Autofrettage Pressure Research Articles

Fatigue and Autofrettage Analysis of Steel Sleeve Based on Precision Fit under Ultra High Pressure

Abstract In order to ensure precision fit between steel sleeve and plunger, copper sleeve and the other parts of the ultra high pressure plunger pump, as well as increase the fatigue life of steel sleeve, this paper made the analysis of work process and fatigue life of ultra high pressure autofrettage steel sleeve of plunger pump based on both analytical method and FEM. The elastic-plastic boundary radius, radial stress and plastic zone displacement were obtained based on the elastic-plastic mechanics analysis of ultra high pressure steel sleeve. The functional relationship built related to precision fit among autofrettage pressure, inner diameter expectation and displacement could ensure the machining dimension of inner diameter of steel sleeve and realize the precise control of inner diameter of steel sleeve. The analytical method and FEM for calculating fatigue life was also proposed, which are verified by experiment. The research shows that machining dimension is determined by inner diameter expectation in the autofrettage process to ensure precision fit; autofrettage treatment can reduce the working stress level of the steel sleeve and extend the fatigue life remarkably. These conclusions contribute to guide the design of steel sleeve.

Development of an efficient design optimization strategy for thick-walled cylinders treated with combinations of autofrettage, shrink-fit and wire-winding processes

Shrink-fit, wire-winding, and autofrettage processes are commonly utilized to enhance fatigue strength and durability of thick-walled cylinders across various mechanical applications. In this study, a novel practical design optimization methodology has been developed to determine the optimal configuration of a thick-walled cylinder, incorporating different combinations of shrink-fit, wire-winding, and autofrettage techniques. The objective is to identify the optimal layer thickness, shrink-fit interference, conventional autofrettage pressure, and reverse autofrettage pressure, if applicable, to maximize the compressive residual stress and minimize the tensile residual stress, thereby extending fatigue lifetime of the cylinder. First, different configurations of thick-walled cylinders, subjected to various combinations of reinforcement processes, are identified. A dataset of residual hoop stress profiles through the cylinder thickness is subsequently generated for these configurations based on the same manufacturing process. Neural network regression is effectively utilized to construct a single fitting function for the residual hoop stress profiles. A parametric study is performed to determine the optimal training functions, activation functions, and hyperparameters, achieving a remarkable agreement with the dataset, indicated by a coefficient of determination of over 0.97. A combination of Genetic Algorithm and Sequential Quadratic Programming algorithms is utilized to determine the accurate optimal values. Fatigue life analysis is subsequently conducted to estimate the fatigue lifetime of the optimal configuration. Results suggest that the optimal configuration, involving conventional autofrettage of the inner layer followed by shrink-fitting with a virgin layer and wire-winding the entire assembly, achieves a maximum fatigue life of 88 × 10⁶ cycles under cyclic pressure load of 300 MPa.

Strength and Failure Analysis of Fiber-Wound Composite Gas Cylinder via Numerical Simulation.

Based on the classical grid theory and related regulations, a structure model of a fiber-wound composite gas cylinder was designed in this paper. Based on the design results, a finite element model of a fully wound composite cylinder of an aluminum alloy inner liner with a working pressure of 35 MPa was established based on the ABAQUS software, and its stress distribution under working pressure and minimum burst pressure was analyzed. According to engineering experience, the pressure tolerance of composite cylinders can be improved by proper autofrettage pressure before working pressure, so the influence of autofrettage pressure was analyzed in this paper. The optimum autofrettage pressure was selected by setting the autofrettage gradient, and damage analysis was carried out on the cylinder with nominal working pressure of 35 MPa based on the Hashin failure criterion. The results show the initial damage sequence: matrix stretching occurs before the fiber stretching, and the damage generally starts from the spiral-wound layer. The tensile damage first appears in the transition section between the head and the barrel body, and the damage of the spiral-wound layer develops from the inner layer of the wound layer to the outer layer, while the damage of the circumferentially wound layer develops from the outer layer to the inner layer.

