Research On Application Of Micro-mechanism Based Fracture Theory In Fracture Prediction Analysis Of High-strength Steel And Structures

High strength steel has been widely used in large and super high-rise buildings in recent years due to its advantageous mechanical properties.Thus,investigation on the fracture behavior of the high strength steel becomes a key issue since it is related to the overall safety performance of structures in the field.At present,the researches concerning the fracture behavior of high strength steel are still at its initial stage,where mature and effective fracture mechanics methods are still insufficient for application.In this scenario,it would be of great value to explore the fracture behavior of high strength steel and put forward reasonable fracture mechanics methods,which will lay an important foundation for promoting the application of high strength steel structures.As a kind of advanced failure analysis method in the field of solid mechanics,micro-mechanism based fracture theory can be used as a new method to explain the ductile fracture of structural steel materials.Compared with the early classical fracture mechanics methods,the newly developed micro-mechanism based fracture theory takes the effects of materials' micro-mechanism and the complex multiaxial stress states into account fully and can be used as an accurate method to predict the fracture initiation and extension of the un-cracked solid bodies.Therefore,these new methods can be used as useful tools in evaluating the failure of the large practical steel structure under complex loading conditions.Currently,the researches concerning the application of these new fracture theories have been limited in the field of solid mechanics and other engineering fields.However,the researches concerning its application in the field of steel structure are found to be quite rare.In this context,conducting a systematical investigation on the validation and validity of these new methods in the fracture prediction analysis of structural steel materials and structures would be of great value for engineering application.This thesis takes the Chinese Q460 high-strength steel and weld material as the research objects and uses theoretical,experimental and numerical methods to systematically investigate the validity of the micro-mechanism based fracture theory in the fracture prediction analysis of the high-strength structural steel.Closely surrounding the aspects of the material mechanical behavior,mechanics theoretical aspects and engineering application,the fracture methods were then applied in the failure prediction analysis of the large steel structures under multiaxial loading conditions.This thesis provides effective methods and guides for failure analysis of large engineering structures.The main research works and achievements can be summarized as follows:(1)The ductile fracture characteristics of Chinese Q460 high-strength structural steel under complex multiaxial stress states are studied systematically.Four types of notched specimens are used to obtain the fracture behavior of the Q460 steel under different stress states.Subsequently,the scanning electron microscope is used to observe the micro-mechanism of the material under different stress states.The effects of stress triaxiality and Lode angle parameter on the ductility of the Q460 structural steel are investigated.Finally,the fracture data of the investigated Q460 steel is compared with that of other grade structural steel.This work provides the necessary fracture parameters for the calibration of the fracture model for high strength steel.(2)The validity of the three-dimensional Lode-dependent uncoupled fracture theory in fracture prediction analysis of high-strength steel is systematically investigated.Nine typical uncoupled ductile fracture models are selected as research objects and calibrated by using the experimental results of the Q460 steel notched specimens.The intrinsic characteristics and prediction accuracy of each fracture model are investigated by comparing their threedimensional fracture locus.The calibrated fracture model is then verified by using the test results of the Q460 steel notched specimens.(3)The validity of the Theory of Critical Distances(TCD)in the fracture assessment of the high-strength steel notched components with complex geometrical features is systematically investigated.Initially,the material parameters in the TCD are calibrated by using the experimental results generated by testing the standard notched round bar specimens.Subsequently,the calibrated TCD is applied in assessing the fracture initiation location and ultimate strength of other three notched components.The results of this study prove that the TCD can be used to predict the fracture initiation and ultimate strength of the high-strength steel components under complex multiaxial stress states accurately and lay the foundation for the further application of this theory in the fracture prediction of practical steel structures.(4)The three-dimensional Lode-dependent uncoupled fracture model is applied in the failure analysis of high-strength steel welded specimens.Initially,the fracture model of the high-strength steel weld material is calibrated using the experimental results generated from the round bar and flat-grooved plate specimens.The calibrated ductile fracture model is then applied in the fracture prediction analysis for three types of high-strength steel welded components to capture the damage evolution and simulate the crack propagation process.The performance of the fracture model in assessing the fracture of high-strength steel welded components is fully investigated,which provides the necessary guidance for the application of micro-mechanism based fracture theory in the failure prediction of steel structures.(5)A novel 3D uncoupled fracture model is proposed in this thesis.This model is proposed by simplifying the Xue-Wierzbicki fracture model using both the classical Tresca failure criterion and Swift hardening law.The greatest advantage of this model is that the fracture parameters in the model can be calibrated by directly using the stress-strain curve of the material.Such a feature makes the calibration work being significantly reduced.In the meantime,the proposed fracture model can be directly used to predict the fracture problem of steel structures with different steel grades.The proposed fracture model is finally applied in the fracture analysis of two types of steel tube connections.The validity of the proposed fracture model is verified by comparing the test and numerical analysis results.The new fracture model proposed in this thesis provides a useful tool for evaluating the fracture of large practical steel structures.