Simulation Study On Dynamic Mechanical Properties Of Concrete Biaxial Under Pre-static Loads

Concrete materials are widely used in various construction projects and are inevitably subjected to dynamic loads such as earthquakes,strong winds,and explosions during their service life.It is worth noting that concrete structures are subjected to initial static loads before they are subjected to dynamic loads.If the effect of initial static loads on dynamic strength is ignored,the true load carrying capacity of the material may be over-calculated,making the structural design dangerous.Limited by the complexity of the test process,the multi-axial mechanical test of concrete under pre-static load is difficult to carry out,and numerical simulation can provide an effective way.In this thesis,simulations of uniaxial and biaxial mechanical loading of concrete under pre-static load are carried out with the help of numerical simulation technology to investigate the changes of static and dynamic mechanical properties of concrete under pre-static load.The main research contents and conclusions are as follows.(1)Based on Monte Carlo method and aggregate grading theory,a polygonal aggregate random placement program was written using Python programming language,and then a numerical model of concrete was established using the finite element software Abaqus,and a plastic damage intrinsic model was used to characterize the damage evolution process.Finally,the numerical results of uniaxial compression were compared with the experimental results to verify the reliability of the model in this thesis,which can lay the foundation for the subsequent mechanical analysis.(2)Numerical simulations of uniaxial compressive and uniaxial tensile stresses at different strain rates were carried out for concrete models under different pre-static loads to study the effects of pre-static loads and strain rates on stress-strain curves,peak stresses,peak strains and damage modes of concrete.It is shown that under uniaxial compressive conditions,the compressive strength of concrete decreases linearly with the increase of pre-static load,the peak strain shows a trend of decreasing and then increasing,and the elastic modulus shows a sudden increase at the position of strain rate transition.The precompressive static load causes microcracks inside the concrete,resulting in a large number of transverse cracks and a significant increase in damage at the time of damage.Under uniaxial tensile conditions,the tensile strength of concrete decreases nonlinearly with increasing pre-static load.At high strain rates,a pre-static load threshold exists for uniaxially tensile concrete,and when the prestatic load is less than 20% of the static tensile load,the deterioration effect of the pre-static load on the concrete is weak and the rate effect effect dominates.The pre-static load effect reduces the rate sensitivity of concrete.(3)Numerical simulations of biaxial compressive resistance of concrete under different pre-static loads,different strain rates and different lateral pressures were carried out to analyze the effects of different influencing factors on the mechanical properties of concrete and to analyze the change pattern of the final damage pattern.It is shown that the peak biaxial compressive stress of concrete decreases with the increase of pre-static load and the peak strain has a slight increase.A prediction model for the biaxial compressive strength of concrete considering the joint action of pre-static load and lateral pressure was established.The degree of damage on both sides of concrete will gradually slow down with the increase of pre-static load,and the damage mode will change from oblique shear damage to sheet damage.With the increase of lateral pressure,the concrete is restrained by the increased effect,and the concrete biaxial compression shows obvious ductility characteristics compared with uniaxial compression.The deterioration factors Dc of concrete under biaxial compression are all higher than the values under uniaxial compression,indicating that the deterioration effect of pre-static load is more significant in biaxial compression condition.(4)Numerical simulations of biaxial tension-compression were carried out for concrete under different pre-static loads,different strain rates and different lateral pressures to study the effects of pre-static loads and lateral pressures on the static and dynamic biaxial tensioncompression mechanical properties of concrete,and to analyze the change pattern of the final damage pattern.It is shown that the concrete biaxial tensile-compressive strength is affected differently by the pre-static load under different strain rate conditions.Under the static strain rate,both the peak stress and peak strain of concrete increase with the increase of pre-static load;under the dynamic strain rate,the peak stress decreases nonlinearly with the increase of pre-static load.With the increase of pre-static load,the brittle damage characteristics of concrete become more and more obvious;with the increase of lateral pressure,the number of shear cracks in concrete is gradually increasing.The pre-static load is in the range of 40%-80%of the static tensile load,and the concrete in the biaxial tension-compression condition is affected by the pre-static load to a higher extent than the uniaxial tensile.