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- Saadati, Mahdi, et al.
(författare)
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A numerical study of the influence from pre-existing cracks on granite rock fragmentation at percussive drilling
- 2015
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Ingår i: International Journal for Numerical and Analytical Methods in Geomechanics. - : Wiley. - 0363-9061 .- 1096-9853. ; 39:5, s. 558-570
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to investigate the effect of pre-existing, or structural, cracks on dynamic fragmentation of granite. Because of the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated, and the material parameters are chosen based on previous experiments. The equation of motion is discretized using a finite element approach, and the explicit time integration method is employed. The pre-existing cracks are introduced in the model by considering sets of elements with negligible tensile strength that leads to their immediate failure when loaded in tension even though they still carry compressive loads as crack closure occurs because of compressive stresses. Previously performed edge-on impact tests are reconsidered here to validate the numerical model. Percussive drilling is simulated, and the influence of the presence of pre-existing cracks is studied. The results from the analysis with different crack lengths and orientations are compared in terms of penetration stiffness and fracture pattern. It is shown that pre-existing cracks in all investigated cases facilitate the drilling process.
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- Saadati, Mahdi, et al.
(författare)
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Granite rock fragmentation at percussive drilling - experimental and numerical investigation
- 2014
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Ingår i: International Journal for Numerical and Analytical Methods in Geomechanics. - : Wiley. - 0363-9061 .- 1096-9853. ; 38:8, s. 828-843
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to numerically model the fracture system at percussive drilling. Because of the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated, and the material parameters are defined based on previous experiments. This includes different tests such as direct tension and compression, three-point bending, and quasi-oedometric tests to investigate the material behavior at both tension and confined compression stress states. The equation of motion is discretized using a finite element approach, and the explicit time integration method is employed. Edge-on impact tests are performed, and the results are used to validate the numerical model. The percussive drilling problem is then modeled in 3D, and the bit-rock interaction is considered using contact mechanics. The fracture mechanism in the rock and the bit penetration- resisting force response are realistically captured by the numerical model.
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- Shariati, Hossein
(författare)
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Mechanical modeling of granite subjected to contact loading
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The mechanical properties of Bohus granite subjected to contact loading is investigated based on experimental and numerical results. An elasto-plastic constitutive material model combined with a damage description is employed. The material model parameters are determined based on experimental results. Any kind of inelastic deformation except the tensile failure is described by a linear Drucker-Prager (DP) plasticity model with variable dilation angle. As for the damage description, an anisotropic damage model (DFH model) is considered to account for the tensile failure (i.e. mode I fracture). The resulting constitutive model is implemented numerically to simulate the mechanical behavior of the material under indentation loading up to its load capacity. In paper A, the DP material model parameters are calibrated based on quasioedometric tests performed in an earlier work. It is described how the yield surface and dilation angle are determined from this test. The calibrated material model is implemented numerically in a commercial finite element software. The numerical model is validated based on quasi-static spherical indentation tests performed in this work. The force-penetration (P-h) response of the material is recorded during the indentation tests. Moreover, a high speed camera is utilized to observe the specimen surface around the contact area during the indentation test. It is detected that the observed load-drops in the P-h response correspond to material removals on the specimen surface. The tested specimens are also scanned by X-ray tomography to investigate the fracture pattern. In paper B, the anisotropic DFH damage model is employed in order to predict the fracture pattern observed in the indentation test. The chosen damage model considers the heterogeneity in the material tensile strength. It is described how the statistical distribution of the tensile strength is calibrated. The calibrated DFH model is combined with the DP model and the resulting DP-DFH model is utilized to simulate the P-h response and the fragmentation process of Bohus granite subjected to quasi-static contact loading.
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- Shariati, Hossein
(författare)
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Mechanical modeling of granite subjected to indentation loading
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The understanding of the mechanical response of Bohus granite, as typical of hard rocks, to percussive drilling is important to improve the efficiency of such an operation. The resulting problem includes material modeling of the selected type of rock under indentation loading conditions. An elastoplastic constitutive material model combined with a damage description is employed for this purpose. The material model parameters are calibrated based on experimental results. Linear Drucker-Prager (DP) plasticity model with pressure-dependent dilatancy and an anisotropic damage model (DFH model) are considered to account for the inelastic deformations and the tensile failure (i.e., mode I fracture), respectively. The resulting constitutive model is implemented numerically into a finite element (FE) commercial software to simulate the material behavior under indentation loading up to its load capacity. In Paper A, the DP material model parameters are determined based on quasi-oedometric tests performed in a previous work and the yield surface and dilation angle are determined. The calibrated material model is implemented numerically taking advantage of Abaqus FE software. The established numerical model is then used to simulate quasi-static indentation test and the force-penetration (P-h) response is predicted. Moreover, a high-speed camera is utilized to monitor the surface of specimens, made of Bohus granite, at different load levels in indentation tests. It is detected that the load-drops observed in the P-hresponse are associated with the material removals on the surface. In Paper B, the anisotropic DFH damage model is employed to predict the material fracture pattern subjected to quasi-static indentation loading. The employed damage model considers the heterogeneity in the material tensile strength using Weibull statistics. It is described how the Weibull parameters are calibrated. The calibrated DFH model is combined with the DP model. The resulting DP-DFH model is used to simulate the material elastoplastic response and the fragmentation process. Furthermore, the frictional effects on the P-hresponse, fracture pattern and plastic zone are numerically investigated in Paper C. In doing so, friction is introduced between the indenter and the granite surface in numerical simulations. A parametric study of frictional coefficients is also carried out. Finally, in situ spherical indentation test is performed and monitored by X-ray microtomography in Paper D. The test is then analyzed by Digital Volume Correlation (DVC), with the aim of validating the calibrated constitutive model. FE simulations of the indentation problem, using different constitutive models, namely, elasticity, compressible elastoplasticity (DP) and compressible elastoplasticity with damage description (DP-DFH) are carried out and the results are compared based on DVC residuals. The frictional contact effects are also studied. It is concluded that compressible elastoplasticity should be accounted for to predict the load level and displacement fields beneath the indenter. It is also concluded that frictional effects lead to damage extension. However, the frictional effects on the P-hresponse and the size of the plastic zone underneath the indenter are negligible. Finally, it is shown that the employed damage model can determine the indentation fracture pattern prior to extensive failure of the chosen type of rock.
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