Papers by Author: Chun An Tang

Abstract: 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.

Abstract: In this paper, the bond length effect of FRP plate on bonding performance and the distribution patterns of the stress in FRP plate was investigated using 3D Realistic Failure Process Analysis (RFPA^3D) code to study the debonding mechanism of the FRP plate bonded to concrete block. Numerical simulations show that the progressive debonding of FRP plate bonded to concrete occurs in the concrete on the condition of different bond lengths can be divided into four stages: elastic-deformation stage, elastic-softening stage, elastic-softening-debonding stage and softening-debonding stage. It is also show that the interfacial bond strength and the global slip of FRP-to-concrete increase with the increase of the bond length.

Abstract: Using numerical code RFPA2D (dynamic version), spallation processes of rock-like medium induced by reflection of stress wave with different rise time and decay time are simulated, and based on description and discussion on the failure area in the document [1], the influence of the rise time and decay time of stress wave on spalling process of rock-like material are numerically analyzed and discussed. Results indicate that the comminution zone is affected little by the duration of stress wave regardless of the rise time or decay time, and the fracture zone is affected to a greater extent by the rise time than the decay time, and the spalling zone is affected to some extent by the duration of stress wave regardless of the rise time or decay time, also, the stability of specimen is affected by the duration of stress wave regardless of the rise time or decay time, and the specimen loses stability at a certain decay time earlier than that at the same rise time. Furthermore, the effects of rise time and decay time on the number of failure elements are also investigated. Results show that a shorter rise and decay time increase the number of radial cracks. At longer rise time and decay time, the number of cracks is reduced and the cracks extend longer. In addition, the spalling increases to some extent as the rise time or decay time decreases.

Abstract: A coupled thermo-mechanical model is employed to analyze the thermo-mechanical behavior of a widely used laminated composite subject to temperature decrease at service conditions. Three sets of governing equations, i.e. heat transfer, thermo-mechanical deformation and damage evolution are respectively described in the model. These equations are then assembled into a coupled matrix equation using finite element formulation and then solved simultaneously at each time interval. A numerical model of two layered composites with some preexisting equal-spacing cracks along the interface in the lower layer is set up to investigate the thermal induced crack propagation due to temperature decrease. Results are presented in the form of crack propagation process in stress profiles and discussed. Numerical simulations show that the crack propagation behavior of the composites is closely dependent on the physico-mechanical properties of two layers and preexisting cracks. It is found that thermal induced cracks penetrate into the upper layer and grow in the upper layer due to the low strength of the upper layer when the model is subject to uniform temperature decrease.

Abstract: Rock permeability is important in civil and geo-hydraulic engineering, the mining and petroleum industries, and in environmental and engineering geology. In this paper, considering the mutual hydro-mechanical response between stress-induced permeability and damage, a coupled mathematical model for solid deformation and gas flow in the coal or rock was established and an attempt is made to investigate the rock permeability evolution, fracture patterns, and flow vectors in rock samples at the scale of usual laboratory samples as well as the relation between permeability and stress induced damage in connection with the complete strain-stress process of loaded rocks. Numerical simulations show that the permeability of rock was not constant, closely related to the state of stress, but varied with the stress and strain states in the rocks. Microcracking, resulting from the concentration of stress on relatively weak rock elements, triggers successive crack initiation and propagation that in turn leads to permeability enhancement. Prior to the peak strength, the permeability decreases with increasing load. A dramatic increase in permeability occurs in the post-peak stress-strain region due to the catastrophic collapse of microstructure in rock. Moreover, the permeability of rock in post-peak stress-strain region is much higher that that of in pre-peak region. Such intensive studies of gas flow in stressed heterogeneous rocks are useful as initial approaches to many engineering problems in mining and petroleum industries.

Abstract: 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.

Abstract: 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.

Abstract: The investigation on the behavior of a specimen under uniaxial tension and the process of microfracture attracts considerable interest with a view to understanding strength characterization of brittle materials. Little attention has been given to the detailed investigation of influence of heterogeneity of rock on the progressive failure leading to collapse in uniaxial tension. In this paper, a numerical code RFPA3D (Realistic Failure Process Analysis), newly developed based on a three-dimensional model, to simulate the fracture process and associated fractal characteristic of heterogeneous rock specimen subjected to direct uniaxial tension. Specimens with different heterogeneity are prepared to study tension failure. In a relatively homogeneous specimen, the macrocrack nucleates abruptly at a point in the specimen soon after reaching peak stress. In more heterogeneous specimens, microfractures are found to appear diffusely throughout the specimen, and the specimens show more ductile failure behavior and a higher residual strength. Development of fractal theory may provide more realistic representations of rock fracture. The fractal dimension of distributed AE is computed during the fracture process. For all specimens, the fractal dimension increases as the loading proceeds, and it reaches the peak value when macrocrack nucleates abruptly. It is also found that fractures scatter more diffusely in relatively heterogeneous specimens, and the fractal dimension has a smaller value. The homogenous rock specimens have flat and smooth rupture faces which are consistent with the fractal results.

Abstract: The changing behavior for infrared thermal image omen of the rock with fracture is essential for the geotechnical engineering. In order to study the behavior, the infrared thermal images for the failure process of rock with hole are carried out. The size of the rock sample is 20cm×10cm×2cm with hole at the center of the sample and the diameter of the hole is 1cm. Considering the fact that sample will effect the results of the observation for infrared thermal image during the experiment, the laminated granite sample was used to replace the cylinder or cuboid sample. The achieved results under uniaxial compression indicate that intensity of the micro ruptures have a close relation with the thermal effects. When the main fractures happen, there is a strip of high temperature that will appear at the destructed local area. During loading process, the abnormity of infrared temperature has two kinds of behaviors as follows: (1) temperature rises and falls alternately, rises before the fracture; (2) temperature falls slowly at beginning, and then rises slowly, then rises quickly before the fracture appears. Even for the same rock sample, the behaviors of the infrared phenomenon may be different during failure.

Abstract: Based on physical model of three-point-bending test, the AE characteristics of three-point-bending beams with different relative notch depth during the entire loading period was simulated by using RFPA3D(realistic failure process analysis) code. Simulation results show that the relative notch depth affects the AE characteristics significantly. With increasing relative notch depth, the occurrence of AE events decreases remarkably. The stress distribution figures, elastic modulus photo and AE relative energy time-space distribution figures as well as an analysis on the failure process are also provided. Based on the analysis of simulation results, it is concluded that the heterogeneity of rock and concrete has great influence on the crack propagation path, which leads the crack propagation path becoming curvilinear.