| With the increasing difficulty of shale oil and gas,tight oil and gas,coalbed methane and other unconventional oil and gas fields,the exploration process is facing many difficult challenges.As the "heart" of oil and gas field equipment,fracturing pumps are important supporting hardware.However,as the types of mining environment and working conditions are becoming more and more complicated,at present,the fracturing pump head body often must be replaced due to fatigue failure before the completion of oil and gas field mining,which causes huge losses to unconventional oil and gas field production.Fatigue and fracture damage is one of the important reasons for the main failure of the ultra-high pressure pump head body.It is the frontier,hot spot and difficult point of structural strength,fatigue and fracture research,and it has not been effectively solved so far.Due to the complexity of the working environment,the existing fatigue life prediction method of the pump head body is difficult to be closely integrated with the actual working conditions.In addition,the cost of the pump head body is expensive.According to field failure data and bench test statistics,the fatigue life of the same type of pump head body has a strong dispersion.Therefore,there is an urgent need to establish a reliable mathematical model for predicting the fatigue life of the pump head body.In general,the research system of the low fatigue life of the fracturing pump head and its prediction method is not perfect.It is urgent to analyze the failure mechanism of the fatigue cracking of the complex inner cavity of the fracturing pump head and establish a fatigue life prediction Only by evaluating the model,can the follow-up research work be carried out for the pump head body fatigue life improvement or structural design.In this paper,the following research work is carried out on the research object of the internal cavity structure model of the fracturing pump through the combination of basic experimental research,theoretical analysis and numerical simulation:(1)First,based on the national standards GB / T 3075-1982 and GB / T 3075-2008,the sample design,mechanical performance test and fatigue performance test of the three pump head body materials were carried out,and based on GB / T 24176-2009 The test data was analyzed and analyzed,and the SN curves of the three pump head materials were obtained by fitting.On this basis,further research on fatigue performance testing methods to reduce costs and save time,based on the "probability quantile consistency principle",proposed a new K value(search to be determined coefficient)search method and standard deviation determination method,Combined with matlab,C language was used to compile the plug-in program of K value search.Finally,a method of fitting P-S-N curve with small sample data was established.The method was compared and verified with the P-S-N curve fitted by group method.The results showed that the new method The error is within the acceptable range,which is a small sample test method with high reliability and saving manpower and material resources.(2)Based on the influence factors of the traditional nominal stress method,the notch effect was first studied,and the theoretical calculation formulas of the theoretical stress concentration factor and the fatigue notch coefficient were analyzed,and it was initially obtained that the peak stress is not the only factor for fatigue failure and fatigue The failure is related to the inner grain size;secondly,the stress gradient effect is studied,and the calculation formula of the relative stress gradient of several typical notched parts is summarized and verified;the influence of the surface processing coefficient is analyzed,designed and processed Fatigue specimens were laser-strengthened,and the test results showed that the surface roughness of the parts was a key factor affecting fatigue life,and also verified the initiation mechanism of fatigue cracks and the preliminary judgment of fatigue failure mechanism;the calculation formula of size effect was summarized,And comprehensively modified the influence coefficients of various factors of the traditional nominal stress method to obtain a comprehensive correction coefficient,applying the nominal stress method to the fatigue life prediction of the ultra-high pressure pump head body,the results show that the error between the estimated life and the actual life is 3 times Within the dispersion zone,the nominal stress method was verified Consider fatigue failure mechanism just think the role of peak stress is not enough,but also for the analysis of the various factors of stress field intensity approach may be more appropriate body of the pump head high cycle fatigue life prediction foundation;(3)Based on the notched stress field equations of Neuber,Lazzarin and Filippi,the stress field equations(Glinka,Kujawski,Shin,Xu,Chen,Pan,Creager and Paris)of famous scholars at home and abroad were deduced and summarized,designed and established A total of 24 three-dimensional finite element models of four notch opening angles ? = 60 °,45 °,30 °,0 °,six notch radii(ie ? = 0.015,0.03,0.05,0.1,0.15 and 0.25 mm),The numerical analysis method was used to calculate the maximum stress distribution law at the root bisector of the notch,and the numerical simulation results were compared with the equations proposed by the above scholars.The advantages and disadvantages of various equations under different notch forms were summarized.(4)According to the principle of the traditional stress field strength method,six different geometric sizes are designed and established(six test pieces are composed of central elliptical holes with different sharpness degrees,where ? is the radius of the notched tip and l is half of the plate length,The ratio of plate length to notch radius is divided into six types l / ? = 100,80,50,40,20,10)Three-dimensional finite element model of finite plate,linear elastic stress analysis of six different sizes of models,Elastoplastic stress analysis,combined with numerical calculation results,analyzes and summarizes the deficiencies of the traditional stress field strength method.On this basis,the concepts of "no boundary damage area" and "invisible boundary" are proposed;the mathematical model of the weight function in the traditional stress field strength calculation model is modified,and the establishment of geometric equations and grain size coefficients and other factors are established A new mathematical model of weight function is developed;an improved mathematical model of stress field strength method is established by comprehensively considering the stress distribution function,weight function,field diameter and invisible boundary.The solution method of fatigue life of the new stress field strength method is much more optimized than the original stress field strength method.Finally,multiple sets of stress tests of double holes(round holes and elliptical holes)are performed at the stress level.The test results show that the new model has a fatigue life The estimated accuracy is better than the original model.(5)Based on the dynamic behavior of material memory performance degradation,the dynamic residual SN curve is defined.Combined with the mathematical model combined with Ebbinghaus curve,the mathematical model of the memory function originally defined by the exponential function is improved to a power function.New linear fatigue cumulative damage model.The new model was compared with four models(Miner's Law,Corten Model,Kwofie Model,and Peng Zhaochun's new cumulative damage model).The two-level loading test data of the two materials 30 Ni Cr Mo V12 and 30 Cr Mn Si A were verified.The results show that the new model is in The prediction accuracy is very high.The reliability of the remaining fatigue life assessment of the pump head body(SJB-03 type pump head body)under the high-low loading two-stage working condition was also verified. |
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