Research On Hot Spot Stress Calculating Method Of Tubular Joints

Due to its beautiful appearance and superior mechanical performance,the direct welded circular hollow section structure has been widely used in buildings,bridges,marine,machinery and other engineering structures.However,due to large number of tubes and welds intersecting at the joint,severe and complicatied stress concentration occurs near the intersection,where fatigue crack of intersecting steel tubular joints often initials.The hot spot stress method is one of the commonly used fatigue design methods.This method focuses on the influence of the peak hot spot stress amplitude on the fatigue life.However,tests show that the intersecting joints perform obvious different fatigue life under the same hot spot stress amplitude but different distribution of hot spot stress.The dessertation,on the basis of experimental database and finite element analysis,comprehensively investigates the hot spot stress field near the intersecting weld of CHS-to-CHS T-joint,focusing on the influence of the following factors,e.g.geometric parameters of joints,load types,constraints of chord ends,etc.The dissertation studies the following aspects:Firstly,different types of end faces of members in a tubular joint are investigated comprehensively and equations of end faces are established.Considering the spatial positioning parameters of a member,the coordinate transformation matrix between the local coordinate systems of the members is derived,and mapping solutions of the equation of end faces are established,which greatly simplifies the process of solving the intersection surface and the sweeping vector.On this basis,radial sweeping modeling scheme with the outer wall as the source surface and the end face as the sweeping side face is proposed.The mesh models of lap and overlapped K-joints,spatial KK-joints are established,which verifies the generality of the modeling method.Secondly,The test data of CHS-to-CHS T-joints were collected,including of 267 joints on hot spot stress at key points,9 joints on stress distribution and 2 joints of degree of bending of stress.The finite element model established by the radial sweeping modeling method was applied to the analysis of the test joints.The analysis results are compared with the experimental results which agree well,indicating that the model can well simulate the stress state of CHS-to-CHS T-joints,which proves the reliability of the radial sweeping modeling method.Thirdly,A systematic variable parameter analysis of hot spot stress field of the CHS-to-CHS T-joints under basic load is done,which focuses on the influence of geometric parameters,load types and the constraints of chord end.Based on the results of finite element parameter analysis,the calculation formulas of the hot spot stress concentration factor at the key points,hot spot stress distribution coefficient,and degree of bending of hot spot stress of CHS-to-CHS T-joints under different basic loads are fitted.The comparative statistical analysis with other formulas and experimental data shows that the proposed formula proposed in this dessertation has higher accuracy.Fourthly,parameter analysis of hot spot stress distribution of CHS-to-CHS T-joints under combined loads is done,which focuses on the influence of geometric parameters and load combination.Comparison analysis of stress distribution differences and connections under basic load and combined loads are made,which verifies the reliability of the linear superposition method based on the hot spot stress distribution.Based on the results of finite element parameter analysis,a simplified calculation formula of peak hot spot stress under combined loads are derived.The comparative statistical analysis with linear superposition method,API formulas and FEA results shows that the accuracy and operability of the simplified calculation formula proposed in this dessertation meets the engineering requirements.Finally,Influence of hot spot stress distribution along the intersecting line and the thickness direction on fatigue performance were analysed.Combined with the hot spot stress amplitude,the calculation method of equivalent hot spot stress is proposed,and the fatigue design formula based on the equivalent hot spot stress is established.The comparison and statistical analysis with the test data and the traditional hot spot stress fatigue design formula show that the fatigue design method based on the equivalent hot spot stress has higher reliability.