关键词： Molecular dynamics simulations   Mechanical properties   Stress-strain curve   alpha-graphyne nanotubes   alpha 2-graphyne nanotubes  

摘要： In this paper, molecular dynamics simulations are applied to study the mechanical properties of two types of graphyne nanotubes;including alpha-graphyne and alpha 2-graphyne nanotubes. The effects of the geometrical properties of the nanotubes on the stress-train curve, Young's modulus, fracture strain and maximum stress are studied. It is observed that Young's modulus of the alpha-graphyne nanotubes are significantly smaller than Young's modulus of the alpha 2-graphyne nanotubes with the same geometrical parameters. Similarly, the maximum stress tolerated by the armchair alpha 2-graphyne nanotubes are larger than the armchair alpha-graphyne nanotubes. However, alpha-graphyne nanotubes tolerate larger fracture strains than the alpha 2-graphyne nanotubes. It is shown that increasing the nanotube diameter leads to increase in Young's modulus of the alpha-graphyne nanotubes and alpha 2-graphyne nanotubes. However, increasing the nanotube lengths has an inverse effect on Young's modulus of the considered nanostructures.

关键词： Carbon nanotubes  

摘要： Although carbon nanotube (CNT) is an excellent additive in geopolymer, the effect of its chemical bonding at interfaces is not well understood. This paper explores the effects of interfacial bonding on the structure, dynamics and mechanical properties of CNT-geopolymer nanocomposites via molecular dynamics simulations. Our findings from the structural analysis reveal that the interfacial bonding decreases the thickness of the "interfacial van der waals (vdW) excluded volume" and interphase (inner) region, and condenses the interfacial structure. Remarkably, it is found that the interfacial bonding provides a new "restrictive mechanism" limiting the diffusion of components in the geopolymer matrix. A dynamical heterogeneous character is found as the atom mobility of the inner region is slower than that of the outer region. Furthermore, the pull-out test reveals about 15 times increase in the interfacial shear strength of CNT-geopolymer nanocomposite with 10.0% interfacial bonding concentration, which is beneficial for the enhancement of its mechanical properties and hinders crack formation under tensile loading. The tensile test reveals about 60% and 80% increase in the nanocomposite's Young's modulus and tensile strength, respectively. This study provides, for the first time, valuable insight into the role of the interfacial bonding on the performance of CNT-geopolymer nanocomposites. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Cylinder head   in-cylinder combustion   computational fluid dynamics analysis   transient thermal   thermo-mechanical fatigue  

摘要： A finite element analysis-computational fluid dynamics coupled analysis on the thermo-mechanical fatigue of cylinder head of a turbo-charged diesel engine was performed, and the complete simulation process is illustrated in this paper. In-cylinder combustion analysis and water jacket coolant flow analysis were conducted to provide heat transfer boundary conditions to the temperature field calculation of the cylinder head. Comparing with the conventional finite element analysis of cylinder head by which the heat transfer boundary conditions of the combustion and coolant sides are estimated, the present method coupled the three-dimensional combustion computational fluid dynamics and coolant computational fluid dynamics with the finite element analysis. Both computational fluid dynamics and finite element analysis obtain more accurate boundary conditions on their interface from each other, and thus, the present method improves accuracy of thermo-mechanical fatigue prediction. Based on the measured material performance parameters such as stress-strain curve under different temperatures and E-N curve, creep, and oxidation data material performance, the cylinder head-gasket-cylinder block finite element transient stress-strain field was calculated using ABAQUS. The thermo-mechanical fatigue analysis of cylinder head submodel was performed by using FEMFAT software that is based on the Sehitoglu model to predict the thermo-mechanical fatigue life of cylinder head. By comparing the measured and predicted temperatures of cylinder head, the temperature results showed a good agreement, and the error is less than 10%.

关键词： Microwave hybrid sintering   Alumina-zirconia composite   Fracture toughness   Stress-induced transformation toughening   Activation energy  

摘要： Structural ceramics such as Al2O3 and Al2O3-ZrO2 composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO2 crystal structures on Al2O3-ZrO2 composites, especially on the sintering kinetics, when fabricated by microwave sintering. The microwave hybrid sintering of Al2O3 and Al2O3-ZrO2 composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an alpha-alumina matrix, and the mechanical and thermal properties were studied. It was found that Al2O3 experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO2 was added. Al2O3-c-ZrO2 also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 +/- 11 kJ/mol for pure Al2O3, 319 +/- 71 kJ/mol for Al2O3-c-ZrO2, and 342 +/- 77 kJ/mol for Al2O3-t-ZrO2. These values indicated that the activation energy was increased by the addition of either type of ZrO2, with the highest value shown by Al2O3-t-ZrO2.

