Tensile Behavior Of 400 MPa～500 MPa-grade Anti-earthquake Hot-rolled Ribbed Bar Over Different Strain Rates

HRB400E and HRB500E anti-seismic rebars have good ductility,which can rely the structure to consume energy and improve the seismic performance of reinforced concrete structures.It is also very beneficial to the structure’s resistance to impact and explosion load.In the process of using reinforced concrete structure,it may suffer dynamic loads such as earthquake,explosion and impact,so that the material has different strain rate.Steel is a material which is rate sensitive,and its mechanical properties can change by the increase of strain rate.This strain rate effect of steel has an impact on the performance of the structure,so it is of great significance to research the strain rate effect of steel.HRB400E and HRB500E anti-seismic steel bars are used to improve the energy dissipation capacity of reinforced concrete structures.It is very important to study the strain rate effect on the anti explosion and anti impact design of structures.However,there is no research on strain rate effect of HRB400E and HRB500E seismic reinforcement.In addition,the examination of the static properties of steel loads under dynamic loads is crucial for evaluating the safety of a building structure after dynamic loads.In view of the above reasons,this paper studies the mechanical properties of hrb400e and hrb500e seismic reinforcement under different strain rates.Firstly,the test is carried out through the electronic universal testing machine and Zwick/Roell HTM5020 hydraulic servo high-speed testing machine.The strain rate range of HRB400E and HRB500E steel bars is 0.00025 s^-1～870 s^-1and 0.00025 s^-1～550 s^-1respectively.The stress-strain curves of the two kinds of steel bars under different strain rates are obtained.Secondly,based on the mechanical properties and stress-strain curve of the two kinds of steel bars under the quasi-static strain rate of 0.00025 s^-1,the dynamic increase coefficients of yield strength and ultimate tensile strength of the two kinds of steel bars are studied.For the dynamic increase coefficient of yield strength,the correlation coefficients of Cowper-Symonds model and Malvar model are measured,and the two models are modified to make the calculation of the dynamic increase coefficient of yield strength more accurate.For ultimate tensile strength,the calculation model of dynamic increase coefficient of ultimate tensile strength of HRB400E reinforcement is obtained by modifying the coefficients in Cowper-Symonds model and Malvar model,while the existing calculation model of dynamic increase coefficient of ultimate tensile strength of500MPa reinforcement is also applicable to HRB500E reinforcement.Thirdly,the strain hardening coefficient P of Mander model describing the stress-strain of reinforcement engineering is measured,and the constant P is modified.The fitting effect of the modified model on the calculation curve and test curve of these two kinds of reinforcement is better than that of the original model.Then,the coefficients related to material properties in J-C model are measured.A new constitutive model describing the real stress-strain of material is proposed.Compared the fitting results of J-C model and the proposed model,the proposed model can better fit the real stress-strain curves of HRB400E and HRB500E steel bars.Finally,the residual mechanical properties of hrb400e reinforcement after dynamic tension unloading are studied.The test results show that the yield strength and ultimate tensile strength of hrb400e reinforcement after dynamic tension unloading and strain aging increase more than those after quasi-static tension unloading and strain time effect.