Strength and fatigue study of on-board type III cryo-compressed hydrogen storage cylinder

Due to its better energy storage density and lower costs for storage, cryo-compressed hydrogen (CcH2) storage provides a wide range of research potential. Based on the grid theory, The type III CcH2 storage cylinder's layup scheme is created using the working environment for on-board hydrogen storage. The failure of the composite layer of gas cylinders was determined using the maximum stress criterion, and hydrodynamic bursting tests on small-volume gas cylinders were utilized to confirm that the design method was appropriate. The innermost layer of the ring winding is where the largest fiber stress is found, according to simulation studies. The load-bearing capacity of the cylinder is improved and can reach 115.1 MPa with layers in the order of [90/±11.8/90]. The equivalent stress and fiber stress ratio of the cylinder continuously drop as the autofrettage pressure rises, while the cylinder's fatigue life gradually lengthens.

Fatigue life analysis of high-pressure seamless steel cylinder for hydrogen using autofrettage design

The seamless steel gas cylinders remain widely used for hydrogen refueling stations due to its intrinsic safety, especially for the situations with high pressure over than 60 MPa. When the internal pressure thick wall cylinder is subjected to high pressure even ultra-high pressure, autofrettage process is usually adopted to improve its bearing capacity and fatigue life. In this paper, fatigue life analysis was performed on the seamless steel cylinder for hydrogen using the autofrettage design approach. The stress distribution along the wall thickness direction before and after the autofrettage process were analyzed. Based on the ideal elastic-plastic material, the residual stress caused by the autofrettage pressure were obtained. On this basis, the expressions of residual stress and total stress along the cylinder wall in elastic-plastic deformation region were obtained, which will affect the fatigue crack growth rate (FCGR). Therefore, the fatigue crack growth tests of 34CrMo4 steel under different stress ratios (R) were carried out, especially for R < 0 to simulate the residual stress. Using the FCGR corrected with stress ratio, the fatigue life of the autofrettage high-pressure seamless steel cylinder was estimated and compared with the conditional cylinder. The results showed that fatigue life can be improved using the autofrettage design approach.

Strength optimization design of spherical hulls for deep-sea submersibles: A hydraulic autofrettage approach in external pressure vessels

The spherical pressure hull is the most important structure to ensure the safety of people in a deep-sea submersible. Its current main design trend is to increase the shell thickness to support greater depths and reduce overall stress. However, increased thickness greatly raises the submersible weight and the stress difference between the inner and outer surfaces. To achieve a balance between structural strength and weight, an autofrettage approach of strengthening the external pressure vessels has been studied. Firstly, an analytical solution of the autofrettage pressure of the spherical external pressure hull is derived based on elastic-plastic mechanics. Secondly, a finite element model is established to validate the analytical solutions and investigate the autofrettage process. Both theoretical and numerical results reveal that the overall stress in the spherical hull has been decreased significantly, and the stress distribution between the inner and outer surfaces becomes more even through the autofrettage approach. Additionally, a method has been developed to determine the optimum autofrettage pressure of the spherical hull for deep-sea manned submersibles which is found to be dependent on working pressure and shell size. This study could contribute to the strength optimization design of spherical hulls for deep-sea manned submersibles without increasing weight.

Evaluation of the endurance limit of notch intersections under inner-pressure loading

In this research paper we evaluate the potential of an ultra-high-strength steel for use in cyclically loaded, inner-pressure bearing components. Component-like specimens with intersecting boreholes were subjected to different grades of autofrettage pressure. The study investigates the material behaviour and describes it using a simple approach. Numerical analysis is conducted to determine the effects of autofrettage on endurance limits, crack arrest occurrences and the length of arrested cracks. The numerical results are compared to results from component-like specimen testing and the overall calculation procedure is found to be accurate. This research provides valuable insights into using ultra-high-strength steel for such applications.

