Fracture Toughness Determination and Micromechanics of Rock under Mode I and Mode II Loading

(TYPE=abstract)This thesis work describes a new experimental method for the determination of Mode II (shear) fracture toughness, KIIC, of rock and compares the outcome to results from Mode I (tensile) fracture toughness, KIC, testing using the International Society of Rock Mechanics (ISRM) Chevron-Bend (CB-) method. The fracture toughness describes the resistance of rock to fracturing. This parameter is therefore important when estimating the failure of rock and rock structures using rock fracture mechanics principles. Critical Mode I fracture growth at ambient conditions was studied by carrying out a series of experiments on a clay bearing sandstone at different loading rates, i.e. clip-gage opening rates of 5·10‑6 m/s to 5·10‑10 m/s. The range of loading rates provides macroscopic fracture velocities that have been shown to cause time-dependent fracture growth in other test set-ups. The mechanical data shows that time- and loading rate dependent crack growth occurs in the test material. Crack density measurements on scanning electron microscopy micrographs show constant size of the symmetric fracture process zone (~ 700-800 µm) independent of loading rate. Fracture surface roughness is constant for all loading rates. Acoustic emission location data demonstrates that the fracture process zone has a constant size of 5 mm in width and ~ 20 mm in length. The number of located acoustic emission events decreases with slower loading rates. The fracture propagating in the CB-samples is therefore not a pure Mode I fracture on the microscale. On the macroscale the fracture propagates co-planar under the Mode I loading. Mode I fracture toughness was determined on six rock types, i.e. Flechtingen sandstone, Rudersdorf limestone, Carrara marble, Aspo diorite, Mizunami granite, and Aue granite. KIC is 1.2 MPa m1/2, 1.1 MPa m1/2, 2.4 MPa m1/2, 3.8 MPa m1/2, 2.4 MPa m1/2, and 1.6 MPa m1/2, respectively. The newly developed set-up for determination of the Mode II fracture toughness is called the Punch-Through Shear (PTS-) test. It uses drill core that is available from most engineering site investigations. Notches were drilled to the end surfaces of 50 mm long samples. These act as friction free initial fractures. An axial load punches down the central cylinder introducing a high localised shear load in the remaining rock bridge. To the mantle of the cores a confining pressure may be applied to simulate a normal stress on the shear zone. The application of confining pressure favours the growth of Mode II fractures as large pressures suppress the growth of tensile (Mode I) cracks. The stress intensity factor at the critical loading condition in the PTS- test is calculated using a Displacement Extrapolation Technique (DET) based on Finite Element Modelling (FEM). Comparison of the results to KIIC values from other estimation methods confirmed the results. Mode II loading experiments were carried out on the same six rock types as used in Mode I testing.Unstable macroscopic shear fracture growth is achieved at peak load in the PTS-test. Cyclic loading in the post peak region provides controlled fracture propagation and shows constant compliance change for the different rock types. Variation of displacement rates from 3.3·10-8 to 1.7·10‑3 m/s do not change the calculated critical stress intensity factor for most rock types. Variation of geometrical parameters, i.e. notch depth, notch diameter, notch width, and sample diameter, leads to an optimisation of the PTS- geometry. Increase of confining pressure, i.e. normal load, on the shear zone increases KIIC bi-linear. High slope is observed at low confining pressures (< 30 MPa); at pressures above 30 MPa low slope increase is evident. The maximum confining pressure, P, applied is 70 MPa. KIIC increases for the Aspo diorite from 5.1 (at P = 0 MPa) to 12.4 MPa m1/2 (at P = 70 MPa), for Aue granite from 4.1 to 13.2 MPa m1/2, for Mizunami granite from 4.9 to 14.2 MPa m1/2, for Carrara marble from 3.1 to 7.9 MPa m1/2, for Flechtingen sandstone from 1.9 to 5.4 MPa m1/2, and for Rudersdorf limestone from 2.3 to 6.7 MPa m1/2. With increase of shear stress from axial loading, primary macroscopic wing fractures develop at about 30 % of the maximum stress. They propagate out of the stressed zone and stop. Further elevation of shear stress results in development of a process zone leading to a secondary ‘shear’ fracture. Consequently the energy requirement for the formation of the two types of fractures is different. Increase of confining pressure to above 30 MPa is shown to suppress the wing fractures. Carrara marble develops an asymmetric process zone with two different regimes of preferred microcrack orientation and a straight main separation. The acoustic emission analysis indicates mixed mode cracking on the microscale. Increase of confining pressure changes the orientation of the main fracture and the cracks within the process zone. These tend to reach constant orientation at P = 30-50 MPa. The Punch-Through Shear (PTS-) test provides controlled testing conditions and reproducible results. Five different evaluation approaches give consistent results for the Mode II fracture toughness. The asymmetry of the evolving fracture process zone in Carrara marble was shown. This result is consistent with the prediction from stress field analysis and it has also been observed in field studies of shear zones. The existence of Mode II fracture in rock is a matter of debate in the literature. Comparison of the results from Mode I and Mode II testing, mainly regarding the resulting fracture pattern, and correlation analysis of KIC and KIIC to physico-mechanical parameters emphasised the differences between the response of rock to Mode I and Mode II loading. On the microscale, neither the fractures resulting from Mode I the Mode II loading are pure mode fractures. On macroscopic scale, Mode I and Mode II do exist.