| Many hydraulic structures may be subjected to the cyclic load of high stress, such as concrete dam under the high intensity earthquake, flood discharge structure with large-power discharge, machine foundation concrete in forced vibration, water cushion pool by the impact of high-speed flow, and so on. Because the inner stress field of these structures acutely changes, it is very possible to result in fatigue damage or failure. Hydraulic structures are very important structures. If these structures wreck, the consequences are unimaginable. Therefore, studying the fatigue property of hydraulic concrete is very meaningful in engineering in order to ensure hydraulic structures working safely and stably for long. Especially, earthquake disaster in Wenchuan excited many researchers to initiate seismic damage analysis of hydraulic structures. Because of the difference of use profile, use condition and mixing proportion, the hydraulic concrete has particular mechanics character and fatigue property by comparing with common concrete. As this research is seldom reported at home and abroad, research on fatigue damage of hydraulic concrete was specially carried out in this dissertation at the support of the National Natural Science Fundation (No.51009074). The experimental and theoretic research involves several aspects as follows:(1) The compressive experiment, fatigue experiment and low-cycle fatigue experiment on 120 specimens of hydraulic concrete were performed. The qualitative analysis of experimental data shows that the flexibility of hydraulic concrete is better because the compressive stress-strain curve is relatively flat, and it is beneficial to the resistance of fatigue damage; damage mechanism of compression failure and fatigue failure is basically same, but the quantity, direction and distribution of fracture is different; fatigue life at the given stress level is very discrete, so the reliability theory will be adopted to analyze this data; maximum strain, deformation modulus, and residual strain possess three separate development phases of all the fatigue life.(2) According to the statistical analysis of fatigue life, the results show it follows both lognormal distribution and three-parameter Weibull distribution, but the latter is more suitable by the P-N figure fitting and kolmogorov test. With the decrease of stress level, the scatter of fatigue life increases.(3) By the reliability analysis of fatigue life, the S-N curve, P-S-N curve and fatigue limit strength are obtained. The S-N curve under lognormal distribution and three-parameter Weibull distribution are approximate, but at the interval of high reliability the two are different, and the former make the result pessimism. In the light of self character and damage mechanism, this paper suggests making use of the S-N curve and P-S-N curve under three-parameter Weibull distribution. By summarization and comparison, the fatigue property of hydraulic concrete has advantage over other concrete.(4) By making use of the calculative method of confidence limits of three-parameter Weibull distribution, the confidence limits of fatigue life of hydraulic concrete at the different stress level are calculated. Furthermore, theγ-S-N curve,γ-P-S-N curve, fatigue limit strength and safety life of high reliability and confidence are obtained. These results have reference value for the fatigue reliability design of hydraulic structure.(5) Based on original Miner rule, the limit state equation of fatigue cumulative damage is established, and the dynamic reliability by using Monte Carlo method can be computed. The computational example shows the calculative dynamic reliability can reasonably evaluate fatigue cumulative damage of hydraulic structure.(6) On the basis of three hypotheses, the P-D-εcurve which describes probabilistic relationship between damage and strain of concrete under fatigue load is defined. The important property of this curve that the failure probability of strain equals reliability of damage at every point has been proved. The analysis and treatment of test data indicates that fatigue strain in case of specified damage follows three-parameter Weibull distribution. The P-D-εcurve of hydraulic concrete was obtained. Furthermore, it is easy to calculate threshold strain, limit strain and fatigue damage at the given reliability. This research lays theoretical foundation for the quantitative evaluation of fatigue damage.(7) Based on the distribution of the static compressive strength, a new distribution which fatigue life follows is deduced by the equation model of the original S-N curve, and the solution approach of the new distribution parameters is given. By statistical analysis, low-cycle fatigue life follows this new distribution. Moreover, the S-N curve equation of low-cycle fatigue of hydraulic concrete can be put forward. Because of fully using the relevant data of compressive strength, this method spends fewer specimens, has the higher accuracy, and is of widespread usage.(8) By the process analysis of strength degradation, the new model of strength degradation and predictor method of life period for concrete under low-cycle fatigue load can be proposed. By comparison with the development process of actual strain and Miner criterion, the model and method are suitable to low-cycle fatigue of concrete, and can correctly reflect the damage mechanics of low-cycle fatigue of concrete.(9) With the aid of the theory of damage mechanics, by studying test data of fatigue deformation modulus, development model of fatigue deformation modulus, calculation formula of fatigue damage, and the instability criterion of fatigue damage are obtained for hydraulic concrete. By synthesizing the above research results, the calculative method and process of cumulative damage under low-cycle fatigue load are put forward for hydraulic structures. By means of the Further Development of FLAC-3D Software, based on the finite element method the computational program of cumulative damage under low-cycle fatigue load is given for hydraulic structures. According to computational example, the above method, process and program can rationally reflect the process of fatigue damage and failure mode of hydraulic structures.(10) The method is proposed, which could get compression strength and peak strain of concrete by using stress-strain curve before failure. It is verified by the tests that this method can apply to low-cycle fatigue, has sufficient accuracy, and can achieve actual stress level of fatigue test. On this basis, the development formula of fatigue residual strain and criterion of fatigue failure are obtained. By comparison with estimation of fatigue life, the above formula and criterion are reasonable.
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