Prediction Of Fatigue Life Of Pump Head Material's Notched Specimen Based On The Theory Of Critical Distances

Fracturing technology is a common means to improve oil recovery,and its key equipment is fracturing pump.As the hydraulic end of the whole system,the pumphead bears the effect of high-pressure circulating load in the work,coupled with the corrosion of fracturing fluid,which often leads to the fatigue failure of the pumphead before it reaches its service life.This problem has greatly restricted the exploitation of oil and gas,so how to estimate the fatigue life of fracturing pump more accurately has become a hot research issue.The structure of pumphead is very complex,so it is difficult to directly carry out fatigue test on its body or the equal proportion model.In engineering,the notched specimen with the same stress concentration coefficient as the fatigue failure area of pumphead is often taken as the research object.The problem of predicting the fatigue life of pumphead is transformed into predicting the fatigue life of notched specimen.At present,the commonly used fatigue life prediction methods include nominal stress method,local stress variation method and stress field strength method.These methods are widely used in practical engineering,but they have their own problems.The critical distance method is a new method for fatigue life prediction,which has the advantages of simple application and small error.Therefore,it is of great theoretical and engineering significance to study the application of critical distance method in the fatigue life prediction of various notched parts.In this paper,the theory ofcritical distance is taken as the research object.Based on the idea of critical distance,the correction ideas for point method and line method are put forward,which are applied to various types of notched specimens.The influence of the geometrical parameters of V-notch specimen and double V-notch specimen on the effective stress?_effof TCD is qualitatively analyzed by using the finite element simulation method.The prediction effect of the traditional critical distance method on the fatigue life of various notch specimens is studied,and the correction idea proposed in this paper is verified.The main contents and research results are as follows:1. The mechanical test and fatigue test of a chromium nickel alloy material for pumphead are carried out.The mechanical property parameters and fatigue data of the material are obtained.The S-N curve formula of the material is fitted.The fatigue test of notched parts with stress concentration coefficient K_T=3 is carried out.The reliable fatigue data are obtained at the stress level of 10k N.2. Based on the von Mises and Tresca stress fields in the stress concentration region of notch model,the concept of characteristic length L is extended and defined.Based on the two parameters of ultimate bearing capacity and inherent strength,the correctness of critical distance theory is tested.The critical distance method is used to predict the fatigue life of V-notch specimen,45 steel double u-notch specimen,45 steel half round notch specimen,aluminum alloy double hole specimen and double notch specimen.Compared with the traditional method,the error of critical distance method is smaller,which verifies the applicability of this method.3.30 kinds of V-notch models with different geometric dimensions and 120 kinds of double V-notch models are established.Based on the numerical analysis of ABAQUS,the relationship between the effective stress?_effof TCD and the geometric factors of notch?notch depth H,fillet radius?at the root of notch and gap spacing D?is qualitatively obtained.4. The point method correction formula based on lazzarin and the line method correction idea based on the ratio of the"focusing path"length a to the material characteristic length L are proposed and verified in various test pieces.The results show that the accuracy of the traditional line method can be greatly improved by using the appropriate correction method.