A node-based smoothed finite element method (NS-FEM) for upper bound solution to visco-elastoplastic analyses of solids using triangular and tetrahedral meshes

A node-based smoothed finite element method (NS-FEM) was recently proposed for the solid mechanics problems. In the NS-FEM, the system stiffness matrix is computed using the smoothed strains over the smoothing domains associated with nodes of element mesh. In this paper, the NS-FEM is further extended to more complicated visco-elastoplastic analyses of 2D and 3D solids using triangular and tetrahedral meshes, respectively. The material behavior includes perfect visco-elastoplasticity and visco-elastoplasticity with isotropic hardening and linear kinematic hardening. A dual formulation for the NS-FEM with displacements and stresses as the main variables is performed. The von-Mises yield function and the Prandtl–Reuss flow rule are used. In the numerical procedure, however, the stress variables are eliminated and the problem becomes only displacement-dependent. The numerical results show that the NS-FEM has higher computational cost than the FEM. However the NS-FEM is much more accurate than the FEM, and hence the NS-FEM is more efficient than the FEM. It is also observed from the numerical results that the NS-FEM possesses the upper bound property which is very meaningful for the visco-elastoplastic analyses which almost have not got the analytical solutions. This suggests that we can use two models, NS-FEM and FEM, to bound the solution, and can even estimate the global relative error of numerical solutions.     

Ground penetrating radar response from voids: A demonstration using a simple model

A simple Ricker wavelet model can be used to illustrate some fundamental properties of the ground penetrating radar (GPR) response from both air- and water-filled voids. Reflections from the top and the bottom of a void overlap significantly, and generate one common characteristic of a void response: a “bright spot”, analogous to the “bright spot” observed in seismic exploration. For time delays equal to about half the pulsewidth, the reflected wavelets superimpose to yield maximum reflection amplitudes. The top reflected wavelet becomes completely separated from the bottom reflected wavelet when the void time delay exceeds twice the wavelet pulsewidth. The two wavelet reflections can be individually identified at earlier time delays, approximately equal to the wavelet pulsewidth. This is still substantial and explains why it is difficult to use GPR to infer void thicknesses, especially for air-filled voids.     

Fabrication and functionalization of carbon nanotube films for high-performance flexible supercapacitors

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Carbon nanotube (CNT) films have shown many promising advantages in the development of high-performance flexible supercapacitors in terms of electrode specific area, mechanical reliability under bending and stretching, electron and ionic mobility tailored for high-rate performance etc. In this review, the recent progress in the design, preparation and functionalization of CNT film based electrodes for the fabrication of high-performance flexible supercapacitors are introduced in details, including the synthesis of conductive CNT films for the electrodes of supercapacitors, and the functionalizations of CNT film with other high-capacitance materials by both mixing and in situ growth strategies for high-performance composite electrodes. Furthermore, we also discussed the assembly strategies, prototypes and electrochemical performance of flexible supercapacitors based on CNT film composite electrodes. At last, the challenges and trends of the CNT film based flexible supercapacitors are prospected as well.     

Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition

The origin of radial breathing mode (RBM) in the Raman spectra of multiwall carbon nanotubes (MWNCTs) is discussed. In general, RBM is characteristics of single wall carbon nanotube (SWCNT). With the help of transmission electron microscope (TEM) and Raman spectroscopic studies, it is established that the presence of SWCNT in the cavity of MWCNT is responsible for the appearance of RBM in MWCNT (synthesized by low temperature catalytic chemical vapor deposition technique). The estimated diameter of 8.2 Å (from Raman study) of SWCNT is almost same as that observed (∼8.3 Å) in TEM studies.     

