关键词： dynamical system   chaos   attractor   hyperbolic dynamics   Lyapunov exponent   Smale - Williams solenoid   parametric oscillations  

摘要： Computer verification of hyperbolicity is provided based on statistical analysis of the angles of intersection of stable and unstable manifolds for mechanical systems with hyperbolic attractors of Smale-Williams type: (i) a particle sliding on a plane under periodic kicks, (ii) interacting particles moving on two alternately rotating disks, and (iii) a string with parametric excitation of standing-wave patterns by a modulated pump. The examples are of interest as contributing to filling the hyperbolic theory of dynamical systems with physical content.

关键词： mitral valve   tricuspid regurgitation   valvular annuloplasty  

摘要： Objective: The aim of this study was to evaluate the long-term clinical and hemodynamic influences of prophylactic tricuspid annuloplasty (TAP) in patients with less-than-moderate tricuspid regurgitation (TR) who underwent mitral valve replacement (MVR). Methods: Between November 1994 and December 2010, 293 patients with less-than-moderate TR who underwent primary mechanical MVR were categorized into 2 groups: TAP (n = 151) or no TAP (n = 142). The median age was 51 years (quartile (Q)1-Q3, 43-59 years). The cause of valve pathology was rheumatic in 92.5% of patients (n = 271). The prevalence of preoperative atrial fibrillation was in 73.0%. Using propensity score matching based on demographic information, 91 TAP patients could be matched to 91 no TAP patients. Median follow-up duration was 107 months (Q1-Q3, 76-162 months). Results: There was no early mortality in either group. Early morbidities, including heart block were not different between groups. Although overall survival and freedom from cardiac-related mortality did not differ between groups (P = .519 and P = .115, respectively), freedom from recurrence of moderate or higher TR grade were significantly higher in the TAP group (P = .043). In subgroup analyses, these group differences of TAP were especially prominent in patients with sinus rhythm compared with patients with atrial fibrillation at discharge (P = .047 vs P = .460). Conclusions: Prophylactic TAP for patients with less-than-moderate TR grade who underwent mechanical MVR can prevent late TR progression without increasing early surgical risks. Longer-term follow-up is required to determine the clinical beneficial effect of prophylactic TAP.

关键词： Metallic glasses   Inhomogeneous deformation   Rapid solidification   Molecular dynamics simulation  

摘要： The chemical short range order of metallic glasses is expected to be correlated with their mechanical properties. In this article, classic molecular dynamics simulations of amorphous Zr45Cu45Ag10 alloys were carried out to reveal such links in metallic glasses. Our calculations of Warren-Cowley parameter indicate the growth of chemical short range order during supercooling process, which also depends on the effective cooling rates. The chemical short range ordering is related to the energetic stability of the system. Based on the chemical preference or avoidance for different bonds, the model is separated into Cu-rich regions and Ag-rich regions. Simulated structural relaxation and shear loading process were performed to study how chemical bonds affect the distribution of dynamic and mechanical heterogeneity in our systems. The Cu-rich regions exhibit slower dynamics and higher shear resistance, whereas Ag-rich regions have faster dynamics and prefer to be plastically deformed. (C) 2015 Elsevier Ltd. All rights reserved.

关键词： Molecular dynamics simulation   Nanoindentation   Mechanical property   Polycrystalline   Twin  

摘要： Molecular dynamics (MD) simulations are performed to study the nanoindentation onto three different crystal structures including the single crystalline, polycrystalline, and nanotwinned polycrystalline copper. To reveal the effects of crystal structure and twin-lamellae-thickness on the response of nanoindentation, we evaluate the evolution of crystalline structure, dislocation, strain, indentation force, temperature, hardness, and elastic recovery coefficient in the deformation zone. The results of MD simulations show that the hardness, elastic recovery ratio and temperature of those three nanocrystalline copper strongly depend on crystal structure and twin-lamellae-thickness. It is also revealed that as nanoindenter goes deeper, the extent of plastic zone becomes substantially larger. Initial dislocation always nucleates at the beneath of indenter, and the discrete drops of indentation force observed at certain indentation depths, indicates dislocation bursts during the indentation process. In particular, the twining and detwining are dominant over the dislocation nucleation in driving plasticity in nanotwinned polycrystalline during nanoindentation, which are in good agreement with the previous work. Furthermore, we find that plastic deformation has a strong dependence on crystal structure. The plastic deformation of the single crystalline copper relies on the generation, propagation and reaction of dislocations, that of the polycrystalline copper depends on the dislocation-grain boundary (GB) interactions, and that of the nanotwinned polycrystalline copper relies upon the dislocation-twin boundary (TB) interactions as well as twining/detwining. This work not only provides insights into the effects of crystal structure and two lamellae-thickness on the mechanical properties of copper under nanoindentation, but also shed lights onto the guideline of understanding other FCC nanocrystalline materials. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights re

