Study On Fracture Performance Of Concrete Beams Under Cyclic Loading

As a civil engineering material,concrete has played an important role in infrastructure construction and is widely used in large structures such as dams,bridges,housing construction,etc.It is worth noting that in civil engineering,water conservancy and other engineering fields,the probability of fracture failure of the actual structure under static load is not high.While the fracture damage mainly caused by complex dynamic load effects,such as earthquakes,vehicle loads,water stress,etc.These dynamic loads will cause certain damage to the structure,and internal stress of some structures is lower than its design strength when they are damaged.In view of the current situation of frequent earthquakes in China,it is necessary to study the fracture behavior of concrete structures under earthquakes and one of the most important topics is to study the fracture characteristics of concrete structures under cyclic loading.In addition,the strength and deformation performance of concrete under cyclic loading is the basis of studying the fatigue resistance of concrete structure.So,the research on the fracture characteristics of concrete under the action of cyclic loading has played an important engineering application value for the research on the anti-seismic and anti-fatigue fracture performance of concrete structure.In order to study the fracture behavior of concrete under cyclic loading,three-point bending tests were performed on pre-notched concrete beams.In the experiment,Linear Variable Differential Transformers is used to measure the deflection of the specimen,Clip Gauge meter is used to measure Crack Mouth Opening Displacement(CMOD)and Electronic Speckle Pattern Interferometry(ESPI)is used to measure the whole field of the beam surface.The Load-Crack Mouth Opening Displacement(P-CMOD)curve,the Load-Crack Tip Opening Displacement(P-CTOD)curve and the ESPI displacement contour are obtained through the test.The results show that the measurement results of the Clip Gauge are in good agreement with those of the ESPI,which proves the accuracy of the measurement of the displacement field of the concrete beam by the ESPI technology.According to the P-CTOD curve and the ESPI displacement contour,the fracture of concrete can be divided into three stages: the first stage is the crack initiation stage,the beam is elastically deformed and its nominal stiffness is unchanged;during stable-propagation stage,the beam enters the stage of elastic-plastic deformation and the crack nominal stiffness is gradually reduced to 0;during unstable-propagation stage,the nominal stiffness is reduced to negative value.In addition,the Extended Finite Element Method(XFEM)is used to study the fracture behavior of concrete beams under different parameters under the action of cyclic loading.First,the basic theories and calculation methods of XFEM were elaborated;the common constitutive relations of concrete and the basic content of cohesive crack model were introduced.By comparing the simulation results with the test results,it was found that the two are in good agreement with an error of about 10%,which proves the reliability and feasibility of the numerical model.On this basis,the parametric analysis of the specimen beam was carried out,the influences of crack length to depth ratio,crack offsetting distance and span on the fracture characteristics of the concrete beam were studied;sensitivity analysis of the effects of material parameters(including the maximum principal stress,elastic modulus and viscosity factor)used in the numerical model.Finally,summarize the research content and main conclusions of this paper,it is put forward suggestions and prospects for the next research work.