Study On Shear Strength Of Concrete Based On Couple Stress Theory

The basic mechanical properties of concrete include compressive strength,tensile strength and shear strength.However,there is no standard test method for shear strength so far.In previous studies,the shear strength values obtained by different test methods are quite different,and the analysis shows that the shear exists in the area with bending moment,that is,it is difficult to design a perfect pure shear test method,However,the shear strength can be inferred by combining mechanical theory with finite element numerical analysis.The classical continuum theory has some limitations because it has made many simplifications.Its calculation results can not well fit the test results,so it can not get more accurate shear strength values from the theoretical level.The theory of couple stress is a more generalized continuum theory.It is based on the classical continuum theory.However,it regards every point in the medium as a microstructure with micro size.The intrinsic length of the material is introduced into the constitutive equation.The micro element has not only the stress in the classical continuum,but also the couple stress,Thus,the bending moment under macro stress can be responded,and more accurate shear strength can be obtained.Based on the secondary development of ABAQUS,this paper establishes the finite element numerical calculation model of concrete based on the couple stress theory,adopts the maximum tensile stress failure criterion,and studies the shear strength of concrete by means of numerical simulation and experimental data fitting.The main contents of this paper are as follows:(1)Firstly,the characteristic parameters of concrete are determined by numerical simulation of different concrete strength and different size bending test,mainly the intrinsic size and ultimate tensile stress.The simulation results show that the calculation results of the finite element model based on the couple stress theory can better match the test results,so the maximum tensile stress failure criterion is feasible,the value of the ultimate tensile stress can be taken as the flexural strength of the standard specimen of the concrete flexural test,and the intrinsic size values of different strength concrete are given.(2)According to the intrinsic size and ultimate tensile stress obtained in(1),the fourpoint shear beam is simulated.The effects of different sizes,different notch positions,different loading points and different concrete strength are mainly considered.On this basis,with the help of different normal stress / shear stress ratios and calculated shear stress values,the four-point shear beam is simulated,The shear strength of concrete with different strength is estimated,which is about 0.085 ～ 0.121 times of the cube compressive strength of standard concrete specimen.(3)Through the analysis of the stress distribution on the shear plane,the results show that the compression stress of the direct shear of rectangular beam,the Z-shaped specimen,the improved Z-shaped specimen and the shear box direct shear test is greatly affected,which is far from the pure shear state,especially the calculation results based on the classical continuum theory,combined with the calculation results of four-point shear beam,the final value of concrete shear strength is given,which is about 0.095 times of the cube compressive strength of concrete standard specimen.