Fractal Characteristics Of Cracks In Limonite Concrete Under Different Strain Rates Based On Meso-scale Particle Element Modeling

With the wide application of concrete materials in the environment of frequent dynamic action,such as sea crossing bridges,high-rise buildings,mountainous seismic zones and nuclear power facilities,the dynamic performance of concrete materials has attracted more and more attention.Concrete has obvious rate effect,that is,under the action of different strain rates,the dynamic strength of concrete will increase with the increase of loading strain rate.Many studies have shown that under the action of high strain rate,multiple cracks are generated in the concrete,which are dispersed,and some cracks will pass through the aggregate.This kind of phenomenon will consume more strain energy,so as to improve the dynamic strength of concrete.In order to quantitatively explore the influence of multi crack development on the improvement mechanism of concrete dynamic strength,supported by the National Natural Science Foundation Project(51678286)"Research on rate effect,dynamic damage fractal characteristics and damage mechanism of radiation resistant concrete",this paper uses micromechanics model to simulate concrete Hopkinson compression test.The fractal theory is used to quantitatively describe the crack development of limonite concrete in each stage of simulated loading.Combined with the stress,crack fractal characteristics and energy comparison of limonite concrete under different strain rates.The main research contents and conclusions of this paper are as follows:(1)According to the theoretical calculation and test characteristics of Hopkinson compression test,the loading speed is simulated by adjusting the speed difference,so as to realize the loading effects of multiple groups of different strain rates.Comparing the simulation results with the test results,the stress-strain relationship,dynamic strength improvement law and damage of concrete simulated by this method under different strain rates are consistent with the test results.(2)The development of micro cracks in limonite concrete under different strain rates during the simulation process was recorded.The box counting dimension method is used to quantitatively describe the development process of micro cracks in concrete under different strain rates.According to the change law of crack fractal dimension and the stress level in the process of concrete loading,the development process of micro cracks in concrete under the action of high strain rate is divided into three stages,including micro crack initiation stage,extension stage and fragmentation stage.(3)The relationship between concrete stress and crack fractal dimension and strain rate under different strain rates is analyzed.The research shows that the development of multiple cracks in concrete,aggregate failure and inertia effect have an impact on the dynamic improvement of concrete under different strain rates.With the increase of loading strain rate,the fractal characteristics of concrete multi crack development gradually converge.Compared with the change of energy,the inertia effect has a greater impact on the dynamic improvement of concrete under the action of higher strain rate.