Direct Tensile Test on Brittle Rocks with the Newly Developed Centering Apparatus

Failure of brittle rock normally initiates from tensile fracture due to its extraordinary lower resistance to tension rather than to compression. Intensive study on the deformability and accurate acquirement of the mechanical properties of rock under tensile stress is crucial for the stability analysis of rock slope and rock caverns. A new set of centering apparatuses, which consist of the specimen bonding part and the axial pulling part, were developed in this study. The misalignments when bonding the specimen and conducting the uniaxial tensile test can be effectively eliminated. Direct tensile tests were conducted on various brittle rocks cored from several large-scale hydropower station sites in China. The tensile strength, tensile elastic modulus, maximum tensile strain, and complete stress-strain curves were successfully obtained. The direct tensile test results indicated that the fracture surface formed in the central part of the specimen, which was under uniform tensile stress. The fracture surfaces were mostly perpendicular to the specimen axis. Fresh fractures without accumulation of rock fragments can be observed and characterized as tensile brittle failure. The microscopic failure modes of brittle rocks under the direct tensile test were not unique but consisted of the intragranular and intergranular fractures. The direct tensile strength (DTS) was generally lower than the Brazilian tensile strength (BTS) (DTS/BTS = 0.60 similar to 0.71). The direct tensile test was therefore recommended for the measurement of the tensile mechanical properties in order to ensure the safety of rock engineering.