Papers by Author: Tian Hui Ma

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Authors: Li Song, Hou Quan Zhang, Zheng Zhao Liang, Ming Ruo Jiao, Tian Hui Ma, Juan Xia Zhang, Liexian Tang
Abstract: Six types of numerical specimens containing two notches are set up to numerically investigate the effect of element size on rock shear strength and failure pattern using RFPA2D (rock failure process analysis) code. These specimens are of the same geometrical dimension 180 mm×180 mm and have been discretized into 61×61, 122×122, 183×183, 244×244, 305×305, and 366×366 elements.The width of notches is about 2.95 (180/61) mm and the length is 45mm. The specimens are placed in a direct shear box. A lateral confining pressure with a value of 0.15MPa is invariably loaded in the vertical direction and an increasing horizontal displacement with 0.002mm/step is applied in the horizontal direction. The whole shear failure progress and associated stress field for the specimens are visually represented. Results show that the crack propagation is mostly influenced by the stress field in the vicinity of the notch tip, the required element size would be necessary in order to obtain good results. In general, for a coarse mesh, the stress field close to the notch tip can’t be represented accurately and shear strength obtained by such discretisation is slightly higher than the accurate value. For a fine mesh, the notch tip spreads through a relatively large number of elements and the stress field in vicinity of notch tip is well represented by the finite element approximation, therefore the failure pattern is consistent with real physical fracture mode.
Authors: Ru Wang, Chun An Tang, Shu Hong Wang, Zhi Yuan Wang, Tian Hui Ma
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.
Authors: Tian Hui Ma, Chun An Tang, Tao Xu, Zheng Zhao Liang
Abstract: 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.
Authors: Tian Hui Ma, Ju Ying Yang, Zheng Zhao Liang, Yong Bin Zhang, Tao Xu
Abstract: 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.
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