Application Of Arlequin Algorithm In The Analysis Of Ultra-low Cycle And High Cycle Fatigue Properties Of Welded Joints

Whether high cycle fatigue failure or ultra-low cycle fatigue failure,the location of fatigue often focuses on the location of welding joints,the structure of welded joint has an important influence on the service performance of steel structure..In order to calculate the stress or strain concentration at the crack initiation location,numerical analysis requires high mesh accuracy and large amount of calculation.The structural multi-scale analysis method has the function of taking into account the requirements of different scales,and it can play an important role in the fatigue analysis.In this paper,based on the general finite element program ABAQUS platform and the user-defined element(UEL)function,a multi-scale structural calculation program based on the Arlequin algorithm is developed.Taking T-type welded joints and cross welded joints as objects,the local stress and strain distribution parameters of welded joints are analyzed,and the influence of parameters on ultra-low cycle fatigue and high cycle fatigue strength is discussed.The main research work and conclusions are as follows:Based on the Arlequin algorithm and the user-defined element(UEL)function of ABAQUS software,the stiffness matrix of plate shell coupling element,plate shell-solid coupling element and solid-solid coupling element is derived,and the coupling element realized by Arlequin method on ABAQUS is developed.The accuracy of user-defined cell is verified by an example.The local plastic strain behavior of T-joints and cross joints under repeated loading is calculated on ABAQUS platform by using the Arlequin algorithm coupling element developed in this paper.The calculation parameters are weld toe radius,unmelted length,unmelted width and flatness of weld toe,and the influence of the above parameters on the joint fatigue life is analyzed.According to the numerical calculation results,it was found that weld toe and unmelted area position is the vulnerable position of welded joints,and unmelted end is the damage of area when unmelted length is more than about 20%;the ultra-low cycle fatigue life of the steel structure is more sensitive to the change of the weld toe radius,and that the increase of the weld toe radius can significantly improve the ultra-low cycle fatigue performance of the structure under cyclic loading;also the flatness of weld surfaces is an important factor on the plastic strain history of welding areas,sharp dents will significantly reduce the ultra-low cycle fatigue performance of welded joints whereas a smoother welding surface is beneficial to reduce the local plastic strain amplitude,thereby improving the ultra-low cycle fatigue strength of joints.The stress concentration factors of T-joints and cross joints are calculated on ABAQUS platform by using the coupling element of Arlequin algorithm developed in this paper.The calculation parameters are weld toe radius,weld toe angle,weld height,type of unmelted end,unmelted length,unmelted width,weld toe flatness.The loading conditions are bending and stretching.According to the numerical calculation results,it was found that the radius of weld toe has a great influence on the stress concentration coefficient for welded joints,whether under tensile or bending loads.When the radius of weld toe is 0.2mm or 4.0mm,the difference of stress concentration coefficient is 80%.For the circular depression at the weld toe,with the increase of defect depth,the stress concentration coefficient increases,with the increase of defect radius,the stress concentration coefficient decreases.Whether it is tensile load or bending load,V-shaped defects have the greatest influence on stress concentration.