Research On Damage And Fracture For Concrete Structure Under Multi-Field Coupling

At present, concrete is applied to all the fields of engineering including dam, building, road, bridge, airport, dock, foundation and so on, which need lots of concrete to be built, and the role of concrete material is not replaced by other materials. However, many large concrete structures which are serving have a number of differents diseases, which influence the safety of structure. Therefore, the researches on disease mechanism, damage evolution and fracture failure have become the hot points in the academic field. The failure of concrete is a continuous process, whose stages are producing of damage, appearance of crack and failure, so the research on process of concrete failure should make damage and fracture combined together. In the dissertation, the failure process of damage and fracture be analyzed and researched under multi loading applying to theoretic analysis, numerical simulation and measuring data and the research includes the following aspects:Firstly, from the point of energic view, the continuous process of damage and fracture is studied which accompanies engergic dissipation. In damage mechanics and fracture mechanics, energy dissipation is described by energy release rate, so energy release rate is the link between damage and fracture. Under loading, inside concrete structure, the damage area appears and then a critical state appears and then cracks appear inside the damage area. By the point of view, the equality formula can be built at the time of critical state in the two different processes and the relation is explained by theory.Secondly, the coupling research of damage and fracture is done to concrete material’s crack mode I. The damage evolution of material is a continuous process but the continuity of damage evolution is not considered in some damage models. In the dissertation, according to the comparative analysis of concrete damage models, the suitable concrete damage models are found and the damage variable is introduced into the micro crack formula in the vicinity of concrete crack mode I applying to Loland damage model and piecewise curve damage model. The range of damage area at the tip of crack was confirmed in the situation of different damage variables and the relation was analyzed between damage radius and damage variable.Thirdly, the crack mode I damage radius of concrete material was defined though building finite element model. Applying to Ansys and Msc.Marc finite element software,2D and3D crack finite element models were built to simulate the process of concrete material damage evolution. The softwares was developed further to realize the process of element damage evolution and setting several loading steps made elements deteriorate gradually and the damage radiuses may be acquired in different variables further.Fourthly, the temperature-stress field of mass concrete structure was researched with the background of the real engineering. The hydration heat of mass concrete will bring about temperature stress that lead to the earlier damage and micro crack and influence the normal use of structure seriously. In the dissertation, through numerical analysis and distribution of mass concrete temperature stress field, the position where the damage and cracks perhaps appear is analyzed. In addition, taking the infrastructure of Tianxingzhou Bridge as a research object, the distribution of temperature-stress fields of thin wall wide pier, base of tower and pile cap were calculated using finite element method. In the analysis, the effect of temperature decreasing was simulated when the different style cooling pipes were added into mass concrete in order to provide the basis for temperatue decreasing and earlier crack preventing of mass concrete.Fanally, the mechanism of serving concrete bridge structure disease was researched through the static and dynamic loading test of Sanjiang Bridge. In the investigation, the strength inferred of girder is all above40Mpa and the bridge can satisfy the requirement of normal use by the results of static and dynamic loading test. Therefore, the failure of concrete bridges is in the coupling results of diverse factors and loads.By the research of the dissertation, some new methods and ideas may be provided for the theoretic research of combining damage with fracture and the economic and social profits can be brought for real projects and the basis can be provided for earlier preventing cracks and reparing and strengthening of concrete structure.