Concrete Quasi-experimental Study And Numerical Simulation Of Dynamic Mechanical Properties

As one of the most important structural materials, concrete is applied widely in engineering. Although most of the structures are often exposed to quasi—static loading, however, many of them are exposed to dynamical loading such as strike , earthquake, engineering blast, oscillation of train, landing of plane and raid of cannonball etc. Therefore, dynamical strength and other dynamical response of concrete are very important for the design of engineering structures and are necessary to the analysis of structure stability and safety. It is well known that responses of concrete to dynamical loading include inertia effect and strain rate effect. So, systematical investigation on the strain rate effect of concrete under dynamical loading is of great significance in both theory and engineering practice. The obtained dynamical strength can be used in the aseismic design, stability estimation and analysis of concrete structures.In this thesis, the dynamical mechanical properties of concrete and a series of related key issues have been systematically investigated on the basis of micro- and macro-scopic level through a method combining theoretical analysis, numerical calculation and experimental investigation. A new experimental method for different range of intermediate strain rate via low fatigue loading has been presented, in which three loading wave forms are adopted. So far, the study on the mechanical properties of concrete is focused on the influence of loading speed on the dynamical response under a certain wave form, however, the influence of wave forms on the dynamical effect has not been paid adequate attention. In this thesis, dynamical effect and influence of three different wave forms on the dynamical properties of concrete have been analyzed. The obtained conclusions show that1. Dynamical effect under sine wave loading is worse than that under square wave loading but better than that under triangle wave loading. And it is the same for the loading speed.2. Both the tensile strength and average strain rate of concrete under sine wave loading are far less than those under square wave loading but a little morn than those due to triangle wave loading.3. The tensile strength and average strain rate increase with the frequency and amplitude of fatigue loading.In the field of dynamical investigation, the influence of loading wave forms on the tensile strength and average strain rate of concrete has been achieved through experimental investigation. The results show that the increasing rate of concrete tensile strength between three different wave forms are 1.9 percent and 2 percent respectively. The average strain rate of sine wave is one level in quantity less than that of square wave and a little more than that of triangle wave. Obviously, the experimental result are consistent well with the analytical results. In this experiment, expect for the results mentioned above, the following conclusions have been alsoachieved:The dynamical tensile properties of concrete is closely related to the age of loading and the conserving humidity.1. the dynamical tensile strength of concrete increases with the age of loading. The experimental result show that the increasing rate of the average tensile strength between two kind of concrete specimens whose distinction on age of loading are 8 days reaches 19 percent.2. the dynamical tensile strength of concrete specimens reserved in a humid condition is higher than that reserved in a relatively dry condition. The increased amplitude of strength reaches 19 percent.In the field of microscopic investigation of concrete fracture, numerical simulation is sometimes difficult to solve. It is well known that macroscopic mechanical properties of concrete is closely related to it's microscopic structure. Therefore, in order to simulate the fracture process of concrete numerically, we must be aware of the microscopic structure of concrete such as aggregate and micro-crack as well as macroscopic mechanical response to this structure. It is well known that fracture mechanics is a macroscopic analyzing means that can be used to investigate the growth of crack, while damage mechanics is used to study micro-crack at the micro- and meso-scopic level. In this thesis, classification of all sorts of damage models applied in analyzing the damage of concrete has been presented, And a classification graph has been presented too. Besides, the dynamical damage constitutive equation given in literature[48] is modified. A conclusion that the enlarging coefficient of strain on dynamic stress-strain curve is linearly related to its strain has been derived.At the end of this thesis, a damage kinematic equation for concrete is derived under the supposition that the damage of concrete is identical with the weibull distribution and numerical code, in which the maximal tensile strain is considered as a threshold value of damage, has been developed by using finite element method. After feeding those mechanical values obtained in the experiment into this code, a stress-strain curve can be simulated out smoothly, the obtained curve is very similar to the experimental result.The quasi-dynamical theory and experimental method of concrete proposed in this thesis can provide a new approach for studying the quasi-dynamical mechanical properties of different materials, and can expand the experimental range of intermediate strain rate. It is of extensive prospect not only in experimental mechanics but also in the strength design, stability and safety analysis of concrete structures.