Theoretical And Experimental Investigation On Fatigue Properties Of Rock Under Cyclic Loading

Rock mass in engineering is not only subjected to static load but also affected by dynamic load such as blasting, impact and earthquake etc. Thus, it is a crucial problem to solve in engineering that taking research on the fatigue behavior of rock subjected to cyclic loading and putting forward analyzing and controlling methods for stability. Especially in the last years, with the development of large-scale hydraulic engineering, urban underground transportation engineering, large-span and large-distance tunnels engineering and deep mining engineering, there are many dynamic problems for us.A series of innovative investigations are carried out in theory,experiment and engineering in this paper based on the frame of fatigue analysis of structure. The main contents include:(1) An improved scheme is proposed to overcome the defects of three-points rain flow counting model. The improved model simplified the process of counting by integrating equivalent points compression, peak-valley points detection into cycle counting. Then, the distribution of frequency is obtained by using the improved model to analyze the stress spectrum of vibration. The statistical result indicates that most of fatigue cycles are low amplitude cycles and the proportion of tensile and compressive cycles is very high.(2) A lot of experiments indicate that the axial irreversible deformation of rock subjected to cyclic loading has stable value and three-phases development law. Base on which, a new inverted-S shaped nonlinear fatigue damage cumulative model is presented in this paper. These physical meanings and ranges of all Parameters of this model are discussed. By using this model, a damage calculation process of rock under three-level cyclic loading is demonstrated and a concept of fatigue path is put forward. To our surprise, the basic reason contributing to the fatigue "training" effect can be well explained theoretically by the concept of fatigue path. In sum, this model has relatively higher accuracy by considering the initial damage of material, loads interaction effect and loading sequence effect and can be applied in damage calculation and fatigue life prediction of random or variable amplitude cyclic loading.(3) A Finite Combination Method is proposed in this paper to estimate initial parameters for Levenberg-Marquardt calculation. That is, initial points uniformly distributed in the feasible region of parameters are inputted for Levenberg-Marquardt calculation and those parameters with minimum residual sum of squares are taken as final initial parameters. The fitting result is highly correlated with experimental data, by which this new method is improved effective and the inverted-S model correct.(4) The principle determining the static strength of rock is put forward after taking an analysis about the influence of loading condition on the complete stress-strain curve. Then, the influence of maximum cycling stress, loading amplitude, waveform and frequency on fatigue strength has also been studied. The experimental results show that the rock has a shorter life when being applied higher maximum cycling stress and amplitude or higher loading frequency. Meanwhile, an obvious conclusion can be drawn that the life of rock under sine waveform is bigger than that under triangle waveform but smaller than that under rectangle waveform.(5) The deformation properties of rock is studied and the law of ultimate deformation is verified in this paper. The investigation on evolution law of axial, lateral and volumetric strain shows that a three-phases developing law can be found in these three kinds of strains and the developing curves can be divided into same three phases. Furthermore, an innovative conclusion can be reached that the deformation and deformation rate of lateral and volumetric strain are much larger than those of axial strain.(6) The experimental result of rock indicates that many methods such as elastic modulus method, ultrasonic velocity method, ultimate strain and residual strain methods are all able to represent the damage evolution law of rock. Among them, residual strain method is more suitable for its clear concept and consideration of fatigue initial damage. The damage of rock develops according to three-phases law and the main factors affecting fatigue deformation include stress level and initial damage. The result shows that the higher maximum stress, amplitude and initial damage, the shorter fatigue life for the increase of proportion of the third phase in the whole life.(7) A Modified Local Strain Method applicable for rock is presented by introducing S-N curve and the inverted-S model proposed by author. Then, the approach to residual life and total life estimation is illustrated based on the inverted-S model. Meanwhile, according to the principle of the Modified Local Stain Method, the general steps of estimating fatigue life of rock subjected to random loading are briefly introduced.(8) A new idea of using Plug-in software architecture to construct fatigue analysis system is put forward after comprehensive research on the procedure of fatigue analysis of structure and the requirement of fatigue software development. Then, a solution to saving the cost of system running is proposed by constructing a plug-ins mapping table and using dynamic loading technique. Meanwhile, a method of dynamically allocating symbol IDs for plug-ins by host program is brought forward to put an end to name repeating phenomenon. This method also reduces the coupling degree between host program and plug-ins.