关键词： Liquid alloy Ni64Zr36   Pressure effect   Microstructural evolution   Molecular dynamics simulation   Cluster-type index method  

摘要： To deeply understand the effects of pressure on microstructural evolutions and mechanical properties, a molecular dynamics simulation study has been performed under pressures of 0-50 GPa for liquid Ni64Zr36 alloy during solidification process. Adopting several microstructural analyzing methods, especially the cluster-type index method (CTIM-3) to analyze the local microstructures in the system, it is found that the icosahedral cluster (12012000000) and the defect icosahedral cluster (1208000220), (1228200000) play a key role during the microstructural transitions. All the solidified structures are of amorphous structures consisted of various basic clusters and their combinations with different sizes. It is worth noting that all the glass transition temperature T-g, main bond-types, main basic clusters and elastic constants are almost increased linearly with the increase of pressure. Especially, the amorphous structure formed under 50 GPa would possess the admirable microstructural configuration and excellent mechanical properties of materials. (C) 2017 Elsevier B.V. All rights reserved.

关键词： MOLECULAR simulation methods   LIQUID ammonia   RESIDENCE time (Biogeochemistry)   SOLVATION   COORDINATION number (Chemistry)  

摘要： Structure and dynamics of solvated Ca2+ in liquid ammonia have been investigated using the quantum mechanical charge field molecular dynamics simulation approach, including the ion and its entire first plus second solvation shells in the QM zone. The Ca2+-ammonia interactions were calculated using ab initio HF calculations with Def2-SVP and DZP for Ca2+ and ammonia, respectively. A labile first shell structure at 2.63 A was observed with the dominant coordination numbers being 7 and 8. A number of exchange processes of ammonia ligands occurred throughout the simulation with a first shell mean ligand residence time of 12.9 ps. (C) 2017 Elsevier B.V. All rights reserved.

关键词： POLYMERIC nanocomposites   GUMS & resins   CROSSLINKING (Polymerization)   MOLECULAR dynamics   MECHANICAL behavior of materials   YOUNG'S modulus   DENSITY  

摘要： Finite-element method and micromechanical formulations have been used to predict mechanical properties of epoxy-based nanocomposites in recent years. The results of these investigations, however, show large scatters and do not agree well with experimental measurements. The differences in the results may be due to the fact that these approaches do not take some important material parameters into account, such as resin crosslinking ratio, nanotube/matrix interface, and nanotube agglomeration. The main objective of this article is to investigate the effects of resin crosslinking ratio on mechanical properties of epoxy-based nanocomposites using molecular dynamics (MD). First, models of partially cured epoxy resin were created in Materials Studio software to determine resin properties at different crosslinking ratios. Next, one sample of neat epoxy and four samples of nanocomposite with different resin crosslinking ratios were modeled using MD. These models were analyzed using constant strain method to determine mechanical properties of the five samples. Also, the effects of crosslinking ratio on nanocomposite density were investigated. The simulation results show that nanocomposite Young's modulus increases with resin crosslinking ratio. Resin crosslinking ratio, however, did not have a significant effect on nanocomposite density. Further, molecular simulation results were compared with rule of mixtures and Mori-Tanaka predictions. In addition, the results were compared with those found in the literature and good agreement was observed between the results. (C) 2016 Society of Plastics Engineers

关键词： Mechanical systems with impact and friction   Contact detection algorithm   Mechanical toys   Rolling bearing mechanism  

摘要： In this work, some new results are presented on the dynamics of a class of multibody mechanical systems, involving contact and friction. The main contribution refers to the development of a systematic, accurate and efficient method for detecting contact among the components of a system of solid bodies. For some simple geometries, this task is achieved by employing analytical means. For systems possessing components with complex geometric shapes a more involved numerical methodology is developed. In both cases, once a potential contact point is detected, the common tangent plane and normal vector are located and the penetration depth is calculated, leading to determination of the force arising between the contacting bodies. This information is then passed to a solver, providing the full dynamic response of the system. The validity and numerical efficiency of the methodology developed is first demonstrated by considering a number of examples with relatively small geometric complexity but large traditional value and interesting dynamic response. Some new results are obtained and presented on the dynamics of these systems. Finally, the same methodology is also tested in a more complicated and demanding mechanical application. (C) 2016 Elsevier Ltd. All rights reserved.

关键词： Carbon nanotube   mechanical properties   molecular dynamics   graphene   thermal conductivity  

摘要： Single layer graphene sheets and carbon nanotubes have resulted in the development of new materials for a variety of applications. Though there are a large number of experimental and numerical studies related to these nanofillers, still there is a lack of understanding of the effect of geometrical characteristics of these nanofillers on their mechanical properties. In this study, molecular dynamics simulation has been used to assess this issue. Two different computational models, single layer graphene sheets-copper and carbon nanotube-copper composites have been examined to study the effect of nanofiller geometry on Young's modulus and thermal conductivity of these nanocomposites. Effect of increase in temperature on Young's modulus has also been predicted using molecular dynamics. The effect of nanofiller volume fraction (V-f) on Young's modulus and thermal conductivity has also been studied. Results of thermal conductivity obtained using molecular dynamics have been compared with theoretical models. Results show that with increase in V-f the Young's modulus as well as thermal conductivity of single layer graphene sheets-Cu composites increases at a faster rate than that for carbon nanotube-Cu composite. For the same V-f, the Young's modulus of single layer graphene sheets-Cu composite is higher than carbon nanotube-Cu composite.

