关键词： Superhard TiN- and ZrN-based heterostructures   Nanocomposites   First-principles molecular dynamics   Dynamic stability   Ideal strength   Stress-strain relationships  

摘要： First-principles quantum molecular dynamics investigations of TiN(001)/AlN and ZrN(001)/AIN heterostructures with one and two monolayers (1 ML and 2 ML) of AIN interfacial layers were carried out in the temperature range of 0-1400 K with subsequent static relaxation. It is shown that the epitaxially stabilized cubic B1-AIN interfacial layers are preserved in all TiN(001)/AIN heterostructures over the whole temperature range. In the ZrN(001)/AlN heterostructures, the B1-AIN(001) interfacial layer exists at 0 K, but it transforms into a distorted one at 10 K consisting of tetrahedral AlN4, octahedral AIN6, and AIN5 units. The thermal stability of the interfaces was investigated by studying the phonon dynamic stability of the B1-AlN phase with different lattice parameters. The calculations showed that the B1-AIN interface should be unstable in ZrN(001)/AIN heterostructures and nanocomposites, and in those based on transition metal nitrides with lattice parameters larger than 4.4 A. Electronic band structure calculations showed that energy gap forms around the Fermi energy for all interfaces. The formation of the interfacial AIN layer in TiN and ZrN crystals reduces their ideal tensile and shear strengths. Upon tensile load, decohesion occurs between Ti (Zr) and N atoms adjacent to the 1 MLAIN interfacial layer, whereas in the case of 2 ML AIN it occurs inside the TiN and ZrN slabs. The experimentally reported strength enhancement in the TiN/AIN and ZrN/AlN heterostructures is attributed to impeding effect of the interfacial layer on the plastic flow. (C) 2014 Elsevier B.V. All rights reserved.

关键词： MATERIALS -- Mechanical properties   GRAPHENE   MOLECULAR dynamics   ORIENTATION   CHIRALITY  

摘要： The mechanical properties of graphene may be strongly affected by defects. In our work, the effects of the orientation and tilting angles of Stone-Thrower-Wales (STW, 5-7-7-5) defects on the mechanical properties of graphene have been investigated based on molecular dynamics simulations. When there is one centred STW defect including STW-1 defect and STW-2 defect, our study reveals that the orientation with respect to the chirality governs the mechanical properties of graphene. For STW-1 defective graphene, the strength of the armchair direction is much lower than that of zigzag direction. While the circumstance for STW-2 defective graphene is opposite, the strength of the armchair direction is much higher than the strength of the zigzag direction. Furthermore, when there is more than one STW defect, the mechanical properties of graphene depends on the tilting angle of STW defects. The breaking strength of graphene decreases with the increasing tilting angle. Our present work could provide significant insights into the effect of STW defects on the mechanical properties of graphene. (C) 2014 Elsevier Ltd. All rights reserved.

关键词： Osteopontin   Hydroxyapatite   Mechanical property   Interface   Molecular dynamics  

摘要： Bone is characterized with an optimized combination of high stiffness and toughness. The understanding of bone nanomechanics is critical to the development of new artificial biological materials with unique properties. In this work, the mechanical characteristics of the interfaces between osteopontin (OPN, a noncollagenous protein in extrafibrillar protein matrix) and hydroxyapatite (HA, a mineral nanoplatelet in mineralized collagen fibrils) were investigated using molecular dynamics method. We found that the interfacial mechanical behavior is governed by the electrostatic attraction between acidic amino acid residues in OPN and calcium in HA. Higher energy dissipation is associated with the OPN peptides with a higher number of acidic amino acid residues. When loading in the interface direction, new bonds between some acidic residues and HA surface are formed, resulting in a stick-slip type motion of OPN peptide on the HA surface and high interfacial energy dissipation. The formation of new bonds during loading is considered to be a key mechanism responsible for high fracture resistance observed in bone and other biological materials. (C) 2014 Elsevier Ltd. All rights reserved.

关键词： Bileaflet mechanical heart valve   Immersed boundary method   Hemodynamics   Left ventricle   Cardio-vascular flows   Validation  

摘要： A computational study of the flow-structure interaction of a bileaflet mechanical heart valve in the mitral position is presented. Flow in a simple model of the left ventricle is simulated using an immersed boundary method, and the dynamics of the valve leaflets are solved in a fully-coupled manner with the flow. Simulations are conducted for two distinct valve orientations and multiple valve hinge locations, and the performance of the valve is compared in terms of metrics associated with leaflet motion, mitral regurgitation, and mechanical energy losses through the valve. Results indicate that a bileaflet mechanical heart valve with a more centrally located hinge, and implanted in the anatomical orientation provides the best overall performance. The fluid and leaflet dynamics, as well as the clinical implications underlying these findings are discussed.

