37,907 research outputs found
Experimental studies on the shear resistance of original coal-shale joint
Purpose. Experimental study and theoretical modeling of the shear resistance of original coal-shale joint.
Methods. A two-segment model was developed to describe the shear resistance-shear displacement curves obtained from the direct shear tests of eight coal-shale joints by letting the two segments of the fitting curve pass through the peak point of each curve.
Findings. The two-segment model well describes the shear resistance variation of the coal-shale joints during the shear process, and there exist good relationships between the fitting parameters and the shear testing parameters. The initial slope of the softening part of the shear resistance – shear displacement curve can help to predict whether the coal pillar will burst drastically when it fails. Moreover, the normal displacement was very small in the pre-peak range which indicates that the complete detachment of original coal-shale joint surfaces has not occurred before the peak.
Originality. Direct shear tests were conducted on original coal-shale specimens for the first time, and a two-segment model is developed to describe their shear resistance-shear displacement curves. The initial slope of the softening part of the shear resistance – shear displacement curve is proposed to predict the burst tendency of coal pillar. Different from unbonded rock joints, the detachment of the original coal-shale joint occurs just after the peak.
Practical implications. The conclusions may have some help to understand the shear resistance mechanism of original coal-shale joint and to provide some new ideas of maintaining coal pillar stability.Мета. Експериментальне дослідження та теоретичне моделювання опору зсуву початкового шару вуглистого сланцю.
Методика. Розроблено модель для опису кривих опору зсуву й зсувного зміщення, отриманих у результаті випробувань на прямий зсув восьми шарів вуглистого сланцю. Модель включає два сегменти апроксимуючої кривої, що проходять через вищі точки описуваних кривих.
Результати. Запропонована модель достовірно описує зміну опору на зсув шарів вуглистого сланцю в про-процесі зсуву, при цьому визначено наявність тісного зв’язку між параметрами апроксимації та параметрами випробування на зсув. За початковим нахилом кривої в частині ослаблення опору зсуву й зсувного зміщення можна передбачити, чи відбудеться значне руйнування цілика вугілля при його обваленні. Нормальне зміщення є незначним у передпіковому діапазоні, що свідчить про те, що в цей час не відбувається повного відриву поверхонь шару вуглистого сланцю.
Наукова новизна. Випробування на прямий зсув проводилися вперше на зразках початкового вуглистого сланцю. Двохсегментна модель була розроблена спеціально для опису кривих опору зсуву та зсувного зміщення досліджуваних шарів. За початковим нахилом кривої в частині ослаблення опору зсуву й зсувного зміщення можна передбачити характер руйнування цілика. На відміну від незв’язаних шарів породи, відділення початкового шару вуглистого сланцю відбувається після пікових значень досліджуваних кривих.
Практична значимість. Висновки можуть допомогти зрозуміти механізм опору зсуву в первісному шарі вуглистого сланцю й окреслити нові підходи до збереження стабільності вугільного цілика.Цель. Экспериментальное исследование и теоретическое моделирование сопротивления сдвигу первоначального слоя углистого сланца.
Методика. Разработана модель для описания кривых сопротивления сдвигу и сдвигового смещения, полученных в результате испытаний на прямой сдвиг восьми слоев углистого сланца. Модель включает два сегмента аппроксимирующей кривой которые проходят через высшие точки описываемых кривых.
Результаты. Предлагаемая модель достоверно описывает изменение сопротивления на сдвиг слоев углистого сланца в процессе сдвига, при этом определено наличие тесной связи между параметрами аппроксимации и параметрами испытания на сдвиг. По начальному наклону кривой в части ослабления сопротивления сдвигу и сдвигового смещения можно предсказать, произойдет ли значительное разрушение целика угля при его обрушении. Нормальное смещение незначительно в предпиковом диапазоне, что свидетельствует о том, что в это время не происходит полного отрыва поверхностей слоя углистого сланца.
Научная новизна. Испытания на прямой сдвиг проводились впервые на образцах первоначального углистого сланца. Двухсегментная модель была разработана специально для описания кривых сопротивления сдвигу и сдвигового смещения исследуемых слоев. По начальному наклону кривой в части ослабления сопротивления сдвигу и сдвигового смещения можно предсказать характер разрушения целика. В отличие от несвязанных слоев породы, отделение первоначального слоя углистого сланца происходит после пиковых значений исследуемых кривых.