Water dynamics inside a cathode channel of a polymer electrolyte membrane fuel cell

The present study focuses on the investigation of water dynamics inside a polymer electrolyte membrane fuel cell using two different modelling approaches: Eulerian two-phase mixture and volume of fluid interface tracking models. The Eulerian two-phase mixture model has provided overall information of species distribution inside a fuel cell and identified that the liquid water usually accumulates under the land area. The volume of fluid interface tracking model has then been implemented to investigate the emergence of water droplets from the gas diffusion layer into the cathode channel and the subsequent removal of water from the channel. Further, the effects of the location of water emergence in the cathode channel on the dynamic behavior of liquid water have been investigated. The present study shows that the water emerging into the channel near the side walls greatly reduces the surface water coverage of the channel. In order to control the water path into the channel near side walls, a further discussion has been provided that a gas diffusion layer design based on hydrophilic fibres distributed inside a hydrophobic fibre matrix could provide a precisely controlled water path through the gas diffusion layer.     

Carbon nanotube assembly at near-industrial natural-fiber spinning rates

A large number of carbon nanotube (CNT) applications require that CNTs be assembled into fibers and films at speeds viable for industrial production processes. Most of the state-of-art CNT thread formation techniques of CNT fiber production in meters per second (m/s) are still an order of magnitude lower than typical industrial processes. Here, we report a modified dry spinning process with improved drawing rates, where catalyst-free CNT films can be drawn at 15.93 m/s, an order of magnitude higher than most similar dry spinning processes reported in literature. We achieved this speed by using spinnable CNT arrays that are completely detached from the substrate and the corresponding catalyst nanoparticles. The detached drawable CNT arrays are free-standing and they can be transferred onto other flat substrates. This allows the formation of CNT threads with higher diameter uniformity; also the Si substrates can be re-used. Additionally, we successfully attached single-layer CNT ribbons from different batches in order to produce arbitrarily long CNT fibers and threads.     

A Multiwalled‐Carbon‐Nanotube‐Based Biosensor for Monitoring Microcystin‐LR in Sources of Drinking Water Supplies

A multiwalled carbon nanotube (MWCNT)‐based electrochemical biosensor is developed for monitoring microcystin‐LR (MC‐LR), a toxic cyanobacterial toxin, in sources of drinking water supplies. The biosensor electrodes are fabricated using vertically well‐aligned, dense, millimeter‐long MWCNT arrays with a narrow size distribution, grown on patterned Si substrates by water‐assisted chemical vapor deposition. High temperature thermal treatment (2500 °C) in an Ar atmosphere is used to enhance the crystallinity of the pristine materials, followed by electrochemical functionalization in alkaline solution to produce oxygen‐containing functional groups on the MWCNT surface, thus providing the anchoring sites for linking molecules that allow the immobilization of MC‐LR onto the MWCNT array electrodes. Addition of the monoclonal antibodies specific to MC‐LR in the incubation solutions offers the required sensor specificity for toxin detection. The performance of the MWCNT array biosensor is evaluated using micro‐Raman spectroscopy, including polarized Raman measurements, X‐ray photoelectron spectroscopy, cyclic voltammetry, optical microscopy, and Faradaic electrochemical impedance spectroscopy. A linear dependence of the electron‐transfer resistance on the MC‐LR concentration is observed in the range of 0.05 to 20 μg L^?1, which enables cyanotoxin monitoring well below the World Health Organization (WHO) provisional concentration limit of 1 μg L^?1 for MC‐LR in drinking water.     

Dynamic analysis in solid mechanics by an alternative boundary element procedure

A new procedure for eigenvalue and transient dynamic analyses in solid mechanics is presented here using a boundary integral approach. Contrary to previous work, it is shown in this paper that by applying a special class of co-ordinate functions one can obtain the mass matrix using boundary values. As a consequence, the free vibration case is reduced to an algebraic eigenvalue problem, while transient vibrations can be solved by using a direct time integration procedure.     

Nano-scale roughness effects on hysteresis in micro-scale adhesive contact

A multi-scale model enables investigation of the effect of surface roughness on energy loss in adhesive contact. Fully atomistic simulation of nano-scale indentation predicts size-dependent force–distance trends that are introduced as roughness into a micro-scale finite element model through randomly distributed non-linear hysteretic springs. The multi-scale model predicts that the energy loss, quantified by the hysteresis loop formed by loading and unloading data, increases with increasing indentation depth and surface roughness. This behavior is discussed in terms of an analytical model of a simply connected system.