Research On Embedded Defect Crack Propagation And Life Assessment Of Railway Steel Bridge Node Welds

At present,welded integral joints are commonly used in railway steel bridges,which are constructed by factory welding and on-site assembly.Affected by the welding process,it is easy to introduce defects such as slag inclusions,pores,and microcracks in a large number of welds of steel bridge nodes.In service,the joint is the most common fatigue failure component of steel bridges due to long-term complex high stress conditions and welding defects.Therefore,the node performance is related to whether the steel bridge can be safely put into service until its design period.The key to fatigue performance evaluation of steel bridge joints is to accurately predict the fatigue crack propagation process of joint welds.Research on fatigue crack propagation of welds focuses on weld toe surface cracks,which are formed by weld toe surface defects.However,the actual welding defects are mostly located in the middle of the weld and are buried,which is not easy to detect.This kind of defect has internal stress concentration under cyclic high stress conditions,and easily develops into initial fatigue cracks.Because the naked eye cannot directly detect this type of embedded crack,it becomes a great safety hazard for steel bridges.Based on such facts,this article uses ANSYS software to perform fatigue assessment of the internal cracks in the weld of a steel bridge joint.Defects and initial flaky cracks are introduced into the weld of dangerous joint.Based on the theory of fracture mechanics,the spatial propagation of buried cracks is simulated and their residual life is evaluated.Contrastive analysis puts forward relevant engineering anti-fatigue design opinions.(1)A method for embedded defect crack propagation simulation and life evaluation is proposed.A simulation example of crack propagation inside a cuboid weld was established.Based on the multi-degree-of-freedom crack propagation model,the space propagation behavior of the crack tip and multiple discrete points was used to represent the crack evolution process.Using the K criterion as the fracture criterion,the spatial evolution law of embedded defect cracks under biaxial tensile load was explored.The results show that the defect crack propagation is divided into two stages:embedded crack paopagation and penetrating crack paopagation.When the embedded crack penetrates the weld,it enters the rapid expansion stage.According to the design concept of damage tolerance,this stage can be used as an inspection point.Compared with the fatigue test results of embedded cracks,the crack surfaces show a nearly circular expansion evolution.At the same time,they all experienced internal propagation of the weld,penetration of the weld,and fracture,verifying the effectiveness of the simulation calculation method for embedded cracks.(2)A validated simulation method for crack propagation of embedded defects was used to evaluate the fatigue performance of welds on steel bridge joints.Cumulative fatigue damage caused to steel bridge nodes by a marshalling freight train is proposed based on the recent changes in the transportation of the steel bridge of the Poyang Lake Railway on the Tongjiu Railway.The fatigue danger nodes of the steel bridge were determined through theoretical and driving dynamic response analysis,and a multi-scale finite element model of the steel bridge was established based on the sub-model method.Calculating the dynamic response of dangerous nodes by displacement interpolation method,we can know that the nodes are under asymmetric spatial stress.According to the response results,dangerous welds were selected and defects and flaky cracks were introduced.The results show that there is a high stress concentration at the defects,which promotes the material to yield and accelerates crack propagation.(3)The rain flow counting method and the stress equivalent transformation principle were used to convert the joint boundary dynamic stress into a constant amplitude load,and input it into the solid model of the cracked joint as an external load.Exploring the evolution law of embedded crack propagation of joint welds under multiaxial asymmetric loads based on ANSYS active meshing method,using the appropriate bearing criterion as the crack failure criterion and calculating the remaining crack life by the fracture mechanics method.Simulations show that under multiaxial stress conditions,the embedded crack propagation surface appears as a wavy or zigzag curved surface,showing an open expansion.Uneven stress concentration in each direction of the cracked surface leads to the shape of the "tip",which accelerates the crack’s propagation in that direction.When the crack penetrates the weld,the equivalent stress intensity factor at the crack tip is close to the fracture toughness.Therefore,it is necessary to distinguish between open cracks on the surface of the weld to prevent embedded cracks from penetrating the weld and causing fracture failure.(4)Comparative analysis of defect crack propagation and life based on measured strain data of joint members.According to the existing safety warning system of the steel bridge,the field intelligently measures the strain time history data of each boundary member of the lower chord node.Simulation and comparative analysis of embedded crack propagation considering axial force of member.The results show that the initial load change can change the "spout" of the embedded crack propagation surface.Therefore,by appropriately changing the initial load and boundary conditions,the "tip mouth" morphology of the crack surface can be adjusted to achieve the effect of suppressing the propagation.The difference between the life of the measured data and the theoretical calculation of the model is 26.1%,which shows that the entire set of embedded crack propagation and life analysis methods for railway steel bridges can accurately reproduce the actual fatigue damage of steel bridges.Based on the above analysis,it is known that embedded crack evaluation and reinforcement are the key steps to ensure the safety of the structure from service to the design period.Controlling the initial crack propagaion and balancing the stress concentration in all directions on the crack surface are important measures to improve life.