Study On Multiaxial Fatigue Assessment Of Tubular K-joints Based On Zero Point Structure Stress Method

As one of the most important structural forms of welded structures in offshore engineering,tubular joints have attracted much attention.Due to the interaction of wind,wave and current,the stress state of tubular joints is generally multiaxial and alternative.At present,the fatigue strength is generally checked by uniaxial fatigue theory in marine engineering industry,but the multiaxial fatigue life method is seldom studied.The weld around tubular joints is not only in multiaxial stress state,but also has large stress gradient and stress concentration.Life assessment based on uniaxial fatigue theory often leads to large dispersion of prediction results,which can no longer meet engineering requirements.Therefore,it is of great significance for the safety of marine structures to study and analyze the multiaxial fatigue characteristics of the tubular joints and to establish a multiaxial fatigue assessment method for the welded tubular joints of the marine engineering.In this paper,a new method of fatigue assessment stress calculation,Zero Point Structural Stress method,is introduced to solve the problems existing in the application of the traditional hot stress method to the evaluation of the fatigue strength of tubular joints.Combined with the latest research results of multiaxial fatigue,a multiaxial fatigue assessment method for tubular K-joints is studied by combining theoretical analysis with finite element method.Firstly,a fine finite element model considering the weld is established,and the calculation accuracy of the model is verified.Based on the Zero Point Structural Stress method,the stress distribution of the tubular K-joints is analyzed and the stress distribution in the thickness direction of each angle position in the intersecting line is given.Through the finite element numerical calculation results,the stress distribution law of tubular K-joints under axial balanced load is obtained based on the Zero Point Structure Stress method.This stress distribution law is compared with the stress distribution law based on the Hot Spot Stress method,which verified the feasibility of our method.Secondly,the effects of geometric parameters on the SCF_ZP and ZP of tubular K-joints are studied,and the parameter equation of the stress concentration coefficient of the Zero Point Structure Stress under the axial balanced load is fitted.The distribution of Zero Point Structure Stress concentration coefficient and the Zero Point position of the tubular K-joints under combined load is studied,and the multiaxial stress state of the tubular K-joints under the combined load is analyzed.Finally,the multiaxial fatigue life assessment of tubular K-joints is carried out by using the Zero Point Structural Stress applied with the traditional multiaxial criterion and the MWCM method respectively.A multiaxial fatigue assessment process and method for the welded tubular joints of marine engineering are established.At the same time,the multiaxial assessment results are compared with the uniaxial CIDECT assessment results,indicating that the multiaxial fatigue assessment method established in this paper is correct.The research ideas and research methods in this paper can provide a reference for the multiaxial fatigue assessment of offshore engineering tubular joints.