| The oil well pipe will be damaged by corrosive medium and alternating load in service condition,and the interaction between the two will accelerate the failure,reduce the fatigue performance of the oil well pipe,and affect its service life.Therefore,it is of great practical significance to study the corrosion fatigue behavior of oil well pipe materials and to analyze the reliability of their life.This paper focuses on the life prediction and life reliability analysis of oil well pipe materials under the interaction of alternating load and corrosive environment.Based on the experimental analysis and the existing fatigue life prediction model,a set of corrosion fatigue reliability research method for oil well pipe materials is established,which provides theoretical and data support for the safe service of oil well pipe.The test material was C110 casing steel commonly used in oil exploitation.Corrosion fatigue tests are carried out under different stress levels,Cl~-concentration and loading frequency.Combined with the experimental method and analytical method as the specific implementation method,and using the knowledge of statistics and probability theory,the corrosion fatigue life assessment and reliability analysis of C110 steel were carried out,and the main conclusions were as follows:(1)The cyclic hardening/softening characteristics of C110 steel are closely related to the concentration of Cl~-.The lower the concentration of Cl~-the more significant the cyclic hardening phenomenon of C110 material is at the initial stage of cyclic loading.With the increase of Cl~-concentration,the softening rate of the material in the later cycle is faster.(2)The corrosion fatigue life of C110 steel is related to stress amplitude,Cl~-concentration and loading frequency.The results show that under the same stress ratio,the corrosion fatigue life of C110 steel increases significantly with the decrease of the stress level,and the decrease range of the corrosion fatigue life of C110 steel decreases gradually,and the corrosion fatigue life curve tends to be flat.At the same frequency,the general trend of corrosion fatigue life of C110 steel is inversely proportional to Cl~-concentration.In different concentrations of corrosion solutions,the corrosion fatigue life of C110 steel increases with the increase of loading frequency,and the increase increases continuously.By comparing the influence of Cl~-concentration and load cycle frequency on the corrosion fatigue life of C110 steel,it can be concluded that there is a certain competitive relationship between corrosion damage and fatigue damage.(3)The corrosion fatigue life of C110 steel is log-normal distribution by testing the normal distribution of corrosion fatigue life.The expression of the median stress life was obtained by fitting the median corrosion fatigue life obtained by the group method.Basquin model and Weibull three-parameter model are used to fit the corrosion fatigue life of C110 steel.The comparison shows that Basquin model has higher fitting accuracy and is more suitable for describing the S-N curve of corrosion fatigue of C110 steel.(4)Basquin model was used to further obtain the P-S-N curves of C110 steel with 95%confidence level and different reliability under different loading stresses. |
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