Study On The Corrosion Fatigue Resistance Degradation Model And System Reliability For Transmission Towers

Since UHV transmission towel-line system becomes a focus in power grid constructions,high strength steels such as Q420 B have been widely used.Considering that the transmission tower is a typical wind load sensitive structure with harsh wind environment,the resistances of its components are declining under the coupling effect of the acid rain environment and wind-induced vibration,which may cause the structure fail to meet the design safety standard in later period of service.Therefore,it is of great significance to evaluate the safety of transmission tower under extreme occasional loads in later period of service.In view of this,the corrosion fatigue test about Q420 B steel material and angel steel components,the residual strength degradation model and the system reliability analysis methods are studied in this research,the residual strength degradation model for transmission towel components and the global reliability analysis method are developed,which can provide experimental data foundation and theoretical analysis basis for subsequent related research.This study mainly includes the following aspects(1)Corrosion fatigue coupling loading device is designed to realize the simultaneous exertion of corrosion and fatigue effect,the corrosion fatigue life test and corrosion fatigue residual strength degradation test are conducted for both Q420 B steel material and angel steel component,where the fatigue and corrosion-fatigue test mode are applied to simulate the actual service status of transmission tower components.According to the test results,the corrosion fatigue life model and the residual strength degradation data for Q420 B steel material and angel steel component are obtained,which can provide experimental foundation for the residual strength degradation model.Besides,the influences of the PH of corrosive solution in salt fog-box and spray interval mode on the corrosion-fatigue life are discussed.(2)As for the residual strength degradation model,the difficult in the classical model under multistage stress level are illustrated,then,a m odel based on the instantaneous degration velocity are introduced,which can overcome the above-mentioned difficulties.However,this model has applicability problems and ignores the residual strength degradation test data.Therefore,in this research,new parameters ? and S0 are introduced to this model,and the improved model has wider applicability and can make full use of both informations about the SN curve and the residual strength degradation test data,the parameter fitting method according to different type of informations for the improved model are also proposed.Next,the improved model are validated based on the test data from both existing researches and this study,and the fitting precision of the improved model with different type of information are compared.Finally,the residual strength degradation model for Q420 B steel material and angel steel component are built according to the improved model and the residual strength degradation data in this research.(3)As for the system reliability analysis,first,a unified algebraic equation based on the Mehler's formula for equivalent correlation coefficient is proposed,which is applicable for different incomplete probability systems.Second,introducing linear reference variables and reasonable truncated terms technology,an effective simplified method for equivalent correlation coefficient is proposed.Third,considering existing system reliability analysis methods can not handle the static load with variable value,the shakedown theory is introduced,the shakedown state is set to be the limit state,and the concept of global reliability is defined.A practical and effective analysis method for global reliability is proposed,which can deal with member structures under static load with variable value,it is worth pointing out that,since static load with constant value is a special case of static load with variable value,the proposed method is also applicable for the classical system reliability analysis.Finally,considering the residual strength degradation in engineering structures,the residual strength degradation model with time-dependent distribution parameters and equivalent extreme-value event are introduced,and the proposed method are expanded to time-dependent reliability analysis domain.(4)Based on the proposed time-dependent global reliability analysis method and residual strength degradation model,the time-dependent global reliability of a transmission towel is evaluated.First,wind-induced vibration time history analysis for the structure is conducted.Then,based on the results of wind-induced vibration time history analysis and the residual strength degradation model of Q420 B angle steel,the resistance degradation model of each component for transmission tower is established,and corrosion effect is also considered.Finnaly,the time-dependent global reliability of the transmission towel for different time interval is compared.Through this example,the complete process of establishing the component resistance degradation model and calculating time-dependent global reliability by the proposed method is illustrated.Finally,the main conclusions and innovation points are summarized,and the directions for future study are also discussed.