| Concrete is one of the most widely used building material in civil engineering.However,some key issues concerning the study of concrete have not been effectively solved,such as the constitutive relationship of concrete materials and nonlinear analysis of concrete structures.How to reflect the whole process of the concrete structure under complex stress states scientifically and effectively,and how to use the scientific understanding of the concrete problem to design and control the mechanical response of the concrete structure in the project effectively is still a great challenge.This paper makes some theoretical explorations of the nature of mechanical behavior of concrete and uses MATLAB to write procedures about concrete yield criteria,constitutive relations,and data processing.Furthermore,FORTRAN is used to define concrete constitutive model for secondary development of ANSYS.The work is dedicated to serving in engineering practice by exploring the scientific understanding of concrete problems.In this paper,the mechanical properties of concrete under multi-axial stress state are studied in three aspects: experimental research,theoretical analysis and numerical simulation.The main research results are as follows:(1)The main disadvantages of the concrete structure design method based on the specification are discussed.It is pointed out that the concrete structure design method based on the specification could not truly reflect the mechanical response of the concrete under multi-axial stress conditions.The research status of the concrete failure criterion is summarized,including the classical strength theory,the three-parameter/ four-parameter/ five-parameter failure criterion.In addition,the research status of concrete constitutive relationship is summarized,including the elasticity theory,the nonlinear elastic theory,the elastic-to-plastic theory,the rheology theory,the inner-time theory,the fracture theory and damage mechanics theory.(2)The true tri-axial mechanical performance test of concrete is introduced in detail,including concrete mix design,test condition design,true tri-axial test equipment principle,and test procedure.Some experiments are carried out such as uniaxial compression,biaxial compression,tri-axial compression and splitting tests of the concrete,and the true tri-axial test data of the concrete is analyzed.The results show that the peak stress of the concrete specimen increases with its confining stress level rises for the multi-axial compression conditions.However,the peak stress decreases with its confining stress value exceeding the elastic limit strength of the uniaxial compression of concrete.Moreover,the multi-axial compressive strength values of concrete have a random distribution.(3)Whether the friction-reducing cushion between the loading plate and the concrete specimen influences the concrete strength value in the process of true tri-axial test of concrete is studied,the finite element analysis software ANSYS is used to carry out the numerical simulation of contact nonlinearity.The results of numerical simulation shows that it is necessary to set the friction-reducing cushion.(4)The characteristics of the yield surface in stress space are introduced,and the characteristics of the concrete failure surface in the principal stress space are summarized.The modified fourparameter failure criterion of concrete is raised and the undetermined coefficient in the function expression of the modified failure criterion is determined by regression analysis of the true triaxial test data.In addition,the hardening rule of concrete is established and the initial yield surface of concrete is fitted based on the gradient descent algorithm.(5)The matrix expression of the anisotropic incremental model is deduced.The plastic flow law of concrete is established based on the incremental plasticity theory.The damage criterion and damage evolution law of concrete is established based on the damage energy release rate.The operator separation algorithm is used to establish the constitutive model of concrete under multi-axial stress.The stress-strain curve obtained by numerical simulation is compared with which obtained by true tri-axial test.The analysis results show that there is a certain deviation between the theory curve and the test curve,however,the two curves are more and more relevant with the confining stress value of the concrete specimen increases.(6)The numerical simulation program of concrete constitutive model is developed using MATLAB based on elastic-to-plastic damage mechanics theory.FORTRAN is used to define concrete constitutive model for secondary development of ANSYS.The experimental data of the three-point bending notch beam in the literature are quoted,and finite element calculation results are compared with experimental data.The analysis results show that there are strain localization problems in the finite element analysis of concrete structures.After modifying element properties in the local part and adjusting mesh size reasonably,for the load-deflection curve,finite element calculation results and experimental data fit well in the elastic stage and around the load peak point,while large deviation still exists in the descending stage. |
