Research On Fracture Toughness And Failure Prediction Of Low Alloy High Strength Structural Steel Based On Local Approach

Low alloy high strength steel is commonly used in pressure vessels,bridges,and large steel structure engineering,but because of factors,such as service environment and under impact load,the brittle fracture of the material suddenly causes a lot of major accidents.Material internal non-uniform microstructure and defects,as well as the cleavage fracture behavior is more complex,lead to the fracture toughness of standard sample measured has a great discreteness.Therefore,the use of a single fracture parameters describing material characteristics of the crack tip region is inaccurate and makes it more difficult to predict the fracture behavior of the structure.Therefore,it is necessary to realize the deficiency of the traditional design idea,and propose a new cleavage fracture local approach model,which considers the local damage criterion of the fracture control region.In this paper,based on local theory,the fracture mechanism and failure prediction of high strength steel Q390,which is high strength steel,and commonly used in domestic.The main contents of this thesis include:(1)The pendulum impact test of Q390 was carried out,fracture micro-mechanisms were analyzed by electron microscope scanning(SEM)as well.Research the impact behavior of low alloy high strength structural steel.Compare the ductile-brittle transition curve obtained by tangent hyperbolic function and Bohzmann function.Estimate the characteristic temperature of the impact test,about T28J=-49.07℃ and T41J=-41.48℃,and temperature of the ductile-brittle transition temperature of the material is about-30℃ and the temperature of fracture toughness near the ductile-brittle transition temperature is about-65℃.(2)Based on the curve method,research material distribution rule of fracture toughness,calculate T0=-45℃ of the material curve by the ductile-brittle transition temperature curve and empirical formula,draw the fracture toughness dispersion zone of 90%confidence interval of the material;Predict the fracture toughness test temperature according to the impact test result,use this temperature to judge the effectiveness of the main curve reference temperature.Calculate the reference temperature T0 of the material in accordance with the procedure specified in ASTM E1921 to verify the accuracy of the empirical formula above and prove that this standard also applies to low alloy structural steels in domestic.(3)Simulation of the impact test process by finite element simulation,and the impact test results compared to verify the correctness of simulation,and the J integral increases with the increase of impact velocity;Finish establishment of three-point bending model to simulate fracture toughness test to calculate the crack tip Weibull stress and fitted the constitutive relation between fracture toughness and Weibull stress;Research the influence of different specimen size and the number of test simple,crack length and temperature on the J integral.Calculate the local approach model parameters of the Weibull distribution.(4)Analyze the influence of different temperature and sample data size on local parameter:temperature has a small influence on local approach model parameters below-40℃;model parameter increases with temperature rise above-40℃.Test data of the sample size determines discrete degree of local approach model,the number of samples exceeds 200,the local parameters of the calibration method fluctuate less,and the local parameters of the random sampling calibration are within the 50%confidence interval.