Evaluation of the effective mechanical properties of single walled carbon nanotubes using a spring based finite element approach

The development of a finite element formulation that is appropriate for the computation of Young’s and Shear modulus of single walled carbon nanotubes (SWCNTs) is the purpose of this paper. The method utilizes the atomistic microstructure of the nanotubes. According to the three-dimensional atomic nanostructure of SWCNTs, nodes are defined at the atom locations. Appropriate spring-type elements interconnect these nodes to simulate properly interatomic interactions. This approach is implemented via the use of three-dimensional spring-like elements each node of which obeys to three translations and three rotations. In this way, molecular mechanics theory can be applied directly while the atomic bonds are modeled by using exclusively physical variables such as bond stretching, bond angle bending and torsional rotation resistance force constants. With the proposed method, the Young’s and shear modulus of numerous SWCNTs were determined. The effect of the nanotube radius and thickness on the mechanical behavior of SWCNTs was tested and demonstrated. The numerical results show good agreement with other corresponding values which are available in the literature.     

On the estimation of mechanical properties of single-walled carbon nanotubes by using a molecular-mechanics based FE approach

Carbon nanotubes (CNTs) have attracted considerable attention in scientific communities due to their remarkable mechanical, thermal and electrical properties (high stiffness, high strength, resilience, etc.). In particular, mechanical properties of single wall nanotubes (SWNTs) have a Young’s modulus of about 1 TPa if normalized to their diameter showing why they are widely considered as reinforcing elements in advanced low weight composite structures. The determinations of mechanical properties of SWNT are currently investigated both experimentally and theoretically. However, to determine CNTs mechanical properties in a direct experimental way is a challenging and not economical task because of the technical difficulties and the costs involved in the manipulation of nanoscale objects. Due to the handling difficulty, estimation of mechanical properties using computer simulations are being performed by several author with different approaches.     

Multiscale Nonlinear Constitutive Modeling of Carbon Nanostructures Based on Interatomic Potentials

Continuum-based modeling of nanostructures is an efficient and suitable method to study the behavior of these structures when the deformation can be considered homogeneous. This paper is concerned about multiscale nonlinear tensorial constitutive modeling of carbon nanostructures based on the interatomic potentials. The proposed constitutive model is a tensorial equation relating the second Piola-Kirchhoff stress tensor to Green-Lagrange strain tensor. For carbon nanotubes, some modifications are made on the planar representative volume element (RVE) to account for the curved atomic structure resulting a non-planar RVE. Using the proposed constitutive model, the elastic behavior of the graphene sheet and carbon nanotube are studied.     

Carbon nanotube-reinforced composites: frequency analysis theories based on the matrix stiffness

Strong and versatile carbon nanotubes are finding new applications in improving conventional polymer-based fibers and films. This paper studies the influence of matrix stiffness and the intertube radial displacements on free vibration of an individual double-walled carbon nanotube (DWNT). For this, a double elastic beam model is presented for frequency analysis in a DWNT embedded in an elastic matrix. The analysis is based on both Euler–Bernoulli and Timoshenko beam theories which considers shear deformation and rotary inertia and for both concentric and non-concentric assumptions considering intertube radial displacements and the related internal degrees of freedom. New intertube resonant frequencies and the associated non-coaxial vibrational modes are calculated. Detailed results are demonstrated for the dependence of resonant frequencies and mode shapes on the matrix stiffness. The results indicate that internal radial displacement and surrounding matrix stiffness could substantially affect resonant frequencies especially for longer double-walled carbon nanotubes of larger innermost radius at higher resonant frequencies, and thus the latter does not keep the otherwise concentric structure at ultrahigh frequencies. Therefore, depending on the matrix stiffness, for carbon nanotubes reinforced composites, different analysis techniques should be used while the aspect ratio of carbon nanotubes has a little effect on the analysis theory which should be selected.     

