Modeling the surface and interior structure of comet nuclei using a multidisciplinary approach

The goal was to investigate the structural properties of the surface of comet nucleus and how the surface should change with time under effect of solar radiation. The basic model that was adopted was that the nucleus is an aggregate of frosty particles loosely bound together, so that it is essentially a soil. The nucleus must mostly be composed of dust particles. The observed mass ratios of dust to gas in the coma is never much greater than unity, but this ratio is probably a much lower limit than that of the nucleus because it is vastly easier to remove the gaseous component by sublimation than by carrying off the dust. Therefore the described models assumed that the particles in the soil were frost covered grains of submicron basic size, closely resembling the interstellar grains. The surface properties of such a nucleus under the effects of heating and cooling as the nucleus approaches and recedes from the Sun generally characterized

Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism

Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs. Graphic abstract: [Figure not available: see fulltext.] © 2021, The Author(s)

Measurement of hardness and elastic modulus by depth sensing indentation: Improvements to the technique based on continuous stiffness measurement

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An Investigation of Youth Swimming Skills and Method of Instruction

Drowning is a leading cause of death for US children. Teaching youth to swim in a formal setting from certified instructors is a consistent drowning prevention recommendation. Purposes for this investigation was to examine type of swimming instruction and ability to swim and compare to attitudes toward swimming among US youth. Methods were similar to previous USA Swimming studies in 2008 and 2010. YMCA associations in five cities were used to recruit adolescent survey respondents (n=600) aged 12-18 years. Results showed African American youth had the lowest rate of formal swimming instruction (29%) compared to White (32%) and Hispanic (42%) peers. Free/reduced lunch qualifiers reported a 23% formal instruction rate as compared to 43% of non-qualifiers. Formally instructed youth were 2.35 times more likely to report being a skilled swimmer (86%) compared to informally instructed youth (72%). Formal swimming instruction is recommended, and interventions need to target underserved populations

Prominent crista terminalis mimicking a right atrial mass: case report

The crista terminalis is a normal anatomical structure within the right atrium that is not normally visualised in the standard views obtained while performing a transthoracic echocardiogram. In this case report, transthoracic echocardiography suggested the presence of a right atrial mass in a patient with end stage renal disease. However, subsequent transesophageal echocardiography revealed that the right atrial mass was actually a thick muscular bridge in the right atrium consistent with a prominent crista terminalis. An understanding of the anatomy and the echocardiographic appearance of a prominent crista terminalis will minimize the misdiagnosis of this structure avoiding unnecessary expensive additional tests

TokaMaker: An open-source time-dependent Grad-Shafranov tool for the design and modeling of axisymmetric fusion devices

In this paper, we present a new static and time-dependent MagnetoHydroDynamic (MHD) equilibrium code, TokaMaker, for axisymmetric configurations of magnetized plasmas, based on the well-known Grad-Shafranov equation. This code utilizes finite element methods on an unstructured triangular grid to enable capturing accurate machine geometry and simple mesh generation from engineering-like descriptions of present and future devices. The new code is designed for ease of use without sacrificing capability and speed through a combination of Python, Fortran, and C/C++ components. A detailed description of the numerical methods of the code, including a novel formulation of the boundary conditions for free-boundary equilibria, and validation of the implementation of those methods using both analytic test cases and cross-code validation is shown. Results show expected convergence across tested polynomial orders for analytic and cross-code test cases

Parental Factors That Influence Swimming in Children and Adolescents

Swimming can be an important source of physical activity across the life-span. Researchers have found that parents influence physical activity behaviors of their children. The purpose of this study was to determine what parental factors influenced the number of days that children swam. Survey respondents (n = 1,909) from six cities across the United States were surveyed at local YMCAs. Children were found to swim significantly more if their parents encouraged them to swim, members of the family knew how to swim and swam with them, or their parents were not afraid of the children drowning or afraid of drowning themselves. The number of times that parents swam was the strongest predictor of the number of times children swam and explained 41% of the variance. A cyclical, familial pattern was found which included encouragement, fear of drowning, and swimming frequency. Introducing strategies to reduce fear of drowning may prove to be an effective intervention to increase physical activity in children

An explanation for the shape of nanoindentation unloading curves based on finite element simulation

Current methods for measuring hardness and modulus from nanoindentation load-displacement data are based on Sneddon`s equations for the indentation of an elastic half-space by an axially symmetric rigid punch. Recent experiments have shown that nanoindentation unloading data are distinctly curved in a manner which is not consistent with either the flat punch or the conical indenter geometries frequently used in modeling, but are more closely approximated by a parabola of revolution. Finite element simulations for conical indentation of an elastic-plastic material are presented which corroborate the experimental observations, and from which a simple explanation for the shape of the unloading curve is derived. The explanation is based on the concept of an effective indenter shape whose geometry is determined by the shape of the plastic hardness impression formed during indentation

Nanoindentation of Soft Films on Hard Substrates:The Importance of Pile-Up

Nanoindentation is used for measuring mechanical properties of thin films. This paper addresses potential measurement errors caused by pile-up when soft films deposited on hard substrates are tested this way. Pile-up is exacerbated in soft film/hard substrate systems because of the constraint the substrate exerts on plastic deformation of the film. To examine pile-up effects, Al films 240 and 1700 nm thick were deposited on hard glass and tested by standard nanoindentation. In Al/glass, the film and substrate have similar elastic moduli; thus, any unusual behavior in nanoindentation results may be attributed to differences in plastic flow alone. SEM examination of nanoindentation hardness impressions in the film revealed that common methods for analyzing nanoindentation data underestimate the true contact areas by as much as 80%, which results in overestimations of the hardness and modulus by as much as 80 and 35%, respectively. Sources of these errors and their effect on measurement of hardness and elastic modulus are discussed, and a simple model for the composite hardness of the film/substrate system is developed. This model could prove useful when it is not possible to make indentations shallow enough to avoid substrate effects

Cracking During Nanoindentation and its Use in the Measurement of Fracture Toughness

Results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined in nanoindentation experiments are reported. The method is based on the radial cracking which occurs when brittle materials are deformed by a sharp indenter such as a Vickers or Berkovich diamond. In microindentation experiments, the lengths of radial cracks have been found to correlate reasonably well with fracture toughness, and a simple semi-empirical method has been developed to compute the toughness from the crack lengths. However, a problem is encountered in extending this method into the nanoindentation regime with the standard Berkovich indenter in that there are well defined loads, called cracking thresholds, below which indentation cracking does not occur in most brittle materials. We have recently found that the problems imposed by the cracking threshold can be largely overcome by using an indenter with the geometry of the comer of a cube. For the cube-corner indenter, cracking thresholds in most brittle materials are as small as 1 mN ({approximately}0.1 grams). In addition, the simple, well-developed relation between toughness and crack length used for the Vickers indenter in the microindentation regime can be used for the cube-corner indenter in the nanoindentation regime provided a different empirical constant is used