Study on time-dependent behavior and stability assessment of deep-buried tunnels based on internal state variable theory

The creep model based on thermodynamics with internal state variables theory can simulate complex time-dependent deformation of rock mass, describe process of energy dissipation of material system, and can be used to evaluate the long-term stability of underground structures quantitatively. In this paper, the creep model proposed by author is improved further and recast to be central difference equation. The redevelopment interface of FLAC^3D is used to develop a new calculation program, which is based on thermodynamics for vsico-plasticity (PTV-P). Program validation has been conducted by comparing the results from FLAC^3D and Matlab software under uniaxial compression condition. Then the developed program has been applied to analyze the time-dependent behavior of deep-buried double tunnels. The integral values of energy dissipation rate and its time derivative in domain can be calculated and are used to evaluate the long-term stability of tunnels quantitatively, and the evaluation criterion is also proposed. Moreover, the contour map of energy dissipation rate is used to exhibits the local non-equilibrium region clearly.     

Surface oxidation of molten AZ91D magnesium alloy in air

The surface of a stagnant AZ91D alloy melt has been exposed to air under two oxidation conditions: at 610°C for 30 min and at 650°C for 10 s. After oxidising, the samples were sand quenched. The macroscopic morphology of the surfaces exhibited regions of non-nodular growth (in a layer manner) and regions of nodular growth. Additionally, the 610°C 30 min condition presented a region where localised combustion took place. The microscopic morphology of the surfaces and the nature of the surface reaction products have been analysed using field emission gun SEM and X-ray diffraction techniques, respectively. These results showed that all these morphologies, i.e. sponge-like, nodular and layer, were porous and therefore non-protective with respect to the evaporation and oxidation of Mg and Zn from the surface of the AZ91D alloy melt. In all these regions, MgO was the main oxidation product along with traces of ZnO and AlN.     

Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

Abstract

The quasi-two-dimensional molecular conductor α-(BEDT-TTF)₂I₃ exhibits anomalous transport phenomena where the temperature dependence of resistivity is weak but the ratio of the Hall coefficient at 10 K to that at room temperature is of the order of 10⁴. These puzzling phenomena were solved by predicting massless Dirac fermions, whose motions are described using the tilted Weyl equation with anisotropic velocity. α-(BEDT-TTF)₂I₃ is a unique material among several materials with Dirac fermions, i.e. graphene, bismuth, and quantum wells such as HgTe, from the view-points of both the structure and electronic states described as follows. α-(BEDT-TTF)₂I₃ has the layered structure with highly two-dimensional massless Dirac fermions. The anisotropic velocity and incommensurate momenta of the contact points, ±k₀, originate from the inequivalency of the BEDT-TTF sites in the unit cell, where ±k₀ moves in the first Brillouin zone with increasing pressure. The massless Dirac fermions exist in the presence of the charge disproportionation and are robust against the increase in pressure. The electron densities on those inequivalent BEDT-TTF sites exhibit anomalous momentum distributions, reflecting the angular dependences of the wave functions around the contact points. Those unique electronic properties affect the spatial oscillations of the electron densities in the vicinity of an impurity. A marked behavior of the Hall coefficient, where the sign of the Hall coefficient reverses sharply but continuously at low temperatures around 5 K, is investigated by treating the interband effects of the magnetic field exactly. It is shown that such behavior is possible by assuming the existence of the extremely small amount of electron doping. The enhancement of the orbital diamagnetism is also expected. The results of the present research shed light on a new aspect of Dirac fermion physics, i.e. the emergence of unique electronic properties owing to the structure of the material.     

Synthesis and characterization of EuBa2Ca2Cu3O9−x: The influence of temperature on dielectric properties and charge transport mechanism

High dielectric constant materials have a crucial importance for various microelectronic applications such as memory devices, supercapacitors etc. Among other insulators, perovskite structured oxide materials attract great interest not only for their high dielectric constants but also their unique electrical and magnetic properties such as superconductivity etc. From this point of view, a new Europium based copper oxide layered material with perovskite structure (EuBa₂Ca₂Cu₃O_9−x coded as Eu-1223) has been synthesized by solid state reaction method in this work. The physical and chemical properties of Eu-1223 have been determined by FTIR, SEM, XRF, XRD, TGA and DTA techniques. The influence of temperature on impedance and dielectric properties of Eu-1223 has been investigated by impedance spectroscopy measurements performed within the frequency interval of 5 Hz–13 MHz between 298 K and 408 K temperatures. It has been found that the Eu-1223 material has high dielectric constants at each temperature operated. In addition, Eu-1223 sample behaves as a colossal dielectric material up to 300 kHz for 408 K due to observation of dielectric constant values which are greater than 10³. Furthermore, it has been revealed that Eu-1223 material can be used as thermally sensitive resistors in electronic circuits due to its decreasing resistance with increasing temperature. Moreover, it has been observed that the relaxation frequency of the system shifts from 46.5 kHz (low frequency radio wave band) to 1.57 MHz (mid frequency radio wave band) as the temperature increasing from 298 K to 408 K. According to dc conductivity investigations, the variation of dc conductivity with the inverse of temperature satisfies linear relationship that indicates a thermally activated nearest neighbor hopping conduction. On the other hand, it has been determined that ac conductivity has frequency dependent relation which obeys ω^s for the high frequency region. Furthermore, the frequency exponent, s, which takes values between 0.7 and 0.4, shows a decreasing behavior with increasing temperature. In conclusion, ac charge transport mechanism has been predicted as correlated barrier hoping for Eu-1223.     

