Study On Fracture Toughness Test Of Ferrite Steel

Ferrite steel is one of the most commonly used steel materials in production and life at present,and during the service of steel material and structure,a series of fracture problems usually occur,resulting in inestimable losses.Fracture toughness is one of the most important indexes to evaluate the integrity of materials or structures.The fracture toughness of materials is not only related to the properties of materials,but also depends on other factors such as the loading mode or constraint level of the research object.The fracture toughness of materials has been a hot topic in the field of fracture mechanics in recent years.In this paper,the fracture toughness of Q345B steel,a typical material of ferrite steel,is solved by different methods.Through the comparison of different fracture toughness solving methods,a more suitable fracture toughness solving method is obtained,which provides a foundation for the integrity assessment of materials and structures and the prediction of fracture toughness of corresponding structures.The main research contents of this paper are as follows:(1)The basic fracture mechanics theory of fracture toughness is summarized and the linear elastic fracture mechanics theory and elastic-plastic fracture mechanics theory are introduced in detail.Various test methods for testing fracture toughness are summarized,including three-point bending test,compact tensile test and charpy impact test.The finite element analysis method is introduced,and the constitutive equation model is given.(2)The basic mechanical properties of Q345B steel at room temperature and low temperature were summarized,and the fitting curves of yield strength and tensile strength with temperature were obtained.The charpy shock absorption work of materials at different temperatures was obtained by numerical simulation of charpy shock test.The simulated values were analyzed and sorted out with those obtained by predecessors,and the fitting formula was obtained by fitting the charpy shock absorption work and temperature.The fracture toughness curves were obtained by empirical formula method and master curve method respectively.(3)The displacement and load curves of crack tip are obtained by simulating three point bending test and compact tensile test.The fracture toughness of metallic materials was measured by quasi-static fracture toughness testing technique,and the validity of fracture toughness value was determined.(4)The critical value of J integral in three point bending test and compact tensile test was determined by finite element analysis and simulation.By analyzing the constraint levels of different components and solving the values of the constraint parameters A₂ of different specimens,the J-A₂ method is used to transform the J integral critical value of different specimens,and the errors generated in the transformation process are analyzed,so as to verify the feasibility of J-A₂ method in the transformation of fracture toughness.