Multiscale fiber reinforced composites based on a carbon nanofiber/epoxy nanophased polymer matrix: Synthesis, mechanical, and thermomechanical behavior

Vacuum assisted resin infusion molding (VARIM) was used to produce multiscale fiber reinforced composites (M-FRCs) based on carbon nanofibers dispersed in an epoxy resin. Flexural, interlaminar shear strength (ILSS) and thermomechanical tests are presented for the 0.1 wt% and 1 wt% M-FRCs and compared with the neat fiber reinforced composites (FRCs). Flexural strength and modulus increased (16–20%) and (23–26%), respectively for the 0.1 wt% and 1 wt% M-FRCs when compared to the neat FRCs. ILSS properties increased (6% and 25%) for the 0.1 wt% and 1 wt% M-FRCs, respectively when compared to neat FRCs. The glass transition temperatures (T_g) of both M-FRC samples were 25 °C higher than the neat FRC. Coefficients of thermal expansion (CTE) of the M-FRC samples improved compared to the neat FRC. The improved T_g and CTE properties in the M-FRC samples are attributed to synergistic interactions between the CNF/PNC matrix and glass fibers.     

Multiscale failure analysis of fiber reinforced concrete based on a discrete crack model

In this work the capabilities of an interface model to predict failure behavior of steel fiber reinforced cementitious composites (SFRCCs) are evaluated at both macro and mesoscale levels of observation. The interface model is based on a hyperbolic maximum strength criterion defined in terms of the normal and shear stress components acting on the joint plane. Pre-peak regime is considered linear elastic, while the post-peak behavior is formulated in terms of the fracture energy release under failure mode I and/or II. The well-known ??Mixture Theory?? is adopted for modeling the interactions between fibers and the surrounding cementitious composite. The effects of both the axial forces on the fibers induced by normal relative displacements, as well as the dowel action due to tangential relative displacements in the interfaces are considered in the formulation of the interaction mechanisms between fibers and cementitious composites. After describing the interface model, this work focuses on numerical analyses of SFRCC failure behavior. Firstly, the validation analysis of the interface model is performed at the constitutive level by comparing its numerical predictions against experimental results available in scientific literature. Then, the sensitivity of the interface theory for SFRCC regarding the variation of main parameters of the composite constituents is evaluated. Finally, the attention is focused on Finite Element (FE) analysis of SFRCC failure behavior at meso and macroscopic levels of observation. The results demonstrate the capabilities of the interface theory based on the Mixture Theory to reproduce the main features of failure behavior of SRFCC in terms of fiber content and involved fracture modes.     

基于离散单元法的群楼拆除爆破仿真模拟

基于青岛开发区一商场爆破工程,利用颗粒离散元法建立离散元网格实体模型,用多面体离散元法模拟分析拆除爆破中建筑物的倒塌过程。研发的网格实体模型详细模拟了建筑物从结构局部失稳到整体完全倒塌的整个过程,确定了爆堆轮廓线和爆堆尺寸,直观给出了爆破设计的效果。触地振动、冲击力变化分析结果表明:所给出的爆破方案可达到预期拆除爆破的效果,触地振动符合《爆破安全规程》要求;该模型可优化爆破设计,指导爆破施工,为建筑物拆除爆破的灾害预测与安全评估提供理论依据。     

基于离散单元法的群楼拆除爆破仿真模拟

基于青岛开发区一商场爆破工程,利用颗粒离散元法建立离散元网格实体模型,用多面体离散元法模拟分析拆除爆破中建筑物的倒塌过程。研发的网格实体模型详细模拟了建筑物从结构局部失稳到整体完全倒塌的整个过程,确定了爆堆轮廓线和爆堆尺寸,直观给出了爆破设计的效果。触地振动、冲击力变化分析结果表明:所给出的爆破方案可达到预期拆除爆破的效果,触地振动符合《爆破安全规程》要求;该模型可优化爆破设计,指导爆破施工,为建筑物拆除爆破的灾害预测与安全评估提供理论依据。     

Numerical algorithms for prediction of mechanical properties of single-walled carbon nanotubes based on molecular mechanics model

