SciCombinator

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Concept: Fracture mechanics

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The physical risk factors leading to distal radial fractures are poorly understood. The goal of this study was to compare postural stability between older adults with and without a prior distal radial fragility fracture.

Concepts: Osteoporosis, Bone fracture, Fracture, Fracture mechanics, Management, Distal radius fracture

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To compare the biomechanical stability of various pin configurations for pediatric supracondylar humeral fractures under varus, internal rotation, and extension conditions. After electronic retrieval, 11 biomechanical studies were included. Stiffness values of pin configurations under different loading conditions were extracted and pooled. There were no statistically significant differences between two cross pins and two divergent lateral pins on the basis of the ‘Hamdi method’ (P=0.249-0.737). An additional pin did not strengthen two-pin construct (P=0.124-0.367), but better stabilized fractures with medial comminution (P<0.01). Isolated lateral pins are preferable because of a better balance of a lower risk of nerve injury and comparable fixation strength. Limitations such as differences in experimental setup among recruited studies and small sample size may compromise the methodologic power of this study.

Concepts: Bone, Medicine, Statistics, Sample size, Statistical significance, Fracture mechanics, Biomechanics, Statistical hypothesis testing

25

The natural course of several isolated and nonisolated orbital roof fractures is reported, by showing four cases in which a “wait and see” policy was followed. All four cases showed spontaneous repositioning and stabilizing of the fracture within less than a year. This might be explained by the equilibrium between the intraorbital and intracranial pressures.

Concepts: Bone fracture, Fracture, Thermodynamics, Fracture mechanics

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The effects of ice formation and accretion on external surfaces range from being mildly annoying to potentially life-threatening. Ice-shedding materials, which lower the adhesion strength of ice to its surface, have recently received renewed research attention as a means to circumvent the problem of icing. In this work, we investigate how surface wettability and surface topography influence the ice adhesion strength on three different surfaces: i) superhydrophobic laser-inscribed square pillars on copper, ii) stainless steel 316 Dutch-weaved meshes, and iii) multi-walled carbon nanotube (MWCNT)-covered steel meshes. The finest stainless steel mesh displayed the best performance with a 93% decrease in ice adhesion relative to polished stainless steel, while the superhydrophobic square pillars exhibited an increase in ice adhesion by up to 67% relative to polished copper. Comparison of dynamic contact angles revealed little correlation between surface wettability and ice adhesion. On the other hand, by considering the ice formation process and the fracture mechanics at the ice-substrate interface, we found that two competing mechanisms governing ice adhesion strength arise on non-planar surfaces: i) mechanical interlocking of the ice within the surface features that enhances adhesion, and ii) formation of micro-cracks that act as interfacial stress concentrators, which reduce adhesion. Our analysis provides insight towards new approaches for the design of ice-releasing materials through the use of surface topographies that promote interfacial crack propagation.

Concepts: Fracture, Fracture mechanics, Carbon nanotube, Materials science, Steel, Stainless steel, Contact angle, Nickel

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Evaluating risk of fatigue fractures in cardiovascular implants via non-clinical testing is essential to provide an indication of their durability. This is generally accomplished by experimental accelerated durability testing and often complemented with computational simulations to calculate “fatigue safety factors”. While many methods exist to calculate fatigue safety factors, none have been validated against experimental data. The current study presents three methods for calculating fatigue safety factors and compares them to experimental fracture outcomes under axial fatigue loading, using cobalt-chromium test specimens designed to represent cardiovascular stents. Fatigue safety factors were generated by calculating mean and alternating stresses using a simple Scalar Method, a Tensor Method which determines principal values based on averages and differences of the stress tensors, and a Modified Tensor Method which accounts for stress rotations. The results indicate that the Tensor Method and the Modified Tensor Method consistently predicted fracture or survival (to 10 million cycles) for specimens subjected to experimental axial fatigue. In contrast, for one axial deformation condition, the Scalar Method incorrectly predicted survival even though fractures were observed in experiments. These results demonstrate limitations of the Scalar Method and potential inaccuracies. A separate computational analysis of torsional fatigue was also completed to illustrate differences between the Tensor Method and the Modified Tensor Method. Because of its ability to account for changes in principal directions across the fatigue cycle, the Modified Tensor Method offers a general computational method that can be applied for improved predictions for fatigue safety regardless of loading conditions.

