Behavior Of Notch Fatigue Of S135 Drill Dipe In Organic Salt Drilling Fluid

A drill pipe is an important component in oil and natural gas exploitation and geological exploration,but fatigue is an important failure form of a drill pipe.In recent years,downhole environment also become more serious and complex especially in deep wells and ultra-deep wells and the failure of drill pipe caused by coupling stress and environment occurs frequently.The drill pipes often serve in complex environment involving multiple factors,such as cyclic loading,corrosive medium and ion,notch,and temperature.In the complex environment of multiple factors,the sensitivity of the material fatigue,the fracture strength and the mechanism are largely different from those under single factor and the probability of drill pipe failure was higher in the complex environment of multiple factors.Therefore,we tested and evaluated of corrosion fatigue sensitivity of high-strength drill pipe steel in the complex environment of alternating load,corrosion medium,notch and temperature,and studied the fatigue performance and made it clear the mechanism of crack initiation and propagation,which was a great significance for assessing safe fatigue lives of high-strength drill pipe in the complex environment.In this paper,the S135 material of drill pipe was used as the research object.The fatigue samples such as smooth and notched specimens that were designed to conduct test were obtained on a joint of drill pipe,to study the notched fatigue behavior of S135 in organic salt drilling fluid at different temperatures.The main research contents were as follows:Firstly,based on S135 fatigue test,it was optimized and evaluated applicably for five kinds of probability distribution model(such as normal distribution,logarithmic normal distribution,Weibull-2/3 distribution and exponential distribution)and three kinds of S-N models(Basqiun S-N,exponential S-N and three-parameter power function S-N),selecting S-N models suitable for different environments and processing datum.Secondly,in view of the stress concentration phenomenon of the threaded joint of the drill pipe and the geometric shape and stress concentration factor(Kt)of the threaded joint,two kinds of smooth and notcheds amples were designed and conducted to test in air,to study effects of Kt on the fatigue performance of S135 and R(root radius)on the fatigue notch sensitivity and to observe the fracture morphology using scanning electron microscopy(SEM),which revealed the mechanism of crack initiation and propagation and provided some references and theoretical guidances for the optimization design of the threaded joint of high-strength drill pipe.Thirdly,according to the working environment of the drill pipe,the fatigue tests were designed in the complex environment of alternating stress,corrosive medium(organic salt drilling fluid),notch and temperature.It was studied that the influence law of notch,corrosive medium,temperature and alternating stress on 5135 fatigue lives and the relationship of fatigue frachure for between multiple factors,and the SEM was used to observed the fracture morphology of corrision fatigue under coupling multi-factor in complex environment and deeply analyzed for the mechanism of crack initiation and propagation.However,article established the testing method of rotating bending fatigue under coupling stress and environment,revealed the relationship of fatigue fracture as well as the mechanisms of fatigue failure of high-strength 5135 steel under coupling multiple-factor in complex environment.