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排序方式: 共有761条查询结果,搜索用时 15 毫秒
741.
We investigate friction memory effects in the chaotic stick-slip motion of a plate shearing a granular bed. By analyzing separately trajectories' portions having increasing or decreasing instantaneous velocity, it is found that there are two quantitatively distinct granular friction-velocity curves for positive or negative acceleration, which cross one another in the weakening region. Interpreting acceleration dependence as an indirect consequence of contact ageing, we try to explain these effects by including rate-and-state (RS) friction equations in the stochastic model describing the plate motion. Preliminary results on a study case show that the main experimental features can be reproduced in this way, although quantitative agreement is partial. From the value obtained for the RS parameters we conclude that sliding friction decorrelation takes place at the length-scale of the solid-on-solid micro-contacts between grains and plate. In addition, the contemporary presence of noise and RS effects influences the average friction curve at large shear rate. 相似文献
742.
Mari Grobbelaar-Van Dalsen 《Journal of Mathematical Analysis and Applications》2009,360(2):577-587
In this paper we consider a linear three-dimensional structural acoustic model which takes account of displacement, rotational inertia and shear effects in the flat flexible structural component of the model. Thus the deflections of the structural component of the structure are governed by the Reissner–Mindlin plate equations. We show strong stabilization of the coupled model without incorporating viscous or boundary damping in the equations for the gas dynamics and without imposing geometric conditions. It turns out that damping is needed in the interior of the plate, to which end Kelvin–Voigt damping is introduced in the plate equations. As our main tool we use a resolvent criterion for strong stability due to Tomilov. 相似文献
743.
Magnetorheological elastomers (MRE) are known as smart materials. However, the magnetorheological (MR) effect of MRE is not high enough at present, which limits its engineering applications. Prior studies have shown that magneto-induced shear storage modulus and MR effect were mainly determined by the performance of the ferromagnetic particles. In this paper, MRE samples were prepared by carbonyl iron particles (CIP) of different compositions based on silicon rubber under external magnetic field. Their microstructures were observed using an optical digital microscope and a scanning electron microscope. The dynamic mechanical properties of MRE samples were measured using a modified dynamic mechanical analyzer under varying magnetic field strength and frequency. The results show that the carbon content of CIP have a greater impact on the dynamic mechanical properties of MRE. The magneto-induced shear storage modulus and MR effect can be increased by selecting CIP of low carbon content. In addition, the damping property is also significantly influenced by the carbon content of the CIP. This study is expected to provide guidance for fabrication of high performance MRE. 相似文献
744.
A. ZellC. Wagner 《Physica A》2012,391(3):464-473
We present experimental results of the flow of dilute and semi-dilute polymer solutions in co-rotating Taylor-Couette cylinders. The experimental set-up consists of a modified Mars II rheometer (Thermo Scientific) with two drive units that are mounted opposite each other. The rotational velocities of the inner and outer cylinders are chosen in a way such that the angular velocity has a 1/r profile and the flow is free of vorticity, but the direction of elongation is not constant, but rotates with the flow. Our particle image velocimetry (PIV) measurements show that for polymer solutions without shear thinning the flow is indeed free of vorticity and is equal to a stagnation point flow at a given position and a given instant in time. In contrast, torque measurements reveal that the stresses are identical to the stresses that are present in a plane shear flow. Thus, we find that for polymer solutions a flow with vorticity and a constant direction of elongation is equal to a flow without vorticity in which the direction of elongation is rotating. Finally, we show that for shear thinning solutions the flow velocity becomes non-monotonic through the gap and resembles a pluglike profile which is known from the Poiseuille flow. 相似文献
745.
Johnson CL Chen DD Olivero WC Sutton BP Georgiadis JG 《Magnetic resonance imaging》2012,30(2):205-212
In magnetic resonance elastography (MRE), shear waves at a certain frequency are encoded through bipolar gradients that switch polarity at a controlled encoding frequency and are offset in time to capture wave propagation using a controlled sampling frequency. In brain MRE, there is a possibility that the mechanical actuation frequency is different from the vibration frequency, leading to a mismatch with encoding and sampling frequencies. This mismatch can occur in brain MRE from causes both extrinsic and intrinsic to the brain, such as scanner bed vibrations or active damping in the head. The purpose of this work was to investigate how frequency mismatch can affect MRE shear stiffness measurements. Experiments were performed on a dual-medium agarose gel phantom, and the results were compared with numerical simulations to quantify these effects. It is known that off-frequency encoding alone results in a scaling of wave amplitude, and it is shown here that off-frequency sampling can result in two main effects: (1) errors in the overall shear stiffness estimate of the material on the global scale and (2) local variations appearing as stiffer and softer structures in the material. For small differences in frequency, it was found that measured global stiffness of the brain could theoretically vary by up to 12.5% relative to actual stiffness with local variations of up to 3.7% of the mean stiffness. It was demonstrated that performing MRE experiments at a frequency other than that of tissue vibration can lead to artifacts in the MRE stiffness images, and this mismatch could explain some of the large-scale scatter of stiffness data or lack of repeatability reported in the brain MRE literature. 相似文献
746.