Concepts: Scientific method, Algorithm, Mathematics, Fracture mechanics, Hypothesis, Mechanics, Calculation, Tensor

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We evaluated the prevalence of osteoporosis using the osteoporosis diagnostic criteria developed by the National Bone Health Alliance (NBHA), which includes qualified fractures, FRAX score in addition to bone mineral density (BMD). The expanded definition increases the prevalence compared to BMD alone definitions; however, it may better identify those at elevated fracture risk.

Concepts: Osteoporosis, Bone, Bone fracture, Fracture, Bone density, United States, Fracture mechanics, New York City

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Fluids trapped as inclusions within minerals can be billions of years old and preserve a record of the fluid chemistry and environment at the time of mineralization. Aqueous fluids that have had a similar residence time at mineral interfaces and in fractures (fracture fluids) have not been previously identified. Expulsion of fracture fluids from basement systems with low connectivity occurs through deformation and fracturing of the brittle crust. The fractal nature of this process must, at some scale, preserve pockets of interconnected fluid from the earliest crustal history. In one such system, 2.8 kilometres below the surface in a South African gold mine, extant chemoautotrophic microbes have been identified in fluids isolated from the photosphere on timescales of tens of millions of years. Deep fracture fluids with similar chemistry have been found in a mine in the Timmins, Ontario, area of the Canadian Precambrian Shield. Here we show that excesses of (124)Xe, (126)Xe and (128)Xe in the Timmins mine fluids can be linked to xenon isotope changes in the ancient atmosphere and used to calculate a minimum mean residence time for this fluid of about 1.5 billion years. Further evidence of an ancient fluid system is found in (129)Xe excesses that, owing to the absence of any identifiable mantle input, are probably sourced in sediments and extracted by fluid migration processes operating during or shortly after mineralization at around 2.64 billion years ago. We also provide closed-system radiogenic noble-gas ((4)He, (21)Ne, (40)Ar, (136)Xe) residence times. Together, the different noble gases show that ancient pockets of water can survive the crustal fracturing process and remain in the crust for billions of years.

Concepts: Fracture, Sun, Fracture mechanics, Mineral, Fluid, Noble gas, Deformation, Residence time

12

Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts.

Concepts: Fracture, Structure, Hierarchy, Fracture mechanics, Fractography, Materials science, Steel, Fracture toughness

11

A (Mo0.85Nb0.15)Si2 crystal with an oriented, lamellar, C40/C11b two-phase microstructure is a promising ultrahigh-temperature (UHT) structural material, but its low room-temperature fracture toughness and low high-temperature strength prevent its practical application. As a possibility to overcome these problems, we first found a development of unique “cross-lamellar microstructure”, by the cooping of Cr and Ir. The cross-lamellar microstructure consists of a rod-like C11b-phase grains that extend along a direction perpendicular to the lamellar interface in addition to the C40/C11b fine lamellae. In this study, the effectiveness of the cross-lamellar microstructure for improving the high-temperature creep deformation property, being the most essential for UHT materials, was examined by using the oriented crystals. The creep rate significantly reduced along a loading orientation parallel to the lamellar interface. Furthermore, the degradation in creep strength for other loading orientation that is not parallel to the lamellar interface, which has been a serious problem up to now, was also suppressed. The results demonstrated that the simultaneous improvement of high-temperature creep strength and room temperature fracture toughness can be first accomplished by the development of unique cross-lamellar microstructure, which opens a potential avenue for the development of novel UHT materials as alternatives to existing Ni-based superalloys.

Concepts: Fracture mechanics, Materials science, Metallurgy, Object-oriented programming, Surface normal, Deformation, Fracture toughness, Crystallite

10

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

Concepts: Mars, Fracture mechanics, Diurnal temperature variation, Speed, Space exploration, Impact crater, Asteroid, Meteoroid