Numerical modelling of the rock fragmentation process by mechanical tools
Sammanfattning: The rock fragmentation process by mechanical tools is investigated using numerical method. The rock and tool interaction code (R-T2D) is first developed on the basis of the rock failure process analysis (RFPA) model and the finite element analysis (FEA) method. Then a series of numerical tests for the typical physical-mechanical and fracture mechanical experiments are conducted to calibrate the R-T2D code. Finally, the rock fragmentation processes in mechanical fragmentation, such as rock cutting, rock drilling and rock crushing are modelled using the R-T2D code. On the basis of the obtained results, it is pointed out that in rock cutting, there are a number of peculiarities in cutting heterogeneous brittle rock. In rock drilling, the rock fragmentation process is in fact a chipping process caused by side cracks. The side crack is initiated from the crushed zone or bifurcated from Hertzian cracks, and propagates approximately parallel to the free rock surface but in a curvilinear path driven by the tensile stress associated with the expansion of the crushed zone during the loading process. The length of side crack can be approximately predicted according to a formulated semi-empirical and semi-theoretical relationship among the side crack length, the rock properties and the drilling force. The crushed zone under mechanical tools is in fact a zone with a high density of microcracks, where microcracking is pervasive and rocks behave in a ductile cataclastic manner with stress satisfying the ductile failure surface of the double elliptic strength criterion. In the simultaneous loading by multiple button- bits, the interaction and coalescence of side cracks induced by the neighbouring button-bits with an optimum line spacing provide a possibility of forming largest rock chips, controlling the direction of subsurface cracks and consuming a minimum specific energy. In rock crushing, two kinds of fracture patterns are recognized: the quasi-uniaxial compressive fracture pattern and the quasi-triaxial compressive fracture pattern.
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