Research On Fracture Simulation And Dynamic Behavior Of Concrete

At mesoscale, concrete may be regarded as a three-phase composite consisting of mortar matrix, coarse aggregate and interfacial transition zone (ITZ) between them. The macroscopic mechanical behaviour of the concrete depended on the properties of its components. It has been noted that the physical explanation of concrete macroscopic mechanical behaviour could be given by adopting a multi-scale analysis approach. In view of this, the mesoscale mechanical model for fracture simulation and the dynamic behavior of concrete were thoroughly studied in this paper base on the mesoscale physics of concrete. The major contributions are summarized as follows:1. A one-dimensional stochastic micromechanical damage model was adopted to study the failure process of concrete specimen subjected to uniaxial tension. A balance was taken into account between the dissipated energy in damage evolution and the released elastic strain energy of the specimen during the loading process. Within the frame of energy principle, the stability analysis of the failure process was conducted and a stability criterion was derived. The critical state that's a transition from uniform damage to local damage, the stress drop phenomena caused by instable failure and the size effect law of critical state were analyzed. These investigations provide useful information and suggestion for the mesoscale mechanical model research.2. A mesoscale numerical model for concrete was developed by incorporating into the model framework two key components:1) an improved modeling of probability volume element and2) the composite interface damage model.The mesoscale element that belongs to aggregate, matrix or interfacial transition zone can be considered as a composite at lower length scale. By exploiting this observation, the distribution difference between elastic modulus and tensile strength of mesoscale element was studied based on Voigt and Reuss prediction. Structural resistance could be considered as the product of all random variables in structural reliability theory. Accordingly, the product of strength and elastic modulus was assumed to be a comprehensive property parameter for mesoscale element, and an improved modeling of probability volume element was proposed. Moreover, the distribution difference derived ahead can be reflected in the improved modeling.A composite interface damage model was developed for the element including the interfacial transition zone but not located in a single material phase, which was considered as a composite element in a broader sense. Using the modified Voigt-Reuss averaging scheme, the influence of the interfacial transition zone was smeared into the composite element. The elastic constants of the composite element were defined in terms of the constitutive properties of both the adjacent materials and the interfacial transition zone, as well as the geometry of the homogenized element. The damage of the composite element was felt in the damage of its each component.3. Numerical analyses of direct dynamic tensile test and direct dynamic compressive test were performed by employing the mesoscale mechanical model developed above without considering the strain-rate effects of material properties. But the effect of inertia, which is essentially a structural feature of the response, was included in the mesoscale numerical simulations. It was found that the predicted results from these tests show correctly strain-rate dependence of tensile and compressive strength and correctly strain-rate effects on concrete fracturing behaviour, which indicates that the observed dynamic properties of concrete can be attributed to two elementary factors:the heterogeneity of concrete and stress wave propagation.The two elementary factors were further investigated with reference to the dynamic failure process of a one-dimensional heterogeneous bar. The physical explanations on mechanisms of concrete dynamic properties described below may be drawn from the study. Because of the spatio-temporal characteristics of stress wave propagation, the stress heterogeneity in a dynamically loaded specimen was different from a static loaded specimen, which leads to the difference of failure modes and evolutions between the dynamically loaded specimen and the static loaded specimen. Furthermore, the local stresses in a dynamically loaded specimen are higher, than a static loaded specimen for most materials. Thus, more materials are damaged so as to form more fractures in the dynamically loaded concrete specimen. The increase of strength with strain rate can also be largely attributed to the stress heterogeneity cause by stress wave propagation and the heterogeneity of concrete, and a formula of strength dynamic increase factor was derived.