Autofrettage Process Research Articles

Material Modeling for Autofrettage Stress Analysis Including the “Single Effective Material”

Analytical and numerical stress analyses of the autofrettage process have made great strides in the last few years. The major challenge is no longer the stress analysis process but the incorporation of “real” material behavior, including Bauschinger effect. This means that material properties may vary at every radial location within the tube. In this paper, it is demonstrated that finite element analysis (FEA) may be accomplished using a “user programmable feature (UPF)” within a nonlinear FEA or, more simply using an elastic modulus and Poisson’s ratio adjustment procedure (EMPRAP) within a linear-effective FEA. The results of these two methods are shown to be in agreement with each other and with an independent numerical analysis. It is further demonstrated that the numerical solutions may be obtained using a single “fictitious” material. This is called a single effective material (SEMAT). While this requires a very small number of iterations for accurate convergence, it dramatically reduces the material-modeling challenges. Furthermore, SEMAT may be implemented into an analytical procedure thereby permitting highly accurate modeling of a real material whose unloading behavior varies with radius. Comparisons indicate that this is a robust, accurate procedure.

A Study on the Tool Geometry Effects in the Swage Autofrettage Process

Swage autofrettage is one of the methods used for increasing life time of tubes and thick wall cylinders under the dynamic loading condition. In this paper swage autofrettage process of thick walled tubes are simulated using ABAQUS commercial code and effect of tool design on the residual stress distribution are investigated. For validating numerical results some experiments were performed and results of experimental and numerical methods showed good agreement with each others.

Residual Stress Measurements of Explosively Clad Cylindrical Pressure Vessels

Tantalum refractory liners were explosively clad into cylindrical pressure vessels, some of which had been previously autofrettaged. Using explosive cladding, the refractory liner formed a metallurgical bond with the steel of the pressure vessel at a cost of induced strain. Two techniques were employed to determine the residual stress state of the clad steel cylinders: neutron diffraction and mechanical slitting. Neutron diffraction is typically nondestructive; however, due to attenuation along the beam path, the cylinders had to be sectioned into rings that were nominally 25 mm thick. Slitting is a destructive method, requiring the sectioning of the cylindrical samples. Both techniques provided triaxial stress data and useful information on the effects of explosive cladding. The stress profiles in the hoop and radial directions were similar for an autofrettaged, nonclad vessel and a clad, nonautofrettaged vessel. The stress profiles in the axial direction appeared to be different. Further, the data suggested that residual stresses from the autofrettage and explosive cladding processes were not additive, in part due to evidence of reverse yielding. The residual stress data are presented, compared and discussed.

Electro-thermo-mechanical behaviors of FGPM spheres using analytical method and ANSYS software

A hollow sphere made from functionally graded piezoelectric material (FGPM) such as PZT_4 has been considered. One-dimensional analytical method for electro-thermo-mechanical response of symmetrical spheres is used. For asymmetric three-dimensional analysis, ANSYS finite element software is employed in this study. Loading is combination of internal and external pressures, a distributed temperature field due to steady state heat conduction and a constant electric potential difference between its inner and outer surfaces for analytical solution. In three-dimensional solutions closed and open spheres with different boundary conditions subjected to an internal pressure and a uniform temperature field are studied. All mechanical, thermal and piezoelectric properties except the Poisson’s ratio are assumed to be power functions of radius. It has been found from analytical solution that the induced radial and circumferential stresses of an imposed electric potential is similar to the residual stresses locked in the homogeneous sphere during the autofrettage process of these vessels. It has been concluded from the three-dimensional analysis that the magnitudes of effective stresses at all node points are higher for the clamped–clamped boundary condition and are lower for the simply–simply supported condition.

Autofrettage Pressure Analysis of Carbon-fiber Wound Composite Gas Cylinders

Despite the widening use of the carbon fiber wound composite gas cylinder, the research on its structure is mainly focused on the experiment method. In this paper, the APDL language in ANSYS was used to create a model of carbon fiber wound aluminum alloy liner composite gas cylinders with the thickness and the wound angle of both heads varied. The autofrettage process of the cylinder was analyzed and the range of autofrettage pressure value was determined by the DOT CFFC standard. It is expected that the finite element analysis results would be conducive to the engineering practice.