The objective of this paper is to develop the numerical algorithms for the prediction of mechanical properties of single-walled carbon nanotubes (SWCNTs). By using the energy method, the analytical expressions are obtained and the five independent variables algorithm is developed for the prediction of the elastic properties of SWCNTs via a molecular mechanics model in which the geometrical relationship of carbon nanotube is introduced. It can be found that due to the introduction of the geometrical approximate conditions some errors may exist in the calculation of mechanical properties of SWCNTs in terms of the five independent variables algorithm. Therefore, two improved algorithms, i.e., eigenvalues modified method (EMM) and eigenvalues and eigenvectors modified method (EEMM) are proposed to analyze the possible errors in the numerical results. It is found that the results obtained by the three kinds of algorithms are almost consistent with one another, but EMM and EEMM are preferred to be used because they have properties similar to those of the finite element method, where the consistent equation works just as the constraint condition to void the singularity of the element stiffness matrix. The computational results also reveal that both the surface Young’s modulus and Poisson’s ratio depend on the diameter of carbon nanotubes, and finally converge to the values of the graphite sheet with an increase in the tube diameter in the inverse trends. For SWCNTs with approximately the same diameters, the surface Young’s modulus is in direct and Poisson’s ratio is in inverse proportion to chiral angles, respectively.     

Modeling of damage evolution in soft-wood perpendicular to grain by means of a discrete element approach

The anisotropy of wood within the radial–tangential (RT) growth plane has a major influence on the cracking behavior perpendicular to grain. Within the scope of this work, a two-dimensional discrete element model is developed, consisting of beam elements for the representation of the microstructure of wood. Molecular dynamics simulation is used to follow the time evolution of the model system during the damage evolution in the RT plane under various loading conditions. It is shown that the results are in good agreement with experiments on spruce wood, and that the presented discrete element approach is applicable for detailed studies of the dependence of the microstructure on mesoscopic damage mechanism and dynamics of crack propagation in microstructured and cellular materials like wood.     

Analysis of a triangulation based approach for specimen generation for discrete element simulations

Discrete element methods are emerging as useful numerical analysis tools for engineers concerned with granular materials such as soil, food grains, or pharmaceutical powders. Obviously, the first step in a discrete element simulation is the generation of the geometry of the system of interest. The system geometry is defined by the boundary conditions as well as the shape characteristics (including size) and initial coordinates of the particles in the system. While a variety of specimen generation methods for particulate materials have been developed, there is no uniform agreement on the optimum specimen generation approach. This paper proposes a new triangulation based approach that can easily be implemented in two or three dimensions. The concept of this approach (in two dimensions) is to triangulate a system of points within the domain of interest, creating a mesh of triangles. Then the particles are inserted as the incircles of these triangles. Extension to three dimensions using a mesh of tetrahedra and inserting the inspheres is relatively trivial. The major advantages of this approach include the relative simplicity of the algorithm and the small computational cost associated with the preparation of an initial particle assembly. The sensitivity of the characteristics of the particulate material that is generated to the topology of the triangular mesh used is explored. The approach is compared with other currently used methods in both two and three dimensions. These comparisons indicate that while this approach can successfully generate relatively dense two-dimensional particle assemblies, the three- dimensional implementation is less effective at generating dense systems than other available approaches. The research presented in this paper made use of software developed by other researchers. For the two-dimensional study the program Triangle developed by Jonathan Shewchuk was used. The three-dimensional analysis used the Geompack++ program developed by Barry Joe as well as an implementation of the Jodrey and Tory (1985) algorithm by Monika Bargiel and Jacek Moscinski called NSCP3D.     

A discrete stress–strength interference model based on universal generating function

Continuous stress–strength interference (SSI) model regards stress and strength as continuous random variables with known probability density function. This, to some extent, results in a limitation of its application. In this paper, stress and strength are treated as discrete random variables, and a discrete SSI model is presented by using the universal generating function (UGF) method. Finally, case studies demonstrate the validity of the discrete model in a variety of circumstances, in which stress and strength can be represented by continuous random variables, discrete random variables, or two groups of experimental data.     

