Research On Fatigue Crack Growth Mechanism Of Damaged Steel Tubes

Mixed-mode fatigue cracking of the steel tube members,which have large torsional stiffness,is a typical and unavoidable phenomenon due to multi-axial fatigue load combinations such as tension and torsion.However,there are few studies on the propagation mode and life calculation of composite cracks in bridge steel tube structures.The primary focus of this study is to simulate the crack growth behavior of damaged steel tube under uniaxial and multi-axial proportional load and predict the fatigue life of mixed-mode cracks with the equivalent mode I stress intensity factor range based on fatigue test of damaged steel tube and the interactive platform of FRANC3D and ANSYS.The main research work and analysis results are as follows:(1)The finite element model was established to calculate the stress intensity factor K_I of circumferential crack in typical length of damaged steel tube,which on mode I crack growth behavior,under uniaxial tensile load.Considering the influence of circular tube radius and wall thickness,the fitting formula of F_I,the geometric correction coefficient of K_I,was obtained by regression analysis method.(2)Based on the fatigue test results,the fatigue crack growth behavior of damaged steel tube under uniaxial tension and pure torsion was studied with the comparation of finite element models.As a result,the numerical simulation method can accurately simulate the fatigue crack behavior of the damaged steel tube.The mode I SIF range?K_I,calculated by F_I fitting formula,can be used to predict the fatigue life of damaged steel tube which on uniaxial nominal stress states.For the damaged members subjected to pure torsional load,it is found that the effective stress intensity factor range?K_effcan be used to predict the fatigue life which is mainly on mode I crack growth behavior.(3)Based on the fatigue test results,the fatigue crack growth behavior of damaged steel tube under tension-torsion proportional load was studied with the comparation of finite element models.The fatigue life of mixed-mode crack was predicted based on the equivalent mode I stress intensity factor range?K_I^eq by analyzing the SIF of the mixed-mode cracks.As a result,the ratio of stress intensity factor|K_II/K_I|and|K_III/K_I|of mixed-mode cracks under tensile-torsional proportional load are affected by the ratio of tensile-torsional load,diameter-thickness ratio and initial damage.In addition,the equivalent range of mode I stress intensity factor?K_I^eq can be used to predict fatigue life,thus the mode I stress intensity factor is still dominant in mixed-mode cracks,and the results are accurate and have certain redundancy.The results gained would have reference value for the fatigue crack propagation prediction and life assessment of damaged steel tubular structures.