关键词： vibration energy harvester   SSHI   self-powered   mechanical switch  

摘要： To increase output power for piezoelectric vibration energy harvesters, considerable attention has recently been focused on a self-powered synchronized switch harvesting on inductor (SSHI) technique employing an electrical and mechanical switch. However, there are two technical issues: in a medium or highly coupled harvester, the piezoelectric coupling force, which increases as the SSHI's voltage increases, will reduce the harvester's displacement and the resulting output power, and there are few reports comparing the performance of electrical switch SSHI (ESS) and mechanical switch SSHI (MSS) that include consideration of the piezoelectric coupling force. We developed a simulation technique that allows us to evaluate the output power considering the piezoelectric coupling force, and investigated the performance of stopper-based MSS and ESS, both numerically and experimentally. The numerical investigation predicted the following: (1) the output power for the ESS is lower than that for the MSS at acceleration lower than 3.5 m s(-2);and (2) intriguingly, the output power for the MSS continues to increase, whereas the peak-peak displacement remains constant. The experimental results showed behaviour similar to that of the numerical predictions. The results are attributed to the different switching strategies: the MSS turns on only when the harvester's displacement exceeds the gap distance, while the ESS turns on at every maximum/minimum displacement.

关键词： TRIP   alpha '-martensite   epsilon-martenite   Austenite   Folding   Simulation  

摘要： In this paper, experimental and phenomenological modeling of the mechanical behavior and austenite-martensitic transformation after deformation in 304L TRIP steel is investigated. Mechanical behavior and the evolution of the microstructure after folding test were studied 25 A degrees C. Scanning electron microscopy (SEM), Vickers hardness measurements, and X-ray diffraction test were conducted in order to characterize the deformation response and microstructure evolution of 304L TRIP steel. The analysis results indicate that both the microstructure and mechanical properties of the 304L TRIP steel varied gradually during the tests, the later showed too that the most favorable way for the formation of martensite is tension. Secondly, a phenomenological constitutive model, developed by Iwamoto et al., was selected and modified to include the microstructure evolution (austenite to martensite) after deformation. The validity and the limitation of the simulations are checked against the experimentally established stress-strain relation and the evolution of martensite phase. The simulation results show that the volume fraction of martensite is proportional to the strain and it is very important in the tended zone than the compressed zone. The results above present a foundation for the folding control technology.

关键词： Ti-6A1-4V   Corrosion fatigue   Stress corrosion cracking   Aggressive environment  

摘要： The high strength-to-mass ratio and the appreciable corrosion resistance make the Ti-6A1-4 V alloy used in aerospace, automotive, maritime and biomedical applications. In this work the quasi-static behavior of this light alloy and its axial fatigue resistance in air, air + beeswax coating, paraffin oil, 3.5 wt% NaCI solution and various water-methanol solutions are presented in order to understand the effects of the medium and of the load. The contribution of the stress concentration factor K-t, calculated by means of finite element modelling, is investigated. A SEM (scanning electron microscopy) analysis was carried out in order to evaluate the micro structural modifications due to the environments and cyclic loads.

关键词： Geopolymer   CSH   ReaxFF   Fracture   Interface   Composite  

摘要： This paper for the first time used reactive molecular dynamics (MD) simulation to study the mechanical behavior of geopolymer binder (GB) and calcium silicate hydrate (CSH) composites. Specifically, GB-CSH composites with different Ca/Si ratios for the CSH phase were constructed and their mechanical behavior in terms of ultimate tensile strength, fracture toughness and strain energy release rates were investigated. It was observed that the Ca/Si ratio of CSH greatly affects the mechanical response of the composite. Increasing the Ca/Si ratio from 1.2 to 1.65 created more disorder in the silica layers in CSH and decreased the mechanical properties of the composite. However, the completely glassy CSH at a Ca/Si ratio of 2.0 showed slight improvement of mechanical properties due to high three-dimensional (3D) tetrahedral network. A detailed analysis of bond evolution and bond angle distribution showed their direct correlation with the observed mechanical response. The failure of the GB-CSH composites were always governed by the breaking of Si-O bonds within the CSH, near the GB-CSH interface. Low Ca/Si ratio structures showed more SieO bond breakage, while in high Ca/Si ratio structures, deformation mobilization was accompanied by significant change in bond angles within GB.

关键词： Chocolate   Conching   Lecithin   Phospholipids   Molecular dynamics  

摘要： The structural properties of phospholipid layers and micelles at sucrose crystal cocoa butter interfaces were investigated by coarse-grained molecular dynamics simulations to understand the molecular mechanisms and structures vital for the chocolate conching process. Influences of the different hydrophilic head groups phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidit acid of lecithin phospholipids as well as influences of the degree of saturation of aliphatic chains were investigated. Phospholipid monolayers at sucrose cocoa butter interfaces were shown to have similar molar composition as soy lecithin, indicating that all phospholipids adsorb with similar probability. Phospholipids with saturated aliphatic chains have smaller areas per lipid in the monolayer on the sucrose cocoa butter interface than unsaturated phospholipids. It was shown that phospholipids that are not adsorbed in the monolayer assemble in spherical, cylindrical, and wormlike micelles in cocoa butter, depending on the phospholipid concentration. Wormlike micelles were shown to be able to build a hydrophilic network in the cocoa butter medium. This is proposed as an explanation for the increasing yield values at higher lecithin concentrations in chocolate manufacturing which have been reported in several studies. The resulting structures of phospholipids on sucrose surfaces were related to experimental measurements from the literature. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： competition   context dependence   demography   ecological forecasting   interspecific interactions   relative abundance   species composition  

