Mode I Fracture Toughness of Rock Under Confining Pressure

Stoeckhert, F. (Bochum University of Applied Sciences) | Brenne, S. (MeSy Solexperts GmbH) | Molenda, M. (Geoexperts) | Alber, M. (Ruhr University Bochum)

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ABSTRACT: Mode I fracture toughness was determined under confining pressure for various rock types using sleeve fracturing laboratory experiments. In these experiments, a confining pressure was applied to rock cores with central axial boreholes. A polymer sleeve was pressurized inside the central borehole until the specimens failed by tensile fracturing. Numerical modeling incorporating fracture mechanics was used to obtain a relation between tensile fracture growth and injection pressure. The calculated injection pressure at specimen breakdown is only depending on the fracture toughness of the tested material, the specimen dimensions and the magnitude of external loading. The latter two are known parameters in the experiments, thus, the fracture toughness can be calculated from the breakdown pressures. Fracture toughness magnitudes determined from the sleeve fracturing experiments are in good agreement with those from Chevron bend tests. The results indicate a positive correlation between fracture toughness and confining pressure for most tested rock types. 1 INTRODUCTION 1.1 Linear elastic fracture mechanics Fracture toughness is a material parameter to describe the strength in the framework of linear elastic fracture mechanics (LEFM). More precisely, it defines a materials' resistance to fracture propagation and is therefore usually used in problems dealing with discrete fractures in a material. Such problems are often encountered in geoscience, for example faulting of rock, hydraulic fracturing or the stability of geotechnical structures in hard rock. Fracture propagation geometries can be subdivided into three principal modes: tensile (mode I), in-plane shearing (mode II) and out-of-plane tearing (mode III) (e.g. Anderson 2005). We are focusing on the tensile mode I in this work. LEFM are a theoretical concept to dea lwith stability and propagation of fractures inside a continuum. There are some mathematical difficulties when considering the stress distribution at the tips of sharp fractures, as loading of such fractures results in a stress singularity, i.e. infinite stresses. To overcome this problem, a parameter called stress intensity is introduced (Irwin 1957). This parameter describes the intensity of the singular fracture tip stress field. Using this parameter a failure criterion similar to the stress-strength concept can be defined: A fracture propagates when the stress intensity K at a fracture tip reaches a critical value KC. This critical stress intensity is also called fracture toughness. For a purely tensile fracture problem, these parameters are expressed by KI, which is the stress intensity resulting from pure mode I loading, and KIC, which is the fracture toughness for tensile loading of the fracture, respectively.

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