Flux-line cutting in hard superconductors

The effect of flux-line cutting upon the magnetic behavior of hard superconductors subjected to a dc bias magnetic field H_z and a transverse field H_y undergoing half-wave oscillations of large amplitude is investigated theoretically. We have applied both the generalized double critical-state model and the elliptic flux-line-cutting one to interpret available experimental results for VTi ribbons. The predictions of these models are compared and discussed.     

Secondary Peak on Asymmetric Magnetization Loop of Type-II Superconductors

Asymmetric magnetization loops with a second peak effect were parameterized by the extended critical-state model. The magnetic field distribution in a sample is considered. An expression is suggested for a peak of the critical current density and corresponding depression on field dependence of the depth of surface layer with equilibrium magnetization. These functions determine the width and the asymmetry of a magnetization loop. The asymmetry of the secondary peak height on magnetization branches for increasing and decreasing field is reproduced on the computed magnetization curves.     

Flux-Line Cutting in Hard Superconductors

No Heading The effect of flux-line cutting upon the magnetic behavior of hard superconductors subjected to a dc bias magnetic field H_z and a transverse field H_y undergoing half-wave oscillations of large amplitude is investigated theoretically. We have applied both the generalized double critical-state model and the elliptic flux-line-cutting one to interpret available experimental results for VTi ribbons. The predictions of these models are compared and discussed.PACS numbers: 74.25.Ha, 74.25.Sv, 74.25.Qt     

AC Loss Evaluation of MgB2 Superconducting Windings Located in a Stator Core Slot with a Finite-Element Method

AC losses in stator windings of fully superconducting motors with an MgB₂ wire are numerically evaluated by means of a finite-element method using edge elements for a self-magnetic field. The physical properties of the MgB₂ wire for numerical calculations are obtained from the corresponding experiments with an existing wire. It is assumed that the voltage?Ccurrent characteristics of the MgB₂ wire are given by Bean??s critical-state model, in which the critical current density is independent of the local magnetic field. The influences of core slot size and turn number of windings on the AC losses are discussed quantitatively toward the optimum design of the stator winding with the MgB₂ wire.     

Dielectric breakdown model for a conductive crack and electrode in piezoelectric materials

The strip dielectric breakdown (DB) model introduced by Zhang and Gao [T.Y. Zhang, C.F. Gao, Fracture behavior of piezoelectric materials, Thero. Appl. Fract. Mech. 41 (2004) 339–379] is used to study the generalized 2D problem of a conductive crack and an electrode in an infinite piezoelectric material. The energy release rate and stress intensity factors are derived based on the Stroh formalism, and then they are applied as failure criteria to predict the critical fracture loads. It is found that the DB strip may take the shielding effect on a conductive crack or electrode. For the case of an electrode, the local energy release rate and stress intensity factor become zero when DB happens ahead of the electrode tip. For the case of a mode-I conductive crack in a transversely isotropic piezoelectric solid, the results based on the DB model show that the critical stress intensity factor linearly increases as the applied electric field parallel to the poling direction increases, while it linearly decreases as the applied electric field anti-parallel to the poling direction increases. Finally, the upper and lower bounds of the actual critical fracture loads are proposed for a conductive crack in a piezoelectric material under combined mechanical–electrical loads.     

Time reversibility of the discrete element method

An algorithm is presented for discrete element method simulations of energy-conserving systems of frictionless, spherical particles in a reversed-time frame. This algorithm is verified, within the limits of round-off error, through implementation in the LAMMPS code. Mechanisms for energy dissipation such as interparticle friction, damping, rotational resistance, particle crushing, or bond breakage cannot be incorporated into this algorithm without causing time irreversibility. This theoretical development is applied to critical-state soil mechanics as an exemplar. It is shown that the convergence of soil samples, which differ only in terms of their initial void ratio, to the same critical state requires the presence of shear forces and frictional dissipation within the soil system.     

Flux-Pinning-Induced Stress and Deformation Analyses of a Long Rectangular Superconducting Bicrystal

The flux-pinning-induced stress and deformation of a long rectangular superconducting bicrystal with an arbitrary aspect ratio are analyzed based on the critical-state model and the finite element numerical method. The flux and current distributions in the superconductor with a low-angle grain boundary (GB) are obtained based on a constant GB critical current density assumption. The distributions of the stresses within the superconductor are obtained for different magnetization stages. The deformation and especially the shape distortion in the irreversible magnetostriction of the superconductor are analyzed. In addition, the relation between the maximum stress on the grain boundary and the ratio of the GB critical current density to the grain critical current density is discussed when the applied magnetic field is reduced to zero.     

Crack problems in magnetoelectroelastic solids. Part I: exact solution of a crack

This paper presents an exact treatment on the problem of an elliptic hole or a crack in a magnetoelectroelastic solid subject to the farfield loadings. First, based on the extended version of Eshelby-Stroh’s formulation, the general solution of an elliptical hole is obtained according to exact boundary conditions at the rim of the hole. Then, when the hole degenerates into a crack, explicit solutions are given for the field intensity factors and electric-magnetic fields inside the crack. It is shown that all the singularities of fields are dependent on the applied mechanical loads, not on the applied electric-magnetic loads. Due to its explicitness, the present solution for a crack can also serve as a benchmark to test the validity of various analysis approaches or assumptions to more complicated crack problems in magnetoelectroelastic media.     