摘要： 1. What controls the distribution and abundance of organisms? This question, at the heart of the dynamics of ecological communities, would have been familiar to the earliest ecologists. Having lain effectively abandoned for many years, community dynamics today is a vibrant research topic of great conceptual interest with practical import for conservation, ecological management, ecosystem services and the responses of ecological communities to climate change. *** describe how modern coexistence theory can be used to predict community dynamics through the use of demography. We explore the challenges that limit the deployment of this demographic framework, and the tools from phylogenetic and functional ecology that have been used to surmount them. 3. Finding existing tools not altogether sufficient, we propose the use of hard functional traits and physiological tolerances of environmental conditions and low resource availability to extend the demographic framework so that the dynamics of a broader range of ecological communities can be accurately predicted. 4. We illustrate these new approaches with two case studies. Given the urgent need to accurately forecast the dynamics of ecological communities, we hope that many ecologists will adopt these tools.

关键词： Mechanical properties   ellipsoidal nanoshells   molecular dynamics   single crystal copper  

摘要： Single crystal copper ellipsoidal nanoshells under outer normal tensile loadings are investigated by the molecular dynamics method. Normal stress and Mises stress are introduced to describe the mechanical properties. The uniform thickness nanoshells, the variable thickness nanoshells and the variable radius nanoshells are simulated to elucidate the effect of thickness on yielding behaviors and other mechanical properties. Potential energies, stresses and dislocations of nanoshells are discussed in the paper. The dislocations of these nanoshells form an octagon or that with an external quadrangle. The variable thickness nanoshells break this shape slightly. The potential energies of nanoshells have stable stages and then increase. The outer normal stresses and Mises stresses of different models differ from eath other. The thickness of nanoshells affects the elastic stage and the variable thickness nanoshell has different mechanical properties with others. When the radiuses of nanoshells with the same thickness are different, their dislocation shapes are the pressed octagon. Thier normal yield stresses are different, but their Mises yield stress are same. Also, the outer shape determines the trend of curves. The structure of a sphere is steadier than that of an ellipsoid.

关键词： metal   composite   simulation  

摘要： Boron nitride nanotubes (BNNTs) have been utilized to strengthen various engineering materials especially metal matrix composites thanks to their extraordinary high tensile strength, elastic modulus, and failure strain. In this paper, single- and multi-walled BNNTs were therefore used to combine with aluminum (Al) metal matrix. Mechanical characteristics and deformation mechanism of nanocomposites reinforced with long (continuous) and short (discontinuous) BNNTs were then investigated for different loadings including uniaxial tension and compression and different boundary conditions based on molecular dynamics simulations. It was found that long BNNTs remarkably improved tensile mechanical properties of the matrix and effectively enhanced elastic modulus and strength of the nanocomposites by 82% and 79A%, respectively. They could provide effective barriers to propagation path of dislocations formed inside the matrix. Diameter and wall number of the reinforcement did not leave considerable impacts on the nanocomposite behavior while its atomic fraction remarkably influenced the material response.

关键词： Molecular dynamics simulation   Perfluorosulfonic acid   Side chain   Morphology   Mechanical deformation  

摘要： The side chain (SC) length of perfluorosulfonic acid (PFSA) reportedly influences the proton conductivity and mechanical strength of the membranes in the application of proton exchange membrane. Here, we conducted a series of molecular dynamics simulations to explore the effect of SC length on the morphology and mechanical properties of the PFSA membrane. The results of these simulations pointed to the stronger aggregation of the aqueous domain in the longer SC membranes. This leads to the swollen network structure of the aqueous domains at high water contents, rather than the tortuous layer structure observed in the shorter SC membrane. Furthermore, the mechanical simulation found that the longer SC membranes possessed lower and higher mechanical strengths than the shorter SC membranes at lower and higher water contents, respectively, resulting from the lower ionic strength and larger polymer domains. These findings will be useful for designing new materials for fuel cell applications. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： Molecular dynamics   Polyvinylidene fluoride   Carbon nanotube   Mechanical property   Microstructure  

摘要： Molecular dynamics (MD) simulation was used to investigate the microstructure and mechanical properties of pure polyvinylidene fluoride (PVDF) and PVDF/carbon nanotube (CNT) composites with and without a polarization process. After annealing, the trans percentages of non-polarized PVDF and composites are improved by about 25% compared to the initial model of alpha-phase PVDF. For PVDF and composites with polarization process, the PVDF trans percentages can be further enhanced by around 10% compared to those without the polarization process. The mechanical properties of pure PVDF can be improved by blending the PVDF with CNTs and with the polarization process. The local shear strain analysis shows that the strength of PVDF around the CNTs have been enhanced, leading to higher brittleness of the composites at increasing CNT weight fractions. During the tensile process, the trans percentage of PVDF increases in the non-polarized cases with increasing strain, but are less sensitive to polarized cases.