Research On Crack Propagation And Fatigue Life Of Corssbores Structure Of Fracturing Pumphead

As the exploitation of conventional oil and gas becoming more and more difficult,the fracturing technology is used widely in the stimulation of conventional oil and gas and exploitation of unconventional oil and gas.Fracturing pumphead as the key equipment of fracturing technology has a great influence on the performance of fracturing technology.Pumphead as a core component of fracturing pump,the crack failure at the intersecting line has become its most common failure mode and greatly decreases its fatigue life.Therefore,in this paper the crossbores structure,the key failure structure of pumphead,is selected as the main research object.Based on the liner elastic fracture mechanics,the fracture parameters and stress field of crack tip,the fatigue crack propagation process is researched in this paper.And the relative results can provide a reference for the structure design and life prediction of pumpheadFirstly,the research on stress distribution of crossbores without crack is performed by theory analysis and numerical simulation.The results show that,the maximum stress is always locating at the middle point of intersecting line where the crack is most likely generated.The calculation equation of maximum circumferential stress of crossbores is built based on its theory solution and numerical solution.Based on the analyzed model of crossbores without crack,the plug-in program of crack analysis on crossbores is developed by Python.Considering the characteristics of crossbores and crack,four key parameters,thickness-diameter ratio Y,crossbores diameter ratio Q,crack length a?which is normalized as??,axis ratio of elliptical crack face b/a,are introduced to research their influence on stress intensity factor K_I of crack tip.The results show that the distribution of K_I along the crack front is mainly influenced by the Q and b/a.And the influence of b/a is more obvious.The K_Imax?maximum K_I along the crack front?normally locates around the two chamber inner surfaces.When b/a=1,the K_Imaxmax locates at the inner surface of sidehole,and when b/a?1,the K_Imaxmax locates at where the short axis of crack face is.The magnitude of K_I along the crack front is in inverse proportion to Y and is in direct proportion to Q and a.When the crack face is elliptical,as the increase of b/a,the change of magnitude of K_I around the short is slight while that around the long axis is obvious.In addition,the relationship between the stress filed of crack tip of crossbores with crack and the stress distribution of crossbores without crack is built by redefinition the nominal stress?₀.Generally,the fracture failure of components always begins from the place where the K_Imaxmax is,so a research on K_Imaxmax is also performed in this paper.Based on the orthogonal design method and variance analysis,the significance of parameters on K_Imaxmax is obtained.The results show that,the influence of?Y,Q,??on K_Imaxmax is highly significant while the influence of?b/a?is not significant.And the influence order of parameters on K_Imaxmax is Y>Q>?>?b/a?.Based on the results,the full-test calculations including 216 sets data of K_Imaxmax about factors Y,Q,?are performed.And the calculation equation of K_Imaxmax is built consequently.For problems considering the crack,the influence of all kinds of parameters on the crack propagation process is essentially derived from the stress distribution of crack tip.In this paper,the stress filed of crack tip of crossbores is analyzed by theory analysis and numerical simulation.The results show that,for crossbores with complicate inner chamber structure,the influence of stress high order terms t₁₁1 and t₃₃3 cannot be neglected.But the influence of stress high order term of opening stress is not obvious and can be neglected.Based on this results,the fracture criterion of crossbores is built according to the critical stress model.According to the experimental standard GB/T21143-2007 and GB/T6398-2000,the CT sample is adopted in the experiments to measure the relative fatigue crack propagation parameters of material 30CrNi2MoVA which is used in the pumphead.Through introducing the one-side tolerance factor,the material p-?-J-?a curve and p-?-da/dN-?K curve with specified reliability p and confidence?are obtained.And based on the curves,the fracture toughness K_Icc and fatigue crack propagation parameters C and m with specified p and?are obtained.Furthermore,the material fatigue crack growing curves and threshold value K_thh under the loads with different stress ratio R are measured.And the function relationships between da/dN,K_thh and R are respectively obtained.Based on the fatigue crack growing parameters(K_IC,C,m,K_th)measured by the experiments,the fatigue crack growing curve?a-N curve?of corssbores and pumphead are calculated by combining the submodel method and remeshing method.The results show that,the crack type of crossbores is mainly the mode-I.During the crack growing process,the crack tip around the two chamber inner surface grows faster,while the crack tip embedded in the structure grows slower.Consequently the shape of the crack face is gradually developed into a triangle.The results show that,various randomness existing in the process of experiments has greatly influence on the predicted fatigue life.And as the increase of stress ratio,the influence of random factors become more and more significant.Whether the initial crack shape is circle or elliptical crack,during the crack propagation process,the crack face would develop into a similar shape with each other.Based on the researches on crossbores,the analysis model of 5ZB2800 type fracturing pumphead is built.The stress distribution and crack propagation progress are studied in this paper.The results show that,the change of crack face during the crack propagation of pumphead is similar with that of crossbores,but the change of K_I of pumphead is more complicated.In the high stress zone of pumphead,K_I increases with the crack length a;in the low stress zone,K_I decreases with the crack length a.But around the step structure at the inner face of pumphead,the K_I appears fluctuations due to the discontinuity of the structure.Combining the calculation equation of K_Imaxmax and material fatigue crack growing parameters,the calculation function on a-N curve of pumphead and crossbores is built.But the function is more suitable the situation when the crack length a<0.5T_w.Based on the linear elastic fracture mechanics,the fracture parameters,stress filed of crack tip and the fatigue crack propagation are presented in this paper.The influences of four key parameters on the stress intensity factor K_I are obtained and the calculation equation of maximum K_I is built.The material fatigue crack propagation parameters of30CrNi2MoVA are measured by the standard experiments,which laid the foundation of the research on fatigue crack propagation and fatigue crack propagation life.Combining the Paris equation,material parameters and K_Imaxmax calculation equation,the calculation method of a-N curve of crossbores is built and applied into the crack propagation of pumphead.