Realistic Failure Process Analysis Code Research Articles

Identification methods for anomalous stress region in coal roadways based on microseismic information and numerical simulation

It is believed that the microseismicity induced by mining effect and gas gradient disturbance stress is a precursor to the essential characteristics of roadway unstability. In order to effectively identify and evaluate the stability of coal roadways in the process of mine development and extraction, a microseismic monitoring system was deployed for the study of the stress evolution process, damage degree and distribution characteristics in the tailgate and headgate. The mine under study is the 62113 outburst working face of Xin Zhuangzi coalmine in Huainan mining area. The whole process of microfractures initiation, extension, interaction and coalescence mechanisms during the progressive failure processes of the coal rock within the delineated and typical event clusters were investigated by means of a two dimensional realistic failure process analysis code (RFPA2D-Flow). The results show that the microseismic events gradually create different-sized event clusters. The microseismicity of the tailgate is significantly higher than that of the headgate. The study indicates that the greater anomalous stress region matches the area where microfractures continuously develop and finally connect to each other and form a fissure zone. Due to the mine layout and stress concentration, the ruptured area is mainly located on the left shoulder of the tailgate roof. The potential anomalous stress region of the coal roadway obtained by numerical simulation is relatively in good agreement with the trend of spatial macro evolution of coal rock microfractures captured by the microseismic monitoring system. The research results can provide important basis for understanding instability failure mechanism of deep roadway and microseismic activity law in complex geologic conditions, and it ultimately can be used to guide the selection and optimization of reinforcement and protection scheme.

Microseismic Signal Characterization and Numerical Simulation of Concrete Beam Subjected to Three-Point Bending Fracture

To study the generation mechanism and failure mode of cracks in mass concrete, microseismic monitoring is conducted on the fracture processes of the three-point bending roller compacted concrete (RCC) beam of Guanyinyan hydropower station. The spectrum characteristics of microseismic signals in different deformation and failure stages of the concrete beam are analyzed, and the identification method of the fracture stages and crack propagation precursors of concrete beam is established. Meanwhile, the Realistic Failure Process Analysis code (RFPA) is adopted to simulate and analyze the entire failure processes of concrete beam from its cracks initiation, development, propagation, and coalescence, until macroscopic fractures formation subjected to three-point bending test. The relation curve of the load, loaded displacement, and acoustic emission (AE) of concrete beam in the three-point bending test is also obtained. It is found that the failure characteristics of concrete beam obtained from numerical experiments agree well with the field physical test results. The heterogeneity of concrete is the major cause of zigzag propagation paths of beam cracks subjected to three-point bending tests. The results lay foundation for further exploring the formation mechanism of dam concrete cracks of Guanyinyan hydropower station.

Damage evolution analysis of SRM under compression using X-ray tomography and numerical simulation

The soil–rock mixture (SRM) is a special inhomogeneous multiphase geomaterial, the study of the SRM’s damage characteristic is crucial to the stability of engineering. This paper explores the use of high-resolution X-ray tomography and Realistic Failure Process Analysis code (RFPA2D) to identify the damage mechanism and the cracking process of SRM during compressive loading. X-ray tomography images were used to extract defects of SRM specimen under different compressive loadings, also damage characteristics of region of interest (ROI) were analysed by X-ray tomography. Numerical simulation was conducted to describe the damage characteristics with damage element number (DEN) recorded during compression. Results show that X-ray tomography and numerical simulation based on RFPA2D are complementary good methods to research the damage evolution of SRM. Damage characteristics of SRM are progressive; cracking begins from rock–soil jointed interfaces, and then propagates into soil matrix, and the growth of crack-shear band and macro-pore finally result in the failure of SRM.

Failure precursor of surrounding rock mass around cross tunnel in high-steep rock slope

The stability of the surrounding rock mass around cross tunnel in the right bank slope of Dagangshan hydropower station, in the southwestern China, was analyzed by microseismic monitoring as well as numerical simulations. The realistic failure process analysis code (abbreviated as RFPA3D) was employed to reproduce the initiation, propagation, coalescence and interactions of micro-fractures, the evolution of associated stress fields and acoustic emission (AE) activities during the whole failure processes of the surrounding rock mass around cross tunnel. Combined with microseismic activities by microseismic monitoring on the right bank slope, the spatial-temporal evolution and the micro-fracture precursor characteristics during the complete process of progressive failure of the surrounding rock mass around cross tunnel were discussed and the energy release law of the surrounding rock mass around the cross tunnel was obtained. The result shows that the precursor characteristic of microfractures occurring in rock mass is an effective approach to early warn catastrophic damage of rock mass around cross tunnel. Moreover, the heterogeneity of rock mass is the source and internal cause of the failure precursor of rock mass.