M. Niwa H. Kawakami T. Kanamori T. Shinbo A. Kaito S. Nagaoka 《Journal of membrane science》2004,230(1-2):141-148
In this study, we focused on the shear stress effects within a spinneret during hollow fiber spinning on the formation of the hollow fibers and their gas transport properties. We fabricated asymmetric polyimide hollow fibers with a completely defect-free thin skin layer using a dry/wet phase inversion process. The apparent calculated skin layer thickness of the hollow fiber was 280 nm and the O2 permeance was 2.9×10−5 cm3 (STP)/(cm2 s cmHg). Interestingly, the skin layer thickness was reduced at the high shear rate. In addition, the gas permeances and selectivities of the hollow fibers increased with the increasing shear rate. We concluded that the oriented skin layer of the hollow fiber induced by shear stress had a significant influence on the formation of the skin layer and its gas transport properties. From the ATR-IR spectra results, it was clear that the surface skin layer of the hollow fiber was parallel oriented. 相似文献
747.
This Note proposes an extension to composite section of the non-uniform (out-of-plane) warping beam theory recently established for homogeneous and isotropic beam by R. El Fatmi (C. R. Mecanique 335 (2007) 467–474). For the present work, which constitutes a first step of this extension, the cross-section is assumed to be symmetric and made by orthotropic materials; however, Poisson's effects (called here in-plane warping) are also taken into account. Closed form results are given for the structural behavior of the composite beam and for the expressions of the 3D stresses; these ones, easy to compare with 3D Saint Venant stresses, make clear the additional contribution of the new internal forces induced by the non-uniformity of the (in and out) warpings. As first numerical applications, results on torsion and shear-bending of a cantilever sandwich beam are presented. 相似文献
748.
G. -X. Wang S. -X. Cai X. -C. Rao X. -Y. Deng R. Guidoin 《Colloids and surfaces. B, Biointerfaces》2003,28(4):11-278
Theoretical analysis has shown that that the tensile stress in the upper cell membrane of the vascular endothelium could accumulate upstream to a very high level despite of the identical shear environments. This phenomenon is called cell membrane tension accumulation (CMTA). To verify the theoretical analysis, the secretion of endothelin-1 (ET-1) by a paired human umbilical vein segments with different lengths (10 and 15 cm, respectively) were measured. The results clearly showed highly significant differences in the secretion rates of ET-1 between the 10 cm-long vein (segment A) and the 15 cm-long vein (segment B) under the same shear stress level of 0.48 N/m2. When exposed to a shear stress of 0.48 N/m2 for 24 h, segment B secreted ET-1 at an average rate of 34.9154±0.9830 pg/cm2 h, almost 14% higher than the average rate of 30.6274±0.4912 pg/cm2 h recorded by segment A (P<0.01). The present study, therefore, confirms that CMTA does in fact occur in the blood vessel. This phenomenon affects the secretion of ET-1 by vascular endothelial cells, and may be more important than shear stress in its effect on the metabolism and biological function of endothelial cells. 相似文献
749.
《Physics and Chemistry of Liquids》2012,50(2-3):115-130
Abstract The infinite frequency shear modulus, G∞, and compressional modulus, K∞, of the Lennard-Jones, LJ, fluid have been determined over essentially the whole phase diagram at densities below the solid-fluid coexistence line using PY, HNC, and Rogers and Young (RY) closures of the Ornstein-Zernike relation. At low density PY is best at reproducing simulation G∞, and K∞, whereas close to the coexistence line, above the critical temperature, the RY closure is best and is remarkably accurate. Agreement is poorest for all three closures below Tc in the liquid phase. 相似文献
750.