| The realization of numerical models built on real rock structures is of great importance for the study of fine fracture processes and macroscopic mechanical behavior of rocks.In previous numerical studies of rock breaking by TBM cutter,the key steps of rock modeling were often done in a more abstract way,for example,using mathematical statistics of random mineral distribution models.Numerical simulations of rock breaking processes based on such models have a large deviation from actual experimental comparisons.Numerical modeling that respects the real structure of rock specimens and the numerical simulation of rock breaking by TBM cutters are useful for discovering rock breaking patterns and calibrating numerical experimental parameters.In this paper,a numerical model representing the fine mineral grains of rocks is established with a minimum number of meshes by adapting the Voronoi mesh generation strategy using the rock surface texture information,and a numerical simulation study of rock breakage by TBM cutters is carried out on this model using the coupled NDDA-ABAQUS method.Starting from a series of pre-processing of the acquired rock images,including: image graying,channel conversion,threshold segmentation,image noise reduction,image erosion and Canny edge detection to enhance the images,the boundaries of the images are extracted as the basis for Voronoi mesh generation.The Voronoi adaptive mesh generation technique is then implemented to obtain a high-quality mathematical mesh that takes into account the image boundary fit and the regularity of the mesh shape.In the framework of NDDA-ABAQUS method,the established numerical model was calibrated and verified by comparing the experimental results of rock breakage by TBM cutters provided by the mechanical rock breaking test platform of Beijing University of Technology.The macroscopic process of rock breaking and crack spreading pattern obtained from the model were compared in detail,and the accuracy and completeness of the technical system for the fine rock damage portrayal were successfully demonstrated.Subsequently,the simulation results of rock breaking by TBM cutters under different conditions are presented visually for a variety of actual working conditions(including combinations of four representative confining pressures and six indentation depths),so as to summarize the correlation law between the main control factors and the breaking effect in the process.In order to quantitatively describe the rock breaking effect in a multi-dimensional way,a new index system consisting of response parameters such as acoustic emission number,total crack length,crack breaking rate,mode I crack percentage and specific energy is introduced in this paper,which provides a comprehensive dynamic characterization of rock breakage under the TBM cutter action and the breakage efficiency of the cutter.Based on the above numerical results,under the condition that the cutter penetration speed and depth are certain,we find that when the ratio of cutter width and critical surface is small,the inhibitory influence of confining pressure on the fracture development produced by local fracturing of the cutter is smaller and is no longer the dominant factor affecting rock fragmentation.Under such working conditions,the cutter spacing should be appropriately reduced to lower the weight of the influence of the confining pressure in the design.In the early stage of damage,due to the pressure from the hob,the damage type is mainly mode I tension damage;with the crack expansion and penetration to form rock fragments,the damage type gradually transforms into mode II shear damage.And with the increase of cutting depth,the specific energy of rock breakage by cutter shows a trend of first decreasing and then slowly increasing.This means that in practical engineering,the average breaking efficiency of the whole process of rock breakage can be improved by appropriately increasing the depth of cut of the cutter. |
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