| Fatigue is the process of progressive, localized and permanent material damage, which often leads to sudden failures with little or no plastic deformation or causes severe damage in such civil engineering structures as continuously reinforced concrete pavements, concrete bridges, concrete crane beams, concrete sleepers, concrete offshore platforms, safety hulls of nuclear power plants, foundations of rotating machinery etc.. These structures may be subjected to fatigue loading caused by vehicle, crane, nuclear power, wind, wave and vibration. Concrete in such structures works in the complex states of multi-axial stresses, whereas, the mechanism of fatigue failure is still not clearly understood in these stress states. Literature on the experiments and the theories of failure prediction of concrete under multi-axial cyclic loadings is extremely scarce, which is the drawback on the design and analysis of these structures. The thesis is an important part of the project of National Natural Science Foundation-Failure criterion of concrete under multi-axial variable-amplitude fatigue loading (No. 50078010), the contents of which are summarized as follows:(1) The testing facility composing with the uni-axial MTS closed-loop system and the lateral loading frames is applied to conduct the experimental program to cubic specimens (and prism specimens) under fatigue load with constant confined pressure. In the first part of this thesis the experiment system is stated, and some key experiment techniques such as the methods to determine the initial strength, technique to measure the fatigue damage are studied.(2) An experimental research program is carried out with the aim of investigating the deformation characteristics of normal strength concrete under biaxial multistage cyclic compression with constant confined stress. A fatigue equation is gained by modifying the classical Aas-Jakobsen's S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The deformation characteristics including the development of longitudinal and transverse strains, the secondary creep rate, under the states of constant-amplitude and variable-amplitude cyclic compressive loads, and deformation modulus of concrete in terms of cyclic number, are gained from the analysis of experiment results. In addition, An ultrasonic pulse velocity method is used to measure the damage growth during cyclic loading of concrete, and whose results imply the progress of failure of concrete under biaxial cyclic compressive loads with constant confined stress. The analysis of the microcosmic destructive mechanism of concrete under the present stress states is conducted, too.(3) A study on the cumulative fatigue damage of plain concrete due to biaxial compression is conducted basing on the experiment results of concrete specimens subjected to constant and variable amplitude cyclic loading with constant confined pressure. The applicability and precision of Miner's rule and Corten-Dolan formula are discussed. In addition, the longitudinal maximum strains, residual strains, deformation modulus of concrete and the ultrasonic pulse velocity are used to define the damage, which induces different lawsthe cumulative fatigue damage following. An average damage variable is proposed in present study for taking into account all of factors mentioned above, and the law of cumulative damage and its evolution were obtained. In this part, the equivalent cumulative damage theory is modified to predict the residual fatigue life of concrete under variable amplitude cyclic loading.(4) The fatigue tests of concrete on biaxial tension-compression under constant -amplitude cyclic loading with constant confined stress are described in this part of the thesis. The relationship of S-N and the fatigue strength envelopes of concrete under tension-compression loads with constant confinement are obtained firstly, basing on the analysis of the results of present experiments and fatigue te...
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