On the mechanical behavior of granite: Constitutive modeling and application to percussive drilling

Abstract: The mechanical behavior and fragmentation response of rock materials is investigated in this work. In particular, Bohus granite is selected with application to percussive drilling. It is well known that rock behaves totally different in compression and tension and dynamic loading conditions and high strain rates under the percussive drilling process makes the material behavior even more complicated. The KST-DFH material model is shown in this work to be appropriate in order to constitutively describe granite at dynamic fragmentation. It consists of a plasticity model in compression and a damage model in tension. The yield surface locus is a quadratic function of the mean pressure in the principal stress space and the damage model is anisotropic.Several experiments are performed in order to define the mechanical behavior and dynamic response of granite and calibrate the KST-DFH model parameters for this material. The material model is implemented as in a commercial finite element program and validated based on dynamic tests such as Edge-on Impact (EOI) and spalling test using Hopkinson bar. The numerical tool is then used to model the rock response during the percussive drilling process. In doing so, only one spherical tool button and just the first impact are considered for simplicity. The anticipated fracture mechanism in percussive drilling is captured and the penetration stiffness obtained is in agreement with practical drilling experiments.In paper A, the experimental work is described and the granite mechanical response is explained. In particular, the influence from pre-existing cracks and defects is examined in great detail. In paper B, the experimental results are used to calibrate the material model parameters. The numerical tool discussed earlier is employed to investigate the rock fracture mechanisms at percussive drilling. In paper C, the effect of pre-existing, or structural, cracks on dynamic fragmentation of granite is investigated in detail. These cracks may be the result from former impact of the drill bit, or by means of other unconventional methods such as microwave and laser that are used to increase the effectiveness of the percussive drilling process. In paper D, the dynamic tensile behavior of granite samples is investigated. Spalling tests using a Hopkinson bar are performed and a strain rate of order 102 1/s is obtained. This experimental technique involves the same order of strain rate as present in rock materials during percussive drilling. A dynamic tensile strength of 18.9 MPa is obtained at a strain rate of 70 1/s. This is more than twice the tensile strength of the specimen (with the same size) at quasi-static conditions, which is 8 MPa.