关键词： Nanoimprinting   Molecular dynamics   Amorphous Ni-Zr alloys   Mechanical behavior  

摘要： In this work, the molecular dynamics simulation is used to investigate the mechanical behavior of amorphous Ni-Zr alloys during the nanoimprinting process. These simulations include the alloy composition, working temperature, radial distribution function, and the recovery ratio. It has been shown that the hardness and the reduced modulus of amorphous Ni-Zr alloy grow with an increasing proportion of Zr atoms. Moreover, due to the kinetic energy of atoms the deformation degree of amorphous Ni-Zr alloys can be enhanced by using the higher sample temperature. The short-range order of individual fractal-like amorphous Ni-Zr alloy deformation mechanism has been also demonstrated. In addition, it has been shown that the pileup index also increases with temperature, i.e. atoms bonding energy at higher temperatures decreases, resulting in the material softening and the thermal expansion. And, the recovery ratio is found to be the alloy composition loading dependent. (C) 2015 Elsevier B.V. All rights reserved.

摘要： The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine system that regulates numerous physiological processes. Disruptions in the activity of the HPA axis are correlated with many stress-related diseases such as post-traumatic stress disorder (PTSD) and major depressive disorder. In this paper, we characterize "normal" and "diseased" states of the HPA axis as basins of attraction of a dynamical system describing the inhibition of peptide hormones such as corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) by circulating glucocorticoids such as cortisol (CORT). In addition to including key physiological features such as ultradian oscillations in cortisol levels and self-upregulation of CRH neuron activity, our model distinguishes the relatively slow process of cortisol-mediated CRH biosynthesis from rapid trans-synaptic effects that regulate the CRH secretion process. Crucially, we find that the slow regulation mechanism mediates external stress-driven transitions between the stable states in novel, intensity, duration, and timing-dependent ways. These results indicate that the timing of traumatic events may be an important factor in determining if and how patients will exhibit hallmarks of stress disorders. Our model also suggests a mechanism whereby exposure therapy of stress disorders such as PTSD may act to normalize downstream dysregulation of the HPA axis.

关键词： Graphite   HOPG   Irradiation defects   AIREBO   Molecular dynamics  

摘要： Molecular dynamics simulations of single crystal graphite have been performed, incorporating point and extended defect structures formed as a result of thermally annealed radiation damage. These AIREBO potential calculations have simulated the changes to crystal dimensions, elastic moduli and thermal expansion due to the formation of different representative morphologies of extended irradiation-induced defects (vacancy and interstitial aggregates) and over a range of atomic displacement fractions. We find that, in addition to the simulation method reproducing the important mechanical and thermal properties of virgin graphite, the property changes caused by the formation of extended defects in the graphite crystal structure qualitatively agree with experimental observations at different irradiation/annealing temperature regimes. The results of these calculations provide a direct insight into how the underlying atomic scale defects and dislocations created by radiation damage can lead to material property changes, and demonstrate how this computationally efficient simulation method can be employed to reproduce crystallite changes in large-scale models of polygranular graphite. (C) 2015 Elsevier B.V. All rights reserved.

关键词： 多体动力学   机械工程领域   机械臂  

摘要： 文章主要分析了多体动力学在机械工程领域中的应用,通过建立多体动力学模型,并对模型各方面进行详细的分析,并提出了相关的实际应用案例来阐述多体动力学在机械工程领域中的应用效果。