Study On Mechanical Properties And Constitutive Model Of The 600MPa Seismic Steel Bar After High Temperature

As a new type of high strength steel bar,600 MPa steel bar has the characteristics of good mechanical properties,excellent machinability,high safety,energy saving and environmental protection.In view of the above excellent comprehensive performance,600 MPa steel bar has been widely used in some developed countries abroad,while in China,the performance research and engineering application of 600 MPa steel bar is still in the initial stage.In order to make the 600 MPa steel bar applied to the engineering construction as soon as possible,the mechanical properties and constitutive model of the 600 MPa steel bar after high temperature cooling were studied in this paper.This study can evaluate the safety of concrete structure with 600 MPa steel bar after high temperature,and provide basis for the damage degree evaluation and repair and reinforcement of building structure after fire.At the same time,it also provides theoretical guidance and experimental basis for the application of 600 MPa steel bar in engineering practice.The main contents and conclusions of the study are as follows:(1)The high temperature test of the 600 MPa steel bars was carried out,and the surface characteristics of the steel bar samples were observed after being heated at different temperatures.It was observed that when the temperature was low,the surface color of the steel samples did not change much after cooling in different ways.With the increase of the heating temperature,the surface of the steel bar samples cooled by air cooling and in the furnace appeared red brown,while the surface of the steel bar samples cooled by water was dark black.When the heating temperature was about 1000?,the surface of the reinforcing steel sample was seriously carbonized,and the oxide layer appeared peeling phenomenon.(2)The tensile test of 600 MPa steel bar after high temperature cooling was carried out and the full stress-strain curves were obtained.Based on the test results,the variation of mechanical parameters of steel bar samples with temperature under different cooling conditions after high temperature was analyzed.The results showed that the ultimate strength and yield strength of decreased with the increase of heating temperature,while the elastic modulus varied little with temperature.(3)The effects of three cooling methods on the mechanical properties of 600 MPa steel bar samples after high temperature were analyzed.According to the measured stress-strain curves of the steel bar samples and the change of mechanical properties with the heating temperature,the calculation model between the mechanical parameters and the heating temperature of the steel bar samples under different cooling conditions after high temperature was put forward.(4)Based on the stress-strain curve characteristics of 600 MPa steel bar samples after high temperature,and combined with the characteristics of common tensile constitutive models of steel,a constitutive model of 600 MPa steel bar under different cooling conditions after high temperature was proposed.By comparing the proposed constitutive model,it was found that the two-fold line constitutive model was suitable for the simplified simulation of steel bar samples after high temperature,the three-fold line constitutive model could better describe the stress-strain relationship of steel bar samples with obvious yield steps after high temperature,and the Ramberg-Osgood model was the best to simulate the strengthening section of the stressstrain curve of the steel bar samples after high temperature.