Fatigue Life Prediction Of Pump End Material Based On Corrosion

Corrosion fatigue is one of the main failure modes of fracturing pumps in oil and gas fields.It is found that the inner surface of the fracturing pump end is covered with corrosion pits,which aggravates the stress concentration and promotes the crack initiation and development.Therefore,it is necessary to analyze the influence of corrosion environment on the fatigue life of the pump head and its materials by studying the effect of corrosion pits on the stress concentration.In this paper,the equivalent stress distribution of the complete model of triplex pump end under the actual working condition is analyzed,and the stress concentration factor at the dangerous place is obtained,and the notched specimen model with the same stress concentration factor under 100 MPa working pressure is established.The P-S-N curve of a pump body material is fitted by the improved geometric method and the improved Projection-Bootstrap-Bayes method,which is the basis of the fatigue life prediction of the specimens with corrosion pits and notches.The influence of the size,location and number of corrosion pits on the stress concentration is analyzed by using the finite element analysis software,and the method of predicting the stress concentration factor of single corrosion pit and multiple corrosion pits with different sizes on the specific notched specimen is proposed.Finally,the P-S-N curve of the material is modified by the formula,and the relationship between the fatigue life and the stress concentration coefficient of the notched specimen with corrosion pit is obtained,and the fatigue life prediction is completed.In order to obtain the more accurate stress concentration factor of the dangerous part of the fracturing pump end,the paper analyzes the stress of the three cylinder pump under six working conditions.The results show that the life prediction of the pump head body is conservative if the three cylinder pump is simplified to half cylinder model.The maximum stress on the surface of the inner cavity is the largest among the three drain cylinders when the working cylinders pass through after the fracturing fluid is collected,so the stress concentration factor at the dangerous position is selected to establish the notched specimen model.In order to improve the fitting accuracy of P-S-N,an improved method suitable for the fatigue test data of this paper is proposed by improving the geometric method and Bayes method,and the corresponding P-S-N curve is fitted.In this paper,the process of improving the geometric method by using sample clustering method,fuzzy mean and fuzzy variance is proposed.Other fatigue test data are consulted.The improved geometric method is applied to the data,and the fitting accuracy is found to be higher than the original geometric method.At the same time,using projection method and Projection-Bootstrap method as prior information,this paper proposes Projection-Bootstrap-Bayes method,and finds that its fitting accuracy is higher than projection method and projection Bayes method.In the end of the paper,the improved geometry method and bootstrap Bayes method are used to fit the P-S-N curve.By the life prediction software,it is found that the predicted fatigue life of the pump head body is in line with the actual production.In order to understand the effect of corrosion pit on stress concentration of notched specimens,the change rule of stress concentration coefficient is studied from the angle of size,location and number of corrosion pit.Taking the size of corrosion pit as the research object,it is found that the width has little effect on the stress concentration coefficient at the notch.Therefore,to some extent,the stress concentration coefficient with the aspect ratio of 1 under each length can be used to characterize the value of all aspect ratios under each length.Taking the location of corrosion pit as the research object,it is found that any corrosion pit mainly affects the stress fringe area of the notch.Taking the number of corrosion pits as the research object,it is found that the multi-directional distribution model of three corrosion pits can be simplified as the vertical distribution model of two corrosion pits,and the effect of multi-directional distribution of multiple corrosion pits on stress concentration can be simplified as the effect of two corrosion pits on stress concentration.At the end of this paper,the stress correction formula from smooth specimen to notched specimen with corrosion pit is proposed,and the fatigue life of the notched specimen with corrosion pit is estimated.The relationship between the fatigue life and the stress concentration coefficient of the notched specimen with corrosion pit is obtained.