一种基于静力位移测量值的车辐式屋盖结构索力识别方法

该文从车辐式张拉屋盖结构的组成特点出发,提出了一种基于静力位移测量值的索力识别方法。首先通过测量屋盖结构的几何位形,建立有限元模型。然后给真实结构施加特定荷载并测量加载点位移。给有限元模型施加同样的荷载并计算其位移。通过比较计算和实测的位移值,修正并迭代有限元模型中拉索的预应力;当计算位移与实测位移一致时就获得了拉索的真实内力。该方法只需要测量加载的自由度所对应的位移,能够在通用有限元软件ANSYS上进行索力识别,计算结果准确。     

Numerical study on fatigue crack growth in railway wheels under the influence of residual stresses

Accurate prediction of fatigue crack growth on railway wheels and the influence of residual stresses by finite element method (FEM) modeling can affect the maintenance planning. Therefore, investigation of rolling contact fatigue and its effect on rolling members life seem necessary. The objective of this paper is to provide a prediction of rolling contact fatigue crack growth in the rail wheel under the influence of stress field from mechanical loads and heat treatment process of a railway wheel. A 3D nonlinear stress analysis model has been applied to estimate stress fields of the railway mono-block wheel in heat treatment process. Finite element analysis model is presented applying the elastic–plastic finite element analysis for the rail wheel under variable thermal loads. The stress history is then used to calculate stress intensity factors (SIFs) and fatigue life of railway wheel. The effect of several parameters, vertical loads, initial crack length and friction coefficient between the wheel and rail, on the fatigue life in railway wheels is investigated using the suggested 3-D finite element model. Three-dimensional finite element analysis results obtained show good agreement with those achieved in field measurements.     

The Paramagnetism Carried by Paramagnetic Ions and its Effect on the Superconductivity in High T C Superconductors

Extended Brillouin function with a revision is applied to describe the paramagnetism carried by the rare-earth Gd^{3 +} ions in GdBa₂Cu₃O_6+δ and the Re²⁺ ions in (Hg_0.9Re_0.1)Ba₂Ca₂Cu₃O _g+δ. We believe that the paramagnetism depends on the internal flux density of the sample instead of the applied field. At the field below H _cl the paramagnetism has no contribution to total magnetization; at the field over H _cl the paramagnetic magnetization has different values when in field-increasing and field-decreasing process. The width of the magnetization hysteresis loop Δ M is broadened by the paramagnetism. The effect of the paramagnetism due to paramagnetic ions on the magnetization relaxation rate and the magnetization critical-current density J _c based on the Bean critical-state model is also discussed. The temperature dependence of the paramagnetism is shown in this paper.     

3D-Modeling Numerical Solutions of Electromagnetic Behavior of HTSC Bulk above Permanent Magnetic Guideway

This paper presents a 3D-modeling numerical method using finite element method (FEM) to simulate the electromagnetic behavior of high-temperature superconductors (HTSC). The models are formulated by the magnetic field vector method (H-method). The resolving code was written by FROTRAN language. The electromagnetic properties of HTSC are described though Kim critical-state model. The magnetic fields and current distribution in the bulk HTSC in the applied non-uniform external magnetic fields generated by the permanent magnetic guideway (PMG) are obtained using the proposed method. The magnetic levitation forces by the interaction between the bulk HTSC and the PMG are calculated. In order to validate the method, measurement of the vertical force between a bulk YBaCuO(YBCO) and a PMG is obtained. The measurement and simulation results show good matching. This method could be used in the HTSC magnetic levitation transportation system optimization design.      

AC electric field-induced alignment and long-range assembly of multi-wall carbon nanotubes inside aqueous media

The present work examines the behavior of multiwall carbon nanotubes (MWCNT) inside AC electric fields created by three-dimensional electrodes. The response of carbon nanotubes stably suspended in water with the aid of a nonionic surfactant is monitored by combining microscopic observations with on-line measurements of the suspension resistivity. It is found that polarization effects induced by the externally applied AC electric field on MWCNTs can cause their unidirectional orientation and end-to-end contact that result in formations of spatially distributed, long-range, three-dimensional and electrically conducting structures that span the entire gap between the electrodes. The length of the formed structures, which in the present case was approximately 30 times larger than that of an individual carbon nanotube, can be controlled by adjusting the spacing between the electrodes. The influence of main experimental parameters, namely, MWCNT concentration, applied voltage, AC field frequency, and electrode surface topography on the suspension behavior is experimentally examined. Results are demonstrated for applied voltage values, AC field frequencies, and carbon nanotube concentrations in the range 4-40 Vptp, 10 Hz-5 MHz, and 0.001-2.0 wt%, respectively. While higher electric field strengths accelerate the formation of aligned structures, higher frequency values were found to result in suspensions that exhibit smaller electrical resistivity. Carbon nanotube dispersions exposed to an AC electric field exhibit a 100-fold or more decrease in their electrical resistivity, even when carbon nanotube concentrations as low as 0.005 wt% are used.     

