压实对土体空间结构影响的分形描述

为了准确地模拟土体颗粒在压实过程中空间结构的变化,计算土体压实所能达到的最终状密实态,文章采用分形几何原理,建立了土粒径分布、颗粒空间结构与土体变形参数之间的函数关系,提出了土体空间结构分形维数计算公式,计算分析了土体孔隙通道分形维数值;分析表明,随着压实进行孔隙通道的曲折程度增大,孔隙通道的长度变短,土体密实增加;通过Gaudin-Schuhmann粒度分布方程可以定量地由粒径的分布来判断压实所能达到的终极状态,从而更清晰地分析土体微观结构与压实性能之间关系。

玄武岩纤维增强珊瑚骨料混凝土的孔结构相关三轴力学响应及强度准则

本文研究了玄武岩纤维增强珊瑚骨料混凝土(BFCAC)的三轴力学性能,分析了玄武岩纤维(BF)和孔结构对BFCAC三轴力学性能的影响,建立了BFCAC的强度准则。结果表明,BFCAC的峰值偏应力和弹性模量随着围压非线性增加,两者的围压效应随BF掺量的增加而增大。围压和BF掺量的增加均提高了BFCAC的临界应变和延性变形,但较大围压作用弱化了BF对珊瑚骨料混凝土延性变形的影响。BFCAC的泊松比随BF掺量和围压增大分别增大和减小。BF和围压提升了BFCAC的体积收缩变形,降低了其体积膨胀变形。围压一方面使BFCAC由劈裂破坏逐渐转变为剪切破坏和挤压塑性流动破坏,另一方面,围压也提高了BF与珊瑚骨料混凝土基体间的粘结性能。BFCAC破坏时,裂缝均贯穿珊瑚骨料,且在较高围压时,珊瑚骨料发生孔隙坍塌。BFCAC的小孔不具有分形特征,中孔和大孔的分形特征较为明显,且其孔结构分形维数随BF掺量的增加而增大。孔结构分形维数与三轴力学性能的关系表明,BFCAC的三轴力学性能不仅受BF本身的影响,也受BF对CAC孔结构的影响。建立了BFCAC的改进非线性M-C强度准则,不仅能够准确计算BFCAC的三轴强度变化规律,而且能够有效体现BF和孔结构特征对BFCAC三轴强度的影响。

氯盐环境下混凝土冻融循环及孔结构演化研究

盐浓度在混凝土盐冻破坏过程中起着关键作用,也是影响其微观结构演化的主要因素。为研究氯盐溶液的浓度对混凝土冻融破坏的影响,并对冻融破坏进行微观尺度定量表征,通过试验对比分析了不同浓度NaCl溶液中混凝土冻融循环过程中的质量损失和相对动弹性模量变化规律,并运用压汞法测试了不同冻融次数下混凝土试件的孔隙率和孔径分布。在此基础上,结合孔结构分形维数对孔结构演化过程进行了定量表征。结果表明:在浓度约3%的NaCl溶液中混凝土冻融破坏最为严重;孔结构分形维数与混凝土冻融循环过程中的质量损失具有高度线性相关性,可以体现混凝土宏观破坏程度。

海洋环境中玄武岩/聚丙烯纤维增强混凝土氯离子扩散性能

采用自然浸泡法模拟海洋水下区环境,研究了玄武岩/聚丙烯纤维增强混凝土(BPFRC)的氯离子扩散性能.通过固液萃取法和电位法测试了不同侵蚀时间下BPFRC中的氯离子含量,分析了纤维种类、掺量和混杂形式对氯离子含量分布、表面氯离子含量(C_(s))和氯离子扩散系数的影响;此外,采用Rapid Air 457测定了BPFRC的孔径分布,并计算了其孔结构分形维数.结果表明:BPFRC中的氯离子含量随着侵蚀龄期的增加而增大;当纤维体积分数为0.10%时,玄武岩纤维对混凝土中氯离子含量的降低作用大于聚丙烯纤维,适量的混杂纤维能够减小混凝土中的氯离子含量,过量的混杂纤维则增大了混凝土中不同深度处的氯离子含量;BPFRC中的C_(s)在侵蚀初期增长较快、后期增长较慢,与侵蚀时间为幂函数关系;BPFRC的孔结构表现出明显的分形特征,分形维数范围为2.301~2.446,分形维数与氯离子扩散系数具有较强的正相关性.

煤岩超微孔隙结构特征及其分形规律研究

煤岩超微孔隙结构对煤的吸附和强度性能起到非常重要的决定作用.为了对其进行精确测定,采用了高精度压汞仪对来自8种不同硬度的煤样进行压汞法实验,测定得出超微孔隙结构的所有特征参数.根据压汞法基本原理和分形几何学理论建立了切合实际的煤孔隙分形维数计算模型,利用孔隙特征参数计算出各硬度的孔隙结构分形维数.研究发现:煤孔隙结构具有很好的分形特征,煤体越松软,分形性越好,用分形规律研究煤岩孔隙结构越精确;随着煤体硬度的增加,孔隙分形维数不断降低,煤体抗压强度不断增大;建立硬度与孔隙分形维数之间的定量关系式,可以用硬度定量描述煤的吸附性和抗压强度.研究结论对于煤层瓦斯的运移、瓦斯抽放以及瓦斯突出均有着极为重要的意义.

