An approach to multi-body interactions in a continuum-atomistic context: Application to analysis of tension instability in carbon nanotubes

AbstractThe tensile strength of single-walled carbon nanotubes (CNT) is examined using a continuum-atomistic (CA) approach. The strength is identified with the onset of the CNT instability in tension. The focus of this study is on the effects of multi-body atomic interactions. Multiscale simulations of nanostructures usually make use of two- and/or three-body interatomic potentials. The three-body potentials describe the changes of angles between the adjacent bonds – bond bending. We propose an alternative and simple way to approximately account for the multi-body interactions. We preserve the pair structure of the potentials and consider the multi-body interaction by splitting the changing bond length into two terms. The first term corresponds to the self-similar deformation of the lattice, which does not lead to bond bending. The second term corresponds to the distortional deformation of the lattice, which does lead to bond bending. Such a split of the bond length is accomplished by means of the spherical–deviatoric decomposition of the Green strain tensor. After the split, the continuum-atomistic potential can be written as a function of two bond lengths corresponding to the bond stretching and bending independently. We apply an example exponential continuum-atomistic potential with the split bond length to the study of tension instability of the armchair and zigzag CNTs. The results of the study are compared with those obtained by Zhang et al. (2004. J. Mech. Phys. Solids 52, 977–998) who studied tension instability of carbon nanotubes by using the Tersoff–Brenner three-body potential, and with recent experimental results on the tensile failure of single walled carbon nanotubes

Quantifying perception of nonlinear elastic tissue models using multidimensional scaling

Simplified soft tissue models used in surgical simulations cannot perfectly reproduce all material behaviors. In particular, many tissues exhibit the Poynting effect, which results in normal forces during shearing of tissue and is only observed in nonlinear elastic material models. In order to investigate and quantify the role of the Poynting effect on material discrimination, we performed a multidimensional scaling (MDS) study. Participants were presented with several pairs of shear and normal forces generated by a haptic device during interaction with virtual soft objects. Participants were asked to rate the similarity between the forces felt. The selection of the material parameters – and thus the magnitude of the shear\ud and normal forces – was based on a pre-study prior to the MDS experiment. It was observed that for nonlinear elastic tissue models exhibiting the Poynting effect, MDS analysis indicated that both shear and normal forces affect user perception

Kinetics of twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in many HCP materials including magnesium (Mg) and its alloys. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension–compression asymmetry, and sometimes non-Schmid effects. Dynamic loading can introduce further complexity in the deformation behavior. The growth of twins takes place by the motion of the twin boundary (TB). Tension twins in Mg can accommodate considerable amounts of plastic deformation as they grow, and this affects the overall rate of plastic deformation. Detailed understanding of the kinetics of TB motion will enable us to work towards achieving the overarching goal of microstructural design of materials for performance. We undertake an experimental approach to gain insight into the kinetics of TB migration under dynamic loading. To achieve this goal we performed normal plate impact recovery experiments with microsecond pulse durations on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several meter per second. These velocities are several orders of magnitude higher than those measured in Mg under quasi-static loading conditions. Further, twin nucleation and growth processes are investigated by conducting experiments with different durations of the loading pulse. This is achieved by using Mg specimens of different thicknesses. Electron back scattered diffraction is used to characterize the nature of the twins, microstructure, and twin fraction evolution. Detailed crystallographic analysis of the twins enables us to correlate the TB velocities to the twin variants

Observations and models for needle-tissue interactions

The asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. In this study we present a mechanics-based model that calculates the deflection of the needle embedded in an elastic medium. Microscopic observations for several needle- gel interactions were used to characterize the interactions at the bevel tip and along the needle shaft. The model design was guided by microscopic observations of several needle- gel interactions. The energy-based model formulation incor- porates tissue-specific parameters such as rupture toughness, nonlinear material elasticity, and interaction stiffness, and needle geometric and material properties. Simulation results follow similar trends (deflection and radius of curvature) to those observed in macroscopic experimental studies of a robot- driven needle interacting with different kinds of gels. These results contribute to a mechanics-based model of robotic needle steering, extending previous work on kinematic models

A Consistent Scaling Framework for Simulating High Rate Brittle Failure Problems

AbstractWe develop a material model that includes non-linear kinematics, a Mie-Grüneisen equation of state, and material failure based on an interacting microcrack damage model. This material model is well suited for simulating hypervelocity impacts on brittle materials. By accounting for the subscale distribution of cracks within the material, we are able to produce a material model that does not require assumptions about how material strength scales with specimen size. This allows us to calibrate the model at laboratory scales and then apply the model under conditions that are not achievable in a laboratory setting such as asteroid impacts

Risk profiles and one-year outcomes of patients with newly diagnosed atrial fibrillation in India: Insights from the GARFIELD-AF Registry.

