单轴压缩下砂岩声发射及分形特征研究

为研究砂岩在单轴压缩下声发射及分形特征,在MTS815程控伺服刚性试验机上,对早古生代志留系下统小河坝组(S1x)砂岩进行单轴压缩下的声发射试验,结合声发射特征参数、应力应变关系以及分形特性,对岩石整个失稳破坏过程进行分析。结果表明:小河坝组砂岩声发射及分形特征、力学特性在岩石损伤破裂过程中具有一致性,以应力比0.6~0.8之间振铃计数率的变化趋势段作为岩体失稳与破坏的识别方法;相空间维数的选取对关联维数有一定影响,随相空间维数m的增加,关联维数D呈现出增加-稳定-发散的发展过程;声发射序列在应力比为0.3~0.5时,关联维数D值最大;声发射序列关联维数曲线表现为"波动-上升-下降-波动"的演化模式,在较高应力比0.7~0.8时,关联维数D在介于0.9~1.4之间小幅波动。据此,可为工程中岩体失稳与破坏的监测预报提供一定的理论基础与数据支撑。     

Thermoelectric properties of nanotubular and fractal carbon deposits

The structure and thermo emf of nanotubular and fractal carbon layers deposited from an electric discharge plasma were studied. The thermo emf of such nanotubular and fractal deposits is almost tenfold higher as compared to that of polycrystalline carbon, which is explained by the nanostructural state of carbon in the deposits studied.     

The measurement and interpretation of dielectric properties

The advantages of frequency response analysers in the measurement of dielectric properties are described and methods of data processing are pointed out which make it possible to obtain from them the maximum yield of physically meaningful information. Identification of physical mechanisms responsible for dielectric relaxation is facilitated by a general classification of response types, based on wide ranging experience and supported by a modern many-body theory of dielectric behaviour.     

Determination of the pore size distribution of correlated mesoporous networks

In the present work we study how the adsorption desorption hysteresis loop of a mesoporous disordered medium represented by a 3-dimensional Dual Site-Bond Model (DSBM) is affected by percolation. Site and bond distributions are assumed to be gaussians. The behavior of the threshold pressure for the evaporation processe suggests a method to determine the site and bond distributions from experimental adsorption-desorption hysteresis curves. Traditional methods developed for non-correlated networks are tested and evaluated against our simulation results showing the discrepancy mainly for highly correlated networks. Results of the prediction capability of our method are shown. Received: 30 March 2000     

用圆孔内单边裂纹平台巴西圆盘和实验-数值-解析法确定砂岩的动态起裂和扩展韧度

在I型(张开型)动态断裂实验中,利用大直径(?100 mm)分离式霍普金森压杆径向冲击圆孔内单边裂纹平台巴西圆盘试样。考虑了材料惯性效应和裂纹扩展速度对动态应力强度因子的影响,用实验-数值-解析法确定了高加载率和高裂纹扩展速度情况下,砂岩的动态起裂韧度和动态扩展韧度。由动态实验获取试样的动荷载历程,采用裂纹扩展计(Crack Propagation Gauge,CPG)测定试样断裂时刻和裂纹扩展速度,获得裂纹扩展速度对应的普适函数值。然后将动荷载历程带入到有限元软件中进行动态数值模拟,求出静止裂纹的动态应力强度因子历程,再用普适函数值对其进行近似修正。最后根据试样的起裂时刻和穿过CPG中点的时刻,由相应的动态应力强度因子历程分别确定砂岩的动态起裂和动态扩展韧度,它们分别随动态加载率和裂纹扩展速度的提高而增加。     

A unified fractal approach for the interpretation of the anomalous scaling laws in fatigue and comparison with existing models

In this paper, a critical reexamination of the fractal models for the analysis of crack-size effects in fatigue is proposed. The enhanced ability to detect and measure very short cracks has in fact pointed out the so-called anomalous behavior of short cracks with respect to their longer counterparts. The crack-size dependencies of both the fatigue threshold and the Paris’ constant C are only two notable examples of these anomalous scaling laws. In this context, a unified theoretical model seems to be missing and the behavior of short cracks can still be considered as an open problem. A new generalized theory based on fractal geometry is herein proposed, which permits to consistently interpret the short crack-related anomalous scaling laws within a unified theoretical framework. The proposed model is used to interpret relevant experimental data related to the crack-size dependence of the fatigue threshold in metals. As a main result, the model gives an explanation to the experimentally observed variability in the slope of the asymptote of the scaling law for the fatigue threshold in the short crack regime.     

