Abstract

We studied the initiation and propagation of mode II fractures in granite and sandstone under confining pressure to investigate the controls on shear fracture propagation in rocks. An asymmetric loading set up was used to induce a fracture in cylindrical rock samples under confining pressure between 0-20 MPa. We achieved quasi-static fracture propagation with a refined AE feedback displacement control. This technique prolongs the fracturing process up to 42 hours, provides a higher AE resolution and thereby allowed the distinction of two different stages in shear fracture propagation. Granitic samples form vertical fractures in the strain strengthening stage that branch and stop propagating at peak stress. Simultaneously at peak stress a distinct diagonal fracture nucleates on the loaded side of the vertical fracture. During strain weakening we observed stable growth of this second diagonal fracture until the sample lost its integrity. On the other hand sandstone samples only form the diagonal fracture during the strain weakening stage. Analysis of AE source type and hypocenter as well as microstructural analysis indicate that porosity, either intrinsic (sandstone) or deformation inflicted (granite), primarily influences this fracture nucleation and propagation behavior for both rock types.

1. INTRODUCTION

Fractures form as microscopic cracks coalesce into a planar structure, which can be captured by monitoring acoustic emissions (AEs). Macroscopically, rocks will fracture in the mode (I or II) corresponding to the mode of loading determined by the orientation of the fracture relative to the stress state. However at a microscopic scale, both modes of fracturing can be found in what may appear to be a pure mode of fracturing at the macroscopic scale [1, 2]. According to the acoustic emissions observed during rock fracturing experiments, shear-, tensile-, and compression-events all occur during macroscopic mode II fracture propagation [3]. On a microscopic scale on the other hand, increased confining pressure suppresses the occurrence of mode I fractures and supports the occurrence of mode II fractures [4]. These observations demonstrate the complexity of rock fracturing on a microscopic scale and thereby raised a controversy about the validity of macroscopic fracture modes. The existence of a fracture criterion for mode II is still debated in the literature [5, 6]. To study this, many authors used acoustic emissions to stabilize the fracture process [1, 7, 8, 9, 10]. By controlling the load in response to the intensity of the observed AEs, the rate of fracturing was varied and fracture propagation could be visualized in detail.

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