Democracy and non-profit housing. The tensions of residents’ involvement in the Danish non-profit sector

Resident democracy as a special form of participatory democratic set-up is fundamental in the understanding, and self-understanding, of the non-profit housing sector in Denmark. Through a case study, the paper explores how resident democracy is perceived and narrated between residents and employees at a housing association. The study indicates that the meta-story of democracy is disconnected from practice and the lived lives of residents. Three analytical tensions structure the analysis, which relate to the conditions for realizing the democratic ideal embedded in the structure of the sector. The tensions are related to representative versus participatory democracy; collectivity versus individuality; and service versus welfare. The tensions elucidate how resident democracy is squeezed between different logics, which result in an ambiguous setting for practising democracy. Based on the results, the article discusses conditions for prospective democracy in the Danish non-profit housing sector.     

Asynchronous distributed state estimation based on a continuous‐time stochastic model

In this paper, the problem of state estimation in an asynchronous distributed multi‐sensor estimation (ADE) system is considered. In such an ADE system, the state of a plant of interest is estimated by a group of local estimators. Each local estimator based, for example, on a Kalman filter, performs fusion of data from its local sensor and other (remote) processors to compute possibly best state estimates. In performing data fusion, however, two important issues need to be addressed, namely, the problem of asynchronism of local processors and the one of unknown correlation between asynchronous data in local processors. Consequently, there are two main contributions proposed in this paper. The first is a method to deal with asynchronous discrete‐time data based on a continuous‐time stochastic plant model. The second contribution is an asynchronous distributed data‐fusion algorithm. Simulated experiments illustrate the effectiveness of the proposed ADE approach. Copyright © 2011 John Wiley & Sons, Ltd.     

土压平衡盾构机刀盘电机控制系统的设计

为解决盾构机刀盘电机控制系统中存在的程序控制点多、逻辑复杂，易导致系统调试和维护不便的问题，将模块化编程技术应用于刀盘电机控制系统程序设计中。开展了刀盘电机控制系统结构分析，建立了刀盘运行状态和各个辅助系统之间的关系，提出了“将刀盘电机运行分成若干个状态，将各个辅助系统的条件采集过程设置为子程序”的状态机编程思想，子程序负责采集系统信息供给状态机决策使用，状态机完整描述了刀盘电机可能经过的各个状态并直接决定刀盘电机在该状态下所执行的动作。将该系统应用于实验型土压平衡盾构机中，并进行了大量的实验。研究结果表明，该系统完全满足土压平衡盾构机刀盘电机控制的需要，状态机编程可使整个程序构架更加清晰明了，易于调试和后期维护。     

Modified dynamic surface approach with bias torque for multi-motor servomechanism

This paper presents a modified neural dynamic surface control (DSC) with an adaptive bias torque for the multi-motor servomechanism (MMS) with backlash, friction and other disturbances. By introducing a continuous hybrid differentiator to replace the first-order filter in each step, a modified DSC is developed to improve the load tracking precision of MMS. However, when the MMS enters the backlash band, only DSC cannot guarantee the load tracking performance. Thus, an adaptive bias torque is firstly proposed based on the prescribed performance function technique to compensate the backlash nonlinearity and guarantee the load tracking performance of MMS. In addition, the unknown dynamics including the friction and other disturbances are approximated by using wavelet echo state networks where the weights are all updated online. By means of Lyapunov stability theory, the semi-globally uniformly ultimately bounded (SGUUB) property of all signals in the closed-loop system is proved. Finally, simulations and experimental results based on a four-motor servomechanism are presented to show the effectiveness of the proposed approach.     

Target tracking with fast adaptive revisit time based on steady state IMM filter

Time is a valuable resource in phased-array radar and proper use of it enables the radar to track more targets simultaneously. Appropriate usage of time in the tracking means the radar can minimize the revisit rate of the tracked targets while the accuracy of targets position estimation is still acceptable. In this paper, a new method for determination of target revisiting time is proposed which has relatively low computational cost and uses steady state filters with Interactive Multiple Models structure. The proposed algorithm allows the radar designer to determine the filter parameters by some criteria such as the desired accuracy and targets maneuver. The performance of the proposed method is compared with the conventional filters for standard maneuverable flying targets. Simulations show the better accuracy and reliability of the proposed method beside its less target revisit time and computational load.     

Maximum likelihood estimation of noise covariance matrices for state estimation of autonomous hybrid systems

A critical aspect of developing Bayesian state estimators for hybrid systems, that involve a combination of continuous and discrete state variables, is to have a reasonably accurate characterization of the stochastic disturbances affecting their dynamics. Recently, Bavdekar et al. (2011) have proposed a maximum likelihood (ML) based framework for estimation of the noise covariance matrices from operating input–output data when an EKF is used for state estimation. In this work, the ML framework is extended to estimation of the noise covariance matrices associated with autonomous hybrid systems, and, to a wider class of recursive Bayesian filters. Under the assumption that the innovations generated by an estimator form a white noise sequence, the proposed ML framework computes the noise covariance matrices such that they maximize the log-likelihood function of the estimator innovations. The efficacy of the proposed scheme is demonstrated through the simulation and experimental studies on the benchmark three-tank system.