Study On Meso-mechanism Of Concrete Fatigue Failure Based On Finite-discrete Element Method

As the most popular engineering material today,concrete is widely used in various complex structures,such as dams,offshore pile foundations,highways,etc.,where the concrete will withstand fatigue loads such as impact,salt erosion and driving for a long time.The research on the fatigue behavior of concrete helps to fully consider the structural use scene in the structural design and improve the safety of the concrete structures.At this stage,the research on the fatigue behavior of concrete mainly analyzes the changes of mechanical properties under the two points bending and compression conditions from the macroscopic point of view.However,the tensile properties of concrete are much lower than the compression properties.Also from a mesoscopic point of view,the mesoscopic multiphase structure of the concrete,the heterogeneity of the material,and the defects in the self-hydration process may affect the fatigue behavior and macroscopic mechanical properties of concrete.Based on the finite-discrete element method,the fatigue model is introduced into the cohesive element model,and the multi-disciplinary research method is used to analyze the distribution of microcracks and the development of macroscopic cracks under the different condition.The effect of heterogeneity on the fatigue behavior of concrete is studied.In order to determine the parameters of loading frequency and fatigue model,this paper uses numerical test parameters from previous literatures to analyze the sensitivity of these parameters.The optimum loading frequency is determined in consideration of test accuracy and calculation cost.According to the sensitivity analysis of the fatigue model parameters,combined with the results of the experiment,the suitable fatigue model parameter values for the numerical model of this paper were selected based on the fatigue life.Firstly,the fatigue tensile test of the basic numerical specimens was carried out.The fatigue behavior of concrete under different stress levels was analyzed from the changes of fatigue life,microcrack morphology and microcrack quantity,etc.: with the increase of stress level,the fatigue life of concrete is reduced,and the number of internal microcracks increases when the concrete specimen is broken.Considering in mesoscale,the particle size of the concrete aggregate relative to the size of the specimen can not be neglected,the size of the basic specimen is expanded by 1.5 times and 2 times respectively.The influence of the size effect on the fatigue behavior of the concrete is studied.The research results show that the tensile fatigue life of concrete decreases with increasing of the size of specimen.In the mesoscopic scale,concrete consists of cement mortar,aggregate and interfacial transition zone(ITZ).In order to study the influence of material heterogeneity on the fatigue behavior of concrete,the fracture strength of the cohesive element in ITZ and cement mortar was changed respectively,and the fatigue tensile test was carried out.The test results show that with the increase of the fracture strength of the cohesive elements in the ITZ region,the fatigue life of the concrete increases,the microcrack distribution area is wider when the specimen is fatigue-damaged,the total amount of microcrack and the ratio of the ITZ microcrack in the concrete decrease.But,the fracture strength of the mortar cohesive element increases,the fatigue life decreases,and the total amount of microcracks in the concrete specimen increases when the specimen is fatigue-damaged,and the microcrack ratio in the ITZ region also increases.The total amount of microcracks of concrete specimens under different fracture strength ratio between ITZ and cement mortar is studied.The results show that the fracture strength of ITZ cohesive element and cement mortar cohesive element is closer(the larger the ratio),the total amount of microcracks is less when the concrete specimen is fatigue-damaged.It can be seen that in the fatigue behavior of concrete,the mechanical properties of the cohesive element in ITZ region predominate.In order to study the influence of concrete meso-structure on the fatigue behavior,considering these three factors that are aggregate shape,pore and aggregate distribution,and the fatigue tensile test under different stress levels was designed.The results show that the fatigue performance of polygonal aggregate is weaker than circular aggregate concrete specimens;and the distribution of pores has a significant effect on fatigue life and the shape of microcracks and the location of macroscopic cracks change significantly.The distribution of aggregates affects the distribution of microcracks and macrocracks when the specimen is fatigue-damaged.On the whole,macroscopic cracks of concrete specimens when the specimen is fatigue-damaged are more likely to develop along larger aggregates.