BACKGROUND: The Global Anticoagulant Registry in the FIELD-Atrial Fibrillation (GARFIELD-AF) is an ongoing prospective noninterventional registry, which is providing important information on the baseline characteristics, treatment patterns, and 1-year outcomes in patients with newly diagnosed non-valvular atrial fibrillation (NVAF). This report describes data from Indian patients recruited in this registry. METHODS AND RESULTS: A total of 52,014 patients with newly diagnosed AF were enrolled globally; of these, 1388 patients were recruited from 26 sites within India (2012-2016). In India, the mean age was 65.8 years at diagnosis of NVAF. Hypertension was the most prevalent risk factor for AF, present in 68.5% of patients from India and in 76.3% of patients globally (P < 0.001). Diabetes and coronary artery disease (CAD) were prevalent in 36.2% and 28.1% of patients as compared with global prevalence of 22.2% and 21.6%, respectively (P < 0.001 for both). Antiplatelet therapy was the most common antithrombotic treatment in India. With increasing stroke risk, however, patients were more likely to receive oral anticoagulant therapy [mainly vitamin K antagonist (VKA)], but average international normalized ratio (INR) was lower among Indian patients [median INR value 1.6 (interquartile range {IQR}: 1.3-2.3) versus 2.3 (IQR 1.8-2.8) (P < 0.001)]. Compared with other countries, patients from India had markedly higher rates of all-cause mortality [7.68 per 100 person-years (95% confidence interval 6.32-9.35) vs 4.34 (4.16-4.53), P < 0.0001], while rates of stroke/systemic embolism and major bleeding were lower after 1 year of follow-up. CONCLUSION: Compared to previously published registries from India, the GARFIELD-AF registry describes clinical profiles and outcomes in Indian patients with AF of a different etiology. The registry data show that compared to the rest of the world, Indian AF patients are younger in age and have more diabetes and CAD. Patients with a higher stroke risk are more likely to receive anticoagulation therapy with VKA but are underdosed compared with the global average in the GARFIELD-AF. CLINICAL TRIAL REGISTRATION-URL: http://www.clinicaltrials.gov. Unique identifier: NCT01090362

Kinetics of a fast moving twinning dislocation

Constitutive models for plastic deformation in materials subjected to high rate loading conditions require kinetic descriptions of moving dislocations and moving interfaces. In materials that exhibit twinning, the velocities with which a twinning dislocation and a twin boundary can propagate has implications on the material behavior at high strain rates‑specifically the rate of plastic deformation and the rate sensitivity. In this study, we focus our attention on motion of a twinning dislocation in a face-centered cubic nickel. We use molecular dynamics simulations to simulate a moving twinning dislocation and investigate the effects of changing shear stress. Results suggest the material speeds have an influence on the velocities with which a twinning dislocation can propagate. Velocities from simulations will be related to observations from impact experiments

Thermomechanical Properties of Polycrystalline Vanadium in Compression

The mechanical behavior of commercially pure polycrystalline vanadium (99.8%) has been characterized in compression over a strain rate range of 104 to 6x103 s-1. In addition, the temperature dependence of the flow stress of vanadium has been examined over a range of temperatures from 25 to 700 °C for a strain rate of 103 s-1. Optical metallography of the deformed specimens revealed no twins, deformation bands or microcracks, suggesting that the deformation proceeded largely by crystallographic slip. The strain rate and temperature dependence of the strain hardening rate and flow stress is indicative of a BCC material, although a pronounced local peak in the flow stress was observed at about 400 °C.Le comportement mécanique de polycristaux de Vanadium (pureté commerciale : 99,8%) a été caractérisé en compression pour des taux de déformation allant de 104 à 6.103 s-1. L'évolution de la limite d'élasticité en fonction de la température a par ailleurs été analysée sur une plage de 25 à 700 °C. L'observation en microscopie optique des échantillons déformés a montré l'absence de mâcle, de bande de déformation ou de micro-fissure, ce qui suggère un mode de déformation par glissement cristallographique. A l'exception d'une anomalie de limite élastique vers 400 °C, le taux de déformation et l'évolution de la limite d'élasticité en fonction de la température est caractéristique des métaux cubiques centrés

DYNAMIC BEHAVIOR OF ELASTOHYDRODYNAMIC LUBRICANTS IN SHEARING AND COMPRESSION

An experimental investigation has been conducted into the response of elastohydrodynamic (EHD) lubricants subjected to the high shear rates and pressures encountered in EHD lubrication and over comparable time scales. New techniques have been developed to measure the dynamic compressibility of EHD lubricants at pressures as high as 1 GPa, and the response to dynamic shearing at shear rates as high as 4.0 x 104 sec-1, over timescales of about 10-4 sec, and for a range of initial temperatures