Aug 21

Fracture strength and fatigue parameters of three ceramic materials submitted to dynamic fatigue were evaluated. A machinable leucite-reinforced dental ceramic, aluminum oxide, and yttria-stabilized zirconia (YSZ) were tested. The inert strength of the materials was determined in air (25 degrees C) at stressing rates of 70, 250, 400 MPa/s for Porcelain, Alumina and YSZ respectively. The data was analyzed using a two-parameter Weibull distribution. The Weibull modulus (m) and the characteristic of fracture (sigma0) parameters were determined for each material. Specimens were also tested in 3-point bending at different stressing rates in distilled/deionized water at 37 degrees C (dynamic fatigue) in order to calculate the fatigue parameters n and ln B. The strength for each material was characterized using Strength-Probability-Time (SPT) diagrams for 1 day, 1 year and 10 years. YSZ showed a high-fracture strength sigma0 (1,459 MPa) at a failure probability of 63.2% and high resistance to subcritical crack growth. YSZ and alumina showed better resistance to slow crack growth than porcelain, indicating less susceptibility to strength degradation by stress corrosion. Lifetime predictions after 10 years indicate a reduction of 50%, 36% and 29% in strength for porcelain, alumina and YSZ respectively. YSZ seems to be a very promising material for long-term dental and biomedical applications.
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PMID:Dynamic fatigue and strength characterization of three ceramic materials. 1727 77


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