Research On Size Effect Of Impact Properties And Prediction Of Fracture Toughness Of Q690 High Strength Steel

Ferritic structural steel exhibits a distinct ductile-brittle transition when it is serviced under low temperature or nuclear radiation conditions,greatly increasing the risk of brittle fracture,under low stress state.Charpy impact test is a test method for determining the impact properties and the ductile-brittle transition temperature of a material,and has the advantages of being simple,efficient,fast and economical.Since the impact toughness of the material has a significant size effect,the test results of the subsize sample cannot be directly converted into the standard sample result,and its application is limited.At the same time,the fracture toughness and impact toughness of the material can be used to characteriz its ability to resist fracture.Establishing inherent correlation between the two propertities can achieve low-cost prediction of fracture toughness of the material.In this paper,Q690 steel was used,the size effect on impact fracture toughness and the fracture behaviour were studied by simulation and experimental methods.The main research contents of the thesis are as follows:(1)Different sizes of specimens were studied by experiment.The results show that the impact toughness value of the sample decreases with the decrease of the sample size under room temperature.The impact toughness value of the sample increases with the decrease of the sample size under low temperature.The characteristic temperature of ductile-brittle transition decreases as the sample size decreases,and standard samples are more susceptible to cleavage in low temperature.(2)Establish the dynamic elastoplastic model and the ductile and brittle damage failure model of Q690 steel.The tensile tests at different loading rates show that the yield strength and ultimate strength of Q690 steel increase with the increase of loading rate,and the dynamic elastoplastic model of the material considering the influence of temperature and strain rate is established.Based on the fracture morphology,the parameters of the GTN damage model and the cleavage stress of the Q690 steel were determined.(3)The impact test was simulated by the damage model.Stress field and plastic zone evolution of the specimen were obtained by FEM,and reveal the mechanism of the size effect of the impact toughness.Based on the evolution of the plastic zone of different size samples,a modified model of the impact toughness size effect was established,and the experimental data was used to verify the modified model.(4)The relationship between the absorbed energy and fracture toughness in the ductile-brittle transition range was established.the fracture toughness at different temperatures was obtained by FEM.According to Charpy impact test,the reference temperature of the master curve of Q690 steel was obtained,and the dispersion band of the fracture toughness of master curve was calculated.It was verified that the fracture toughness by FEM accorded with the master curve in the ductile-brittle transition temperature range.The relationship between fracture toughness and absorbed energy in the ductile-brittle transition range is established,and the fracture toughness can be predicted by absorbed energy.