Research On Fatigue Damage And Mechanical Characteristics Of Brittle Rock Under Cycle Dynamic Loading

In recent years, with the rapid development of national economy, there are more and more various large-scale infrastructure construction and geotechnical engineering, for example, high and steep rocky slope engineering, rail and road tunnel project, the city subway project, large nuclear power plant project, large hydropower project, deep and long underground tunnels, deep mines open and other projects. Depend on technical characteristics of efficiency, economy and shortcut, mechanical shock and Blasting are widely used in these projects, which can bring enormous social and economic benefits. No matter in what way for rock mass excavation, stress wave produced by impact propagates in the surrounding rock, and inevitably causes a certain degree of damage and destruction of the rock mass away from the blasting operating point, thus threatening the stability of engineering rock. Therefore, in order to guide the engineering practice reasonably, as well as full use of the carrying capacity of rock according to the fatigue damage characteristics of surrounding rock, to reduce unnecessary waste. Studying the fatigue mechanical characteristics of rock under cycle dynamic loading, evolution law of damage accumulation and failure mode has important practical significance.In the long geological history of the development process, rock is the product formed by a variety of complex geological tectonism, it’s also a complex mixture of minerals from a variety of grains, cement and other components, and its structure generally exist a lot of geological defects. As a natural material, it’s a natural engineering material and widely found in nature, plays a very important role in human economic activity. Mechanical properties of rocks under static or quasi-static cyclic loading have made extensive research, and the theoretical system is more perfect. However, the research on mechanical properties of rock under cycle dynamic loading is obviously insufficient, the theoretical research obviously lags behind the engineering practice.According to the insufficiency that research on fatigue damage and mechanical characteristics of brittle rock under cycle dynamic loading, this paper uses SHPB dynamic load test device to carry out cyclic compression test on Lawrence granite. Based on the test results, variation of stress-strain curve and the dynamic characteristics of strength and deformation are analyzed, and revealing influence law of the test conditions and the sample itself and other factors on the mechanical fatigue characteristics. The evolution law of damage accumulation and failure mode are analyzed. The research results can provide a theoretical basis for engineering practice.The main research work and conclusions are shown as follows:(1) To study the mechanical characteristics of rock specimens under cycle dynamic loading, this paper improves experimental technique, mainly applies to the sleeve through the rock specimens, to control axial strain and deformation.(2) The stress-strain curves of rock specimens under single impact process, mainly divided into the following five stages:compaction phase, elastic stage, accelerated deformation phases, the first unloading stage, the second unloading stage. Under cycle dynamic loading, the elastic modulus of rock specimens continuously decrease, the ability to resist deformation continuously weaken, and mechanical properties continuously damage, but the process of cumulative damage has a mutation, that is the damage of the early stage is slow, the mechanical properties drastically decrease when the specimens are about to destroy. The internal cracks of rock specimens continuously initiate and propagate, deformation has a mutation. In early stage, the specimens mainly happen elastic deformation, and have no residual deformation, but in later stage, the strain drastically increases and the specimens drastically destroy.(3) Impact pressure and porosity do not affect the overall variation of fatigue mechanical properties of rock, but will affect the damage process of local phase, the injury severity and speed of the macroscopic mechanics properties. Comparing with rock porosity, impact pressure has greater impact on fatigue mechanical properties of rock. In other words, the fatigue mechanical properties of rock are sensitive to impact pressure.(4) Under cycle dynamic loading, the variation of acoustic velocity and the variation of strain are roughly corresponding. Overall trend constantly decrease, the velocity decreases slow in early stage, but quickly in later stage. The variation of acoustic velocity embodies the overall accumulated damage process of the microscopic rock structure. Impact pressure and porosity do not affect the overall trend, but will affect the decreasing amplitude and speed of local damage phase. The possible reasons that the wave velocity of rock specimens remain unchanged during the period of impact is that the cracks is small after impact or crack direction is consistent with P-wave propagation direction.(5) Cumulative damage process of rock under cycle dynamic loading can generally be divided into three stages:the initial stage, the low-speed development stage and accelerated destruction stage. Impact pressure and porosity do not affect the overall evolution law of damage accumulation, but will affect the fatigue life, damage degree and speed of local damage stage, which is basically identical with dynamic variation of macro stress and strain of rock.(6) Under a range of impact pressure, and a range of smaller size, the failure modes of rock under cycle dynamic loading is splitting failure. Its failure mechanism is:under the impact loading, due to the Poisson effect, scale deformation of the specimen also inevitably occurs when the axial compressive deformation occurs, namely laterally elongated linear strain. Since the ability of rock to resist elongation deformation is weak, when the strain reaches a certain value, the rock failure occurs.