Study On The Damage Mechanism And Constitutive Model Of Steel Fiber Reinforced Concrete Under Uniaxial Tension

Steel Fiber Reinforced Concrete(SFRC)is widely used in various fields of civil engineering for excellent physical and mechanical properties.The mechanical properties of concrete materials determine the mechanical behavior of concrete structures.The study of mechanical properties and damage behavior of concrete materials and the establishment of constitutive relationship are of great practical significance for improving the design level.In this paper,under the support of National Natural Science Foundation project "Elasto-plastic damage constitutive relationship of steel-polypropylene hybrid fiber reinforced concrete"(Project No.51608397),the mechanical properties and damage behavior of steel fiber reinforced concrete subjected to uniaxial cyclic tension test and acoustic emission monitoring are studied,and an elasto-plastic damage constitutive model is established.The main work and research results are as follows:(1)An experimental investigation of 42 specimens under uniaxial monotonic and cyclic tension was taken to study the stress-strain behavior of SFRC.The effect of steel fiber volume fraction and aspect ratio on the stress-strain relationship of SFRC was analyzed.The results show that the failure mode of steel fiber reinforced concrete is ductility.The peak strength,peak strain,residual strength and toughness of concrete are improved remarkably with the increase of steel fiber volume fraction and aspect ratio.The monotonic tensile stress-strain curve of SFRC is almost identical to the enveloping curve of stress-strain curve under cyclic loading.Moreover,the relation of the envelope unloading strain and plastic strain is linear.Stiffness degradation process of concrete is alleviated by the increase of steel fiber volume fraction and aspect ratio.The effect of steel fiber on the stress degradation rate of concrete is not obvious,and the stress degradation rate is about0.9.Based on experimental results in current study and other literature,analytical equations are developed to generalize the monotonic and cyclic stress-strain responses of SFRC,which can serve as a reference for engineering applications and modification of related specifications.(2)By analyzing the parameters of acoustic emission signals collected to deeply study the whole damage process of SFRC,and the damage mechanism is revealed from the micro level.The results show that acoustic emission technology can monitor the failure process of concrete very well.Based on the monitoring results,the failure process of concrete can also be divided into four stages: elastic stage,micro-crack propagation stage,macro-crack propagation stage and failure stage.In the elastic stage,there is no acoustic emission signal;in the micro-crack propagation stage,a few acoustic emission signals are generated,accompanied by low acoustic emission signal energy;in the macro-crack propagation stage,the number of acoustic emission signals of concrete specimens increases rapidly,accompanied by high acoustic emission signal energy;in the failure stage,acoustic emission signal decreases gradually as axial displacement increases continuously,and finally the specimen was broken.Acoustic emission damage localization results are in good agreement with the macroscopic crack morphology of specimens.Based on the relationship between average frequency(AF)and RA value,the results show that compared with plain concrete,the number of cracks in the failure process of steel fiber reinforced concrete increases significantly,and with the increase of volume fraction and aspect ratio of steel fiber,the number and proportion of shear cracks increase,which makes concrete break from brittle failure mode to ductile failure mode.Damage expressed by acoustic emission cumulative counts can quantitatively represents the damage evolution process of concrete.(3)Based on the experimental results and existing research,the damage evolution equation is established and the elastic-plastic damage constitutive equation of steel fiber reinforced concrete under uniaxial tension is established by referring to the Code for Design of Concrete Structures(GB 50010-2015).The accuracy of the constitutive equation is proved,which can provide reference for the engineering application of steel fiber reinforced concrete and the revision of relevant codes.