Influence of the pressure holding time on strain generation in fuel injection lines

An influence of the pressure holding time on residual strain generation during the autofrettage process was studied experimentally for the first time in the present work. It is the state of the art that fuel injection lines are held at the autofrettage pressure for only a few seconds in an industrial production. In doing so, it is assumed that a desirable residual stress–strain pattern is generated. However, the results of the experimental investigations outlined in this work indicated that completion of the plastic deformation caused by the autofrettage process and generation of the desirable stress–strain pattern require a much longer period. As shown, a third-order polynomial equation best described the interdependence between the time required for the completion of the process, the corresponding autofrettage pressure and the generated strain state. The method presented can be used as a tool for the determination of the optimal autofrettage process parameters in industrial production of fuel injection lines.

Type3 복합재료 압력용기의 복합재층 손상에 따른 영향성 평가를 위한 해석기법에 관한 연구

Type3 cylinder is a composite pressure vessel fully over-wrapped with carbon/epoxy composite layers over an aluminum liner, which is the most ideal and safe high pressure gas container for CNG vehicles due to the lightweight and the leakage-before-burst characteristics. During service in CNG vehicle, if a fiber cut damage occurs in outer composite layers, it can degrade structural performance, reducing cycling life from the original design life. In this study, finite element modeling and analysis technique for the composite cylinder with fiber-cut crack damage is presented. Because FE analysis of type3 cylinder is path dependant due to plastic deformation of aluminum liner in autofrettage process, method to introduce a crack into FE model affect analysis result. A crack should be introduced after autofrettage in analysis step considering real circumstances where crack occurs during usage in service. For realistic simulation of this situation, FE modeling and analysis technique introducing a crack in the middle of analysis step is presented and the results are compared with usual FE analysis which has initial crack in the model from the beginning of analysis. Proposed analysis technique can be used effectively in the evaluation of influence of damage on composite layers of type3 cylinder and establish inspection criteria of composite cylinder in service.

A theory for swaging of discs and lugs

A practical theory for swaging bored holes within plates and cylinders is proposed which can take into account work-hardening in the presence of small plastic strains based upon equivalent stress-strain data. With the appropriate choice of yield function, this theory applies to the swaging of both thin and thick plates under respective plane stress and plane strain conditions. The theory can be adapted further to the autofrettage of open and closed-ended, thick-walled cylinders where similar plane deformations conditions apply. Here swaging refers to the practice in which an oversized plug or sphere is forced into the bore thereby expanding it permanently to leave a residual circumferential compression in the bore material upon removal of the expanding tool. A similar effect results from applying an initial over-pressure to a long thick-walled cylinder in an autofrettage process. Both treatments are employed to enhance the fatigue resistance when the service loading upon the disc or cylinder amounts to a cyclic, circumferential tension within its bore. Strain gauges bonded to the entry face of the plate are used to monitor the circumferential and radial strain distributions both during and after the swaging process. Experimental results presented for swaging of thin and thin annular discs in aluminium alloy show that the measured residual strain distributions concord with the theory for large discs with a 10/1 diameter ratio. The agreement is less satisfactory with the loss in axial symmetry for parallel-sided lugs with a width to hole diameter ratio of 4/1.