Determination of pore size distribution by suction prosimetry

The theory and practice of suction porosimetry methods (suction plate, pressure membrane and centrifuge) are reviewed together with related methods based on the rate of penetration of wetting liquids, and on static and dynamic fluid/fluid displacement measurements.

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Résumé La porosimétrie par succion repose sur la mesure de la dépression capillaire que suscite un liquide mouillant retenu dans un milieu poreux. C’est donc une méthode complémentaire à celle de la porosimétrie par pression (par ex. pénétration de mercure) où l’on opère avec des liquides mouillants. On expose la théorie et la pratique des méthodes par succion, entre autres les méthodes de la plaque, de la membrane et de la centrifugation. Un groupe de méthodes voisines découle de la vitesse de pénétration des liquides mouillants sous l’action des forces capillaires (par ex. la méthode d’Astbury), et de l’observation dans les conditions statique et dynamique des mouvements gaz/liquide et liquide/liquide.

     

Open pore description of mechanical properties of ceramics

A dependence of Young's modulus of elasticity on open porosity in ceramics is derived from an open-porosity model, which in the literature, is applied to salinity conductivity and fluid permeability in rocks. A random distribution of grain and pore size is assumed. The relation developed,E(p)=E _o(1–"p)^m, whereE is the modulus of elasticity of the porous ceramic,E _o is the theoretical elastic modulus,p is the porosity andm is an exponent dependent on the tortuosity of the structure of the ceramic, adequately describes the dependence of the modulus of elasticity on porosity. The model is applied to the experimental data from several ceramics such as alumina, silicon nitride, silicon carbide, uranium oxide, rare-earth oxides, and YBa₂Cu₃O_7– superconductor, and the value ofm is obtained for each case. We have shown thatm has a value of nearly 2 for sintered ceramics, unless sintering aids or hot pressing have been used during fabrication of the ceramic. Such additional procedures approximately double the magnitude ofm. On joint appointment with: Materials Laboratory, Physics Department, University of the West Indies, Mona, Kingston 7, Jamaica,     

Some properties of solid fractal structures in carbon nanofibers

Experimental data on the self-organization of massive fractal granules with a total volume of up to 1.1 cm³ consisting of carbon nanofibers with transverse dimensions within 50–70 nm and a length of up to 1000 nm are presented. The fractal granules have a density of 1.3 g/cm³, an elastic modulus of 37.4 MPa, and a fractal dimension of D = 2.95. The resistivity of this material is about three orders of magnitude greater than the values typical of pyrolytic graphite. The surface layers in the fractal granule are characterized by a Seebeck coefficient of about S ～ 24 μV/K, whereas the bulk regions have S ～ 11 μV/K.     

The fractal properties of a geometric model of the high-resolution X-ray diffractor

Perfect and mosaic crystals are conventionally used in X-ray monochromators operating in the energy range from several hundred to tens of thousands of electronvolts. The focusing X-ray optics for nonparallel beams employs either cylindrical bent crystals (the methods of Johann [1], Johansson [2], and Cauchois [3]) or crystals with spherical or toroidal [4] bending of the crystallographic planes. Special variants of high-resolution stepped-crystal diffractors [5–8] were developed to study the possibility of high-precision focusing of a monochromatic X-ray radiation. The fractal properties of a geometric model of such a high-resolution steppedcrystal diffractor are considered.     