NMR research on deterioration characteristics of microscopic structure of sandstones in freeze-thaw cycles

In order to study the deterioration characteristics of the microscopic structure of sandstones in freeze-thaw cycles, tests of180 freeze-thaw cycles were performed on sandstone specimens. The nuclear magnetic resonance （NMR） technique was applied tothe measurement of sandstone specimens and analysis of the magnetic resonance imaging. Then, the fractal theory was employed tocompute the fractal dimension values of pore development of rocks after different freeze-thaw cycles. The results show that the massand porosity of rocks grow with the increase of freeze-thaw cycles. According to the NMR T2 distribution of sandstones, the poresizes of rock specimens increase after 180 freeze-thaw cycles, especially that of the medium-sized and small-sized pores. The spatialdistribution of sandstone pores after freeze-thaw cycles has fractal features within certain range, and the fractal dimension ofsandstones tends to increase gradually.

A numerical simulation of seepage structure surface and its feasibility

According to Cubic law and incompressible fluid law of mass conservation, the seepage character of the fracture surface was simulated with the simulation method of fractal theory and random Brown function. Furthermore, the permeability coefficient of the single fracture was obtained. In order to test the stability of the method, 500 simulations were conducted on each different fractal dimension. The simulated permeability coefficient was analyzed in probability density distribution and probability cumulative distribution statistics. Statistics showed that the discrete degree of the permeability coefficient increases with the increase of the fractal dimension. And the calculation result has better stability when the fractal dimension value is relatively small. According to the Bayes theory, the characteristic index of the permeability coefficient on fractal dimension P（Dfi｜ Ri） is established. The index, P（Dfi｜ Ri）, shows that when the simulated permeability coefficient is relatively large, it can clearly represent the fractal dimension of the structure surface, the probability is 82%. The calculated results of the characteristic index verify the feasibility of the method.

Yarn Architecture Analysis of Two-step 3D Braided Composites

A comprehensive study of yarn architecture of two-step rectangle 3D braided composites is presented. Firstly, the braided surface, the shapes of yarns and the intertwining between braider yams and axial yams are analyzed from experimentation. With the microstructure being defined, three levels of unit cell structure are identified, i.e. large unit cell, second unit cell and minimal unit cell. Secondly, based on the minimal unit cell in the interior and on the boundary of the entire cross-section, the deformations of axial yams squashed by braider yams contribute to the increase of the fiber packing factors of axial yams. Finally, the predicted fiber volume fraction of the composites decreases with the increase of linear density of the braider yam and the pitch length. Favorable correlations between the predicted and the experimental results arc found for six groups of the composites.

Analysis of microscopic pore structures of rocks before and after water absorption

Hydrophilic characteristics of rocks are affected by their microscopic pore structures,which clearly change after water absorption.Water absorption tests and scanning electron microscopic（SEM） experiments on rock samples,located at a site in Tibet,China,were carried out Changes of rock pore structures before and after water absorption were studied with the distribution of pore sizes and fractal characteristics of pores.The results show that surface porosities,fractal dimensions of pores and the complexity of pore structures increased because the number of new small pores produced increased or the original macropore flow channels were expanded after rocks absorbed water.There were points of inflection on their water absorption curves.After water absorption of other rocks,surface porosities and fractal dimensions of pores and complexity of pore structures decreased as the original pore flow channels became filled.Water absorption curves did not change.Surface porosity and the pore fractal dimensions of rocks have good linear relationships before and after water absorption.

Morphological Research on Geometrical Scattering Waves of an Underwater Target

In this paper, a new method based on morphologic research named reconstruction cross-component removal （RCCR） is developed to analyze geometrical scattering waves of an underwater target. Combining the origin of the cross-component in Wigner-ViUe distribution, the highlight model of target echoes and time-frequency features of linear frequency-modulated signal can remove cross-components produced by multiple component signals in Wigner-Ville distribution and recover the auto-components of output signals. This method is used in experimental data processing, which can strengthen the real geometric highlights, and restrain the cross components. It is demonstrated that this method is helpful to analyze the geometrical scattering waves, providing an effective solution to underwater target detection and recognition.

