Research On Frequency Domain Method For Fatigue Life Prediction Of Welded Structures Under Random Loads

Welding is one of the most important connection methods of engineering structures,and fatigue fractures are also most likely to occur at welds.With the continuous expansion of the application range of welded structures,the number of engineering accidents that occur at the same time is also increasing,making structural fatigue also It has become a hot issue for designers and engineers.The fatigue life of a welded structure is not only related to the form of the structure itself,welding quality and residual stress,but also closely related to the form of fatigue load.At present,most researches on fatigue focus on constant amplitude loads,while the loads borne by actual engineering structures are often Has greater randomness.Therefore,this paper takes the welded structure as the research object,and proposes a frequency domain prediction method for the fatigue life of the structure considering the welding residual stress under random loads.The main research contents are as follows:First,it summarizes the research status of random fatigue based on frequency domain method and time domain method,and expounds the characteristics of random vibration fatigue.The frequency domain method is widely used in the fatigue life prediction of Gaussian random process due to its advantages of convenient solution and efficient calculation.Random vibration simulation analysis was carried out on the circular groove notch specimen,and its fatigue life was predicted,and the predicted life was compared with the test results.The results show that the frequency domain method can better predict the fatigue life of the circular notch specimen.Secondly,with butt joints and T-shaped joints as the research objects,the double ellipsoid heat source model combined with "life and death element technology" is used to dynamically simulate the welding process of two types of typical joints,and the stress field distribution results after welding cooling are obtained.The distribution curve of the weld toe stress along the weld direction is given,and the distribution law of the transverse residual stress at the weld toe along the specified path is obtained.Then,based on the introduction of the mean square stress concentration factor,the effective stress concentration factor based on the frequency domain is defined,and the method for solving the coefficient is given.This method reflects the influence of stress concentration,residual stress and average stress on fatigue strength.On this basis,a frequency domain method for predicting the S-N curve of the welded structure is established.The method is used to predict the SN curve of the butt joint and the T-joint respectively.Finally,according to the finite element model of butt joints and T-joints,modal analysis and random response analysis are carried out,and the stress power spectrum density of the dangerous parts of the joint is obtained.Combined with the predicted S-N curve equation and the Miner damage accumulation criterion,the frequency domain method is used.The fatigue life of the above joint is predicted.The prediction results of the frequency domain method and the rainflow counting method are compared and analyzed.The results show that the frequency domain method established in this paper is easy to calculate and has good accuracy.