A hybrid electronic-density-functional/molecular-dynamics simulation scheme for multiscale simulation of materials on parallel computers: applications to silicon and alumina

In hybrid electronic-density-functional/molecular-dynamics schemes, a total system is partitioned in real space into the quantum-mechanical (QM) region treated by the electronic-density-functional theory and the molecular dynamics (MD) region in which atoms are interacting through the empirical inter-atomic potential. In the former hybrid scheme [Ogata et al. Comput. Phys. Commun. 149 (2002) 30], appropriate selection of QM atoms for seamless coupling between the QM and MD regions is limited in Si systems, and applications of the scheme to other materials are difficult. Novel hybrid scheme that is free from the limitation and applicable to both Si and alumina systems, is presented.     

A multivariate spatial crash frequency model for identifying sites with promise based on crash types

Many studies have proposed the use of a systemic approach to identify sites with promise (SWiPs). Proponents of the systemic approach to road safety management suggest that it is more effective in reducing crash frequency than the traditional hot spot approach. The systemic approach aims to identify SWiPs by crash type(s) and, therefore, effectively connects crashes to their corresponding countermeasures. Nevertheless, a major challenge to implementing this approach is the low precision of crash frequency models, which results from the systemic approach considering subsets (crash types) of total crashes leading to higher variability in modeling outcomes. This study responds to the need for more precise statistical output and proposes a multivariate spatial model for simultaneously modeling crash frequencies for different crash types. The multivariate spatial model not only induces a multivariate correlation structure between crash types at the same site, but also spatial correlation among adjacent sites to enhance model precision. This study utilized crash, traffic, and roadway inventory data on rural two-lane highways in Pennsylvania to construct and test the multivariate spatial model. Four models with and without the multivariate and spatial correlations were tested and compared. The results show that the model that considers both multivariate and spatial correlation has the best fit. Moreover, it was found that the multivariate correlation plays a stronger role than the spatial correlation when modeling crash frequencies in terms of different crash types.     

An accident waiting to happen: a spatial approach to proactive pedestrian planning

There are about 75,000 pedestrian crashes in the United States each year. Approximately 5000 of these crashes are fatal, accounting for 12% of all roadway deaths. On college campuses, pedestrian exposure and crash-risk can be quite high. Therefore, we analyzed pedestrian crashes on the campus of the University of North Carolina at Chapel Hill (UNC) as a test case for our spatially-oriented prototype tool that combines perceived-risk (survey) data with police-reported crash data to obtain a more complete picture of pedestrian crash-risk. We use spatial analysis techniques combined with regression models to understand factors associated with risk. The spatial analysis is based on comparing two distributions, i.e. the locations of perceived-risk with police-reported crash locations. The differences between the two distributions are statistically significant, implying that certain locations on campus are perceived as dangerous, though pedestrian crashes have not yet occurred there, and there are actual locations of police-reported crashes that are not perceived to be dangerous by pedestrians or drivers. Furthermore, we estimate negative binomial regression models to combine pedestrian and automobile exposure with roadway characteristics and spatial/land use information. The models show that high exposure, incomplete sidewalks and high crosswalk density are associated with greater observed and perceived pedestrian crash-risk. Additionally, we found that people perceive a lower risk near university libraries, stadiums, and academic buildings, despite the occurrence of crashes.     

Effects of H2O on Si fracture: a hybrid quantum-classical simulation

The mechanism of stress corrosion cracking in Si has became recently important, because it occurs in micro-machines. A novel hybrid quantum-mechanical/molecular-dynamics simulation code developed recently for parallel computers is used to perform simulations for a cracked Si-model under tension (mode-I opening) with three H₂O molecules around the crack front to investigate possible effects of both, saturation of dangling bonds of Si with hydrogen atoms and environment molecules on the fracture initiation. Our results demonstrate formation of Si–O–Si bond by chemical reaction, which occurs at the crack tip after dissociation of O atom from H₂O molecule. Moreover, the reaction exhibits no energy barrier and the hydrogen saturation has no effect on H₂O molecule pathway.     