Dynamic restructuring process for self-reconfiguration in the fractal manufacturing system

To meet the rapidly changing customer needs in the manufacturing environment, future manufacturing systems must be dynamically and flexibly reconfigured. The fractal manufacturing system (FrMS) is one of the new manufacturing paradigms that can meet such requirements. The basic component of the FrMS is referred to as a fractal. Consisting of self-similar agents, each fractal autonomously cooperates and negotiates with others to coordinate its tasks. Dynamic restructuring process (DRP) supports reorganization of the system configurations so that the FrMS can be adapted to dynamically changing environments. Although traditional approaches have endeavoured to demonstrate reconfigurability of a manufacturing system, they are not accurate enough to meet the requirements of circumstances such as high-level autonomy in reconfiguring the system architecture. In this paper, therefore, the DRP, which embodies self-reconfigurability of a system, is proposed focusing on the FrMS. To check the effectiveness of the DRP, we have developed the FrMS test bed and conducted the experimentation on the DRP. A simulation study has been conducted to show the effectiveness of the DRP under an illustrative situation.     

Goal-balancing process for goal formation in the fractal manufacturing system

The coordination architecture and mechanisms are proposed for the goal-balancing process (GBP), which is one of the components of goal-formation process (GFP) in the fractal manufacturing system (FrMS). Since each agent in the FrMS generates, achieves and modifies its own goal autonomously during the coordination process with other agents, it is necessary to develop a systematic methodology for the goal-formations in a manufacturing system: GFP takes charge of this methodology, which enables the formation of and the changes in goal through coordination between agents in real-time in the distributed and dynamic systems. The GFP is composed of three sub-processes: the goal-generating process (GGP), the goal-harmonizing process (GHP), and the GBP. The GGP makes and propagates goals for all fractals. The GHP then eliminates or reduces possible conflicts and interferences between goals generated in GGP. The GBP is the post-process of GHP; it refines the fractal's goal, which has been ridded of conflict during the GHP to enhance the global performance of the system, rather than maximizing the performance of each fractal. For the development of the GBP we proposed a mechanism for unit goal-balancing process (UGBP), which is the basic process of GBP that adopts the, fuzzy decision-making, and distributed problem solving approaches.     

Conflict detection and resolution for goal formation in the fractal manufacturing system

The fractal manufacturing system (FrMS) is based on the concept of autonomously cooperating agents referred to as fractals. A fractal is a set of self-similar agents whose goal can be achieved through cooperation, coordination, and negotiation among the agents for themselves. A fractal has fractal-specific characteristics (e.g. self-similarity, self-organization, self-optimization, goal-orientation, and dynamics), and it also has the characteristics of an agent (e.g. autonomy, mobility, intelligence, cooperation, and adaptability) at the same time. In the FrMS, a goal can be regarded as the status which the system aspires to be in. The goal-formation process (GFP) in the FrMS is a process of generating goals and modifying them by coordination between agents. In the GFP, conflicts may occur between goals, which can drive a system to become inefficient. In this paper, a conflict resolution mechanism via agent-based negotiation is proposed for facilitating the GFP. The scheme deals with non-fixed goals. The mobile agent-based negotiation process (MANPro), in which a mobile agent is used for information-exchanging and problem-solving, is used for negotiations in this scheme. The proposed mechanism is illustrated with a goal formation scenario in an exemplary FrMS.     

温度与荷载对砂岩动态力学性能及裂纹的影响

为研究在不同温度与荷载下砂岩的动态力学性能,运用了SHPB实验仪器及配套高温环境箱分别在25(常温)、100、200、300、400、500、600、700℃下对砂岩进行3种不同荷载(0.3、0.5、0.7 MPa)动态压缩实验,并结合SEM设备对高温应力作用下砂岩内部裂纹进行分析。结果表明:当冲击荷载增大时,砂岩所能承受的动态压缩强度显著提高;当所处温度一定时,其破裂情况也随荷载的增大而加剧;当冲击气压一定时,随着温度升高,砂岩的峰值应力表现出平缓、上升、下降的变化趋势,最终在700℃时达到最小值。在应力与常温作用下,试件会先以应力作用影响为主,出现单条裂纹,当温度升高至300℃以上,单条裂纹会有所延伸并形成多条裂纹,出现二次开裂。     