3D modeling of stratified and irregularly jointed rock slope and its progressive failure

Little has been published on the three-dimensional (3D) simulation of the progressive failure of rock slopes, possibly because the process of failure involves a complex, nonlinear evolution from initiation, through propagation and crack. In addition, rock is typically anisotropic, which makes it difficult to identify and describe the slope constituents and failure processes accurately. Despite such difficulties, further study of the fracture process is just as important as analyzing stress fields in 3D rock slope failures. In this paper, the 3D realistic failure process analysis code using finite element programming, and an extended version of numerical centrifugal method, is used to simulate slopes failure with different dip angles. The numerical centrifugal analysis results in this paper are found that the critical failure surface develops along the weak structural surface when the slope dip angle β is below 30°; conversely, the failure surface is formed along the toe of circular sliding when β is above 30°. In addition, it is also found that whether or not including the irregularity of joint into modeling to analyze the 3D slope stability problem will lead to a significant difference in factors of safety, it can reach 8.41 % at the same slope angle. Furthermore, the acoustic emission analyzing reveals deformed location characters of rock slope during the failure processes. With such capabilities, the approach contributes significantly to the in-depth study of the mechanisms of rock slope instability process.

Numerical Simulation on Fracture Formation on Surfaces of Bi-Layered Columnar Materials

Parallel fracture formation on surfaces of bi-layered columnar materials like growing tree trunk has been previously studied numerically. In this paper, numerical results of a continuous transition from parallel to polygonal fracture patterns with principal stress ratio provides the clear convincing theoretical explanation for fracture spacing. We perform three-dimensional simulations of fracture growth in a bi-layered columnar model with an embedded heterogeneous layer under inner radial expansion and terminal tension by finite element approach. As a result of this expansion, the bark stretches until it reaches its limit of deformation and cracks. A novel numerical code, 3D Realistic Failure Process Analysis code (abbreviated as RFPA3D) is used to obtain numerical solutions. In this numerical code, the heterogeneity of materials is taken into account by assigning different properties to the individual elements according to statistical distribution function. Elastic-brittle constitutive relation with residual strength for elements and a Mohr-Coulomb criterion with a tensile cut-off are adopted so that the elements may fail either in shear or in tension. The discontinuity feature of the initiated crack is automatically induced by using degraded stiffness approach when the tensile strain of the failed elements reaching a certain value. Numerical results of a continuous transition from parallel to polygonal fracture patterns with principal stress ratio are obtained by varying simulation parameters, the thickness of the material layer. We find that, except for further opening of existing fractures after they are well-developed (saturation), new fractures may also initiate and propagate along the interface between layers, which may serve as another mechanism to accommodate additional strain for fracture saturated layers.

Numerical Simulation of Fracture Saturation in Multilayer Materials in Symmetric Case

Using Realistic Failure Process Analysis Code RFPA2D, a three layer materials model with a central layer and with the same thickness top and bottom layer, progressive formation of fracture saturation in multilayer materials is simulated in all case by numerical simulation. Our numerical simulation recurrence the phenomenon of fracture saturation in multilayer materials. numerical simulation shown that for the symmetric case the spacing of these fractures at fracture saturation are approximately the same and lineally related to thickness of the fractured layer. We investigate that the critical fracture spacing to layer thickness ratio is function of the thickness ratio of the top and bottom layers to central layer at fracture saturation. numerical simulation shown that for the symmetric case the critical spacing of fractures to layer thickness ratio decreases rapidly and tends to a constant value.

Modeling the Effects of Heterogeneity and Anisotropy on the Excavation Damaged/Disturbed Zone (EDZ)

When modeling the mechanical behavior of underground excavations, it is necessary to include the influence of the rock mass characteristics on the Excavation Damaged/Disturbed Zone (EDZ). In this paper, the Realistic Failure Process Analysis code, RFPA, is used to model the extent of the EDZ. The inhomogeneous characteristics of rock at the mesoscopic level are included by assuming that the material properties of the constituent elements conform to a Weibull distribution; the anisotropy is incorporated as a transversely isotropic medium; the non-elastic characteristic is simulated via an elastic damage-based constitutive law. A finite element program is adopted as the basic stress analysis tool. In this study, a notable feature is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can take place spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. The deformation and failure process of anisotropic rock around excavations of different geometries is analyzed, and compared to experimental tests, showing similar fracture patterns. Additionally, the effect of confining stress and of different material layers is modeled and discussed. It is found that the model clearly illustrates that fracturing, both initiation and propagation, occurs as a combination of the stress concentrations and weakness planes introduced via the transverse anisotropy – which could represent either foliations or ubiquitous joint sets.

Modeling of the Effect of Excavation Mode on the Extent of the Anisotropic Zone Surrounding Deep Tunnels

A series of numerical model tests were performed to investigate the behaviour of the anisotropic rock surrounding circular excavations under high confining pressures. The aim was to provide information on the formation of fractures and failure around deep level rock tunnels under controlled conditions. Solid cubes containing a circular hole were confined to a vertical pressure with same as the confinement in the horizontal directions. In this modeling, the inhomogeneous rock is generated by using Weibull parameters which are related to the microstructural properties determined by crack size distribution and grain size. The fracture angle is assumed to be 45o. The observed failure zone around the excavation was simulated using both the maximum tensile strain criterion and Mohr-Coulomb criterion respectively (as the damage threshold). And RFPA (Realistic Failure Process Analysis) code was used as the calculating tool in this modelling, three opening modes are simulated and compared. Computational model predictions that include crack propagation and failure modes of rock show a good agreement with those of the observation in site. It is pointed out that the damage evolution of EDZ strongly depends on the inhomogeneous, the excavation mode, anisotropic property, and the various loading conditions. Concerning the existence of a weak plane, the amount of displacement at the side wall of the tunnel was quite large, since the shear deformation occurred in EDZ. The model is implemented in RFPA code and is able to represent the change in fracture patterns between the solid and jointed parts. This provides confidence for the application of the numerical model to the design of rock tunnels at great depth.