Post-Autofrettage Thermal Treatment and Its Effect on Reyielding of High Strength Pressure Vessel Steels

The autofrettage process of a thick walled pressure vessel involves applying tensile plastic strain at the bore of the vessel, which reverses during unloading and results in favorable compressive residual stresses at the bore and prolongs the fatigue life of the component. In thick walled pressure vessels this process can be accomplished with either a hydraulic or mechanical overloading process. The Bauschinger effect, which is observed in many of the materials used in thick walled pressure vessels, is a phenomenon, which results in lower compressive residual stresses than those predicted with classic ideal isotropic hardening. The phenomenon is a strong function of the amount of prior tensile plastic strain. A novel idea, which involves a multiple autofrettage processes, has been proposed by the present authors. This process requires a low temperature post-autofrettage thermal treatment, which effectively returns the material to its original yield conditions with minimal effect on its residual stress state. Details of this low temperature thermal treatment are proprietary. A subsequent second autofrettage process generates a significantly lower amount of plastic strain during the tensile reloading and results in higher compressive residual stresses. This paper reports the details of the exploratory tests involving tensile and compressive loading of a test coupon, followed by a low temperature post-plastic straining thermal treatment, and subsequent reloading in tension and compression. Finally results of a full scale safe maximum pressure (SMP) test of pressure vessels are presented; these tests indicate a significant increase (11%) in SMP.

Autofrettage of layered and functionally graded metal–ceramic composite vessels

The residual compressive stresses induced by the autofrettage process in a metal vessel are limited by metal plasticity. Here we showed that the autofrettage of layered metal–ceramic composite vessels leads to considerably higher residual compressive stresses compared to the counterpart metal vessel. To calculate the residual stresses in a composite vessel, an extension of the Variable Material Properties (X-VMP) method for materials with varying elastic and plastic properties was employed. We also investigated the autofrettage of composite vessels made of functionally graded material (FGM). The significant advantage of this configuration is in avoiding the negative effects of abrupt changes in material properties in a layered vessel – and thus, inherently, in the stress and strain distributions induced by the autofrettage process. A parametric study was carried out to obtain near-optimized distribution of ceramic particles through the vessel thickness that results in maximum residual stresses in an autofrettaged functionally graded composite vessel. Selected finite element results were also presented to establish the validity of the X-VMP method.

The Beneficial Contribution of Realistic Autofrettage to the Load-Carrying Capacity of Thick-Walled Spherical Pressure Vessels

Increased strength-to-weight ratio and extended fatigue life are the main objectives in the optimal design of modern pressure vessels. These two goals can mutually be achieved by creating a proper residual stress field in the vessel’s wall by a process known as autofrettage. Although there are many studies that have investigated the autofrettage problem for cylindrical vessels, only a few of such studies exist for spherical ones. Because of the spherosymmetry of the problem, autofrettage in a spherical pressure vessel is treated as a one-dimensional problem and solved solely in terms of the radial displacement. The mathematical model is based on the idea of solving the elastoplastic autofrettage problem using the form of the elastic solution. Substituting Hooke’s equations into the equilibrium equation and using the strain-displacement relations yield a differential equation, which is a function of the plastic strains. The plastic strains are determined using the Prandtl–Reuss flow rule and the differential equation is solved by the explicit finite difference method. The existing 2D computer program, for the evaluation of hydrostatic autofrettage in a thick-walled cylinder, is adapted to handle the problem of spherical autofrettage. The presently obtained residual stress field is then validated against three existing solutions emphasizing the major role the material law plays in determining the autofrettage residual stress field. The new code is applied to a series of spherical pressure vessels yielding two major conclusions. First, the process of autofrettage increases considerably the maximum safe pressure that can be applied to the vessel. This beneficial effect can also be used to reduce the vessel’s weight rather than to increase the allowable internal pressure. Second, the specific maximum safe pressure increases as the vessel becomes thinner. The present results clearly indicate that autofrettaging of spherical pressure vessels can be very advantageous in various applications.

Autofrettage process analysis of a compound cylinder based on the elastic-perfectly plastic and strain hardening stress-strain curve

The autofrettage process enhances the carrying capacity and fatigue lifetime of pressure vessels by increasing their residual stress. A compound cylinder was introduced in order to increase residual stress. An autofrettaged compound cylinder can resist a higher pressure than a single cylinder having the same dimension. This residual stress can be measured through experimental or calculation processes. In this study, residual stress analysis of an autofrettaged compound cylinder was conducted. The elastic-perfectly plastic and strain hardening models were investigated. The residual stress distribution of the autofrettaged compound cylinder with shrink fit tolerance was predicted. Shrink fit is a very efficient way to extend compressive residual stress. The compressive residual stress of the strain-hardening model is smaller than that of the elastic-perfectly plastic model because of the Bauschinger effect. The compressive residual stress of the strain hardening model decreased by up to 80% overstrain level.