关键词： Multifunctional composites   Environmental degradation   Mechanical testing   Extrusion  

摘要： Bilayer biodegradable, eco-friendly films were prepared by co-extrusion film blowing, coupling polylactic acid (PLA) and Bio-flex (R) (BIO). Furthermore, in the PLA layer, carvacrol (CRV) was added as a natural antimicrobial additive, whereas a nanoclay (D72T) was integrated to protect CRV from volatilization and to modulate release. The materials were analyzed by morphological, chemical-physical, mechanical testing. Furthermore, CRV release and degradation tests were performed. The results pointed out that coupling the two matrices allows gathering the stiffness of PLA with the ductility of BIO. Furthermore, the interlayer adhesion is promoted by CRV. D72T exerts a key-role in avoiding CRV volatilization, thus allowing more prolonged release. Degradation tests results highlight that bilayer films, while being particularly resistant in both acidic and neutral environments, showed a rapid degradation under alkaline conditions, which proved to be intermediate between those of the corresponding monolayers.

关键词： Blood flow   Cartesian grid generation   computational fluid dynamics   mechanical heart valve   non-Newtonian flow   shear stress  

摘要： In this paper, we perform a numerical analysis for simulating steady, two-dimensional, laminar blood flow through our proposed design, known as the Butterfly mechanical heart valve, where the leaflets are fully opened. Blood has been assumed to be Newtonian and non-Newtonian fluid using the Casson model for shear-thinning behavior. A non-uniform Cartesian grid generation technique is presented to generate a two-dimensional grid for the irregular geometry of the Butterfly valve. The governing Navier-Stokes equations of flow, written in a stream function-vorticity formulation, are solved by the finite difference method with hybrid differencing of the convective terms. The computed results show that the blood's non-Newtonian nature significantly affects the flow field with the existence of recirculation and consequently stagnation causing thrombus formation, as well as an increase of the shear stress along the wall, which contributes to hemolytic blood damage. The results demonstrate that the model is capable of predicting the hemodynamic features most interesting to physiologists. It can be used to assess thromboembolic problems occurring with heart valves and in the design of cardiac prostheses.

摘要： Background: The authors' previous studies have found that spinal protein kinase C gamma expressing neurons are involved in the feed-forward inhibitory circuit gating mechanical allodynia in the superficial dorsal horn. The authors hypothesize that nerve injury enhances the excitability of spinal protein kinase C gamma expressing interneurons due to disinhibition of the feed-forward inhibitory circuit, and enables A beta primary inputs to activate spinal protein kinase C gamma expressing interneurons. Methods: Prkcg-P2A-tdTomato mice were constructed using the clustered regularly interspaced short palindromic repeats and clustered regularly interspaced short palindromic repeats-associated nuclease 9 technology, and were used to analyze the electrophysiologic properties of spinal protein kinase C gamma expressing neurons in both normal conditions and pathologic conditions induced by chronic constriction injury of the sciatic nerve. Patch-clamp whole cell recordings were used to identify the nature of the dynamic synaptic drive to protein kinase C gamma expressing neurons. Results: A beta fiber stimulation evoked a biphasic synaptic response in 42% (31 of 73) of protein kinase C gamma expressing neurons. The inhibitory components of the biphasic synaptic response were blocked by both strychnine and bicuculline in 57% (16 of 28) of neurons. Toll-like receptor 5 immunoreactive fibers made close contact with protein kinase C gamma expressing neurons. After nerve injury, the percentage of neurons double-labeled for c-fos and Prkcg-P2A-tdTomato in animals walking on a rotarod was significantly higher than that in the nerve injury animals (4.1% vs. 9.9%, 22 of 539 vs. 54 of 548,P < 0.001). A beta fiber stimulation evoked burst action potentials in 25.8% (8 of 31) of protein kinase C gamma expressing neurons in control animals, while the proportion increased to 51.1% (23 of 45) in nerve injury animals (P = 0.027). Conclusions: The Prkcg-P2A-tdTomato mice the authors construct

