On Tension Failure of 2-D Rock Specimens and Associated Acoustic Emission

Summary To understand the failure mechanism of quasi-brittle materials like rock under tensile stress, observations on the failure process of granite and marble plate specimens under tension are summarized and presented. Micro- and macro-failure properties of rock plates under uniaxial tension have been characterized by using an acoustic emission technique. Acoustic emission signals associated with micro-fractures are captured to locate the sources. An algorithm based on arrival time difference is developed for this purpose. The results reveal clearly the failure processes of rock which include initiation, nucleation and propagation of micro-fractures when the axial stress is close to the peak strength of rock. It is believed that the difference in heterogeneity between granite and marble specimens leads to different fracture shapes and different behaviors of associated acoustic emissions. Numerical simulation of acoustic emissions for two-dimensional tensile test is also carried out. The simulated characteristics are in good agreement with the experimental results.     

Ammonia toxicity in fish

Ammonia is present in the aquatic environment due to agricultural run-off and decomposition of biological waste. Ammonia is toxic to all vertebrates causing convulsions, coma and death, probably because elevated NH₄⁺ displaces K⁺ and depolarizes neurons, causing activation of NMDA type glutamate receptor, which leads to an influx of excessive Ca²⁺ and subsequent cell death in the central nervous system.

Present ammonia criteria for aquatic systems are based on toxicity tests carried out on, starved, resting, non-stressed fish. This is doubly inappropriate. During exhaustive exercise and stress, fish increase ammonia production and are more sensitive to external ammonia. Present criteria do not protect swimming fish. Fish have strategies to protect them from the ammonia pulse following feeding, and this also protects them from increases in external ammonia, as a result starved fish are more sensitive to external ammonia than fed fish.

There are a number of fish species that can tolerate high environmental ammonia. Glutamine formation is an important ammonia detoxification strategy in the brain of fish, especially after feeding. Detoxification of ammonia to urea has also been observed in elasmobranches and some teleosts. Reduction in the rate of proteolysis and the rate of amino acid catabolism, which results in a decrease in ammonia production, may be another strategy to reduce ammonia toxicity. The weather loach volatilizes NH₃, and the mudskipper, P. schlosseri, utilizes yet another unique strategy, it actively pumps NH₄⁺ out of the body.     

沟道流域坡面与沟谷侵蚀演化关系——以晋西王家沟小流域为例

以往坡、沟侵蚀关系研究主要限于水沙关系的探讨 ,有关原型尺度小流域坡、沟地貌侵蚀演化关系的定量研究甚少 ,尤其是坡、沟地貌特征对流域切割程度的综合影响与交互作用的定量研究尚属空白。本文在 3S技术支持下 ,根据晋西王家沟小流域 196 7年 1∶5 0 0 0正摄影像图 (DOM )、高程数字化模型 (DEM )和同期、同比例尺地形图 ,利用正交多项式回归分析方法 ,定量分析了坡、沟地貌特征对流域切割程度的影响与交互作用。研究表明 ,沟谷地在流域地貌演化及地表切割程度中起主导作用 ,在沟间地面积相同时 ,沟谷地面积约为0 1km2 是影响流域切割裂度侵蚀演化强度最大的区域 ,流域面积愈小影响流域地表破碎的程度最大。其沟道流域坡、沟地貌演化机制可用坡、沟水流动能的地貌指标的综合影响与交互作用来解释     

Influence of Heterogeneity of Mechanical Properties on Hydraulic Fracturing in Permeable Rocks

Summary Numerical simulations of circular holes under internal hydraulic pressure are carried out to investigate the hydraulic fracture initiation, propagation and breakdown behavior in rocks. The hydraulic pressure increases at a constant rate. The heterogeneity of the rocks is taken into account in the study by varying the homogeneity index. In addition, the permeability is varied with the states of stress and fracture. The simulations are conducted by using a finite element code, F-RFPA^2D, which couples the flow, stress and damage analyses. The simulation results suggest that the fracture initiation and propagation, the roughness of the fracture path and the breakdown pressure are influenced considerably by the heterogeneity of rocks. The hole diameter elongation and the stress field evolution around the fracture tip during the fracture propagation can also provide useful information for the interpretation of the hydraulic fracturing behaviour.     

