Recently, great attention has been paid to develop effective and cost efficient drilling technology. Studying the microscopic surface cracks is necessary to understand the mechanical properties of damaged rocks. The surface cracks have strong effect on the strength of the rock and play a major role in the fracture behaviour. To quantify these effects, we have established a new methodology using fractal dimension of the surface cracks. In this procedure, we calculate the fractal dimension of the surface cracks before and after applying a thermal shock on the rock surface. These thermal shocks are used to modify the surface cracks and weaken the rock. The images from the surface of the samples are taken with a stereomicroscope and the calculation of fractal dimensions are done by an in-house Matlab code.
This new method allows estimations of the tensile strength of the rock based on optical micrographs. The method also provides constitutive relationship for building of material models, which can be used for FEM simulations to predict the rate of penetration during drilling operations.
Drilling causes major part of costs in geoengineering applications including tunnelling, mining, and excavation operations. Estimations of the drilling speed and tool wear are often based on ad-hoc material models, which cannot predict the changes in the rate of penetration based on rock type and rock microstructure. Good understanding of the rock-bit interactions allows designing new tools and better equipment. This work facilitates understanding and predicting the rock-tool interactions for a damaged rock mass in, for example, percussive drilling where the effect of previous impacts affects the properties and drillability of the rock.
Ahmad Mardoukhi, Tampere University of Technology
University of Potsdam, Sintef Materials and Chemistry, Tampere University of Technology