Experimental And Theoretical Research On Fatigue Properties Of Plain Concrete Under Triaxial Cyclic Loading With Constant Lateral Pressure

It is widely concerned in fatigue of concrete structures. In this paper, concrete triaxial compression fatigue experiments and tension-compression fatigue experiments with constant confinements in two lateral directions have been carried out using a self-modified loading apparatus. Based on the analysis of the test data, the quantitative principles of concrete fatigue deformation and the calculation method of fatigue strength are determined. Moreover, the multi-axial fatigue damage mechanics and process are discussed more extensively, and a nonlinear cumulative damage model to predict the remaining fatigue life and a constitutive model to describe concrete fatigue characteristics are established. The thesis is an important part of the project of National Natural Science Foundation-Fatigue failure criterion of concrete under multi-axial variable-amplitude fatigue loading (No. 50078010), the contents of which include seven chapters as follows.The first chapter is preface. It expounds the significance of this study and the relevant literatures. Meanwhile the historical and present background of the research on experiment and theory of plain concrete is introduced systematically. Based on the information, the objectives of this paper are put forward.The second chapter introduces the test system simply at first. Then an experiment is carried out on concrete cube specimens to investigate the deformation characteristics of plain concrete under triaxial constant-amplitude and variable-amplitude compression fatigue loading with constant confinement in two lateral directions. The damage mechanics under multi-axial compressive fatigue loading is analyzed and the damage criterion is determined. The deformation characteristics including the development of the maximum/minimum longitudinal strains, the residual strain, the maximum longitudinal strain creep rate, the deformation modulus and the damage cumulation of concrete in terms of cyclic number are gained. In addition, some experience formulas for them are regressed.The third chapter establishes a unified S-N curve equation to calculate multi-axial fatigue ultimate strength of concrete considering both the effect of confinement and minimum/maximum stress ratio, based on the triaxial fatigue test data in this paper and the test results under uniaxial and biaxial fatigue loading in previous literatures. Moreover, the consistency of calculating condition fatigue ultimate strength with fatigue life distribution and initial ultimate strength distribution is analyzed. Thus the calculation of condition fatigue ultimate strength using common fatigue experiment with small samples is available.The fourth chapter carries out the triaxial tension-compression fatigue experiments on tapered prism specimens subjected to constant-amplitude and variable-amplitude fatigue loading with constant confinement in two lateral directions. The damage mechanics and failure mode are analyzed. A unified S-N curve equation to calculate tension-compression fatigue ultimate strength of concrete is established. In addition, the deformation characteristics including the development of the cyclic stress-strain curve, the longitudinal strains, the secondary stage strain rate, the deformation modulus and the damage cumulation of concrete in terms of cyclic number are obtained.The fifth chapter reviews the applicability and characteristics of previous nonlinear cumulative damage models synthetically, and the inherent limitation of Miner's rule onpredicting remaining fatigue life is also proofed from the point of view of the nonlinear dynamic. Taking into account of the effect of loading history, a new nonlinear cumulative damage model is established and the steps to predict remaining fatigue life with the model is given.The sixth chapter discusses the characteristics of concrete under static and fatigue loading in detail. Considering the complicated characteristics of plasticity and brittleness, a two-surface model is established. The so-called two surfaces are a damage surface established by the co...