Numerical Fatigue Life Evaluation With Experimental Results for Type III Accumulators

Abstract Type III accumulator is widely used in hydrogen stations. A high-cost pressure cycle test is mandatory to ensure the safety of the accumulator in present regulations. To reduce the high cost, the aim is to develop a methodology of numerical fatigue life prediction, where an axisymmetric finite element model for the Type III accumulator is created precisely and actual loading process including autofrettage pressure is simulated. The alternating stress intensity is evaluated based on the instructions in KD-3 of 2015 ASME Boiler &amp; Pressure Vessel Code, Section VIII, Division 3. By comparing stress amplitude distributions with the leak positions after the pressure cycle test, and plotting the results in the design fatigue curve, it could be shown that fatigue life prediction of Type III accumulator can be done by precise finite element analysis of the liner including dome part, where the principal axes of stress change in pressure cycle.

Loading-unloading of spherical and cylindrical cavities in cohesive-frictional materials with arbitrary radially symmetric boundary conditions

Many scenarios in geotechnical engineering involve cavity unloading from a loaded state under different boundary conditions, especially for thick-walled cylinder tests in laboratory conditions. A novel analytical cavity expansion-contraction solution is proposed for modeling thick-walled cylinder tests during a complete loading-unloading process under arbitrary symmetric boundary conditions. The proposed solution has potential implication in investigating ambiguous boundary effects for problems involving cavity expansion-contraction. The mechanical model considers two concentric layers of materials around the cavity, where the inner layer is considered as cohesive-frictional and the circumambient infinite layer is assumed as elastic to simulate the arbitrary radially symmetric boundary condition, including two extreme scenarios with constant-pressure and zero-displacement boundaries. The derived solution provides the evolutions of cavity pressure and distributions of stresses and displacements in both layers during the loading-unloading process, which is then rigorously validated against finite element results. The effects of boundary conditions and size of the cohesive-frictional layer have been comprehensively investigated by analyzing the cavity pressure and the developments of plastic zones during expansion and contraction. The developed solution has been proved to be applicable in geotechnical engineering, with comparisons between analytical results and experimental data for pressuremeter tests in calibration chambers and pile installation tests in a rigid container. The proposed solution might also be helpful in the analysis of autofrettaged pressure vessels.

Fluid End Blocks: Numerical Analysis of Autofrettage and Reautofrettage Based Upon a True Material Model

AbstractFluid end blocks (FEBs) are the most important components of hydraulic fracturing pumps. A potential important application of the hydraulic autofrettage process (HAP) is to strengthen the fatigue-prone FEBs. This creates a favorable compressive residual stress field near to the critical surface locations within the component and serves to increase its pressure-bearing capacity and/or improve lifetime. This requires a fundamental understanding and modeling of the complex mechanics of the HAP in order to accurately predict such residual stresses. The key outstanding modeling issue is the complex material behavior, dominated by the Bauschinger effect and associated with reversed yielding. This effect differs throughout the FEB. It has been modeled for plane axisymmetric cylinders but has not previously been incorporated into FEB analyses. In this paper, a newly developed finite element analysis (FEA)-based user programable function (UPF), featuring true material constitutive behavior, i.e., replicating an existing Bauschinger-effect characterization (BEC), is adopted to accurately simulate the HAP and quantitatively investigate the stress–strain evolution and residual stress fields throughout the FEB. This simulation is then compared with FEA modeling by a traditional bilinear kinematic hardening material model to indicate the importance of the accuracy of the material constitutive model in determining appropriate residual stresses and strains. An autofrettage pressure of 500 MPa generally achieves net compressive hoop stresses at each of four critical crossbore location. Finally, a prospective re-autofrettage sequence is described; approximate modeling suggests an improvement that might permit operation at higher working pressure.