Design of Semi-composite Pressure Vessel using Fuzzy and FEM

The present study attempts to present a new method to design a semi-composite pressure vessel (known as hoop-wrapped composite cylinder) using fuzzy decision making and finite element method. A metal-composite vessel was designed based on ISO criteria and then the weight of the vessel was optimized for various fibers of carbon, glass and Kevlar in the cylindrical vessel. Failure criteria of von-Mises and Hoffman were respectively employed for the steel liner and the composite reinforcement to characterize the yielding/ buckling of the cylindrical pressure vessel. The fuzzy decision maker was used to estimate the thickness of the steel liner and the number of composite layers. The ratio of stresses on the composite fibers and the working pressure as well as the ratio of stresses on the composite fibers and the burst (failure) pressure were assessed. ANSYS nonlinear finite element solver was used to analyze the residual stress in the steel liner induced due to an auto-frettage process. Result of analysis verified that carbon fibers are the most suitable reinforcement to increase strength of cylinder while the weight stayed appreciably low.

A residual stress analysis program using a Matlab GUI on an autofrettaged compound cylinder

A program for the residual stress analysis of an autofrettaged compound cylinder is designed using a Matlab graphical user interface (GUI) and program design technique. The high-pressure vessels are autofrettaged in order to increase their operating pressure and fatigue life. An autofrettage process causes plastic expansion of the inner section of the cylinder, adding residual compressive stress to the bore after relaxation. Such a compound cylinder is produced via a shrink-fit procedure that incorporates a monobloc tube that has previously undergone autofrettage. This paper presents a simple and visual tool to calculate the residual stress and describe the distribution of residual stress for both the elastic-perfectly plastic model and the strain-hardening model.

An Analytical Framework for the Solution of Autofrettaged Tubes Under Constant Axial Strain Condition

Autofrettage is a technique for introducing beneficial residual stresses into cylinders. Both analytical and numerical methods are used for the analysis of the autofrettage process. Analytical methods have been presented only for special cases of autofrettage. In this work, an analytical framework for the solution of autofrettaged tubes with constant axial strain conditions is developed. Material behavior is assumed to be incompressible, and two different quadratic polynomials are used for strain hardening in loading and unloading. Clearly, elastic perfectly plastic and linear hardening materials are the special cases of this general model. This quadratic material model is convenient for the description of the behavior of a class of pressure vessel steels such as A723. The Bauschinger effect is assumed fixed, and the total deformation theory based on the von Mises yield criterion is used. An explicit solution for the constant axial strain conditions and its special cases such as plane strain and closed-end conditions is obtained. For an open-end condition, for which the axial force is zero, the presented analytical method leads to a simple numerical solution. Finally, results of the new method are compared with those obtained from other analytical and numerical methods, and excellent agreement is observed. Since the presented method is a general analytical method, it could be used for validation of numerical solutions or analytical solutions for special loading cases.

A Study on Hydraulic Autofrettage of Thick-Walled Cylinders Incorporating Bauschinger Effect

Pressure vessels which are subjected to cyclic external or internal high pressure are used in many fields of industry and need to be sure of reliability and safety. To ensure of reliability and safety, thick-walled cylinder, such as a cannon or nuclear reactor, is autofrettaged to induce advantageous residual stresses. The compressive residual stress which was introduced by autofrettage process acts to offset the tensile residual stress induced by internal pressure. It increases operating pressure and restrains crack initiation and crack propagation. As the autofrettage level increases, the magnitude of compressive residual stress at the bore also increases. However, the Bauschinger effect reduces the compressive residual stresses with prior tensile plastic strain, and decreases the beneficial autofrettage effect. There are some differences between theoretical solution considering elastic-perfectly material behavior and real autofrettage process results. The purpose of the present paper is to predict the accurate residual stress of SNCM 8 high strength steel using the Kendall model which was adopted by ASME Code. The tensile and uniaxial Bauschinger effect tests of SNCM 8 were performed to evaluate Bauschinger effect factor(BEF), thereafter this constant was used in calculating the residual stress. The residual stress distribution which is considering the Bauschinger effect was profiled using Kendall model, and the results were compared with the analytical and Finite Element analysis. The results were found that residual stress incorporating the Bauschinger effect at bore was smaller than ideal calculation. These results should be considered in designing pressure vessels.