纳米碳包镍/环氧树脂复合材料的NTC效应及其调节

采用磁场辅助固化工艺,制备了纳米碳包镍/环氧树脂复合材料,用SEM观察了复合材料的微观组织,测量了复合材料的直流电学特性,考察了磁场强度、纳米粒子含量、温度对电性能的影响。在无磁场情况下固化时,纳米粒子在环氧树脂中均匀分布;而施加磁场后,纳米粒子在基体中以纤维状线性排列;施加的磁场越大,纤维组织发育得越粗壮,获得的复合材料的电阻率越小。复合材料的I-U曲线呈现非线性特征,表明电导机制依赖于导电粒子之间的隧道传递,电阻率随温度上升而下降,显示奇怪的负温度系数（NTC）效应,且可由填料含量、磁场强度调节。为解释NTC起源,提出一个修正的量子隧道模型。根据该模型,复合材料的NTC效应归因于占统治地位的电子热活化隧道传递;另一方面,环氧树脂的低膨胀率促成了这一效果。      

The influence of Maxwell stresses on the fracture mechanics of piezoelectric materials

The influence of Maxwell stresses on the generalized 2D fracture mechanics problem of piezoelectric materials under combined mechanical and electric loads at infinity is studied. The electrically semi-permeable crack boundary condition is adopted in this paper. Based on the Stroh’s formalism, explicit and closed-form solutions of electric displacement inside the crack, stress and electric intensity factors are obtained. Numerical results are also given to discuss the effects of Maxwell stresses on the stress and electric displacement intensity factors when the interior of the crack and the surrounding space at infinity are filled with different dielectric medium. It is found that the stress intensity factor increases rapidly with increasing value of the applied electric displacement load for the case of the dielectric constant of the surrounding at infinity is smaller than that inside the crack. The electric displacement intensity factor always increases as the applied electric loads or the applied mechanical loads increase.     

The seepage evolution law under the fault creep in right bank of Longyangxia Dam

The present work shows the application of a new methodology, to find out the seepage evolution law in the earth dam under its flow conditions, to a real field case: the Longyangxia Dam, in Qinghai, China. Under the influence of various loads and environmental factors, the creep property of dam foundation fault will be bound to be changes in the evolution of its seepage field. Therefore, the normal compression creep and the tangential shear creep of dam foundation faults are studied in this paper. Moreover, assuming the non-tension capacity in the normal direction, the normal compression creeping model and the tangential shear creeping model of the fault are established. On the basis, the time series evolution model on the permeability coefficient of the fault in dam foundation is built. These methods have already been applied into Longyangxia Dam and the result indicates that the seepage condition is normal when dam foundation seepage coefficient are getting smaller.     

Influence de l'intensité et de la fréquence d'un champ électrique sur la structure et la résistance des couches minces d'aluminium

The influence of d.c. field, a.c. sinusoïdal field and rectangular pulses applied to the substrate during the deposition process of thin aluminium films on their resistivity at high frequencies has been investigated. Increase in the a.c. electric field leads to an increase of film resistivity, while, in the case of d.c. field, the resistivity decreases. Film resistivity also increases with increasing frequency of the a.c. field. The dependence of film resistivity upon the frequency of rectangular pulses reveals a maximum. The electron-microscopic investigation has shown that there is a correlation between the structure and electrical properties of the films.     

Nanospring pressure sensors grown by glancing angle deposition

Arrays of Cr zigzag nanosprings and slanted nanorods, 15-55 nm and 40-80-nm-wide, respectively, were grown on SiO2/Si substrates by glancing angle deposition. The arrays exhibit a reversible change in resistivity upon loading and unloading, by 50% for nanosprings and 5% for nanorods, indicating their potential as pressure sensors. The resistivity drop is due to a compression of nanosprings (by a measured 19% for an applied external force of 10(-10) N per spring), which causes them to physically touch their neighbors, providing a path for electric current to flow between nanosprings. Repeated loading and unloading at large loads (> or =1 MPa) results in irreversible plastic deformation and a degradation of the pressure sensitivity.     

The Numerical Study of Influence of Flux Creep on AC Losses in Superconductors

Numerical simulation of AC losses in superconductors with gaint flux creep was carried out based on the collective creep model of the vortex glass. Influence of physical parameters, including frequency (f) and the amplitude (B _ac) of AC field, current density distribution (j), DC field (B _d), and temperature (T) on AC losses was studied based on a unified method and the Maxwell equations. The frequency-dependent AC losses is the most important among the results which differ from the static models. Also, AC losses derived in the flux creep state are larger than those in the critical-state when the sample is only partly penetrated by the field, which is the second difference between the flux creep model and the static models. Moreover, the dependence of AC losses on B _ac are derived and compared with that based on the Bean model. Preliminary comparison with experiments showed that the procedure could give qualitative understanding and estimation of AC losses in superconductors with giant flux creep.