Neutron diffraction, finite element and analytical investigation of residual strains of autofrettaged thick-walled pressure vessels

Residual strain distributions, positions of elastic-plastic juncture and 1st yielding etc. of autofrettaged thick-walled pressure vessels were revealed quantitatively using Neutron Diffraction (ND), Finite Element (FE) and Analytical solution. FE models were validated with ND results. Relationships between autofrettage pressures (Pmii, Pmao, Paopt) and load bearing capacity versus radius ratio K were respectively established via FE parametric study, by which optimal radius ratio K = 3 rather than K = 4 was identified for some existing vessels. Based on ND and FE results, new formula for calculating optimal juncture radius rjopt was deduced which narrowed the gap between analytical and FE results by 0.5 mm. Four coefficients Cj were proposed to modify conventional equations for calculating juncture radius rj, that facilitated a tool to address residual strains of autofrettaged pressure vessels. These results may be referred to for the design, manufacture, and proper application of the autofrettage technique onto pressure vessels and relevant engineering components.

Fatigue analysis of high-pressure hydrogen storage vessel based on optimum autofrettage pressure

To improve the fatigue performance of high-pressure hydrogen storage vessels, a method is proposed to determine the optimal autofrettage pressure of the vessel using numerical simulation and experimental verification. First, a finite element model considering the actual structure of the vessel is established based on grid theory. Then, the effect of autofrettage pressure on the equivalent alternating stress amplitude of liner under cyclic fatigue load is studied by using the modified equation of Smith–Watson–Topper mean stress. Lastly, the optimum autofrettage pressure is determined according to the DOT-CFFC and CGH2R standards, and the fatigue life is predicted. The results show that under the optimum autofrettage pressure, the bearing capacity of the liner under working pressure and the fatigue life under fatigue cycle load are increased by 74% and 12 times, respectively. The experimental results are in good agreement with the numerical simulation results, with an average error of 6.3%. This research has important reference significance for the preliminary design stage of composite vessels.

Investigation of Cracks on Internal Surfaces of Extruded Cold Worked Thick Walled Pipes of an Age Hardened Al-Alloy

Thick seamless pipes of hardenable aluminum alloys demand close geometrical tolerances as well as high quality surface finish which are met by cold drawing after a series of different thermo-mechanical treatments. To meet the requirements of critical applications the final product undergoes stringent quality inspection procedures. State of the art quality assessment can detect even minor isolated defects. The production facilities develop their quality criteria suitable for specific applications. The present study investigated minute defects on the inside surface of thick seamless pipes, proposed mechanism of their formation and suggested the impact of defects on the end use. The root cause analysis was conducted, and measures were suggested to control the defects. Thick extruded seamless aluminum alloy pipes underwent a series of different thermo mechanical treatments; the final dimensions with required tolerances and the surface finish were achieved by adopting a 2-step cold drawing process. Cold drawing generated residual stresses which resulted in the formation of cracks in the material, preferentially at the defects generated during solidification and/or extrusion processes. The final product underwent stringent quality inspection, and the material was rejected if cracks of size 3 mm or larger were detected. The die scratches or notches generated on the inside surface of the pipes, during extrusion are assumed to grow if subjected to high stresses during subsequent processes, e.g. cold working. Observations at high magnification in SEM helped to determine the morphology of cracks. Radiographic testing did not detect any crack in the bulk material. Particles with faceted features indicated the presence of inclusion. Inclusions were detected in the form of strings along the direction of cold drawing. Energy dispersive spectrometry in SEM was used to determine the composition of inclusion detected in the vicinity of cracks. Almost all the inclusions were rich in silicon, iron, calcium along with carbon; it indicated that the inclusions were trapped particles of fluxes, slag, and brick powder. Particles rich in Ca, Na and/or Cl indicated entrapped flux, Fe and Si were mostly coming from aluminum scrap and refractory powder while presence of carbon indicated entrapped extrusion lubricant. Inclusions rich in a large variety of unwanted elements indicated presence of slag particles. Numerical analysis was conducted to develop a model in FEM in which scratches of different depths were introduced and autofrettage pressure was applied to determine the stresses generated according to the established Von Mises Model; the latter was used to establish the yield criteria. Finite Element Modelling concluded that when cold drawing pressure was applied on a pipe with a single notch of depth 0.3mm or three notches of depth 0.1 or greater at different locations the Von Mises stresses approached the yield strength of the pipe.