자긴가공된 이중후육실린더의 피로수명에 관한 연구

Thick-walled cylinder with high pressure have had wide application in the armament industry. In the thick-walled cylinder, fatigue crack is generated at inner radius and developed toward the outer radius. To prevent generation of fatigue crack, the autofrettage process had been used. The compressive residual stress induced by the autofrettage process extends loading pressure and fatigue life of the thick-walled cylinder. In this study, the residual stress of single and compound cylinder by the autofrettage process was evaluated. The analytical compressive residual stress of single cylinder was good agreement with experimental result at inner radius. The analysis on the residual stress of compound cylinder was conducted. The compressive residual stress at inner radius was increased with the overstrain level. And fatigue life of the compound cylinder with initial crack was evaluated. The considered initial crack shape was straight and semi-elliptical. The fatigue life was extended with the overstrain level. The fatigue life of the compound cylinder with semi-elliptical crack was longer than straight crack. The suitable way to extend fatigue life of the compound cylinder was proposed.

Fracture analysis of hydrogen storage composite cylinders with liner crack accounting for autofrettage effect

Abstract An understanding of fracture behavior is crucial to the safe installation and operation of high-pressure composite cylinders for hydrogen storage. This work has developed a comprehensive finite element model to investigate axial surface flaws in cylinder liners using the fracture mechanics and a global–local finite element technique. Since the autofrettage process has a strong influence on cylinder fracture behavior, it is also considered in this analysis. The simulation process is broken down into three steps in order to precisely extract fracture parameters and incorporate the autofrettage effect. In the first step, the global model performs the autofrettage simulation to study the residual stress with consideration of both material hardening and the Bauschinger effect. In the second step, the global model uses residual stress to compute displacement for the local model. Finally, in the third step, the local model extracts the values of stress intensity factor and J -integral. Comparison is conducted on the fracture parameters with various autofrettage levels and crack shapes. The vicinity of the crack front is also studied by the size and shape of the plastic zone, and the validity of stress intensity factor and J -integral dominances is examined.

Residual stress analysis of an autofrettaged compound cylinder under machining process

AbstractThe autofrettage process is used to manufacture pressure vessels and cylinders that can withstand high internal pressure. A multi‐layered cylinder was assembled by a shrink fit. Autofrettaged compound cylinder can resist higher internal pressure than a thick cylinder having the same dimension and extend its life time. Inner and outer surfaces of the autofrettaged compound cylinder have to be manufactured into exact dimensions. The distribution of residual stress can change after the machining process. The machining procedure of inner and outer surfaces also affects the distribution of residual stress as a function of the machining procedure. In this study, the distribution of residual stresses of an autofrettaged compound cylinder as machining procedure was investigated using analytical and numerical analyses.

Analysis of the autofrettage effect in improving the fatigue resistance of automotive CNG storage vessels

Type II compressed natural gas (CNG) storage vessels for automobiles are becoming widely used. They are not only supplied to automakers in Korea (such as Hyundai Motors), but are being exported overseas in increasing numbers. Autofrettage is a process that produces beneficial residual stresses in a vessel by subjecting it to excessive internal pressure. This strengthens the vessel and improves its fatigue resistance. This paper presents research into the autofrettage process and the residual stresses it produces in type II CNG storage vessels. Finite element analysis and a closed-form equation are used. Fatigue resistance is then analyzed via a fatigue evaluation performed according to ASME section VIII.