Experimental And Numerical Analysis Study On Direct Tensile Test Of Hard Brittle Rocks

In recent years, in the forefront of the field of engineering, including hydropower, traffic, energy extraction and reserve, and disposal of nuclear waste, geotechnical engineering is developing rapidly. The internal crack propagation and failure of rock caused by the tensile stress is also increasing. For the tension failure of hard rock, tensile strain failure criterion should be considered to judge its destruction. The direct axial tensile test is the only way to test the ultimate tensile strain of rock.Because of the difficulties about the rock specimen processing, the attaching to testing machine, and the control of eccentric, research on rock tensile test is carried out rarely. At present, indirect test methods such as Brazil splitting test and bending test are commonly used to test rock tensile strength. In the indirect tensile test process, the stress state of rock is very complex, and the test results must exist deviation. Meanwhile, indirect test methods can not effectively get the tensile stress-strain curves, rock ultimate tensile strain and tensile deformation characteristics. Therefore, developing the new rock axial direct tensile testing device and carrying out rock axial tensile test, to study the mechanical properties and deformation rule of rock play a very important engineering significance, theoretical significance. In this paper, the main research contents include:(1) Developing the new rock axial direct tensile test device which include the bond positing device and tension positing device. Two sets of axial positing device can eliminate bonding eccentricity and tensile eccentricity respectively. The entire device can be used with a universal testing machine to carry out rock direct tensile test. The device is easy to use and install. And it can be used repeatedly.(2) The rock direct tension test for hard brittle rocks of large hydropower station dam areas is carried out for the first time by using the device. The tensile strength, ultimate tensile strain and tensile elastic modulus of the rocks are obtained. The whole process rock tensile stress-strain curves are also obtained.(3) Comparing the direct tensile test results and indirect Brazil splitting test results. Comparative results show that:Rock tensile strength tested by direct tensile test is less than Brazil splitting test result. The ratio between them is 0.60 to 0.71. Carrying out the electron microscope scanning test to study the tensile micro fracture mechanism of rock. The test results show that the tensile micro fracture mechanbism of rock mainly include transgranular fracture and intergranular fracture. The fracture surface is clean without rock slag. And there is no cutting or rubbing marks which is different from splitting test.(5) Numerical simulation of rock direct tensile test is carried out, using the RFPA software. The failure modes of calculation and testing are identical. The tensile stress-strain curves are almost coincide. The difference of tensile strength and ultimate tensile strain obtained from calculation and testing is less than 5%. The calculation results verify the advance of rock direct tensile test device and the reliability of test results.