54th U.S. Rock Mechanics/Geomechanics Symposium,
28 June - 1 July,
physical event cancelled
2020. American Rock Mechanics Association
1 in the last 30 days
18 since 2007
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Brazilian disc test (BDT) is widely used to characterize tensile strength because of the ease of preparing cylindrical disc specimens and simple compression loading setup. However, evaluation of crack initiation location and analysis of nature of induced fracture (tensile or shear) are essential for correct interpretation of results before determination of tensile strength. In this study, 3D-Digital Image Correlation (DIC) system is incorporated during BDT to capture and characterize the complex full field deformation of rock samples from two formations, Berea sandstone and Mancos shale. The displacements and in-plane strain fields are evaluated across the full surface of the specimens using DIC. Variation of failure load, displacement and strain fields and final fracture pattern induced by testing are examined as a function of the orientation of bedding planes to the loading axis. Using DIC, it is possible to capture the sites of fracture initiation which was utilized to determine the validity of each particular experiment for evaluation of tensile strength. The results show that the two materials have very distinct fracture behavior. In Berea sandstone, multiple cracks get initiated during the BDT and failure is caused by coalescence of the cracks. In Mancos shale, a single crack dominates the failure process. The fractures, in both the rock formations, initiated along bedding planes and then grew along loading axis. The fractures observed in the valid tensile test experiments were determined to be tensile based on the strain fields at the induced crack locations. The specimens show higher strength when bedding planes are perpendicular to the loading axis with tensile fractures occurring at peak load.
The knowledge of rock properties and deformation behavior of rock are important for the fields of civil, mining and petroleum engineering. Measurement of tensile strength has been challenging due to the brittle nature of rocks. Brazillian disc test (BDT) has been the most popular way to conduct an indirect tensile test because of its simplicity in loading setup and use of cylindrical specimen (Rocco et al., 1999; Yoshinaka et al., 2008; Pan et al., 2009). Despite extensive use of BDT questions remain about the fracture initiation and validity of BDT for measuring tensile strength.
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