The Study On Residual Stress And Fatigue Life Of Autofrettage Super-high Pressure Pump Head

With unconventional oil and gas resources become the direction of new energy development, the hydraulic fracturing technique put forward the higher performance requirements such as super-high pressure, large displacement and long life to fracturing units. Cross hole inner cavity structure of pump head is complex, and its fatigue life is low and easily produce fatigue cracking under the action of super-high pressure pulsating cyclic. Autofrettage technology is an effective method to improve the fatigue life and carrying capacity of pressure vessel by using residual stress. The hydraulic autofrettage technology treatment of the pump head can produce a favorable residual stress in its inner cavity. Thus, the carrying capacity and fatigue life of inner cavity of pump head can be improved greatly. Therefore, it is of great significance to study the residual stress and fatigue life of super-high pressure pump head.With pump head of 3000 fracturing pump as the object of the research in the dissertation, accurate autofrettage residual stress distribution field of pump head was acquired by the development and application of mixed hardening elastoplastic constitutive UMAT. Based on the analysis of multiaxial fatigue critical plane theory, the optimum autofrettage pressure of pump head was gained. Furthermore, according to theoretical analysis and analogy test, the improvement of fatigue life has carried on the quantitative evaluation after autofrettage process. At last, Influence of autofrettage residual stress on fatigue life of the pump head was studied from the two aspects of crack initiation life and crack propagation life. Thesis’ s main work done is as follows:(1) Autofrettage Residual Stress Analysis Based on the mixed hardening constitutive model.Due to the mixed hardening law is more close to the actual hardening behavior of metal materials, the law is suitable for simulating autofrettage residual stress analysis of the reversed loading. The back stress updating algorithm, the stress compensation updating algorithm and the uniform stiffness matrix updating algorithm were applied to the development of mixed hardening elastoplastic constitutive UMAT. Through reliability verification and comparison with tensile test simulation, it is found that the mixed hardening UMAT can well simulate the strain hardening and Bauschinger effect of the materials, and the calculation accuracy is high. Mixed hardening elastoplastic constitutive UMAT is suitable for accurate analysis of autofrettage residual stress field of pump head.(2) Study on the Optimum Autofrettage Pressure of Pump Head.The failure mode of the pump head is determined as high cycle fatigue under multiaxial stress by the failure analysis for engineering applications and the limit state stress analysis of the pump head. Taking the plastic strain as the criterion, and under the premise that pump head doesn’t occur reverse yield, the optimum autofrettage pressure of pump head was obtained by maximum static strength criterion,and the influence of multiaxial fatigue critical plane and residual stress of the pump head on fatigue life was token into account, and a kind of new method which makes sure optimum autofrettage pressure with the criteria of maximum multiaxial fatigue life of stress concentration point was put forward. The equivalent stress peak value of the inner cavity of pump head was obviously decreased and the stress distribution was more uniform after the optimum autofrettage pressure treatment.(3) Fatigue Life Analysis and Experimental Research of Autofrettage Pump Head.According to stress fatigue analysis theory, the S-N curve of the pump head was got by the test and the maximum likelihood estimation method was used to obtain P-S-N curve of 99.9% confidence level. On this basis, the gap circle was designed to simulate stress concentration of the inner cavity of pump head, and it is found that the autofrettage circle fatigue life is twice than not autofrettage circle by theoretical analysis and top pressure fatigue test. According to ABAQUS and MSC.Fatigue numerical analysis software, the whole life of the pump head was analyzed by using Signed von Mises stress components and considering the effect of autofrettage residual stress and choosing average stress Goodman correction method. Results show that pump head fatigue life can be increased to 3.1 times after the autofrettage process, and the analysis results is the most conservative based on the estimated S-N curve, and the fatigue life was the biggest based on the test S-N curve.(4) Effect of Autofrettage Residual Stress on the Fatigue Crack of the Pump Head.Influence of autofrettage residual stress on fatigue crack of the pump head was studied from the two aspects of crack initiation life and crack propagation life. The pump head autofrettage treatment increases the yield strength of stress concentration point, gets the three-dimensional residual compressive stress, and reduces the stress ratio. The change of three aspects can effectively improve the crack initiation life of materials. When the minimum stress intensity factor of the crack tip is greater than zero, autofrettage residual compressive stress decreases the crack propagation rate by the method of lowering the payload ratio. When the minimum stress intensity factor of the crack tip is less than zero, autofrettage residual compressive stress decreases the crack propagation rate by the method of reducing the driving force of crack propagation.