Buckling analysis of thin-walled metal liner of cylindrical composite overwrapped pressure vessels with depressions after autofrettage processing

Abstract The cylindrical filament wound composite overwrapped pressure vessels (COPV) with metal liner has been widely used in spaceflight due to their high strength and low weight. After the autofrettage process, the plastic deformation of the metal liner is constrained by composite winding layers, which introduce depressions to the metal liner that causes local buckling. To predict the local buckling of the inner liner with depressions of the pressure vessel after the autofrettage process, a local buckling analysis method for the metal liner of COPV was developed in this article. The finite element method is used to calculate the overall stress distribution in the pressure vessel before and after the autofrettage process, and the influence of local depressions on the buckling is evaluated. The axial buckling of the pressure vessel under external pressure is analyzed. The control equation of the metal liner with depressions is developed, considering the changes in the pressure and the bending moment of the liner depressions and its vicinity during the loading and unloading process. Taking the cylindrical COPV (38 L) with aluminum alloy liner as an example, the effects of liner thickness, liner radius, the thickness-to-diameter ratio, autofrettage pressure, and the length of straight section on the autofrettage process are discussed. The results show that the thickness of the inner liner has the most significant influence on the buckling of the liner, followed by the length of the straight section and the radius of the inner liner, while the autofrettage pressure has the least influence.

Effect of autofrettage on the ultimate behavior of thick cylindrical pressure vessels

Major applications of thick cylinders are in the defense sector. Stress distributions especially under elastic-plastic conditions are more complex in thick cylinders. Though the cylinder may not be strained to ultimate in service, assessment of factor of safety is essential. Simulating elastic-plastic material behavior of cylinders using the uni-axial test data is more dependable and the finite element analysis (FEA) software has got this capability. A procedure using a suitable material model is demonstrated in this study utilizing the FEA software Ansys. The instability criterion followed in Svensson's pressure expansion relationship is used in the process of FEA. The predicted failure pressure is very close to the literature test data as well as few theories that have been highlighted. Moreover, thick cylinders are mostly subjected to autofrettage in applications like gun barrels to enhance service pressure and fatigue life. Hence it is attempted to simulate the development of residual stress due to autofrettage and its effect on the ultimate behavior on the pressure vessels. The residual stress developed is superimposed on the stresses due to internal pressure which causes the redistribution of stresses throughout the thickness. There is a marginal increase in burst pressure due to autofrettage. Though the effect could be made significant by increasing the autofrettage pressure, it gives detrimental effects on the other side by developing tensile residual stresses on the external surface. Hence, with the permissible extent of autofrettage, the effects are found to be marginal and this knowledge helps assess the safe loading limits.

High-cycle fatigue behavior of type 4340 steel pressurized blocks including mean stress effect

Mean stress effects in pressurized steel blocks were examined under constant amplitude fatigue loading. The tests were performed to provide experimental data needed to study the effect of mean stress on fatigue lives of subject specimen, and to substantiate the use of analytical expressions to account for the mean stress. The mean stress was the result of subjecting the specimens to an autofrettage pressure which induced compressive residual stresses at the crossbore intersection of the specimens. Fatigue tests were carried out under both tensile and compressive mean stress levels. Test results were compared to several mean stress accounting relationships such as the Smith-Watson Topper, Bergmann and Seeger, modified Goodman, Gerber and Soderberg. Test results indicated that the modified Goodman equation is favorable in accounting for the effect of both tensile and compressive mean stresses on fatigue life (up to a compressive mean stress to ultimate stress ratio of −0.2). The behavior under compressive mean stress to ultimate stress ratio of less than −0.2 indicated that a linear correction relationship was required.