Практическая значимость. Выводы могут помочь понять механизм сопротивления сдвигу в первоначальном слое углистого сланца и обозначить новые подходы к сохранению стабильности угольного целика.The authors gratefully acknowledge funding by China National Natural Science Foundation (51109076) and the key scientific and technological project of Henan Province (152102210316)
Variance-constrained multiobjective control and filtering for nonlinear stochastic systems: A survey
The multiobjective control and filtering problems for nonlinear stochastic systems with variance constraints are surveyed. First, the concepts of nonlinear stochastic systems are recalled along with the introduction of some recent advances. Then, the covariance control theory, which serves as a practical method for multi-objective control design as well as a foundation for linear system theory, is reviewed comprehensively. The multiple design requirements frequently applied in engineering practice for the use of evaluating system performances are introduced, including robustness, reliability, and dissipativity. Several design techniques suitable for the multi-objective variance-constrained control and filtering problems for nonlinear stochastic systems are discussed. In particular, as a special case for the multi-objective design problems, the mixed H 2 / H ∞ control and filtering problems are reviewed in great detail. Subsequently, some latest results on the variance-constrained multi-objective control and filtering problems for the nonlinear stochastic systems are summarized. Finally, conclusions are drawn, and several possible future research directions are pointed out
Tunable near- to mid-infrared pump terahertz probe spectroscopy in reflection geometry
Strong-field mid-infrared pump--terahertz (THz) probe spectroscopy has been
proven as a powerful tool for light control of different orders in strongly
correlated materials. We report the construction of an ultrafast broadband
infrared pump--THz probe system in reflection geometry. A two-output optical
parametric amplifier is used for generating mid-infrared pulses with GaSe as
the nonlinear crystal. The setup is capable of pumping bulk materials at
wavelengths ranging from 1.2 m to 15 m and beyond, and detecting the
subtle, transient photoinduced changes in the reflected electric field of the
THz probe at different temperatures. As a demonstration, we present 15 m
pump--THz probe measurements of a bulk EuSbTe single crystal. A
transient change in the reflected THz electric field can be clearly resolved.
The widely tuned pumping energy could be used in mode-selective excitation
experiments and applied to many strongly correlated electron systems.Comment: 4 pages, 4 figure
Optical spectroscopy study of the collapsed tetragonal phase of CaFe(AsP) single crystals
We present an optical spectroscopy study on P-doped CaFeAs which
experiences a structural phase transition from tetragonal to collapsed
tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of
low frequency spectral weight and emergence of a new feature near 3200 \cm (0.4
eV) in optical conductivity across the transition, indicating an abrupt
reconstruction of band structure. The appearance of new feature is related to
the interband transition arising from the sinking of hole bands near
point below Fermi level in the cT phase, as expected from the density function
theory calculations in combination with the dynamical mean field theory.
However, the reduction of Drude spectral weight is at variance with those
calculations. The measurement also indicates an absence of the abnormal
spectral weight transfer at high energy (near 0.5-0.7 eV) in the cT phase,
suggesting a suppression of electron correlation effect.Comment: 6 pages, 4 figure
Optical study of phase transitions in single-crystalline RuP
RuP single crystals of MnP-type orthorhombic structure were synthesized by
the Sn flux method. Temperature-dependent x-ray diffraction measurements reveal
that the compound experiences two structural phase transitions, which are
further confirmed by enormous anomalies shown in temperature-dependent
resistivity and magnetic susceptibility. Particularly, the resistivity drops
monotonically upon temperature cooling below the second transition, indicating
that the material shows metallic behavior, in sharp contrast with the
insulating ground state of polycrystalline samples. Optical conductivity
measurements were also performed in order to unravel the mechanism of these two
transitions. The measurement revealed a sudden reconstruction of band structure
over a broad energy scale and a significant removal of conducting carriers
below the first phase transition, while a charge-density-wave-like energy gap
opens below the second phase transition.Comment: 5 pages, 6 figure
Time-and event-driven communication process for networked control systems: A survey
Copyright © 2014 Lei Zou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.In recent years, theoretical and practical research topics on networked control systems (NCSs) have gained an increasing interest from many researchers in a variety of disciplines owing to the extensive applications of NCSs in practice. In particular, an urgent need has arisen to understand the effects of communication processes on system performances. Sampling and protocol are two fundamental aspects of a communication process which have attracted a great deal of research attention. Most research focus has been on the analysis and control of dynamical behaviors under certain sampling procedures and communication protocols. In this paper, we aim to survey some recent advances on the analysis and synthesis issues of NCSs with different sampling procedures (time-and event-driven sampling) and protocols (static and dynamic protocols). First, these sampling procedures and protocols are introduced in detail according to their engineering backgrounds as well as dynamic natures. Then, the developments of the stabilization, control, and filtering problems are systematically reviewed and discussed in great detail. Finally, we conclude the paper by outlining future research challenges for analysis and synthesis problems of NCSs with different communication processes.This work was supported in part by the National Natural Science Foundation of China under Grants 61329301, 61374127, and 61374010, the Royal Society of the UK, and the Alexander von Humboldt Foundation of Germany
Optical spectroscopy study on single crystalline LaFeAsO
Millimeter-sized single crystals of LaFeAsO were grown from NaAs flux and the
in-plane optical properties were studied over a wide frequency range. A sizable
electronic correlation effect was indicated from the analysis of the
free-carrier spectral weight. With decreasing temperature from 300 K, we
observed a continuous suppression of the spectral weight near 0.6 eV. But a
spin-density-wave gap formation at lower energy scale was seen only in the
broken-symmetry state. We elaborate that both the itinerancy and local spin
interactions of Fe\emph{3d} electrons are present for the FeAs-based systems;
however, the establishment of the long-range magnetic order at low temperature
has a dominantly itinerant origin.Comment: 4 figures, 5 page
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