Wave propagation analysis in laminated composite plates with transverse cracks using the wavelet spectral finite element method

This paper presents a newly developed wavelet spectral finite element (WFSE) model to analyze wave propagation in anisotropic composite laminate with a transverse surface crack penetrating part-through the thickness. The WSFE formulation of the composite laminate, which is based on the first-order shear deformation theory, produces accurate and computationally efficient results for high frequency wave motion. Transverse crack is modeled in wavenumber-frequency domain by introducing bending flexibility of the plate along crack edge. Results for tone burst and impulse excitations show excellent agreement with conventional finite element analysis in Abaqus (R). Problems with multiple cracks are modeled by assembling a number of spectral elements with cracks in frequency-wavenumber domain. Results show partial reflection of the excited wave due to crack at time instances consistent with crack locations. (C) 2014 Elsevier B.V. All rights reserved.

Unzipping Carbon Nanotubes at High Impact

The way nanostructures behave and mechanically respond to high impact collision is a topic of intrigue. For anisotropic nanostructures, such as carbon nanotubes, this response will be complicated based on the impact geometry. Here we report the result of hypervelocity impact of nanotubes against solid targets and show that impact produces a large number of defects in the nanotubes, as well as rapid atom evaporation, leading to their unzipping along the nanotube axis. Fully atomistic reactive molecular dynamics simulations are used to gain further insights of the pathways and deformation and fracture mechanisms of nanotubes under high energy mechanical impact. Carbon nanotubes have been unzipped into graphene nanoribbons before using chemical treatments but here the instability of nanotubes against formation, fracture, and unzipping is revealed purely through mechanical impact. defect

Fabrication and characterization of gold coated hollow silicon microneedle array for drug delivery

In this paper, we present the fabrication and characterization of Ti and Au coated hollow silicon microneedles for transdermal drug delivery applications. The hollow silicon microneedles are fabricated using isotropic etching followed by anisotropic etching to obtain a tapered tip. Silicon microneedle of 300 mu m in height, with 130 mu m outer diameter and 110 mu m inner diameter at the tip followed by 80 mu m inner diameter and 160 mu m outer diameter at the base have been fabricated. In order to improve the biocompatibility of microneedles, the fabricated microneedles were coated with Ti (500 nm) by sputtering technique followed by gold coating using electroplating. A breaking force of 225 N was obtained for the fabricated microneedles, which is 10 times higher than the skin resistive force. Hence, fabricated microneedles can easily be inserted inside the skin without breakage. The fluid flow through the microneedles was studied for different inlet pressures. A minimum inlet pressure of 0.66 kPa was required to achieve a flow rate of 50 mu l in 2 s with de-ionized water as a fluid medium. (C) 2014 Elsevier B.V. All rights reserved.

Isotropic finite volume discretization

Finite volume methods traditionally employ dimension by dimension extension of the one-dimensional reconstruction and averaging procedures to achieve spatial discretization of the governing partial differential equations on a structured Cartesian mesh in multiple dimensions. This simple approach based on tensor product stencils introduces an undesirable grid orientation dependence in the computed solution. The resulting anisotropic errors lead to a disparity in the calculations that is most prominent between directions parallel and diagonal to the grid lines. In this work we develop isotropic finite volume discretization schemes which minimize such grid orientation effects in multidimensional calculations by eliminating the directional bias in the lowest order term in the truncation error. Explicit isotropic expressions that relate the cell face averaged line and surface integrals of a function and its derivatives to the given cell area and volume averages are derived in two and three dimensions, respectively. It is found that a family of isotropic approximations with a free parameter can be derived by combining isotropic schemes based on next-nearest and next-next-nearest neighbors in three dimensions. Use of these isotropic expressions alone in a standard finite volume framework, however, is found to be insufficient in enforcing rotational invariance when the flux vector is nonlinear and/or spatially non-uniform. The rotationally invariant terms which lead to a loss of isotropy in such cases are explicitly identified and recast in a differential form. Various forms of flux correction terms which allow for a full recovery of rotational invariance in the lowest order truncation error terms, while preserving the formal order of accuracy and discrete conservation of the original finite volume method, are developed. Numerical tests in two and three dimensions attest the superior directional attributes of the proposed isotropic finite volume method. Prominent anisotropic errors, such as spurious asymmetric distortions on a circular reaction-diffusion wave that feature in the conventional finite volume implementation are effectively suppressed through isotropic finite volume discretization. Furthermore, for a given spatial resolution, a striking improvement in the prediction of kinetic energy decay rate corresponding to a general two-dimensional incompressible flow field is observed with the use of an isotropic finite volume method instead of the conventional discretization. (C) 2014 Elsevier Inc. All rights reserved.

Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions

Colloidal systems with competing interactions are known to exhibit a range of dynamically arrested states because of the systems' inability to reach its underlying equilibrium state due to intrinsic frustration. Graphene oxide (GO) aqueous dispersions constitute a class of 2D-anisotropic colloids with competing interactions long-range electrostatic repulsion, originating from ionized groups located on the rim of the sheets, and weak dispersive attractive interactions originating from the unoxidized graphitic domains. We show here that aqueous dispersions of GO exhibit a range of arrested states, encompassing fluid, glass, and gels that coexist with liquid-crystalline order with increasing volume fraction. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces. This can be realized by changing the ionic strength of the medium. We observe at low salt concentrations, where long-range electrostatic repulsion dominates, the formation of a repulsive Wigner glass, while at high salt concentrations, when attractive forces dominate, the formation of gels exhibits a nematic to columnar liquid-crystalline transition. The present work highlights how the chemical structure of GO hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet gives rise to a rich and complex array of arrested states.

Transverse orthotropic elastic moduli of bundled coated thin elliptical tubes

Light weight structures with tailored mechanical properties have evolved beyond regular hexagonal/circular honeycomb topology. For applications which demand anisotropic mechanical properties, elliptical-celled structures offer interesting features. This paper characterizes the anisotropic in-plane elastic response of coated thin elliptical tubes in different array patterns viz, close-packed, diagonal and rectangular patterns under compression. This paper also extends earlier works on elliptical close-packed structure to a more general case of coated tubes. Theoretical framework using thin ring theory provides formulae in terms of geometric and material parameters. These are compared with a series of FE simulations using contact elements. The FE results are presented as graphs to aid in design. (C) 2014 Elsevier Ltd. All rights reserved.

Dynamical facilitation governs glassy dynamics in suspensions of colloidal ellipsoids

One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. Although the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times. In particular, recent experiments and simulations have highlighted the importance of facilitation using simple model systems composed of spherical particles. However, an overwhelming majority of liquids possess anisotropy in particle shape and interactions, and it is therefore imperative to examine facilitation in complex glass formers. Here, we apply the DF theory to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Furthermore, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical heterogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids.

Synthesis and photocatalytic performance of quasi-fibrous ZnO

The combustion of oxidizer zinc nitrate and fuel oxalic acid results in quasi-fibrous zinc oxide. The processing parameters including oxidizer to fuel ratio, time and temperature were optimized for the resultant crystal structure and morphology. Pure hexagonal phase formation does not depend on the fuel ratio, but a stoichiometric ratio of oxidizer to fuel at 450 degrees C and 30 min results in highly crystalline ZnO with 3 mu m length and 0.5 mu m width. This quasi-fiber originates from partial fusion of near spherical, similar to 60 nm particles during the rapid rate of reaction in the combustion process. Transmission electron microscopic analysis confirms the anisotropic primary particle orientation and pore distribution within the developed quasi-fibrous particles. The degradation of methyl orange was assessed by degrading the dye in the presence of the synthesized ZnO (2.95 eV) under both UV and visible light. Quasi-fibrous zinc oxide exhibits effective photocatalytic efficiency under visible light irradiation.

Semipolar and nonpolar GaN epi-films grown on m-sapphire by plasma assisted molecular beam epitaxy

We hereby report the development of non-polar epi-GaN films of usable quality, on an m-plane sapphire. Generally, it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode. However, we could achieve good quality epi-GaN films by involving controlled steps of nitridation. GaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. The films grown on the nitridated surface resulted in a nonpolar (10-10) orientation while without nitridation caused a semipolar (11-22) orientation. Room temperature photoluminescence study showed that nonpolar GaN films have higher value of compressive strain as compared to semipolar GaN films, which was further confirmed by room temperature Raman spectroscopy. The room temperature UV photodetection of both films was investigated by measuring the I-V characteristics under UV light illumination. UV photodetectors fabricated on nonpolar GaN showed better characteristics, including higher external quantum efficiency, compared to photodetectors fabricated on semipolar GaN. X-ray rocking curves confirmed better crystallinity of semipolar as compared to nonpolar GaN which resulted in faster transit response of the device. (C) 2014 AIP Publishing LLC.