孔隙水压力对岩石裂纹扩展影响的数值模拟

应用岩石破坏过程渗流-应力-损伤(FSD)耦合分析软件F-RFPA2D,通过对孔隙水压作用下岩石试件加载破坏过程的数值模拟,对孔隙水压力大小和梯度对岩石试样中裂纹的萌生和扩展进行了数值模拟研究。模拟结果再现了孔隙水压力作用下裂纹萌生扩展的全过程,表明孔隙水压力大小和梯度对岩石中裂纹的萌生和扩展模式都有很大的影响。     

An Examination of ENSO’s Effect on the Monthly and Seasonal Climate of Hong Kong from a Statistical Perspective

This study aims to examine the effect of El Nino and La Nina on the monthly and seasonal climate of Hong Kong against the ENSO-neutral situation from a statistical perspective. Monthly and seasonal temperature and rainfall of Hong Kong and monthly number of tropical cyclones (TCs) coming within 500 km of the city over the 59-yr (1950-2008) period are examined under three ENSO situations, namely El Nino, La Nina, and ENSO-neutral. It is found that, compared with the ENSO-neutral situation, El Nino tends...     

An enhanced engineering perspective of global climate systems and statistical formulation of terrestrial CO2 exchanges

This paper designs a comprehensive approach based on the engineering machine/system concept, to model, analyze, and assess the level of carbon dioxide (CO₂) exchange between the atmosphere and terrestrial ecosystems, which is an important factor in understanding changes in global climate. The focus of this article is on spatial patterns and on the correlation between levels of CO₂ fluxes and a variety of influencing factors in eco-environments. The engineering/machine concept used is a system protocol that includes the sequential activities of design, test, observe, and model. This concept is applied to explicitly include various influencing factors and interactions associated with CO₂ fluxes. To formulate effective models of a large and complex climate system, this article introduces a modeling technique that will be referred to as stochastic filtering analysis of variance (SF-ANOVA). The CO₂ flux data observed from some sites of AmeriFlux are used to illustrate and validate the analysis, prediction, and globalization capabilities of the proposed engineering approach and the SF-ANOVA technique. The SF-ANOVA modeling approach was compared to stepwise regression, ridge regression, and neural networks. The comparison indicated that the proposed approach is a valid and effective tool with similar accuracy and less complexity than the other procedures.     

Connection among spacecrafts and ground level observations of small solar transient events

An overview of the results of the search for small solar transient events, in association with muon enhancements (deficits) registered at ground-level by the Tupi muon telescopes, is presented. Among the events, there are three interplanetary shocks and two solar flares of small scale whose X-ray emission flux is much smaller than 10^???5 W m^?2 at 1 AU (GOES-Tupi connection). Two of the interplanetary shocks are cataloged as corotating interaction region and the third shock is due to the passage of a CME (coronal mass ejection) ejecta (ACE and SOHO-Tupi connection) in the Earth’s proximities. In most cases, the particles excess (deficit) coming from these events have only been observed with spacecraft instruments. However, the Tupi telescopes are located at sea level and within the South Atlantic Anomaly (SAA), a region where the shielding effect of the magnetosphere is not perfectly spherical and shows a ‘dip’. This fact enables the muon telescopes to achieve a low rigidity of response to primary and secondary charged particles (≥?0.1 GV). Muon excesses (deficits) with significances above 4σ have been found. These events observed at ground admit a temporal correlation with solar transient events observed by spacecrafts, which suggests strongly a real connection between them. Details of these observations are reported.     

Homologous gravitational collapse in Lagrangian representation

The classical problem of spherical homologous gravitational collapse with a polytropic equation of state with γ=4/3 is examined in Lagrangian fluid coordinate. The fluid velocity v(t)=dr(t)/dt=ηdy/dt is derived from the evolution function y(t) where η=r(0) is the radial fluid label in Lagrangian formulation. The evolution function y(t), which describes the collapse time history of a finite pressure cloud, is solved which happens to be identical to the well established parametric form of Mestel (Mon. Not. R. Astron. Soc., 6:161–198, 1965) for cold cloud collapse. The spatial structure is described by a nonlinear equation of the density profile function q(z) with z=aη. Due to the nonlinearity, the collapse profile is highly non-uniform in space. For moderate values of q(0), the solutions are homologous. For large q(0), homology is broken leading to the formation of a central core and a central cavity. The stellar envelope bounding the central cavity collapses under the additional external gravity of the central core, generating eventually a sequence of cavity-shell structure in the envelope, until the entire mass of the original cloud is accounted for.     

Celestial n-Body Coupling in the Lunar Orbit Theory

Making use of the fact that, in the solar system, the angular momentum is carried predominantly by the planets while the mass is beared almost entirely by the Sun, an iterative scheme is devised to solve approximately the n-body contributions of the lunar orbit problem. The scheme envisages the Moon-Earth-Sun three-body subsystem as being nested in the grand Earth-Jupiter-Sun system. In the planetocentric representation, the orbital motion of the Sun about the solar system center of mass is transmitted to the third body via the second primary body in both the grand and nested three-body systems. This revised version was published online in July 2006 with corrections to the Cover Date.