关键词： Composite beam   Nonlinear vibrations   Thermal loading   Bifurcations   Chaotic oscillations  

摘要： Large amplitude vibrations of a Timoshenko beam under an influence of temperature are analysed in this paper. In the considered model the temperature increases instantly and the heat is uniformly distributed along the beams length and cross-section. The mathematical model, represented by partial differential equations takes into account thermal and mechanical loadings. Next, the problem is reduced by means of the Galerkin method, considering the first three natural vibration modes of a simply supported beam in the both ends. The influence of the temperature on amplitudes and localisation of the resonance zones and stability of the solutions is studied numerically and analytically by the multiple time scale method. The bifurcation points, existence of unstable lobes and transition from regular to chaotic oscillations are shown.

关键词： Dielectric spectroscopy   Liquid-glass transition   Relaxation dynamics  

摘要： Dielectric and mechanical spectroscopy methods have been employed to describe the temperature dependencies of the segmental and macromolecular relaxation rates in epoxy/hydroxy functionalized polybutadiene. Dielectric studies on the dynamics of segments of the polymer as well as the mobility of small ions trapped in the system have been carried out both as a function of temperature and pressure under isobaric and isothermal conditions, respectively.

关键词： POLYETHYLENE synthesis   MOLECULAR weights   POLYMERIZATION   PHASE transformations (Physics)   CHEMICAL kinetics   MATERIALS -- Mechanical properties  

摘要： The synthesis of linear ultra high molecular weight polyethylene, with a "pseudoliving" catalyst in various conditions results in samples of different molecular weights (M-w ranging between 2 to 35 million g/mol), all having a reduced number of entanglements to an extent that allows the solid-state uniaxial deformation of such high molar masses without melting. The solid-state processing of these materials shows a dear relationship between mechanical properties and molecular weight. For the adopted polymerization conditions, stretching forces required for the uniaxial deformation increase with the increasing molar mass, ultimately limiting the achievable maximum draw ratio in the polymers having M-w > 10 million g/mol. The increase in the stretching force is attributed to the increasing number of entanglements between the crystals with the molar mass. The estimation of entanglements is established with the help of melting kinetics involved in the "disentangled" crystals, and rheological response of the polymer melt obtained just after melting of the crystals. In spite of the increase in the stretching forces with the increasing molar mass, tensile modulus increases with the increasing draw ratio and the molecular weight. However, above the molar mass of 10 million g/mol, the stretching force required increases to the level that the uniaxial deformation becomes difficult thus limiting the tensile strength.

关键词： Plasticity   Graphite   Shock wave effects   Molecular dynamics   Friction  

摘要： We study the shock-induced hot spot formation mechanism of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine-based explosives by molecular dynamics, compare different kinds of desensitizers and different shock velocities. A set of programs is written to calculate the physical picture of shock loading. Based on the simulations and analyses, the hot spots are found at the interface and are heated by plastic work in three ways: the interface intrinsic dissipation, the pore collapse, and the coating layer deformation. The work/heat transition rate is proved to be increasing with a loading speed. (C) 2014 AIP Publishing LLC.

关键词： computational fluid dynamics   heart failure   left ventricular assist device  

摘要： Many end-stage heart failure patients are not eligible to undergo heart transplantation due to organ shortage, and even those under consideration for transplantation might suffer long waiting periods. A better understanding of the hemodynamic impact of left ventricular assist devices (LVAD) on the cardiovascular system is therefore of great interest. Computational fluid dynamics (CFD) simulations give the opportunity to study the hemodynamics in this patient population using clinical imaging data such as computed tomographic angiography. This article reviews a recent study series involving patients with pulsatile and constant-flow LVAD devices in which CFD simulations were used to qualitatively and quantitatively assess blood flow dynamics in the thoracic aorta, demonstrating its potential to enhance the information available from medical imaging.

关键词： 呼气末正压通气(PEEP)   复苏后综合征(PRS)   心率(HR)   血氧饱和度(SaO2)   血管外肺水(EVLW)   胸腔内血容量(ITBV)   每搏输出量变异度(SVV)   全心射血分数(GEF)  

摘要： 目的观察不同水平的呼气末正压通气（PEEP）对复苏后综合征（PRS）患者血流动力学的影响。方法记录15例实施机械通气的PRS患者在不同PEEP水平时的心率（HR）、血氧饱和度（SaO2）、血管外肺水（EVLW）、胸腔内血容量（ITBV）、每搏输出量变异度（SVV）、全心射血分数（GEF），采用单因素方差分析和一元线性回归模型对数据进行分析。结果不同水平PEEP时EVLW、SVV、SaO2比较差异有统计学意义（P均〈0．05）。其中EVLW与PEEP呈显著正相关（r=0．572，P=0．013），SVV与PEEP呈显著正相关（r=0．498，P=0．021），SaO2与PEEP无显著相关性（r=-0．219，拟合度=0．048，P=0．032），而GEF、ITBV、HR与PEEP呈非线性相关。结论PEEP可影响PRS患者的血流动力学，使实际测得的EVLW、SVV值偏高。