Abstract
The rock fragmentation response to drilling, in particular percussive drilling, is important in order to improve the efficiency of such an operation. The resulting problem includes spherical contact between the drill bit and the material and therefore, a numerical analysis of frictional effects in quasi-static spherical indentation of Bohus granite is presented. The frictional coefficient between the indenter and the granite surface is accounted for in numerical simulations. A previously determined constitutive law is used for the purpose of numerical analyses. The latter consists of a Drucker-Prager plasticity model with variable dilation angle coupled with an anisotropic damage model. Since the tensile strength is random, Weibull statistics was considered. Using a frictionless contact model, the stress state of Bohus granite corresponding to the first material failure occurrence, observed in indentation experiments, was numerically determined. However, the frictional effects, which are of interest in this study, may lead to changes in the numerically established stress state and consequently the Weibull parameters should be recalibrated. The so-called Weibull stress decreases from 120 MPa for a frictionless contact to 75 MPa for frictional contact, and the Weibull modulus from 24 to 12. It is numerically observed that the predicted force-penetration response, using the new set of Weibull parameters, is not influenced by friction. Conversely, the predicted fracture pattern, in the case of frictional contact, is similar to the case of frictionless contact, but its size is somewhat larger. Last, a parametric study analyzing the dependence of the friction coefficient is carried out and no significant changes are detected. The novelty of the present findings concerns the fact that both an advanced damage description in combination with an advanced plasticity model, both implemented for finite element analyses, is used to analyze frictional effects at granite indentation.
Highlights
A large number of studies on the behavior of quasi-brittle materials under quasi-static or dynamic loadings is found in the literature [1,2,3,4,5,6,7,8,9,10,11,12,13]
Quasi-static spherical indentation was already numerically simulated for a frictionless contact [25], and the numerical results were compared to experimental data
Frictional effects are investigated in indentation tests
Summary
A large number of studies on the behavior of quasi-brittle materials under quasi-static or dynamic loadings is found in the literature [1,2,3,4,5,6,7,8,9,10,11,12,13]. The DFH model accounts for tensile failure due to high stress state on the rock surface outside the contact area. This stress state may lead to cracking of the rock, and subsequently material removals on the specimen surface. These material removals were observed in the images provided by high-speed camera monitoring, and they corresponded to load-drops in the force-penetration response. It was suggested that the stress state at the first load-drop could be used to determine the tensile strength of the tested material. This is believed to be sufficient for the present purpose but for future and more detailed studies, advanced models such as the ones by [22,23] could be useful to include in the analysis
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