Rock Direct Tension And Compression Deformation Behavior Of The Test Study

It is well known that rocks bear compressive stress rather than tensile stress in most cases. As a consequence, the deformation parameters and the constitutive models of rocks deduct from only compressive tests in the practical rock engineering. However, some researches have shown that Young's modulus in direct tension, E_T, is much smaller than the modulus in compression, E_C, for some rocks. For such rocks, large error may be produced if their constitutive models and the deformation parameters such as Young's modulus and Poisson's ratio are obtained from only compressive tests, and the larger difference between E_C and E_T, the greater error would be produced.Unfortunately, most investigations on the deformation behavior of rock are conducted from only compression so far. The constitutive models of rocks and failure criterion are very incomplete under tension and compression. Actually, only bimodularity is recommended to constitutive models of rocks in tensile and compressive tests. More researches are then needed to investigate the deformation behavior of rocks under both tension and compression.In the research works involved in this paper, the compression-tension cyclic loading tests with different loading routes and stress levels were porformed with four rock types. It was developed by Kunming University of Science and Technology that the testing apparatus can conduct both compression and direct tension tests with the same rock sample.Following conclusions are obtained: (1) For three tested rock types, Young's modulus in direct tension, E_C, are much smaller than the modulus in compression, E_T. For another type of rock, Young's modulus in tension is nearly equal to the modulus in compression. (2) As stress is applied in tension and compression cyclic loading tests, the values of Young's modulus and the Poisson's ratio in tension continously decrease, while the values tends to increase up to the stage of incipient failure in compression. (3) The rock, whose E_T is nearly equal to E_C, can be basically regarded as elastic material. For the rocks of which E_T is much less than E_C, the deformation in compression is generally elastic, while the deformation in tension is prominent plastic. (4)In the loading cycles among tension and compression, the slope of stress-strain curve in the initial tensile load is the same as the slope of the curve in compression. The stress-strain curves between the loading stage of tension and compression are generally continuous and smooth. (5)With the applied load increasing in tension, the modulus of unloading in tension is 1.5 to 3.0 times higher than the modulus of loading. While the modulus of unloading is only about 5～40% higher than the modulus of loading in compression.