Numerical Simulation on Fracture Formation on Surfaces of Bi-Layered Materials

Fracture formation on surfaces of bi-layered materials is studied numerically. A simplified two-layered materials model like growing tree trunk is present. This work is focused on patterns of fractures and fracture saturation. We consider the formation of crack pattern in bark as an example of pattern formation due to expansion of one material layer with respect to another. As a result of this expansion, the bark stretches until it reaches its limit of deformation and cracks. A novel numerical code, 3D Realistic Failure Process Analysis code (abbreviated as RFPA3D) is used to obtain numerical solutions. In this numerical code, the heterogeneity of materials is taken into account by assigning different properties to the individual elements according to statistical distribution function. Elastic-brittle constitutive relation with residual strength for elements and a Mohr-Coulomb criterion with a tensile cut-off are adopted so that the elements may fail either in shear or in tension. The discontinuity feature of the initiated crack is automatically induced by using degraded stiffness approach when the tensile strain of the failed elements reaching a certain value. The different patterns are obtained by varying simulation parameters, the thickness of the material layer. Numerical simulation clearly demonstrates that the stress state transition precludes further infilling of fractures and the fracture spacing reaches constant state,i.e. the socalled fracture saturation. It also indicates that RFPA code is a viable tool for modeling fracture formation and studying fracture patterns.

Digital Image Based Modeling of Rock Failure at Meso-Scale

This paper presents a new meso-mechanical analysis method of rock failure. The actual inhomogeneity of rock at meso-scale level is represented by processing the image of rock section and incorporated into Realistic Failure Process Analysis code (abbreviated as RFPA2D). Here, this numerical tool is employed to study the fracture phenomena of granite sample considering the interface strength between mineral grains. Numerical results show that interface strength has significant influence on the strength of sample and its failure mode. The larger the interface strength is, the more brittle rock samples become and the strength is bigger. With the interface strength increasing, failure mode gradually varies from intergranular frature to transgranular fracture.

Influence of Different Loading Modes on Rock Tensile Strength

A typical mechanical character of rock is that the tensile strength is far less than the compressive strength. Meanwhile, the test data of tensile strength is very dispersive. Because the direct tensile tests always result in failure due to the difficulty in clamping the rock sample, the splitting test is used to determine the tensile strength of rock. There are four kinds of loading modes in the splitting test in actual laboratory test: angle pad splitting, round pad splitting, aclinic loading platen splitting, arc loading platen splitting. In this paper, the direct tensile test, the splitting test and the influence of different loading modes on rock tensile strength were studied. In order to study the stress distribution, the progressive splitting failure process was numerically modeled under the four kinds of loading cases by the Realistic Failure Process Analysis code (RFPA2D). Results show that the stress states under angle pad splitting, round pad splitting are similar to the stress states under diametrical compressive state. Regarding that the round pad splitting test is easy to implement, and its numerical results are also stable relatively, the round pad loading mode was suggested to be adopted.

Numerical Simulation of Short Cracks in Fiber-Reinforced Ceramics

In this paper, a numerical code, Realistic Failure Process Analysis code (RFPA), was used to perform a microscopic analysis of a crack in a fiber-reinforced ceramic, when the crack length is the same order of magnitude as the fiber spacing. The numerical results performed in the paper shown the failure process of fiber-reinforced ceramic subjected to tension loading, which indicate that the reinforcing fibers in a ceramic composite have a significant effect in inhibiting crack propagation even during the stages of the development of crack. Moreover, the fiber evidently increased the load-carrying capacity.

Numerical Simulation and Analysis on Crack Path Deviation in Brittle Solid

Ductile tearing of brittle solid with initial crack is studied numerically. This work is focused on the simulation and analysis of crack path deviation for the respective configuration and the study of relation between crack length and the fracture resistance, and that the configuration of fracture surface is also observed. The simulating tool is a novel numerical code, 3D Realistic Failure Process Analysis code (abbreviated as RFPA3D). The simulating investigation has shown that the fracture resistance decreases with the increase of the angle between the crack ends and horizontal direction. Due to heterogeneity of materials distribution, crack path deviation towards the softer zone or elements is observed. The topography of the non-planar fracture surface has plotted. Concluding the simulation made in the present study, the problem of crack extension in the brittle solid can be handled by RFPA3D. Fracture resistance can be predicted and the crack path deviation be simulated, if the model parameters have been carefully identified and the mesh design is adequate. So this work is beneficial to choose safe load-spots and predicate the direction of the crack path deviation.