Numerical evaluation of the viscoelastic and viscoplastic behavior of composites

This paper is concerned with the development of homogenization methods in order to obtain the effective material parameters of viscoelastic and viscoplastic composites. In the case of a viscoelastic behavior, the constitutive laws at the heterogeneous scale are transposed into a LAPLACE-CARSON domain where the equations become similar to those of linear elasticity and classical homogenization procedures can be applied. Afterward, the results have to be converted by an inverse transformation. Mostly, this procedure is combined with analytical or semi-analytical models such as the self-consistent or Mori-Tanaka model. Since the application of this approach is limited to simple structured composites it is not suitable for materials with a complex internal architecture. In order to avoid this limitation, this paper is focused on the development of a homogenization procedure which is based on the LAPLACE-CARSON transformation and a numerical solution of the field problem. For heterogeneous material with viscoplastic or nonlinear viscoelastic properties, an additional linearization step is required. A new algorithm is proposed in order to combine this affine formulation with the numerical homogenization method. Finally, these procedures are used to estimate the effective material behavior of different types of composites. With that, the accuracy of the results and efficiency of the methodologies is assessed

Microscale Modeling of Magnetoactive Composites Undergoing Large Deformations

This paper is concerned with the development of a material model for the constituents of a magnetoactive composite. Special attention is paid to magnetorheological elastomers which are synthesized from a soft polymeric matrix material with embedded magnetizable particles. Because the particles interact under an applied magnetic load, a coupled magneto-mechanical field problem has to be solved. The mechanical properties of the polymer matrix motivate the consideration of large deformations. We present the balance equations with boundary conditions and an appropriate material model. The corresponding boundary value problems are solved by the Finite-Element-Method. A weak numerical coupling scheme enables the staggered solution of two subproblems, the stationary magnetic and mechanical one. The coupling between both is realized by a surrounding iterative loop

Body composition evaluated by skinfolds, bioimpedance and body mass index in adults

El objetivo de este estudio fue comparar la composición corporal por pliegues cutáneos (DC) por bioimpedancia eléctrica(BIA) y el índice de masa corporal(IMC). Se hizo un trabajo de campo con 153 militares usando los siguientes equipos: balanza Wiso; estadiómetro WCS; adipómetro Cescorf científico y Malton BF-900 para bioimpedancia eléctrica. La densidad corporal se consiguió por la ecuación de Jackson&Pollock y la clasificación delporcentaje de grasa corporal siguiendo Pollock&Wilmore. Para verificar la correlación se usó el test de Spearman. Los resultados promedios (± desviación estándar) hallados para edad, peso, estatura e IMC, fueron: 19 años (±1,8 años), 70,9Kg (±9,55), 1,74 metros (±0.06) y 23,9 kg/m² (±2,76) respectivamente. El porcentaje de grasa corporal por DC y BIA fueron respectivamente 12,78 (±5,45); 16,29 (±4,02). Concluí que la composición corporal por el método de DC, BIA e IMC presentó datos que se correlacionan, siendo que el uso de la bioimpedancia fue más indicada en grupos cuyo porcentaje de grasa corporal variaba entre 18-20%.The aim of this study was to compare body composition by skinfold thickness (DC) by bioelectrical impedance analysis (BIA) and body mass index (BMI). Research was carried out with 153 soldiers with the equipment: balance Wiso; WCS stadiometer; adipometer Cescorf Malton scientific and BF-900 to bioelectrical impedance analysis. Body density was obtained by the equation of Jackson&Pollock and classification of the percentage of fat by following Pollock&Wilmore. To check the correlation was used Spearman's test. The average results (±standard deviation) found for age, weight, height and BMI were: 19 years (±1.8years), 70.9 kg (±9.55), 1.74 meters (±0.06) and 23.9 kg/m² (±2.76), respectively. The fat percentage by BIA and DC were respectively 12.78 (±5.45) and 16.29 (±4.02). Concluded that body composition by the method of DC, BIA and BMI have data that correlate, and the use of bioimpedance was indicated in most groups whose fat percentage ranged around 18-20%.Gracias CAPES para la inversión financiera en forma de beca en el Programa de Posgrado en Ingeniería Eléctrica e Informática Industrial (CPGEI) de la Universidad Tecnológica Federal de Paraná (UTFPR)

Phase Space Tomography of Matter-Wave Diffraction in the Talbot Regime

We report on the theoretical investigation of Wigner distribution function (WDF) reconstruction of the motional quantum state of large molecules in de Broglie interference. De Broglie interference of fullerenes and as the like already proves the wavelike behaviour of these heavy particles, while we aim to extract more quantitative information about the superposition quantum state in motion. We simulate the reconstruction of the WDF numerically based on an analytic probability distribution and investigate its properties by variation of parameters, which are relevant for the experiment. Even though the WDF described in the near-field experiment cannot be reconstructed completely, we observe negativity even in the partially reconstructed WDF. We further consider incoherent factors to simulate the experimental situation such as a finite number of slits, collimation, and particle-slit van der Waals interaction. From this we find experimental conditions to reconstruct the WDF from Talbot interference fringes in molecule Talbot-Lau interferometry.Comment: 16 pages, 9 figures, accepted at New Journal of Physic

