Fracture Analysis of Brittle Materials Based on Nonlinear FEM and Application in Arch Dam with Fractures

Current fracture analysis models based on fracture mechanics or continuum damage mechanics are still limited in the application to three-dimensional structure. Based on deformation reinforcement theory coming from elastoperfect plastic theory, unbalanced force is proposed to predict initiation and propagation of cracks. Unbalanced force is the driving force of time-dependent deformation according to Perzyna’s viscoplasticity theory. It is also related to the damage driving force in viscoplastic damage model. The distribution of unbalanced force indicates cracks initiation area, while its direction predicts possible cracks propagation path. Uniaxial compression test of precrack specimen is performed as verification to this method. The trend and distribution of cracks are in good agreement with numerical results, proving that unbalanced force is feasible and effective for fracture analysis. The method is applied in fracture analysis of Xiaowan high arch dam, which is subjected to some cracks in dam due to the temperature control program. The results show that the deformation and stress of cracks and the stress characteristics of dam are insensitive to grouting of cracks. The existing cracks are stable and dam heel is still the most possible cracking position

Long-term stability analysis of the left bank abutment slope at Jinping I hydropower station

The time-dependent behavior of the left bank abutment slope at Jinping I hydropower station has a major influence on the normal operation and long-term safety of the hydropower station. To solve this problem, a geomechanical model containing various faults and weak structural planes is established, and numerical simulation is conducted under normal water load condition using FLAC3D, incorporating creep model proposed based on thermodynamics with internal state variables theory. The creep deformations of the left bank abutment slope are obtained, and the changes of principal stresses and deformations of the dam body are analyzed. The long-term stability of the left bank abutment slope is evaluated according to the integral curves of energy dissipation rate in domain and its derivative with respect to time, and the non-equilibrium evolution rules and the characteristic time can also be determined using these curves. Numerical results show that the left bank abutment slope tends to be stable in a global sense, and the stress concentration is released. It is also indicated that more attention should be paid to some weak regions within the slope in the long-term deformation process

Elucidating Meta-Structures of Noisy Labels in Semantic Segmentation by Deep Neural Networks

The supervised training of deep neural networks (DNNs) by noisy labels has been studied extensively in image classification but much less in image segmentation. So far, our understanding of the learning behavior of DNNs trained by noisy segmentation labels remains limited. In this study, we address this deficiency in both binary segmentation of biological microscopy images and multi-class segmentation of natural images. We classify segmentation labels according to their noise transition matrices (NTM) and compare performance of DNNs trained by different types of labels. When we randomly sample a small fraction (e.g., 10%) or flipping a large fraction (e.g., 90%) of the ground-truth labels to train DNNs, their segmentation performance remains largely the same. This indicates that DNNs learn structures hidden in labels rather than pixel-level labels per se in their supervised training for semantic segmentation. We call these hidden structures "meta-structures". When we use labels with different perturbations to the meta-structures to train DNNs, their performance in feature extraction and segmentation degrades consistently. In contrast, addition of meta-structure information substantially improves performance of an unsupervised model in binary semantic segmentation. We formulate meta-structures mathematically as spatial density distributions and quantify semantic information of different types of labels, which we find to correlate strongly with ranks of their NTM. We show theoretically and experimentally how this formulation explains key observed learning behavior of DNNs

Effect of dynamic cyclic shear on frictional strength weakening of a plane joint

Understanding and predicting the frictional behavior of rock joints subject to dynamic cyclic loading is essential for the seismic safety of many rock engineering problems. In this paper, cyclic shear experiments were conducted based on a shaking table apparatus. A series of cyclic friction tests covering various earthquake-relevant frequencies was conducted on dry planar joints of granitic rock. The friction behavior especially the dynamic evolution of the frictional strength under cyclic loading was investigated, with the influence of loading frequency on the frictional strength weakening and energy dissipation quantified. Based on the experimental results, we propose a phenomenological model characterizing the dynamic frictional behavior of planar granite joints under cyclic loading. This empirical formulation relates the evolution of joint frictional strength to the number of cycles of dynamic loading. As the frequency increases, the frictional strength and dissipated energy density both show a pronounced reduction, while the strength weakening ratio and the critical number of cycles (at which the residual strength is reached) increase in a logarithmic fashion. The frictional strength evolution is described by the product of the peak frictional strength and a damage function, which is further related to the number of cycles and the frequency. In addition, a close inspection of fracture surface morphology after the cyclic loading was conducted to understand the role of surface wear and gouge formation in the frequency-enhanced dynamic friction weakening of rock joints.ISSN:0267-7261ISSN:1879-341

Seepage field changes of ship‐lock slope of TGP under raining and percolation

The unsteady changes of seepage field of slope of TGP shiplock under raining and percolation are anlyzed by saturated seepage model. The computation suggests that raining and percolation do have some effect on the seepage field of high slope，but they don’t lead to high pressure water head in the shallow slope and vicinity of vertical wall of shiplock for the existence of drainage holes. Therefore, raining and percolation have slight influence on the stability of high slope

The dissolution mechanism and karst development of carbonate rocks in karst rocky desertification area of Zhenfeng–Guanling–Huajiang County, Guizhou, China

The phenomenon of rocky desertification is obvious in karst plateau of Guizhou, China, and the distribution area of slight–medium–severe rocky desertification is very large in Zhenfeng–Guanling–Huajiang, Guizhou. To explore the cause of formation and development of rocky desertification in the region, the karst development mechanism and characteristics were studied by the method of indoor dissolution test and microscopic analysis. The results show that the dissolution and karst features of carbonate rocks are closely related to geological structure, hydrogeological conditions, and lithology. The dissolution rate is the most affected by the chemical composition and mineral composition, followed by the pore structure and the particle feature. The dissolution rate of carbonate is in positive proportion with the calcite content, while it decreases with increasing content of dolomite. In the same chemical composition, the rock with the finer grains has a faster surface dissolution rate. In addition, the erosion amount will change with the pore aperture and connectivity. Karst morphology is mainly affected by the microstructure and mineral assemblage characteristics of carbonate rocks. In this rocky desertification area, bare rock over a large area will be further eroded, which will form the stone bud, stone forest. At the same time, the hydrodynamic effect of groundwater will erode the rock deeply, and the depression, karst funnel, or deep valley will appear gradually

Leaching Kinetics and Mechanism of Laterite with NH4Cl-HCl Solution

Following the growing demand for Ni and Co and the dwindling supplies of sulfide nickel ore, attention has turned toward the more efficient exploitation and utilization of laterite ore. Using ammonium chloride acid solution to leach is an effective method. Our research concerned investigations on the leaching mechanism and leaching kinetics of laterite. XRD was used to demonstrate the leaching mechanism through analysis of the pattern of the leaching residue and raw ore, showing that acid concentration affects the leaching process more significantly than other factors, and that valuable metals are mainly released from goethite and serpentine. The leaching order of these materials are as follows: Goethite > serpentine > magnetite and hematite. The leaching kinetics were analyzed and this leaching process followed a shrinking core model controlled by a combination of interfacial transfer and diffusion across the solid film. Leaching data fitted to the kinetic equation perfectly, and the apparent activation energies for the leaching of nickel, cobalt, and iron were calculated to be 4.01 kJ/mol, 3.43 kJ/mol, and 1.87 kJ/mol, respectively. The Arrhenius constants for Ni, Co, and Fe were 204.38, 16.65, and 7.12 × 10−3, respectively, with reaction orders of Ni (a 1.32, b 0.85, c 1.53), Co (a 1.74, b 1.12, c 1.22), and Fe (a 2.52, b −0.11, c 0.94)