Fatigue life prediction and verification of high-pressure hydrogen storage vessel

A fatigue life prediction method is developed for the high-pressure hydrogen storage vessel based on theoretical research and experimental verification. Firstly, the finite element model of vessel was built considering wound angle of head, thickness and number of the composite layer, then simulation was performed. The optimum range of autofrettage pressure was obtained by FEA with consideration of the DOT-CFFC and CGH2R standards. The influence of autofrettage pressure, metal liner thickness, and fiber thickness on vessel fatigue life was discussed under internal pressure cyclic load. Finally, the experimental verification was carried out. It was found that fatigue failure first occurred in middle cylinder. The experiment results agree well with theory analysis. Their average error is 6.33%.

Research on Autofrettage Mechanism in Ultra-High Pressure Thick-walled Vessel

The stress distribution of the Ultra-High Pressure (UHP) vessel is uneven, the inner layer stress value is the maximum and the outer layer stress value is the minimum, resulting in insufficient utilization of the outer layer material and reduced fatigue strength. In order to obtain the pressure when the UHP vessel bursts into failure, this paper proposes a new formula for calculating the burst pressure and uses 30 experimental sample data to verify the accuracy. The error of the calculation result is about 5% lower than the error of the traditional Faupel burst pressure formula by 15%, and is not affected by the tensile strength.The UHP vessel improves the stress distribution and improves the fatigue life using autofrettage technology, reverse yielding does not occur when the best autofrettage pressure is unloaded, and the yield strength is calculated using the burst failure criterion method. Simultaneously, the Shigley method is used to predict the fatigue life of the autofrettaged UHP vessel to prove that its fatigue life has been significantly improved.

The Application of Neutron Diffraction Techniques (NDT) in Measuring Residual Strain-Stresses of Engineering Materials

In order to quantitatively understand the residual strain distributions and benefits of engineering components following special technique treatment, such as Autofrettaging, Hot Isostatic Pressing (HIP) and Dissimilar Material Welding (DMW), the Neutron Diffraction Technique (NDT) has been employed recently to measure residual strain and stress distributions on following three cases of (a) Autofrettaged Aluminum 7075 high pressure vessels; (b) Hot Isostatic Pressed (HIPPED) heavy metals of tungsten clad in tantalum plate and (c) Dissimilar Material Welding (DMW) of 316L austenitic stainless steel and ferritic steel AS508 with Alloy 52 weld filler. This paper reports the recent research findings, including (a) NDT can identify optimal Autofrettage pressure level, by which load bearing capacity of the Autofrettaged pressure vessel increased by 215MPa; (b) NDT is able to reveal residual strains within heavy metals of Tungsten clad in Tantalum plate HIPPED and (c) NDT revealed a maximum residual tensile hoop stress value of about 494MPa in the interface between parent material SA508 and the weld seam. This is vital information for the post weld process and subsequent safe use of the dissimilar materials weld. Other researchers’ successful examples of working with NDT are also briefly reviewed. Future prospective of Engineering Materials Diffractometer (EMD) at CSNS is described too with a view to demonstrating the application and importance of NDT in revealing residual strain-stresses that are inevitable within engineering materials and engineering components following any manufacturing process.

Finalization of minimum Autofrettage pressure for steel made hydraulic cylinder used in industrial power pack applications

Autofrettage is key process in thick cylinder manufacturing, where internal residual stresses were produced by applying pressure at inner bore of cylinder. These residual stresses are also known as Locked-in stresses which will neutralize few of Hoop stresses or Maximum Von-Mises stresses in the cylinder as they are opposite in nature. So there is reduction in working stresses of cylinder due to Autofrettage. In Autofrettage it is very important to decide right pressure for hydraulic cylinder, as low pressure design leads to underutilization of material & high pressure selection leads to problems related to safety & hardening effect. To study and optimize the pressure of Hydraulic Cylinder used for Industrial Power Pack, analytical method is used & same is compared with FEM results.