pentahexoctite: A new two-dimensional allotrope of carbon

The ability of carbon to exist in many forms across dimensions has spawned search in exploring newer allotropes consisting of either, different networks of polygons or rings. While research on various 3D phases of carbon has been extensive, 2D allotropes formed from stable rings are yet to be unearthed. Here, we report a new sp(2) hybridized two-dimensional allotrope consisting of continuous 5-6-8 rings of carbon atoms, named as ``pentahexoctite''. The absence of unstable modes in the phonon spectra ensures the stability of the planar sheet. Furthermore, this sheet has mechanical strength comparable to graphene. Electronically, the sheet is metallic with direction-dependent flat and dispersive bands at the Fermi level ensuring highly anisotropic transport properties. This sheet serves as a precursor for stable 1D nanotubes with chirality-dependent electronic and mechanical properties. With these unique properties, this sheet becomes another exciting addition to the family of robust novel 2D allotropes of carbon.

Thick PECVD Germanium films for MEMS Application

Plasma enhanced chemical vapour deposition (PECVD) of thick germanium (Ge) films (similar to 1 mu m) on silicon dioxide (SiO2) at low temperatures is described. A diborane pretreatment on SiO2 films is done to seed the Ge growth, followed by the deposition of thick Ge films using germane (GeH4) and argon (Ar). Further, the effect of hydrogen (H-2) dilution on the deposition rate is also investigated. The film thickness and morphology is characterized using SEM. Use of high RF power and substrate temperature show increased deposition rate. EDS analysis indicates that these films contain 97-98 atomic percentage of Ge. A recipe for anisotropic dry etching of the deposited Ge films with 10nm/ min etch rate is also suggested.

Effect of Anisotropy of Fibers on the Stress-Strain Response of Fiber-Reinforced Soil

Reinforcing soil with fibers is a useful method for improving the strength and settlement response of soil. The soil and fiber characteristics and their interaction are some of the major factors affecting the strength of reinforced soil. The fibers are usually randomly distributed in the soil, and their orientation has a significant effect on the behavior of the reinforced soil. In the paper, a study of the effect of anisotropic distribution of fibers on the stress-strain response is presented. Based on the concept of the modified Cam clay model, an analytical model was formulated for the fiber-reinforced soil, and the effect of fiber orientation on the stress-strain behavior of soil was studied in detail. The results show that, as the inclination of fibers with the horizontal plane increased, the contribution of fibers in improving the strength of fiber-reinforced soil decreased. The effect of fibers is maximum when they are in the direction of extension, and vice versa. (C) 2014 American Society of Civil Engineers.

Response of shock wave deformation in AA5086 aluminum alloy

The AA5086 aluminum alloy sheets with different starting textures were subjected to shock wave deformation with an input impulse of similar to 0.2 Ns. Microstructural examination indicate no significant change in grain size; however, the evolution of substructure manifesting intra-granular misorientation was evident. The improvement in hardness indicates the absence of recovery and strain hardening during shock deformation. Shock deformed samples show characteristic texture evolution with high Brass {110}< 112 > component. The study demonstrates the viability of high velocity forming of AA5086 aluminum alloy sheet using shock wave. (C) 2014 Elsevier B.V. All rights reserved.

Kinetics of dispersion of nanoparticles in thin polymer films at high temperature

We report the first detailed study of the kinetics of dispersion of nanoparticles in thin polymer films using temperature dependent in situ X-ray scattering measurements. We show a comparably enhanced dispersion at higher temperatures for systems which are otherwise phase segregated at room temperature. Detailed analysis of the time dependent X-ray reflectivity and diffuse scattering data allows us to explore the out-of-plane and in-plane mobility of the nanoparticles in the polymer films. While the out-of-plane motion is diffusive with a diffusion coefficient almost two orders of magnitude lower than that expected in bulk polymer, the in-plane one is found to be super-diffusive resulting in significantly larger in-plane displacement at similar time scales. We discuss the origin of the observed highly anisotropic motion of nanoparticles due to their slaved motion with respect to the anisotropic chain orientation and consequent diffusivity anisotropy of matrix chains. We also suggest strategies to utilize these observations to kinetically improve dispersion in otherwise thermodynamically segregated polymer nanocomposite films.

Low-Wear High-Friction Behavior of Copper Matrix Composites Dispersed With an In Situ Polymer Derived Ceramic

We show that copper-matrix composites that contain 20 vol. % of an in situ processed, polymer-derived, ceramic phase constituted from Si-C-N have unusual friction-and-wear properties. They show negligible wear despite a coefficient of friction (COF) that approaches 0.7. This behavior is ascribed to the lamellar structure of the composite such that the interlamellar regions are infused with nanoscale dispersion of ceramic particles. There is significant hardening of the composite just adjacent to the wear surface by severe plastic deformation.