| The vehicle structure of stainless steel is an effective way to realize the light weight of vehicles and realize the high-speed,heavy-load,energy-saving and safe modern railway transportation.Dissimilar materials spot welded joints of stainless steel and carbon steel can utilize comprehensively the both two advantages,and can also overcome the problem of joint strength decrease,which results from welding deformation due to traditional arc welding.Howerer,anti-fatigue design standard damage and life assessment method for spot welded joints especially dissimilar materials spot welded joints are all borrowed from metal material or typical welded joint such as butt joint borrowed.There is no reliable method according to the characteristics of spot welded joints and the particularity of fatigue problem.This project is carried out to satisfy the actual requirement of the fatigue life prediction and cumulative damage assessment of rail vehicle welded structures.The project combines theories and methods of welding structure,thermodynamics,metallography and statistics,and it regards the temperature-energy-entropy-damage variable as the main line.Firstly,the analysis and study of the temperature evolution during the fatigue process of welded structures lay the foundation of the research.Then,through introducing the thermodynamic method into the fatigue life and damage assessment of welded structures,fatigue performance characterization and damage assessment methods of welded structures based on fatigue damage entropy are proposed and established.As a result,It can provide theoretical support for real-time,on-line,full-field fatigue life monitoring and cumulative damage assessment of in-service welded structures.The main research achievements of the paper are as followed:1.A method for predicting the fatigue limit of dissimilar materials spot welded joints based on the slope turning point of temperature rise was proposed.The hot spot surface of nugget and plastic ring on the side of SUS301L stainless steel of 4mm+4mm SUS301L-Q235B spot welded joints under the high frequency cyclic load was monitored,and it displays the "four stages" temperature evolution characteristics.With the increase of the load level,the peak of fatigue temperature rise gradually increased,and the rate of temperature change,which is the temperature increase per unit become much higher.The fatigue limit predicted by the established model is 5.569 kN,and the fatigue limit obtained by the traditional ladder test is 5.875 kN,so that the error between the predicted value and the experimental value is 5.21%,which has a high consistency.It is proved that the established model can realize the fast prediction of the fatigue limit of dissimilar materials spot welded joints.2.The relationship between temperature evolution and energy dissipation of specimens during fatigue process is studied,and physical parameters related to material properties of the fatigue process of SUS3011-Q235B spot welded joint is integrated into an energy characteristic coefficient Ef.Under the given spot welding process parameters,it is proved that the energy characteristic coefficient Efis a fixed value when the geometry size of the spot welded joints is certain.As a result,a fast prediction method for fatigue life of SUS3011-Q235B spot welded joints based on energy dissipation theory is established.Through the verification test,the error between the fatigue life predicted values of the dissimilar materials spot welded joints and the experimental values are within 20%,and the maximum value of prediction error is only 16.98%,which show high consistency between the fatigue life predicted value and experimental value.So,the fatigue life prediction model of SUS301L-Q235B spot welded joint based on energy dissipation theory has high authenticity and validity.3.The concept of fatigue damage entropy for welding structure is proposed.Refer to the construction of the generalized thermodynamic entropy law,the temperature evolution of spot welded joint fatigue process combined with energy dissipation model and process are combined.With the temperature and energy as strong measure and extensive amount,the concept of fatigue damage entropy is proposed,so that the model of dissimilar materials spot welded joints fatigue damage entropy is established.Under the given welding process parameters and for the same geometry parameters of the spot welded joints,when fatigue fracture occurs,the accumulative fatigue damage entropy is a constant value.And the value has no relationship with the load level,load frequency and fatigue life.Then from the view of continuum damage mechanics and damage variables construction law,the fatigue cumulative damage variable evaluation model for dissimilar materials spot welded joints is established.According to the established model,the predicted value of fatigue life is in good agreement with the test value,and the error is within 15%.It is proved that the model can accurately evaluate the fatigue damage of SUS301L-Q235B dissimilar materials spot welded joints and predict its fatigue life effectively.It opens the new mentality and provide the new method for the cumulative fatigue damage problem of welded structures.4.Starting from the analysis of the factors affecting the fatigue performance,the basic principle of the information entropy is introduced into the research field of welded structures fatigue reliability evaluation.And then a welding fatigue data analysis model based on information entropy is established,and the various influencing factors of spot weld fatigue damage entropy is studied.Moreover,influence degree of the six parameters that affect the fatigue damage entropy for spot welded joints are examined,including the fatigue life,load frequency,thickness,melt nuclear diameter,temperature rising slope and relative temperature rise.The temperature effect of the load level on the fatigue damage has a decisive effect on the fatigue damage entropy,which is also verified the feasibility of the whole research idea that from the evolution rules of temperature,rising to the energy dissipation theory,and then fatigue damage entropy model is established.Then from the perspective of traditional fatigue S-N curve method,and on the basis of nominal stress,structural stress and mesh-insensitive structure stress,the aluminum alloy welded joint fatigue data analysis model based on the information entropy is established.Through calculating and analyzing the information entropy of decision attributes,quantitative contribution of stress concentration,plate thickness and loading mode to the fatigue destruction are researched,which verifies the effectiveness of the welding fatigue data analysis model based on information entropy. |
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