Theory And Experimental Research On Microscopic And Quasi-brittle Damage Model For Concrete In Uni-axial Tension Conditions

With the change of external load and environment, internal micro defect inquasi-brittle materials, such as concrete, will keep on evolving, which will reduce thestructure strength and stiffness and lead to destruction in the end. Studying the evolvingrules of concrete under loading condition has a critical significance to predict theconcrete structure failure, structure safety and reliability. Based on the scientificresearch project_“studying on size effect and strength calculation of quasi-brittle”,which is sponsored by the construction of science and technology plan projects ofChongqing, the dissertation focuses on the damage evolving rules and mechanicalproperty for concrete in uni-axial tension conditions. The results and suggestions are asfollows:Referring to the idea of PBS(Parallel Bar System) and SDM(Stochastic DamageModel), concrete materials can be treated as REV(Representative Volume Element) inseries, and each REV consists of an idea elastic-brittle spring, elastic-plastic bar andrigid plate at both ends. The spring is used to simulate the fracture damage of theconcrete material and the elastic-plastic rod is used to simulate the ductile damage. Themicroscopic quasi-brittle damage model of concrete, which can reflect the interactionbetween the developments of micro-defects in the process of simulating damage, isestablished and it shows a more reasonable description of the distribution and shape ofthe concrete damage on the thermodynamic process and geometry process in thepreclinical state.The character strain of concrete is chosen based on its single axis pulling character;voice launching test and the related theories of modern nonlinear science. It is assumed,according on its traits of heterogeneous matter and on the basis of amendatory parallelstaff model, that yield probability density function and fracture probability densityfunction of all mechanical parameters of the concrete in its different phases be obeyedto Weibull distribution and normal school, afterwards the damage evolvement equation of concrete is deduced. The reasonableness of damage description and parameterselection of the inference model of this paper is demonstrated, and a numerical exampleis a further validation.Nine groups of concrete are tested by the indirect tensile test, which providescredible test data for the comprehensive and in-depth study of the stress-strainrelationship of concrete under uni-axial tension conditions as well as the variation ofmechanical properties of concrete. Besides, the test results and the calculation results ofthe microscopic damage model are compared to verify the reasonableness of themicroscopic quasi-brittle damage model.