Study On Variable-Amplitude Corrosin Fatigue Life And Cumulative Damage Models For Oil Well Pipe Materials

Oil well pipes are an important part of the oil and gas industry.Its service conditions are extremely complex,not only subjected to random loads such as bending and vibration,but also affected by corrosive media in the surrounding environment.The combined effect of mechanical factors and environmental factors often causes oil well pipes to fail due to corrosion fatigue damage,resulting in significant losses.Therefore,it is of great significance to study the fatigue life law of amplitude-corrosion of oil well pipe materials.The cumulative damage model plays an important role in the study of the variable amplitude-corrosion fatigue life law of oil well pipe materials.This article will analyze the cumulative damage model widely used in engineering.Firstly,the multi-sample constant amplitude corrosion fatigue test at four stress levels was carried out on the oil well pipe materials TP140 steel and C110 steel to analyze the probability distribution characteristics of corrosion fatigue life.The fatigue crack initial life model is used to fit the test data to obtain the S-N curve,which is used as the characteristic curve of the material.Then,design the variable amplitude load spectrum,perform the variable amplitude corrosion fatigue test of TP140 steel alternately loaded with large and small loads according to the load spectrum,and perform the multi-level load high-low load and multi-level load low-high load variable amplitude corrosion fatigue of C110 steel Test to obtain the test measurement life of the sample.At the same time,the literature cited the corrosion fatigue test data of the low-carbon and low-alloy steel steel whose corrosion environment is atmospheric.Finally,according to the load spectrum,combined with different cumulative damage models to predict the life of the specimen.The cumulative damage model is analyzed based on the comparison and verification results of the test measured life and the model predicted life.The following conclusions are obtained: The TP140 steel and C110 steel have the same amplitude corrosion fatigue life under the same stress level,which all follow normal distribution and lognormal distribution.In the variable amplitude test with alternating loading of large and small loads,the predicted life of the model obtained by using the Miner criterion and Carten-Dolan theory is basically the same as the average value of the test life of the sample.Its pre-accuracy is higher than that of thenonlinear cumulative damage correction model.However,the application process of the Miner criterion is simpler and easier to operate.In the multi-level load high-low load variable amplitude test,the prediction accuracy of Carten-Dolan theory is higher than that of the other two models,but the error is still large.In the multi-level load low-high load variable amplitude test,the prediction accuracy of the Miner criterion and the nonlinear cumulative damage correction model is higher than that of the Carten-Dolan theory,but the calculation process of the nonlinear cumulative damage correction model is more cumbersome,The principle is more complicated.Simultaneously,the analysis results of the cited test data model also prove the correctness of the above conclusions.