Nonlinear Stiffness of a Magneto-Rheological Damper

The last decade has witnessed an important role of magneto-rheological dampers in the semi-active vibration control on the basis of empirical models. Those models established by fitting experimental data, however, do not offer any explicit expressions for the stiffness and the damping of magneto-rheological dampers. Hence, it is not easy for engineers to get any intuitive information about the effects of stiffness and damping of a magneto-rheological damper on the dynamic performance of a controlled system. To manifest the nonlinear properties of a magneto-rheological damper, this paper presents the hysteretic phenomena and the additional nonlinear stiffness of a typical magneto-rheological damper in terms of equivalent linear stiffness and equivalent linear damping. Then, it gives a brief discussion about the effect of nonlinear stiffness on the vibration control through the numerical simulations and an experiment for the semi-active suspension of a quarter car model with a magneto-rheological damper installed. Both numerical simulations and experimental results show that the additional nonlinear stiffness in the magneto-rheological damper is remarkable, and should be taken into consideration in the design of vibration control.     

纤维金属板层分层尺寸的测量方法研究

导出了纤维金属胶接层板双面裂纹搭接剪切（ＤＣＬＳ）试样分层尺寸和柔度之间的线性关系式，提出了ＤＣＬＳ试样分层尺寸的柔度测量法，并与用光测法测得的分层尺寸进行了对比。结果表明，用柔度法测定ＤＣＬＳ试样的分层尺寸，不仅测量结果准确，而且简单易行，避免了光测法的诸多麻烦和精度低的缺点。     

The finite deformation theory of Taylor-based nonlocal plasticity

Recent experiments have shown that metallic materials display significant size effect at the micron and sub-micron scales. This has motivated the development of strain gradient plasticity theories, which usually involve extra boundary conditions and possibly higher-order governing equations. We propose a finite deformation theory of nonlocal plasticity based on the Taylor dislocation model. The theory falls into Rice's theoretical framework of internal variables [J Mech Phys Solids 19 (1971) 433], and it does not require any extra boundary conditions. We apply the theory to study the micro-indentation hardness experiments, and it agrees very well with the experimental data over a wide range of indentation depth.     

Simulated properties of Kagomé and tetragonal truss core panels

The finite element method has been used to simulate the properties of panels with Kagomé and tetragonal cores under compressive and shear loading. The simulation has been performed for two different materials: a Cu-alloy with extensive strain hardening and an Al-alloy with minimal hardening. It is shown that the Kagomé core is more resistant to plastic buckling than the tetragonal core under both compression and shear. One consequence is that the Kagomé structure has the greater load capacity and a deferred susceptibility to softening. Another is that the Kagomé core is isotropic in shear: contrasting with the soft orientations exhibited by the tetragonal core.     

Theoretical study of the anisotropic electron scattering by steps in vicinal AlGaN/GaN heterostructures

It was found by many experiments that the electron mobilities in vicinal AlGaN/GaN heterostructures are highly anisotropic which was believed to be caused by the steps at the vicinal heterointerfaces. However, there is no theoretical explanation for such experimental results because it is difficult to find an universal expression for the electron relaxation time in all directions. In this work, we started from solving the standard linearized Boltzmann equation and separated the electron relaxation time into x- and y-components (perpendicular and parallel to the steps, respectively). In this way, the y-component of the relaxation time was found to be infinite while the x-component part depended on the form of the scattering potential. Subsequently we were able to find an analytic expression for the anisotropic mobility, and the calculated results were found to be consistent with the experimental values.     

Bulk and interface recombination in planar lead halide perovskite solar cells: A Drift-Diffusion study

A theoretical approach based on Drift-Diffusion equations is presented to study planar mixed lead halide perovskite solar cells. Updated physical parameters such as permittivity, mobility, effective density of states and doping density is employed in simulations. Current-voltage curve data for two experimental sample is imported and through fitting with the model, density of bulk and interface defects is calculated. We obtain the bulk defect density around 10¹⁶ cm⁻³ and surface recombination velocities in the range of 10 cm/s. These values which are in good agreement with experimental measurements and considerably deviated from previous theoretical studies, verify the model and adopted constants. Shockley-Queisser limit is also presented as the ideal device and the effect of bulk and interface defects are presented as loss factors that cause departure from this limit. Our simulations conclude that the overall efficiency of perovskite solar cells is mainly governed by the open-circuit voltage and also identify the interface defects as the major loss factor in these devices.     

Laser fabrication nanocrystalline coatings using simultaneous powders/wire feed

Laser melting deposition (LMD) fabrication is used to investigate feasibilty of simultaneously feeding TC17 wire and the Stellite 20–Si₃N₄–TiC–Sb mixed powders in order to increase the utilization ratio of materials and also quality of LMD composite coatings on the TA1 substrate. SEM images indicated that such LMD coating with metallurgical joint to substrate was formed free of the obvious defects. Lots of the ultrafine nanocrystals (UNs) were produced, which distributed uniformly in some coating matrix location, retarding growth of the ceramics in a certain extent; UNs were intertwined with amorphous, leading the yarn-shape materials to be produced. Compared with substrate, an improvement of wear resistance was achieved for such LMD coating.     

Electrical manipulation of spins in a nanowire with Rashba interaction

We investigate the influence of external electric fields on the spins of a ballistic nanowire in terms of variations of the Rashba parameter and modification of the confinement potential. For a weak Rashba effect, the spins along the confinement direction in a given subband nearly assume full quantization. In the presence of a perpendicular magnetic field, the state of quantization can be manipulated using a transverse electric. This process requires modifications in the spin textures. If an in-plane magnetic field is applied, spins suffer rigid displacement to one edge of the wire and their expectation value becomes independent of the transverse electric field.     

Thermal and dynamic mechanical properties of epoxy resin/poly(urethane‐imide)/polyhedral oligomeric silsesquioxane nanocomposites

Linear isocyanate‐terminated poly(urethane‐imide) (PUI) with combination of the advantages of polyurethane and polyimide was directly synthesized by the reaction between polyurethane prepolymer and pyromellitic dianhydride (PMDA). Then octaaminophenyl polyhedral oligomeric silsesquioxane (OapPOSS) and PUI were incorporated into the epoxy resin (EP) to prepare a series of EP/PUI/POSS organic–inorganic nanocomposites for the purpose of simultaneously improving the heat resistance and toughness of the epoxy resin. Their thermal degradation behavior, dynamic mechanical properties, and morphology were studied with thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and transmission electron microscope (TEM). The results showed that the thermal stability and mechanical modulus was greatly improved with the addition of PUI and POSS. Moreover, the EP/PUI/POSS nanocomposites had lower glass transition temperatures. The TEM results revealed that POSS molecules could self assemble into strip domain which could switch to uniform dispersion with increasing the content of POSS. All the results could be ascribed to synergistic effect of PUI and POSS on the epoxy resin matrix. Copyright © 2010 John Wiley & Sons, Ltd.