Effect of ageing conditions on the pore structural properties of titania aerogels

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of shrinkage reduction admixtures on the pore structure properties of mortars

In order to reduce the shrinkage of concrete, which can cause cracks on structural or non-structural elements, the addition of a shrinkage reducing admixture (SRA) in concrete mixtures is one of the techniques that can be used. The decrease of shrinkage due to SRA is attributed to the reduction of the surface tension of the pore water. This study analyses the changes on the capillary suction and the oxygen permeability of mortar mixtures with and without SRA. This indicates that the addition of a SRA causes another physical effect on the porous structure, which can also affect the shrinkage of concrete.     

Effect of smooth,porous and fractal surface structure on the properties of an interface

Recently, biomaterials research has focused on the creation of new materials. Mixing or doping of well-tried materials should aim to result in new physical or chemical properties, better tissue tolerability and improved long-term stability. We present another way of improving performance: the application of new technologies to well-known materials. The surface structure of a coating depends on the deposition technique. Hence, the transfer of, for example, sputtering technology to biotechnology may solve current problems with well-tried materials. This paper reports the influence of surface structure and area on the properties of pacemaker electrodes. Smooth electrodes were compared with porous electrodes produced by sintering and with fractal electrodes resulting from physical vapour deposition of iridium or titanium nitride. Micrographs revealed a slight enlargement of the surface area by sintering and an enormous enlargement with fractal coating. Analysis of electrochemical properties proved the advantages of fractal coating: the impedance spectra exhibited incomparable low impedances for frequencies down to 1 Hz and no after-potentials could be detected. Clinical results confirmed the superiority of fractal electrodes. The stimulation thresholds were significantly reduced, P-and R-wave amplitudes were increased, and ventricular evoked responses and monophasic action potentials were able to be measured with clarity unknown so far.     

Influence of damage on the fractal properties of concrete subjected to pure tension

In a previous work [1] the authors showed how to acquire the meso-structural characteristics of undamaged concrete-like materials by a peculiar laser equipment [2]. In order to extend the analysis to damaged disordered materials, a new direct tension test equipment has been developed, that minimizes flexural effects by freely rotating boundary conditions. Increasing levels of damage are obtained, after reaching the peak load, by proceeding along the descending strain-softening curve. After the desired damage level is reached, the load is removed and the specimen is cut to permit the laser acquisition of the most damaged zone. The progressive rarefaction of the effective stress-carrying cross section is described by means of fractal concepts. It is worth noting that both the fractal dimension and the measure of the stress carrying cross section decrease after the peak load, and vanish when the specimen is broken apart.

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Résumé Lors d'une précédente étude [1], les auteurs ont démontré la possibilité d'apprécier les caractéristiques méso-structurelles des mattériaux désordonnés, comme le béton, au moyen d'un dispositif laser [2]. Afin de pouvoir étendre l'analyse à des matériaux désordonnés endommagés, il a fallu mettre au point un équipement d'essa à traction directe, pour minimiser les effects de flexion tout en permenttant la rotation des sections de extrémité de l'échantillon. Des niveaux d'endommagement croissants sont obtenus par l'application d'une charge supérieure à la valeur de pic, suivant la courbe décroissante de radoucissement (strain-softening). Une fois le niveau d'endommagement souhaité atteint, la charge est enlevée et l'échantillon est coupé. Le dispositif laser est ainsi capable d'analyser la section la plus endommagée. La raréfaction progressive de la section effective est décrite en utilisant des concepts appartenant au domaine de la géométrie fractale. Il est à la fois possible d'observer la réduction de la mesure de la section effective et la diminution de la dimension fractale après que la charge appliquée a franchi le pic, jusqu'à l'annulation lorsque les deux moitiés de l'échantillon sont complètement séparées.

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Editorial Note Prof. Alberto Carpinteri is a RILEM Senior Member.