Nichtlineare Schalentheorie in konvektiver Beschreibung mit anisotroper Plastizität

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A Mechanical Mass Sensor with Yoctogram Resolution

Nanoelectromechanical systems (NEMS) have generated considerable interest as inertial mass sensors. NEMS resonators have been used to weigh cells, biomolecules, and gas molecules, creating many new possibilities for biological and chemical analysis [1-4]. Recently, NEMS-based mass sensors have been employed as a new tool in surface science in order to study e.g. the phase transitions or the diffusion of adsorbed atoms on nanoscale objects [5-7]. A key point in all these experiments is the ability to resolve small masses. Here we report on mass sensing experiments with a resolution of 1.7 yg (1 yg = 10^-24 g), which corresponds to the mass of one proton, or one hydrogen atom. The resonator is made of a ~150 nm long carbon nanotube resonator vibrating at nearly 2 GHz. The unprecedented level of sensitivity allows us to detect adsorption events of naphthalene molecules (C10H8) and to measure the binding energy of a Xe atom on the nanotube surface (131 meV). These ultrasensitive nanotube resonators offer new opportunities for mass spectrometry, magnetometry, and adsorption experiments.Comment: submitted version of the manuscrip

Reversibly compressible and freestanding monolithic carbon spherogels

We present a versatile strategy to tailor the nanostructure of monolithic carbon aerogels. By use of an aqueous colloidal solution of polystyrene in the sol-gel processing of resorcinol-formaldehyde gels, we can prepare, after supercritical drying and successive carbonization, freestanding monolithic carbon aerogels, solely composed of interconnected and uniformly sized hollow spheres, which we name carbon spherogels. Each sphere is enclosed by a microporous carbon wall whose thickness can be adjusted by the polystyrene concentration, which affects the pore texture as well as the mechanical properties of the aerogel monolith. In this study, we used monodisperse polystyrene spheres of approximately 250 nm diameter, which result in an inner diameter of the final hollow carbon spheres of approximately 200 ± 5 nm due to shrinkage during the carbonization process. The excellent homogeneity of the samples, as well as uniform sphere geometries, are confirmed by small- and angle X-ray scattering. The presence of macropores between the hollow spheres creates a monolithic network with the benefit of being reversibly compressible up to 10% linear strain without destruction. Electrochemical tests demonstrate the applicability of ground and CO2 activated carbon spherogels as electrode materials. © 2019 The Author

Monte Carlo Transmission Line Modeling of Multilayer Optical Coatings for Performance Sensitivity of a Dichroic Filter for the ARIEL Space Telescope

Dichroic beamsplitters, or dichroics, are filters that rely on the optical interference that occurs within thin layers to ensure the transmission and reflection of selective wavelengths of an incident beam of light. These optical components consist of a substrate coated on one or both surfaces with multiple layers of thin films, the spectral design and construction of which determine the isolation of particular wavebands. Discrepancies between the measured and expected spectral performance of optical elements with such coatings can largely be attributed to depositions errors and uncertainties in the refractive indices of the materials. Our model uses two-dimensional transmission line modeling to evaluate the transmittance of light through multilayer coatings deposited on a substrate material for given materials, angle of incidence and polarisation. This model allows us to perform Monte Carlo simulations to obtain statistical information about the tolerance of the coating performance to systematic and random uncertainties from the manufacturing process, as well as from environmental changes in space. With the aid of accurate manufacturing recipes and uncertainty amplitudes from commercial manufacturers, this tool can predict variations in the optical performance that result from the propagation of each of these uncertainties for various hypothetical scenarios. One particular application of this study are the dichroics of the ARIEL space telescope. We compare the predicted optical performance with transmission measurements at cryogenic temperatures for one of the ARIEL dichroics, which show the specification compliance of this prototype after many thermal cycles

Berechnung von Blechumformprozessen auf der Grundlage der Abbildung von Flächen

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Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene

For most optoelectronic applications of graphene a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in graphene occurs through two competing pathways: carrier-carrier scattering -- creating an elevated carrier temperature -- and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy, and fluence over a wide range. We find that sufficiently low fluence ($\lesssim$ 4 $\mu$J/cm$^2$) in conjunction with sufficiently high Fermi energy ($\gtrsim$ 0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier scattering, which leads to efficient carrier heating. Upon increasing the fluence or decreasing the Fermi energy, the carrier heating efficiency decreases, presumably due to energy relaxation that becomes increasingly dominated by phonon emission. Carrier heating through carrier-carrier scattering accounts for the negative photoconductivity for doped graphene observed at terahertz frequencies. We present a simple model that reproduces the data for a wide range of Fermi levels and excitation energies, and allows us to qualitatively assess how the branching ratio between the two distinct relaxation pathways depends on excitation fluence and Fermi energy.Comment: Nano Letters 201