| The welded structure is an important structural form in mechanical engineering structure.The discontinuity of geometry,microstructure,and mechanical properties of welded joints makes the fatigue characteristic of welded structures pretty different from that of non-welded structures.The fatigue performance characterization and evaluation of welded structures have been the center of attention and research in aerospace,rail transportation,and bridge engineering as well as other related fields.In recent years,the evaluation of fatigue from the standpoint of thermodynamic temperature rise evolution has gradually developed as a new train of thought.The energy dissipation of the fatigue process is manifested in the form of selfheating,and entropy is a physical quantity associated with the degree of disorder.The increase of entropy describes that the energy transfer and transformation is toward the direction of the increase of the degree of disorder,and it can be used to characterize the fatigue process accompanied by the movement evolution of non-recoverable microstructure.Therefore,the fatigue performance characterization and evaluation model based on thermodynamic entropy is expected to give a more in-depth interpretation of the mechanism behind fatigue failure of welded structures.The butt joint of weathering steel,which is commonly used in rail vehicles,was taken as the research object in this paper.Combined with theories of thermodynamics,welded structure,macro and micro mechanics,and based on the temperature rise evolution of butt joint,this study is devoted to further investigating the high-cycle fatigue damage mechanism of welded structure,establishing the functional relationship between fatigue limit and energy dissipation or thermodynamic entropy,and finally building an energy dissipation and thermodynamic entropy model for rapid high-cycle fatigue evaluation of butt joints through following the main line of “temperature rise response-energy dissipation-thermodynamic entropy”.(1)The fatigue self-heating characteristic of the butt joints during the high-cycle fatigue process was studied.The fatigue tests of Q460 butt joints with 5 mm and 8 mm plate thicknesses and Q310NQL2-Q345NQR2 butt joints with 2.5 mm+4 mm plate thickness were carried out,respectively.The temperature rise of the sample surface during the test was measured in realtime using an infrared camera.Then,the relationship between the temperature rise and the cycle number of butt joints during the high-cycle fatigue was investigated based on the well-plotted“temperature rise-cycle number” curve from thermographic data analysis.This lays the foundation for further establishing the energy dissipation or thermodynamic entropy model for the high-cycle fatigue evaluation model of butt joints.(2)The energy dissipation model of butt joints for high-cycle fatigue was constructed using the local thermodynamic equilibrium equation,and the fatigue limit was well predicted and evaluated rapidly.The local thermodynamic equilibrium equation suited for the high-cycle fatigue process of the equal thickness butt joint was developed using thermodynamic theory.Thereafter,an energy dissipation calculation model for the equal thickness butt joint was created.Based on this model,a fatigue limit prediction method,defined as the maximum slope transition method,was proposed,and the effectiveness of the model was also verified.The error of the prediction values between the maximum slope transition method and the staircase method is 7.40%,demonstrating that the proposed energy dissipation model could be used to reliably predict the fatigue limit when compared to traditional test results.Furthermore,the error of prediction results between the proposed maximum slope transition method and the bi-linear method is only 0.04%,and these two prediction values are nearly equal,exhibiting that this method can effectively overcome the problem of the slope of the bi-line of fatigue thermographic data not being obvious,thus enabling better applicability.(3)A high-cycle fatigue energy dissipation model of the butt joints based on the energy conservation equation was proposed.Given the asymmetric heat distribution on both sides of the butt joints with unequal thickness,a calculation model of the high-cycle fatigue energy dissipation of the butt joint was deduced from the perspective of the energy conservation equation by using the Q310NQL2-Q345NQR2 butt joints with 2.5 mm+4 mm plate thickness as the research object.Combined with the representative volume element(RVE)and damage theory,the energy dissipation under different load levels was analyzed,and the high-cycle fatigue damage mechanism of the butt joints was explained from the perspective of microstructure movement.Then,the dividing point of recoverable microstructure movement and non-recoverable microstructure movement within the material was used as the feature point for fatigue limit prediction,thereby reaching the rapid and accurate prediction of the fatigue limit of the Q310NQL2-Q345NQR2 butt joint.Simultaneously,to verify the model’s applicability for the equal thickness butt joint,this model was also validated by Q460 equalthickness butt joints.Considering that when the load level exceeds the fatigue limit of the material,there is a threshold of energy dissipation related to damage,a fast prediction model of fatigue life of butt joints with unequal thickness based on energy dissipation was established,thus realizing the rapid prediction of S-N curve.Finally,the slope and intercept of the fitted SN curves based on the predicted and the tested data were compared,and the results showed that the error values of the two fitting lines are 7.53% and 6.07%,respectively,which validates the feasibility of the developed model.(4)Based on fatigue damage entropy,a high-cycle fatigue performance characterization model of the butt joint was created.The entropy balance equation and the calculation expression for entropy production suitable for the butt joint under high-cycle fatigue regime were deduced using the heat balance theory of fatigue process.On this basis,this model considers two types of microstructure motion with entropy increase under high-cycle fatigue regime,one is the anelastic motion that has nothing to do with damage,and the other is the microplastic motion associated with damage accumulation.By analyzing the dependence of thermodynamic entropy production related to microstructure motion and load during the high-cycle fatigue process,a maximum slope transition and bi-line compound method was established based on the bi-linear method and maximum slope transition method.The fatigue limit was rapidly realized using the proposed method.Then,the total entropy related to microplasticity during the high-cycle fatigue process was defined as fatigue damage entropy,and the fatigue damage entropy under different load levels was calculated.The results showed that this value is a constant independent of load.When the entropy associated with cumulative damage reaches this constant(threshold),fatigue failure occurs.Based on this,a rapid fatigue life prediction model based on fatigue damage entropy was constructed and compared with the test results.The results suggested that the slope and intercept errors of the S-N curves fitted by predicted data and test data are 15.50%and 9.99%,respectively,verifying that the model has good accuracy.This enables the fatigue characterization of welded structures in essence and then supports a new train of thought for realizing a rapid and accurate fatigue evaluation. |
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