关键词： Molecular dynamics simulations   alpha-graphyne   alpha(2)-graphyne   Young's modulus   Fracture strain   Maximum stress  

摘要： Molecular dynamics simulations are used to study the mechanical properties of alpha-graphyne and alpha(2)-graphyne. The square and rectangular alpha-graphyne and alpha(2)-graphyne with different dimensions are modelled. It is seen that for both of the considered nanosheets, Young's modulus is smaller than along the armchair direction than along the zigzag direction. Similarly, zigzag alpha-graphyne has larger fracture strain than the armchair one. For the alpha(2) graphyne, however, the fracture strain along the armchair direction is larger than zigzag direction. Besides, it is observed that Young's modulus of the alpha-graphyne is smaller than alpha(2)-graphyne for both of the armchair and zigzag structures. However, the alpha-graphyne possesses larger fracture strain than alpha(2)-graphyne. It is also seen that Young's modulus of the square alpha-graphyne and alpha-graphyne nanosheet is not dependent on the nanosheet dimension. However, fracture strain and maximum stress of these nanosheets decrease by increasing the nanosheet size. Finally, the Poisson's ratio of the alpha-graphyne and alpha(2)-graphyne is obtained as large as 0.9 for some sizes.

关键词： Ball screw system   Thermo-mechanical dynamics model   Dynamic thermal network model   Natural frequency  

摘要： The ball screw system operates under the coupling condition of external loads, preloads, and thermal loads, and its thermo-mechanical characteristics significantly affect its dynamics performance. Therefore, a thermal-mechanical real-time dynamics model is necessary. In this work, the screw systems thermo-mechanical coupling mechanism was first analysed. Second, a new dynamic thermal network model for the screw system with moving heat excitation was proposed using the position of the moving nut obtained by FANUC Open CNC API Specifications (FOCAS), which can rapidly determine the transient temperature field. Based on this model, a new thermo-mechanical coupling real-time dynamics model of the screw system was established. The influence of temperature on dynamics characteristics was analysed using the dynamics model, and the results showed that the system preload caused by thermal elongation of the screw decreases with an increase in temperature. The system preload caused by the thermal deformation of the bearing increases with an increase in temperature. The total preload of the system depends on the interaction between the thermal deformation of the screw and the axial thermal deformation of the bearing. Additionally, the system stiffness and natural frequency variation relative to the increase in temperature and position of the carriage were obtained. Thermo-mechanical dynamics performance experiments of the feed system were carried out to validate the effectiveness of the proposed model.

关键词： Uranium silicides   U3Si2   Nuclear fuel   DFT plus U   BTE   Thermal conductivity  

摘要： Triuranium diSilicide (U3Si2) has been identified as an accident reSistant alternative to uranium dioxide (UO2) based fuels. While experimental studies of U3Si2 are not new, only few studies on the thermal conductivity has been reported. Moreover, theoretical studies of the thermal properties of U3Si2 are scarce. We found that inaccuracies in experimental data were a source of discrepancies in recently reported theoretical results, as such need to be addressed. In this work, the structural, thermal, and mechanical properties of, the non-magnetic and magnetic phases, of U3Si2 were investigated uSing Hubbard corrected denSity functional theory (DFT+U) and molecular dynamics (MD) Simulations. The lattice and electronic thermal properties were determined based on Boltzmann transport equation (BTE) approaches. A conSiderable anisotropy was determined the lattice and electronic contributions to the thermal conductivity from DFT + U/BTE results, in which the electronic contribution is predominant. A relative good agreement between the lattice contributions to the thermal conductivity, predicted from DFT+U and MD computations, was associated to Similarities in the acoustic phonon modes. The present computational results Significantly enhance the understanding of the thermal properties of U3Si2. The good agreement between DFT + U/BTE results with experimental data confirm the accident tolerant characteristic of U3Si2, but also further demonstrating the accuracy of the DFT( + U)/BTE approach for the investigation of thermal properties of advanced nuclear fuels. Crown Copyright (c) 2020 Published by Elsevier B.V. All rights reserved.