NUMERICAL MODELING OF ROCK CUTTING AND ITS ASSOCIATED FRAGMENTATION PROCESS USING THE FINITE ELEMENT METHOD
Jaime, Maria Carolina (2012) NUMERICAL MODELING OF ROCK CUTTING AND ITS ASSOCIATED FRAGMENTATION PROCESS USING THE FINITE ELEMENT METHOD. Doctoral Dissertation, University of Pittsburgh.
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As the need for reaching fuel reserves at greater depths increases, over the past 30 years scientists have been exploring and developing the technology required to efficiently drill rock at highly pressured environments; yet, there are still gaps in the understanding of the physical phenomena involved. One of the basic problems has to do with the cutter-to-rock interaction during the cutting process.
This study employs the Finite Element Method (FEM) to investigate the mechanics of rock cutting because of its flexibility in handling material heterogeneity, nonlinearity and boundary conditions. Using the FEM to model fracturing of a brittle material like rock –and consequently treating its discontinuous chips– is a challenging undertaking that requires the tackling of a sequence of complex problems: As the cutter advances and touches the rock material, a contact problem first arises. This is followed by nonlinear deformation and the determination as to when and whether the rock would fail. Subsequently, the question of how to initiate the fragmentation process has to be resolved if the rock fails. The cycle repeats starting with a new contact problem after new surfaces are generated due to fracture.
At present, few researchers have focused on crack initiation and subsequent crack propagation, but even fewer have accounted for actual chip formation, and none has considered the dynamic interaction amongst chips, newly formed surfaces, and the cutter. One important goal of this study is to advance the modeling such that it is possible to follow the cutter in a complete cutting process in a credible manner.
A framework of three-dimensional FEM modeling was developed so that the fragmentation process observed in laboratory rock scratching tests could be properly simulated. A thorough calibration of the rock material model was carried out, together with extensive sensitivity analyses of contact models, damage based failure and its associated fracture modeling using the commercial software LS-DYNA.
This study was able to obtain ductile failure mode for shallow cuts, and brittle failure for deep cuts as observed in the laboratory, all without a priori setting on the failure modes. Also, cutting force magnitudes and tendencies obtained from the study correlated well with published results of the physical experiments. Moreover, in a limited scope, this study also investigated the effects of applying external hydrostatic pressure on rock cutting. Preliminary numerical results indicate a good comparison with few published data. Lastly, theoretical models for obtaining cutting forces were assessed, providing a better understanding of their limitations and usability.
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