Research On Fatigue Properties Of Welded T-Tube Joints Based On Critical Distance Theory

Pipe structures are widely used in engineering structures such as offshore platforms,bridges and towers.The welded pipe joint is a key part of the structure and is also a weak link of the structure.The fatigue problem of welded pipe joints has always been a hotspot problem in the different engineering fields in the world.It is of great significance to study the fatigue performance of welded high-strength steel pipe joints.For the fetigue analysis of welded pipe structures,four commonly used methods are the nominal stress method,the hot spot stress method,the notch stress method and the fracture mechanics method.Among them,the nominal stress method is the most widely used,but it is too conservative;the hot spot stress method is considered to be a suitable method and has been included in the relevant specifications,but it still needs further impromvement;other methods are not yet mature,and have not applied to the engineering problem.Based on the fatigue test data of welded T-joints of high strength steel subjected to the axial load and the in-plane bending load respectively,the fatigue performance of tubular joints is predicted by point method(PM)and line method(LM)in the critical distance theory.Firstly,the value of the critical distance parameter L is determined,and then the finite element parametric modeling analysis of the pip ejoint is carried out by ANSYS so ftware.For the axle-Joaded T-tube joints,two models are established separately with and without considering the welds.Then,the maximum point of the first principal stress along its maximum gradient direction is selected as the focus path of the critical distance theory.The stress gradient of T-tubular joints subjected to axle toad is extracted by using MATLAB program.The stress gradient of T-shaped tubular joints under the in-plane bending can be obtained by re-inporting the stress data into the model as the thermal toads and displaying the teryperature gradient.That is the stress gradient.Finally,the stress-distance curve is drawn by using ANSYS path commands.The hot spot stress concentration fector(SCF)and the hot spot stress amplitude of tubular joints are calculated by point method and line method,and the accuracy o is verified by the test data.The results show that for the wekiless model,the predicted results of SCF and hot spot stress amplitude by point method and line nethod under the axle toads has a great deviation,and the accuracy of prediction is different For the model with the welds,the line method is more accurate than the point method.Both the point method and the line method can predict the hot spot stress amplitude.The error range can be controlled within±20%.The calculated values of the point method and the line method for the tubular joints subjected to the in-plane bending loads are higher than the experimental values,which indicates that the fatigue design of T-tubular joints based on this method will be too conservative.