| As a simple and reliable metal connection,welding has been widely used in civil engineering,mechanical manufacturing,aerospace and other fields.In practical engineering,fatigue fracture caused by random alternating load is the main failure mode of welded structures.Therefore,it is of great theoretical significance and engineering application value to study the random fatigue performance of welded structures and predict the fatigue life of welded joints.At the same time,the welding residual stress and the stress relaxation behavior in the loading process will produce a certain impact on the fatigue performance of welded j oints.Therefore,based on the butt joint of butt joint and unequal thickness butt joint as the research object,and the fatigue life prediction method of welded joints is established considering residual stress relaxation under random load by using fracture mechanics theory.The main research contents are as follows:(1)The fatigue life prediction of butt joints and butt joints of unequal thickness under random loads is studied in this paper.The rain-flow counting method was selected to deal with the time history of stress,Based on the Miner’s cumulative damage theory,a series of known cyclic stress amplitude at all levels were transformed into an equivalent stress amplitude by the stress equivalent method.Then,the fatigue life prediction model considering residual stress relaxation was used to predict the fatigue life of the joint.(2)The finite element calculation model of heat conduction in the welding process of butt joint and butt joint with unequal thickness was established by three-dimensional finite element theory.The dynamic welding process was simulated by the " life-death element method " and double ellipsoid heat source model,the temperature field and stress field results of the welded joint were obtained,and the distribution law of the residual stress fields of the two joints was given.(3)Based on the fracture mechanics theory,the fatigue life prediction model of welded joints under constant amplitude load considering residual stress relaxation was established.the amplification coefficient of stress intensity factor of surface crack of unequal thickness butt joint was modified.On the basis of considering the influence of residual stress relaxation,the method of solving the transient residual stress intensity factor by weight function method was given.(4)According to the established fatigue life prediction model and the numerical simulation results of residual stress,the MATLAB iterative calculation program was developed to obtain the fatigue life of the welded joint under constant amplitude load,and the correctness of the model was verified by comparing with the experiment.By comparing the fatigue life prediction results of two kinds of joints,the influence of welding residual stress relaxation on fatigue life is quantitatively analyzed.The results show that the fatigue life prediction results are conservative if the residual stress relaxation is not considered,and the residual stress relaxation degree is greater in the early loading stage.(5)The fatigue life prediction model established in this paper is used to predict the fatigue life of welded joints under random load,and the results are compared with those calculated by FE-SAFE software.It is proved that the fatigue life prediction method of welded joints established in this paper can better predict the structural fatigue life,which can be referred for engineering application. |
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