Experimental Study On Creep Behavior In Steel At Elevated Temperature

Steel structure is not fireproof and easily buckles and failures earlier in fire.Creep in steel is obvious at elevated temperature and has a great impact on the deformation of structure in fire.At present, fire resistance design of the steel structure at home and abroad have not yet to consider creep effect at the elevated temperature, because of the lack of enough research results as a support.Therefore, in-depth study on creep properties of steel at elevated temperatures is necessary.Creep tests on low alloy Q345 steel and high-strength Q460 steel were carried out at elevated temperature.On the basis of elevated temperature creep test data, creep model were obtained by numerical simulation.Using general finite element software ANSYS, the creep effect on fire resistance of steel structure was analyzed in this paper.Three aspects as follows were mainly completed in this paper.(1)Material tensile tests and creep tests at elevated temperature: Material tensile tests on 12 specimens at room temperature and creep tests on 55 specimens at elevated temperature of low-alloy Q345 steel were completed, as well as 10 specimens for room temperature material tensile test and 49 specimens for elevated temperature creep test of high-strength Q460 steel.The data obtained in material tensile tests at room temperature include stress-strain curves, yield strength, ultimate strength, elastic modulus and ultimate strain.The creep tests at elevated temperature last more than 1000 hours, and creep- time curves were obtained at elevated temperature.(2)Creep model at elevated temperature: The seven current common creep models were introduced and their advantages and disadvantages were compared.On the basis of the existing model, the parameters of the ANSYS Compound Time-Hardening model, Norton model and Field&Field model were obtained by numerical fitting on the creep test data at elevated temperature.The difference between fitting results by creep model and test data were compared and the fitting results by three models were also compared each other.(3)Finite element analysis: The finite element model of steel column was established to analyze bearing capacity, considering the residual stress, initial imperfection and elevated temperature creep.Stable bearing capacity analysis at room temperature, thermal analysis and the fire resistance analysis were completed on steel column considering elevated temperature creep effect respectively for low-alloy Q345 steel and high-strength Q460 steel.The results of the analysis include the bearing capacity of steel columns at room temperature, temperature-time curve, fire duration curves and critical temperature curves of steel column considering elevated temperature creep effect, and compared with that without considering creep effect.The main conclusions on three aspects are obtained by experiment and analysis as fellows:(1)It can be found from the creep tests that for the low-alloy Q345 steel and high-strength Q460 steel, when the temperature at 400℃ and below, the first phase of creep accounts for major proportion and the second stage creep develops slowly.As the temperature increased gradually, the second stage creep becomes to the main component and the third stage creep quickly appears.Especially under the high stress levels, the creep development is rapid and it’s prone to creep rupture.In particular, within the scope of the 700℃~900℃, superplasticity properties, enhancement toughness and better elongation were seen in steel.(2)By using different creep models calculation, it can be found that three model parameters can effectively predict the creep at elevated temperature and the simulation results are in good agreement with test results.The analysis results used by the parameters of Compound Time-Hardening and Norton model fitted by the same temperature test data show that the fire duration and critical temperature of the same steel column are close and in accordance with a similar regular.It’s recommended to use Norton model for structural analysis, as its simple form, less model parameters and convenient calculation.(3)Through the finite element analysis, it can be found that the results by finite element calculation agree well with the test results and the finite element model was validated.The result of thermal analysis by finite element is in good agreement with calculation results by four specifications and it′s closer to EC3 calculation result.When the steel column temperature is higher than 600℃ or the load ratio is above 0.5, the fire duration of steel column significantly reduced.At the same temperature and load ratio, the calculation results show that the fire duration of low alloy Q345 steel column are longer.Generally the critical temperature of steel column are reduced by about 2%~17%, the critical temperature of high strength steel Q460 steel column are higher 10℃~25℃ at the same load ratio(except 0.1 and 0.9).In a word, creep increases component deformation, weakens the stiffness, accelerates failure, reduces the stable bearing capacity, shortens fire duration and decreases critical temperature.