关键词： borides   hardness   molten-salt boriding   pure titanium   wear properties  

摘要： To modify the surface properties of pure titanium, boride layers had been fabricated by the boron molten-salt diffusion on pure titanium surfaces in the temperature range of 900-1100 A degrees C for 5- to 30-h treatments. The results demonstrated that the boride layers were mainly composed of TiB whiskers and TiB2 layers without the rutile titanium oxide TiO2. Two diffusion models were introduced to model the growth kinetics of boride layers. The parabolic growth constants and the boron diffusion coefficients were obtained. The boron activation energies for TiB2 and TiB were 225.617 and 165.266 kJ mol(-1), respectively. The surface microhardness of the borided titanium decreased with the increase in distance from the surface. The results of wear tests indicated that the wear properties had been improved significantly compared to the pure titanium under dry sliding conditions.

关键词： Hydrodynamic non-contact mechanical seal   Cryogenic   High speed   Two-phase flow   Leakage   Turbopump  

摘要： The non-contact mechanical seal of a high-speed turbopump in a liquid rocket engine operates under very harsh conditions (such as rapid start-up, cryogenic, highspeed, high- pressure and low-viscosity sealing fluid). The performance of the seal is very different to the performance under normal running conditions. In this paper, for the sake of safety, an experiment is carried out with liquid nitrogen as the sealing fluid. The experimental results with liquid nitrogen are expected to provide an equivalent seal performance as would be experienced with liquid oxygen and liquid hydrogen rocket engine. The main performance parameters, including face temperatures, leakage, face friction force, and friction coefficients, are measured in the speed-up, stable, and speed-down stages. The results show that in the speed-up stage, with rapidly increasing speed, the local face temperature rises dramatically to even higher than the vaporization temperature of liquid nitrogen, and a two-phase flow phenomenon occurs. In the start-up and stable stages, the friction coefficients are 0.25 and 0.13, respectively. After the test, it was found that the wear thickness of the rotor was 0.2 mm, and serious point corrosion appeared on the surface of the stator.

关键词： graphene   mechanical properties   molecular dynamics  

摘要： This work reports estimation of mechanical properties, particularly Young's modulus of a single-layered graphene sheet by molecular dynamics (MD) simulation-based four different approaches, viz. tensile modeling, bending modeling, oscillation modeling and equilibrium MD modeling. The Young's modulus is estimated to be of the order of some TPa. The equilibrium MD method has yielded a Young's modulus value lower than the other non-equilibrium methods, due to the absence of any external forcing factor. Among the non-equilibrium MD methods, the bending modeling is found to predict the highest value of Young's modulus. Comparison among different non-equilibrium methods has established the effect of strain rate on the estimated value of the Young's modulus. The MD simulation-based approaches adopted here can be useful for the design of graphene and graphene-based materials in advanced mechanical applications.

关键词： wheel loader   hydrodynamic mechanical power split transmissions (HMPSTs)   torque converter (TC)   fuel economy   design optimization  

摘要： The fuel economy of wheel loaders is deeply affected by the efficiency of their propelling transmissions, however, the torque converter (TC) in existing propelling transmissions is a low-efficiency component and leads to excessive energy consumption. Accordingly, this paper replaces the TC with a hydrodynamic mechanical power split transmission (HMPST) for improving the fuel economy of wheel loader. Based on probability similarity theory, the typical operating mode for the vehicles is constructed, which is used to evaluate the energy consumption performance of the selected solutions. The four reasonable solutions, which can be initially applied to wheel loaders, are selected from among the HMPSTs using the lever diagram. Furthermore, the comparisons on efficiency and loading characteristics between these four solutions and a prototype TC are conducted. The design optimization for all the four solutions is implemented, in order to find the optimal fuel saving solution relative to the prototype TC, and only one solution with pure power split can meet the constraints. Finally, a simulation model of the wheel loader powertrain is established for validating the effectiveness of this optimal solution. The results show that the optimized solution can effectively improve the fuel economy of wheel loaders compared to the prototype TC and provides a novel substitute for current technology.

关键词： rat   isolated cardiac myocytes   nitric oxide   mechanical stretch   fluorescent detection  

摘要： Discrete mechanical stretch of isolated spontaneously contracting cardiac myocytes was employed to examine the kinetics of NO production in these cells. NO oscillations were detected with fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate. The mechanisms underlying stretch-induced changes in NO concentration remain unclear and further studies are needed to evaluate the role of NO oscillation in the regulation of cardiomyocyte function.