Evaluating three conventional tests for determining mode I static fracture toughness of rocks

Summary Fracture toughness is defined as the material resistance against crack extension. Different tests have been presented for determination of mode I fracture toughness of rocks, but results of these tests have considerable differences with each other. In this paper, amongst various methods for determining mode I fracture toughness of rocks, Chevron Bend, Brazilian and Flattened Brazilian tests were chosen for investigation of main causes of difference of fracture toughness values. For all tests, 3-dimensional finite element analysis also was used to find the best test for determining this parameter. Finally, it was concluded that amongst three mentioned tests, the Chevron Bend test is the best for determination of mode I fracture toughness because of the least fracture process zone around the crack tip, fairly good sample preparation, a stable crack extension and the least influence of boundary conditions on fracture toughness values. Introduction Fracture toughness represents the material resistance against crack extension and is one of the most important parameters in hydraulic fracturing test. Because this parameter is one of inherit characteristics of rocks, values of fracture toughness of different tests seems to have a good agreement with each other, but in practice it doesn’t happen. In this paper amongst various methods of mode I fracture toughness determination, Chevron Bend, Brazilian and Flattened Brazilian tests have been selected in order to find main reasons for difference of fracture toughness values. Methodology and Approaches tests were evaluated based on sample preparation, crack extension, crack type, effect of boundary condition and volume of fracture process zone around crack tip according to experimental results and numerical modeling. Results and Conclusions Results showed that Brazilian test is not suitable test because of assuming infinite plate in its equations, crack initiation near loading plates, non-uniform loading condition and effect of boundary conditions on fracture toughness values. In Flattened Brazilian test crack is initiated at the center of sample, fracture toughness is numerically determined based on finite plate assumption and loading conditions is uniform, but fracture toughness values is also affected by boundary conditions. In general, Chevron Bend test is chosen as the best test for determining fracture toughness of rocks according to stable crack growth, which makes natural crack at the point of calculating the fracture toughness, and lower size of the fracture process zone around the crack tip. Also, boundary conditions have no considerable influence on fracture toughness values in this test.