A method for choice of optimum scale on land use monitoring in Tarim River Basin

Optimal scale is one of the important issues in ecology and geography.Based on land-use data of the Tarim River Basin in Xinjiang of China in the 1950s,regarding the area of land use types as the parameter in scale selecting,the histograms of the patches in area are charted.Then,by reinforcing the normalized scale variances(NSV) with 3 landscape indi-ces,the scale characteristics of land use in the Tarim River Basin can be summarized.(1) NSV in the Tarim River up to a maximum at scale of 1:50,000 which is considered appropriate for the Tarim River.(2) Diversity indices of saline land are consistent with NSV's.Diversity indices and NSV of sandy land showed that the appropriate scale is in the same scale domain.There is a significant difference between diversity indices and NSV of forestland and shrub-land.(3) Fractal dimension of sandy land and saline land showed a hierarchical structure at a scale of 1:10,000.Fractal dimension of forestland and shrubland are distributed under the same hierarchical structure in the region.

Iris Feature Extraction and Recognition Based on Fractal Dimension of Grayscale Extremums

This paper presented an individual recognition algorithm for human iris using fractal dimension of grayscale extremums for feature extraction.Firstly,iris region was localized from an eye image with modified circle detector stemmed from Daugman’s integro-differential operator.Then,segmentation was used to extract the iris and to exclude occlusion from eyelids and eyelashes.The extracted iris was normalized and mapped to polar coordinates for matching.In feature encoding,a new approach based on fractal dimension of grayscale extremums was designed to extract textural features of iris.Finally,a normalized correlation classifier was employed to determine the agreement of two iris feature templates,and the feature template was rotated left and right to avoid the interference from rotation of eyes and tilting of head.The experimental results show that fractal dimension of grayscale extremums can extract textural features from iris image effectively,and the proposed recognition algorithm is accurate and efficient.The proposed algorithm was tested on CASIA-IrisV3-Interval iris database and the performance was evaluated based on the analysis of both False Accept Rate(FAR)and False Reject Rate(FRR)curves.Experimental results show that the proposed iris recognition algorithm is effective and efficient.

北川震裂带柳杉林地土壤功能特性试验分析

汶川地震形成了大面积的震裂带,震裂带内水土流失严重。本文重点从土壤结构体分形维数、有机质含量、土壤抗冲性三个方面探讨了北川震裂带柳杉林地土壤结构功能特性,分析了地震裂隙程度对土壤结构体分形维数、有机质含量、土壤抗冲性的影响及其与土壤深度的相关性。结果表明地震裂隙程度对土壤结构体分形维数影响不显著,而地震3 a后强度裂隙样地土壤有机质含量、抗冲性大于中度裂隙或弱度裂隙样地;各样地土壤有机质含量、抗冲性与土壤深度显著负相关,有机质含量与土壤抗冲性存在显著的正相关关系。

Lattice Boltzmann simulation of fluid flow through coal reservoir's fractal pore structure

The influences of fractal pore structure in coal reservoir on coalbed methane(CBM) migration were analyzed in detail by coupling theoretical models and numerical methods.Different types of fractals were generated based on the construction thought of the standard Menger Sponge to model the 3D nonlinear coal pore structures.Then a correlation model between the permeability of fractal porous medium and its pore-size-distribution characteristics was derived using the parallel and serial modes and verified by Lattice Boltzmann Method(LBM).Based on the coupled method,porosity(ф),fractal dimension of pore structure(Db),pore size range(rmin,rmax) and other parameters were systematically analyzed for their influences on the permeability(ф) of fractal porous medium.The results indicate that:① the channels connected by pores with the maximum size(rmax) dominate the permeability,approximating in the quadratic law;② the greater the ratio of r max and r min is,the higher is;③ the relationship between D b and follows a negative power law model,and breaks into two segments at the position where Db ≌2.5.Based on the results above,a predicting model of fractal porous medium permeability was proposed,formulated as k=cfrnmax,where C and n(approximately equal to 2) are constants and f is an expression only containing parameters of fractal pore structure.In addition,the equivalence of the new proposed model for porous medium and the Kozeny-Carman model k=Crn was verified at Db =2.0.

Fractal structure reconstruction for alumina-silicate refractory fiber and simulation of the thermal conductivity

In this paper, it is proved that the intemal porous structure of alumina-silicate refractory fiber has fractal characteristics, which is reconstructed by the computer and the reconstructed structure further proved to have fractal characteristics. Based on the reconstructed structure, the network-thermal-resistance model is established to calculate the thermal conductivity of the fiber. It is shown that the calculated results agree well with the previous experimental ones, proving the correctness of the method.

Projection pressure and Bowen's equation for a class of self-similar fractals with overlap structure

Let {Si}li=1 be an iterated function system （IFS） on Rd with attractor K. Let π be the canonical projection. In this paper, we define a new concept called ＂projection pressure＂ Pπ（φ） for φ ∈（Rd） under certain arlene IFS, and show the variational principle about the projection pressure. Furthermore, we check that the unique zero root of ＂projection pressure＂ still satisfies Bowen＇s equation when each Si is the similar map with the same compression ratio. Using the root of Bowen＇s equation, we can get the Hausdorff dimension of the attractor K.