多壁碳纳米管横向扰动波传播的研究

将N层碳纳米管模拟成N层连续弹性壳模型，研究横向扰动波在单个多壁碳纳米管中轴向传播问题。研究表明，当扰动频率低于多壁碳纳米管的临界频率时，振动模态是同轴的；如果扰动频率至少高于多壁碳纳米管的一个临界频率时，振动模态是非同轴的，并以不同的波速传播，或低或高于单壁连续弹性壳模型的波速。因此，多壁碳纳米管的THz高频波存在着复杂现象，振动基本是不同轴的。特别是当多壁碳纳米管THz高频波以不同的速度传播时，传播速度不仅依赖于频率，而且依赖于非同轴的振动模态。     

基于仿真的电子元器件故障模型研究

  故障知识获取是基于知识的电路故障诊断方法的瓶颈问题,严重限制了电子设备智能诊断系统的发展.提出了基于仿真获得故障知识的方法,介绍了数字/模拟电子元器件的故障模式仿真模型的建立方法.采用FMECA分析法选择仿真对象,有效地减少了故障注入的次数.建立故障仿真模型库,降低了编写仿真文件的难度.通过数模混合电路验证了故障模型的有效性.     

The impact of outlet densities on alcohol-related crashes: a spatial panel approach

BACKGROUND: A number of studies using cross-sectional data have demonstrated that the availability of alcohol, measured by the number and types of alcohol outlets, is directly related to numerous measures associated with drinking and driving. The current study contributes the first observation of relationships over time between alcohol outlet densities on one hand and both automobile crashes and related injuries on the other hand. METHOD: The study examined longitudinal data from 581 consistently defined zip code areas represented in the California Index Locations Database, a geographic information system that coordinates population and ecological data with spatial attributes for areas across the state. Six years of data were collected on features of local populations (e.g., demographics, household size) and places (e.g., retail markets) thought to be related to two measures of automobile crashes (hospital discharges related to car crash injuries geocoded to the zip code of patient residence, and police reports associated with car crashes geocoded to the zip code of crash location). Both crash measures were positively associated with two outlet types: bars, and off-premise outlets. Additionally, restaurants appear to provide a protective effect relative to the residence-based measure. Crash rates were also related to changes in population and place characteristics using random effects models with controls for spatial autocorrelation (nxt=3486 observations). Changes in population and place characteristics of adjacent (spatially lagged) areas were also considered. RESULTS: Over time, both local and lagged population and place characteristics were related to automobile crash-related measures. CONCLUSION: Controlling for cross-sectional differences between zip code areas, changes in numbers of licensed alcohol retail establishments, especially bars and off-premise outlets, affect rates of car crashes and related injuries.     

A consistent approach for fluid-structure-contact interaction based on a porous flow model for rough surface contact

Simulation approaches for fluid-structure-contact interaction, especially if requested to be consistent even down to the real contact scenarios, belong to the most challenging and still unsolved problems in computational mechanics. The main challenges are 2-fold—one is to have a correct physical model for this scenario, and the other is to have a numerical method that is capable of working and being consistent down to a zero gap. Moreover, when analyzing such challenging setups of fluid-structure interaction, which include contact of submersed solid components, it gets obvious that the influence of surface roughness effects is essential for a physical consistent modeling of such configurations. To capture this system behavior, we present a continuum mechanical model that is able to include the effects of the surface microstructure in a fluid-structure-contact interaction framework. An averaged representation for the mixture of fluid and solid on the rough surfaces, which is of major interest for the macroscopic response of such a system, is introduced therein. The inherent coupling of the macroscopic fluid flow and the flow inside the rough surfaces, the stress exchange of all contacting solid bodies involved, and the interaction between fluid and solid are included in the construction of the model. Although the physical model is not restricted to finite element–based methods, a numerical approach with its core based on the cut finite element method, enabling topological changes of the fluid domain to solve the presented model numerically, is introduced. Such a cut finite element method–based approach is able to deal with the numerical challenges mentioned above. Different test cases give a perspective toward the potential capabilities of the presented physical model and numerical approach.